Chapter 7 - Membrane Structure and Function

What happens to a cell placed in a hypertonic solution? Describe the free water concentration inside and out.

(7.3) There will be a net diffusion of water out of a cell into a hypertonic solution. The free water concentration is higher inside the cell than in the solution (where water molecules are not free, but are clustered around the higher concentration of solute particles).

How are phospholipids and proteins arranged in the membranes of cells?

In the FLUID MOSAIC MODEL, where the membrane is a fluid structure with a "mosaic" of various proteins embedded in or attached to a double layer (bilayer) of phospholipids.

If HIV first enters the cell in an endocytotic vesicle, instead of directly fusing with the plasma membrane, then A) HIV infection should be hindered by microtubule polymerization inhibitors such as nocodazole. B) HIV infection should be more efficient at lower temperatures. C) intact cortical actin microfilaments should interfere with HIV infection. D) cells lacking integrins should be resistant to HIV infection. E) addition of ligands for other cell-surface receptors to stimulate their endocytosis should increase the efficiency of HIV infection.

A

What is a phospholipid?

an amphipathic molecule, meaning it has both a hydrophilic region and a hydrophobic region

What holds some membrane proteins in place on the extracellular side?

attachment to fibers of the extracellular matrix (ECM)

How do aquaporins affect the permeability of a membrane?

(7.2) Aquaporins are channel proteins that greatly increase the permeability of a membrane to water molecules, which are polar and therefore do not readily diffuse through the hydrophobic interior of the membrane.

ATP is not directly involved in the functioning of a cotransporter. Why, then, is cotransport considered active transport?

(7.4) One of the solutes moved by the cotransporter is actively transported against its concentration gradient (sucrose, in my example). The energy for this transport comes from the concentration gradient of the other solute, which was established by an electrogenic pump (proton pump) that used energy to transport the other solute across the membrane.

What happens when a cell is immersed in an environment that is hypertonic?

(Hyper means "more" in this case referring to nonpenetrating solutes). The cell will lose water, shrivel, and probably die. This is one way an increase in the salinity (saltiness) of a lake can kill animals there; if the lake water becomes hypertonic to the animals cells, the cells might shrivel and die.

What are the two major populations of membrane proteins?

1) Integral proteins and 2) Peripheral protiens

Fig 7.18 - Describe the steps that occur in the operation of the sodium-potassium pump, which is an example of active transport.

1. Cytoplasmic Na+ binds to the sodium-potassium pump. The affinity for Na+ is high when the protein has this shape. 2. Na+ binding stimulates phosphorylation (the transfer of a phosphate group to the transport protein) by ATP. 3. Phosphorylation leads to a change in protein shape, reducing its affinity for Na+, which is released outside. 4. The new shape has a high affinity for K+, which binds on the extracellular side and triggers release of the phosphate group. 5. Loss of the phosphate group restores the protein's original shape, which has a lower affinity for K+. 6. K+ is released; affinity for Na+ is high again, and the cycle repeats.

Familial hypercholesterolemia is characterized by which of the following? A) defective LDL receptors on the cell membranes B) poor attachment of the cholesterol to the extracellular matrix of cells C) a poorly formed lipid bilayer that cannot incorporate cholesterol into cell membranes D) inhibition of the cholesterol active transport system in red blood cells E) a general lack of glycolipids in the blood cell membranes

A

In most cells, there are electrochemical gradients of many ions across the plasma membrane even though there are usually only one or two electrogenic pumps present in the membrane. The gradients of the other ions are most likely accounted for by A) cotransport proteins. B) ion channels. C) carrier proteins. D) passive diffusion across the plasma membrane. E) cellular metabolic reactions that create or destroy ions.

A

The sodium-potassium pump in animal cells requires cytoplasmic ATP to pump ions across the plasma membrane. When the proteins of the pump are first synthesized in the rough ER, what side of the ER membrane will the ATP binding site be on? A) It will be on the cytoplasmic side of the ER. B) It will be on the side facing the interior of the ER. C) It could be facing in either direction because proteins are properly reoriented in the Golgi apparatus. D) It doesn't matter, because the pump is not active in the ER.

A

Which of the following factors would tend to increase membrane fluidity? A) a greater proportion of unsaturated phospholipids B) a greater proportion of saturated phospholipids C) a lower temperature D) a relatively high protein content in the membrane E) a greater proportion of relatively large glycolipids compared with lipids having smaller molecular masses

A

How does membrane structure result in selective permeability?

A cell must exchange molecules and ions with its surroundings, a process controlled by the selective permeability of the plasma membrane. Hydrophobic substances are soluble in lipids and pass through membranes rapidly, whereas polar molecules and ions generally require specific transport proteins to cross the membrane.

What is selective permeability?

Allowing some substances to cross more easily than others

How do some membrane proteins that actively transport ions contribute to the membrane potential?

An example is the sodium potassium pump. Notice in Figure 7.18 that the pump does not translocate Na+ and K+ one for one, but pumps three sodium ions out of the cell for every two potassium ions it pumps into the cell. With each "crank" of the pump, there is a net transfer of one positive charge from the cytoplasm to the extracellular fluid, a process that stores energy as voltage.

What is the energy source that supplies the energy for most active transport?

As in other types of cellular work, ATP supplies the energy for most active transport. One way ATP can power active transport is by transferring its terminal phosphate group directly to the transport protein. This can induce the protein to change its shape in a manner that translocates a solute bound to the protein across the membrane. One transport system that works this way is the sodium-potassium pump, which exchanges Na+ for K+ across the plasma membrane of animal cells.

A bacterium engulfed by a white blood cell through phagocytosis will be digested by enzymes contained in A) peroxisomes. B) lysosomes. C) Golgi vesicles. D) vacuoles. E) secretory vesicles.

B

In an HIV-infected cell producing HIV virus particles, the viral glycoprotein is expressed on the plasma membrane. How do the viral glycoproteins get to the plasma membrane? A) They are synthesized on ribosomes on the plasma membrane. B) They are synthesized by ribosomes in the rough ER, and arrive at the plasma membrane in the membrane of secretory vesicles. C) They are synthesized on free cytoplasmic ribosomes, and then inserted into the plasma membrane. D) They are synthesized by ribosomes in the rough ER, secreted from the cell, and inserted into the plasma membrane from the outside. E) They are synthesized by ribosomes on the HIV viral membrane, which fuses with the plasma membrane from inside the cell.

B

In what way do the membranes of a eukaryotic cell vary? A) Phospholipids are found only in certain membranes. B) Certain proteins are unique to each membrane. C) Only certain membranes of the cell are selectively permeable. D) Only certain membranes are constructed from amphipathic molecules. E) Some membranes have hydrophobic surfaces exposed to the cytoplasm, while others have hydrophilic surfaces facing the cytoplasm.

B

Proton pumps are used in various ways by members of every domain of organisms: Bacteria, Archaea, and Eukarya. What does this most probably mean? A) Proton pumps must have evolved before any living organisms were present on Earth. B) Proton gradients across a membrane were used by cells that were the common ancestor of all three domains of life. C) The high concentration of protons in the ancient atmosphere must have necessitated a pump mechanism. D) Cells of each domain evolved proton pumps independently when oceans became more acidic. E) Proton pumps are necessary to all cell membranes.

B

Using live-cell fluorescence microscopy, researchers observed that a red fluorescent spot moved from the plasma membrane into the interior of target cells when red fluorescent dye-labeled HIV was added to the cells. What is the best conclusion from these observations? A) The hypothesis that HIV enters the cell via fusion with the target cell plasma membrane is proved. B) The hypothesis that HIV enters the cell via fusion with the target cell plasma membrane is not supported. C) The hypothesis that HIV enters the cell via endocytosis is proved. D) The hypothesis that HIV enters the cell via endocytosis is not supported. E) Neither hypothesis is supported by these results.

B

What would be observed by live-cell fluorescence microscopy if the red fluorescent lipid dye-labeled HIV membrane fuses with the target cell plasma membrane? A) A spot of red fluorescence will remain on the infected cell's plasma membrane, marking the site of membrane fusion and HIV entry. B) The red fluorescent dye-labeled lipids will diffuse in the infected cell's plasma membrane and become difficult to detect. C) A spot of red fluorescence will move into the infected cell's cytoplasm. D) A spot of red fluorescence will remain outside the cell after delivering the viral capsid. E) Fluorescence microscopy does not have enough resolution to visualize fluorescently labeled HIV virus particles.

B

White blood cells engulf bacteria through what process? A) exocytosis B) phagocytosis C) pinocytosis D) osmosis E) receptor-mediated exocytosis

B

A patient has had a serious accident and lost a lot of blood. In an attempt to replenish body fluids, distilled water-equal to the volume of blood lost-is transferred directly into one of his veins. What will be the most probable result of this transfusion? A) It will have no unfavorable effect as long as the water is free of viruses and bacteria. B) The patient's red blood cells will shrivel up because the blood fluid has become hypotonic compared to the cells. C) The patient's red blood cells will swell because the blood fluid has become hypotonic compared to the cells. D) The patient's red blood cells will shrivel up because the blood fluid has become hypertonic compared to the cells. E) The patient's red blood cells will burst because the blood fluid has become hypertonic compared to the cells.

C

According to the fluid mosaic model of membrane structure, proteins of the membrane are mostly A) spread in a continuous layer over the inner and outer surfaces of the membrane. B) confined to the hydrophobic interior of the membrane. C) embedded in a lipid bilayer. D) randomly oriented in the membrane, with no fixed inside-outside polarity. E) free to depart from the fluid membrane and dissolve in the surrounding solution.

C

In a paramecium, cell surface integral membrane proteins are synthesized A) in the cytoplasm by free ribosomes. B) by ribosomes in the nucleus. C) by ribosomes bound to the rough endoplasmic reticulum. D) by ribosomes in the Golgi vesicles. E) by ribosomes bound to the inner surface of the plasma membrane.

C

In receptor-mediated endocytosis, receptor molecules initially project to the outside of the cell. Where do they end up after endocytosis? A) on the outside of vesicles B) on the inside surface of the cell membrane C) on the inside surface of the vesicle D) on the outer surface of the nucleus E) on the ER

C

In the small airways of the lung, a thin layer of liquid is needed between the epithelial cells and the mucus layer in order for cilia to beat and move the mucus and trapped particles out of the lung. One hypothesis is that the volume of this airway surface liquid is regulated osmotically by transport of sodium and chloride ions across the epithelial cell membrane. How would the lack of a functional chloride channel in cystic fibrosis patients affect sodium ion transport and the volume of the airway surface liquid? A) Sodium ion transport will increase; higher osmotic potential will increase airway surface liquid volume. B) Sodium ion transport will increase; higher osmotic potential will decrease airway surface liquid volume. C) Sodium ion transport will decrease; lower osmotic potential will decrease airway surface liquid volume. D) Sodium ion transport will decrease; lower osmotic potential will increase the airway surface liquid volume. E) Sodium ion transport will be unaffected; lack of chloride transport still reduces osmotic potential and decreases the airway surface liquid volume.

C

The CFTR protein belongs to what category of membrane proteins? A) gap junctions B) aquaporins C) electrogenic ion pumps D) cotransporters E) hydrophilic channels

C

The difference between pinocytosis and receptor-mediated endocytosis is that A) pinocytosis brings only water molecules into the cell, but receptor-mediated endocytosis brings in other molecules as well. B) pinocytosis increases the surface area of the plasma membrane whereas receptor-mediated endocytosis decreases the plasma membrane surface area. C) pinocytosis is nonselective in the molecules it brings into the cell, whereas receptor-mediated endocytosis offers more selectivity. D) pinocytosis requires cellular energy, but receptor-mediated endocytosis does not. E) pinocytosis can concentrate substances from the extracellular fluid, but receptor-mediated endocytosis cannot.

C

The movement of potassium into an animal cell requires A) low cellular concentrations of sodium. B) high cellular concentrations of potassium. C) an energy source such as ATP. D) a cotransport protein. E) a potassium channel protein.

C

The sodium-potassium pump is called an electrogenic pump because it A) pumps equal quantities of Na+ and K+ across the membrane. B) pumps hydrogen ions out of the cell. C) contributes to the membrane potential. D) ionizes sodium and potassium atoms. E) is used to drive the transport of other molecules against a concentration gradient.

C

What is the voltage across a membrane called? A) water potential B) chemical gradient C) membrane potential D) osmotic potential E) electrochemical gradient

C

What would be observed by live-cell fluorescence microscopy if HIV is endocytosed first, and then fuses with the endocytotic vesicle membrane? A) A spot of red fluorescence will remain on the infected cell's plasma membrane, marking the site of membrane fusion and HIV entry. B) The red fluorescent dye-labeled lipids will diffuse in the endocytotic vesicle membrane and become difficult to detect. C) A spot of red fluorescence will move into the infected cell's interior. D) A spot of red fluorescence will remain outside the cell after delivering the viral capsid. E) Fluorescence microscopy does not have enough resolution to visualize fluorescently labeled HIV virus particles.

C

Which of the following would increase the electrochemical potential across a membrane? A) a chloride channel B) a sucrose-proton cotransporter C) a proton pump D) a potassium channel E) both a proton pump and a potassium channel

C

How do you think a cell performing cellular respiration rids itself of the resulting CO2?

CO2 is a nonpolar molecule that can diffuse through the plasma membrane. As long as it diffuses away so that the concentration remains low \outside the cell, it will continue to exit the cell in this way. (This is the opposite of the case for O2 - which is a polar molecule, therefore will need the help of transport proteins to diffuse across the membrane - this is called facilitated diffusion.

Why do carrier proteins undergo a subtle change in shape?

Carrier proteins, such as the glucose transporter mentioned earlier, seem to undergo a subtle change in shape that somehow translocates the solute-binding site across the membrane. Such a change in shape may be triggered by the binding and release of the transported molecule. Like ion channels, carrier proteins involved in facilitated diffusion result in the net movement of a substance down its concentration gradient. No energy is thus required: This is passive transport.

What makes a carrier protein different compared to a channel protein

Carrier proteins, unlike channel proteins, can undergo changes in shape that translocate bound solutes across the membrane.

How do cells without walls (animals and some protists) deal with the environment?

Cells lacking walls are isotonic with their environments or have adaptations for osmoregulation. Plants, prokaryotes, fungi, and some protists have relatively inelastic cell walls, so the cells don't burst in a hypotonic environment.

What is the function of the "channel proteins"?

Channel proteins simply provide corridors that allow specific molecules or ions to cross the membrane. The hydrophilic pathways provided by these proteins can allow water molecules or small ions to diffuse very quickly from one side of the membrane to the other. Aquaporins, the water channel proteins, facilitate the massive amounts of diffusion that occur in plant cells and in animal cells such as red blood cells. Certain kidney cells also have a high number of aquaporins, allowing them to reclaim water from urine before it is excreted. If the kidneys did not perform this function, you would excrete about 180 L of urine per day - and have to drink an equal volume of water!

When does cotransport occur?

Cotransport of two solutes occurs when a membrane protein enables the "downhill" diffusion of one solute to drive the "uphill" transport of the other. An example, that I remember, is where hydrogen ions would act as the "downhill" diffusion of one of the solutes. The H+ ions would work together with sucrose molecules (the "uphill" transport, moving it from a lower concentration to a higher concentration of sugar inside the cell).

An organism with a cell wall would most likely be unable to take in materials through A) diffusion. B) osmosis. C) active transport. D) phagocytosis. E) facilitated diffusion.

D

Ions diffuse across membranes through specific ion channels A) down their chemical gradients. B) down their concentration gradients. C) down the electrical gradients. D) down their electrochemical gradients. E) down the osmotic potential gradients.

D

Which of the following processes includes all others? A) osmosis B) diffusion of a solute across a membrane C) facilitated diffusion D) passive transport E) transport of an ion down its electrochemical gradient

D

You are working on a team that is designing a new drug. In order for this drug to work, it must enter the cytoplasm of specific target cells. Which of the following would be a factor that determines whether the molecule selectively enters the target cells? A) blood or tissue type of the patient B) hydrophobicity of the drug molecule C) lack of charge on the drug molecule D) similarity of the drug molecule to other molecules transported by the target cells E) lipid composition of the target cells' plasma membrane

D

Why is facilitated diffusion considered passive transport?

Despite the help of transport proteins, facilitated diffusion is considered passive transport because the solute is moving down its concentration gradient, a process that requires no energy. Facilitated diffusion speeds transport of a solute by providing efficient passage through the membrane, but it does not alter the direction of transport. Some transport proteins, however, can move solutes against their concentration gradients, across the membrane from the side where they are less concentrated (whether inside or outside) to the side where they are more concentrated.

Several epidemic microbial diseases of earlier centuries incurred high death rates because they resulted in severe dehydration due to vomiting and diarrhea. Today they are usually not fatal because we have developed which of the following? A) antiviral medications that are efficient and work well with all viruses B) antibiotics against the viruses in question C) intravenous feeding techniques D) medication to prevent blood loss E) hydrating drinks that include high concentrations of salts and glucose

E

Which of the following is most likely true of a protein that cotransports glucose and sodium ions into the intestinal cells of an animal? A) The sodium ions are moving down their electrochemical gradient while glucose is moving up. B) Glucose entering the cell along its concentration gradient provides energy for uptake of sodium ions against the electrochemical gradient. C) Sodium ions can move down their electrochemical gradient through the cotransporter whether or not glucose is present outside the cell. D) The cotransporter can also transport potassium ions. E) A substance that blocks sodium ions from binding to the cotransport protein will also block the transport of glucose.

E

What are electrogenic pumps?

Electrogenic pumps, such as the sodium-potassium pump and proton pumps, are transport proteins that contribute to electrochemical gradients.

What is facilitated diffusion?

Facilitated diffusion is the passage of molecules or ions down their electrochemical gradient across a biological membrane with the assistance of specific transmembrane transport, requiring no energy expenditure.

Give me an example of a cotransporter in action.

For example, a plant cell uses the gradient of H+ generated by its proton pumps to drive the active transport of amino acids, sugars, and several other nutrients into the cell. One transport protein couples the return of H+ to the transport of sucrose into the cell. (Figure 7.21) This protein can translocate sucrose into the cell against its concentration gradient (more sugar inside the cell than outside), but only if the sucrose molecule travels in the company of a hydrogen ion. The hydrogen ion uses the transport protein as an avenue to diffuse down the electrochemical gradient maintained by the proton pump. Plants use sucrose-H+ cotransport to load sucrose produced by photosynthesis into cells in the veins of leaves. The vascular tissue of the plant can then distribute the sugar to nonphotosynthetic organs, such as roots.

How is endocytosis related to cholesterol in humans?

Human cells use receptor-mediated endocytosis to take in cholesterol for membrane synthesis and the synthesis of other steroids. Cholesterol travels in the blood in particles called low-density lipoproteins (LDLs), each a complex of lipids and a protein. LDLs bind to LDL receptors on plasma membranes and then enter the cells by endocytosis. (LDL's thus act as ligands, a term for any molecule that binds specifically to a receptor site on another molecule). In humans with familial hypercholesterolemia, an inherited disease characterized by a very high level of cholesterol in the blood, LDLs cannot enter cells because the LDL receptor proteins are defective or missing. Consequently, cholesterol accumulates in the blood, where it contributes to early atherosclerosis, the buildup of lipid deposits within the walls of blood vessels. This buildup causes the walls to bulge inward, thereby narrowing the vessels and impeding blood flow.

Sodium-potassium pumps help nerve cells establish a voltage across their plasma membranes. Do these use ATP or produce ATP? Explain.

I believe these pumps use ATP in a process called phosphorylation (where a phosphate that is broken off from the ATP molecule is used by the pump in order to change its confirmation and allow for transport of hydrogen ions across the membrane, from the cytoplasmic fluid to the extracellular fluid.) The pump uses ATP. To establish a voltage, ions have to be pumped against their gradients, which requires energy.

What happens when a cell is immersed in an environment that is isotonic?

If a cell without a wall, such as an animal cell, is immersed in an environment that is isotonic to the cell (iso means "same") There will be no net movement of water across the plasma membrane. Water diffuses across the membrane, but at the same rate in both directions. In an isotonic environment, the volume of an animal cell is stable.

What happens when a cell is immersed in an environment that is hypotonic? (hypo means "less")

If we place an animal cell in a solution that is hypotonic, water will enter the cell faster than it leaves, and the cell will swell and lyse (burst) like an overfilled water balloon.

Give an example of diffusion.

Imagine a synthetic membrane separating pure water from a solution of a dye in water. Diffusion would result in both solutions having equal concentrations of the dye molecules. Once that point is reached, there will be a dynamic equilibrium, with as many dye molecules crossing the membrane each second in one direction as in the other.

How does endocytosis work?

In endocytosis, the cell takes in biological molecules and particulate matter by forming new vesicles from the plasma membrane. Although the proteins involved in the processes are different, the events of endocytosis look like the reverse of exocytosis. A small area of the plasma membrane sinks inward to form a pocket. As the pocket deepens, it pinches in, forming a vesicle containing material that had been outside the cell.

What is exocytosis and endocytosis?

In exocytosis, transport vesicles migrate to the plasma membrane, fuse with it, and release their contents. In endocytosis, molecules enter cells within vesicles that pinch inward from the plasma membrane. The three types of endocytosis are phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Describe what occurs in phagocytosis.

In phagocytosis, a cell engulfs a particle by wrapping pseudopodia around it and packaging it within a membranous sac called a food vacuole. The particle will be digested after the food vacuole fuses with a lysosome containing hydrolytic enzymes.

Describe what occurs in pinocytosis.

In pinocytosis, the cell "gulps" droplets of extracellular fluid into tiny vesicles. It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplets. Because any and all included solutes are taken into the cell, pinocytosis is nonspecific in the substances it transports.

Which type of endocytosis involves ligands? What does this type of transport enable a cell to do?

In receptor-mediated endocytosis, specific molecules act as ligands when they bind to receptors on the plasma membrane. The cell can acquire bulk quantities of those molecules when a coated pit forms a vesicle and carries the bound molecules into the cell.

State one of the rules of diffusion.

In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated. Put another way, any substance will diffuse down its concentration gradient, the region along which the density of a chemical substance increases or decrease (in this case, decreases). No work must be done to make this happen; diffusion is a spontaneous process, needing no input of energy. Note that each substance diffuses down its own concentration gradient, unaffected by the concentration gradients of other substances.

How does the concept of passive transport change in the case of ions.

In the case of ions, we must refine our concept of passive transport: An ion diffuses not simply down its concentration gradient but, more exactly, down its electrochemical gradient. For example, the concentration of Na+ inside a resting nerve cell is much lower than outside it. When the cell is stimulated, gated channels open that facilitate Na+ diffusion. Sodium ions then "fall" down their electrochemical gradient, driven by the concentration gradient of Na+ and by the attraction of these cations to the negative side (inside) of the membrane. In this example, both electrical and chemical contributions to the electrochemical gradient act in the same direction across the membrane, but this is not always so. In cases where electrical forces due to the membrane potential oppose the simple diffusion of an ion down its concentration gradient, active transport may be necessary. In Chapter 48, you will learn about the importance of electrochemical gradients and membrane potentials in the transmission of nerve impulses.

Is there any advantage for a plant cell that is immersed in a hypertonic environment?

In this case, there is no advantage. A plant cell like an animal cell, will lose water to its surroundings and shrink. As the plant cell shrivels, its plasma membrane pulls away from the cell. This phenomenon, called plasmolysis, causes the plant to wilt and can lead to plant death. The walled cells of bacteria and fungi also plasmolyze in hypertonic environments.

Where are integral and peripheral proteins located on the plasma membrane?

Integral proteins are embedded in the lipid bilayer; peripheral proteins are attached to the membrane surface.

What are ion channels?

Ion channels are channel proteins that transport ions across the membrane. Many ion channels function as as gated channels, which open or close in response to a stimulus. For some gated channels, the stimulus is electrical. Other gated channels open or close when a specific substance other than the one to be transported binds to the channel. Both types of gated channels are important in the functioning of the nervous system.

What is the electrochemical gradient?

Ions can have both a concentration (chemical) gradient and an electrical gradient (voltage). These gradients combine in the electrochemical gradient, which determines the net direction of ionic diffusion.

What is cotransport?

It is a single ATP-powered pump that transports a specific solute that can indirectly drive the active transport of several other solutes in a mechanism. The substance that has been pumped across a membrane can do work as it moves across the membrane by diffusion, analogous to water that has been pumped uphill and performs work as it flows back down. Another transport protein, a cotransporter separate from the pump, can couple the "downhill" diffusion of this substance to the "uphill" transport of a second substance against its own concentration gradient (or electrochemical) gradient.

What is an electrogenic pump?

It is a transport protein that generates voltage across a membrane. The sodium-potassium pump appears to be the major electrogenic pump of animal cells. The main electrogenic pump of plants, fungi, and bacteria is a proton pump, which actively transports protons (hydrogen ions, H+) out of the cell. The pumping of H+ transfers positive charge from the cytoplasm to the extracellular solution. By generating voltage across membranes, electrogenic pumps help store energy that can be tapped for cellular work. One important use of proton gradients in the cell is for ATP synthesis during cellular respiration (CR).

What is facilitated diffusion?

It is a type of passive transport, where a transport protein speeds the movement of water or a solute across a membrane down its concentration gradient. Ion channels, some of which are gated channels, facilitate the diffusion of ions across a membrane.

What is the diffusion of a substance across a biological membrane called?

It is called passive transport because the cell does not have to expend energy to make it happen. The concentration gradient itself represents potential energy and drives diffusion. Remember however, that membranes are selectively permeable and therefore have different effects on the rates of diffusion of various molecules. In the case of water, aquaporins allow water to diffuse very rapidly across the membranes of certain cells.

What is passive transport?

It is diffusion of a substance across a membrane with no energy investment.

What is tonicity?

It is the ability of a surrounding solution to cause a cell to gain or lose water. The tonicity of a solution depends in part on its concentration of solutes that cannot cross the membrane (nonpenetrating solutes) relative to that inside the cell. If there is a higher concentration of nonpenetrating solutes in the surrounding solution, water will tend to leave the cell, (due to osmosis - which is the movement of water from an area of higher to lower free water concentration.

What is atherosclerosis?

It is the buildup of lipid deposits within the walls of blood vessels. This buildup causes the walls of the vessels to bulge inward, thereby narrowing the vessels and impeding blood flow. What are the therapies nowadays? I would solve this problem by either gene therapy, where I could fix broken/defective receptors on cells, in order to once again let the LDL's enter the cells in order to prevent this condition. I guess another way to fix this is to cut an individual open and go in using surgical equipment and clearing out the blood vessels (this must be very difficult). Perhaps there is a solution similar to (Drano) that we could intake in order to clear out our pipes/blood vessels.

What is the electrochemical gradient?

It is the diffusion gradient of an ion, which is affected by both the concentration difference of an ion across a membrane (a chemical force) and the ion's tendency to move relative to the membrane potential (an electrical force).

What is diffusion?

It is the movement of molecules of any substance so that they spread out evenly into the available space.

What is osmosis?

It is the movement of water from an area of higher to lower free water concentration. It is the diffusion of free water across a selectively permeable membrane. The movement of water across cell membranes and the balance of water between the cell and its environment are crucial to organisms.

What is diffusion?

It is the spontaneous movement of a substance down its concentration gradient. Water diffuses out through the permeable membrane of a cell (osmosis) if the solution outside has a higher solute concentration (hypertonic) than the cytosol. Water enters the cell if the solution has a lower solute concentration (hypotonic). If the concentrations are equal (isotonic), no net osmosis occurs. Cell survival depends on the balancing water uptake and loss.

What does it mean for a molecule to be "diffusing down a concentration gradient?"

It means that the molecule is diffusing from where it is more concentrated to where it is less concentrated. This diffusion of molecules down its concentration gradient leads to a dynamic equilibrium. The solute molecules continue to cross the membrane, but at equal rates in both directions.

How are plant cells like animal cells in dealing with water?

Like an animal cell, the plant cell swells as water enters by osmosis. However, the relatively inelastic wall will expand only so much before it exerts a back pressure on the cell. At this point, the cell is turgid (very firm), which is the healthiest state for most plant cells. Plants that are not woody, such as most houseplants, depend for mechanical support on cells kept turgid by a surrounding hypotonic solution. If a plant's cells and their surroundings are isotonic, there is no net tendency for water to enter, and the cells become flaccid (limp).

Where are membrane proteins and lipids synthesized?

Membrane proteins and lipids are synthesized in the ER and modified in the ER and Golgi apparatus. The inside and outside faces of membranes differ in molecular composition.

What type of solution are most terrestrial (land dwelling) animals bathed in?

Most terrestrial (land-dwelling) animals are bathed in an extracellular fluid that is isotonic to their cells. In hypertonic or hypotonic environments, however, organisms that lack rigid cell walls must have other adaptations for osmoregulation, which is the control of solute concentrations and water balance.

How has our knowledge about cotransport proteins in animal cells helped us to find more effective treatments for diarrhea, a serious problem in developing countries?

Normally, sodium in waste is reabsorbed in the colon, maintaining, constant levels in the body, but diarrhea expels waste so rapidly that reabsorption is not possible, and sodium levels fall precipitously. To treat this life-threatening condition, patients are give a solution to drink containing high concentrations of salt (NaCI) and glucose. The solutes are taken up by the sodium-glucose cotransporters on the surface of intestinal cells and passed through the cells into the blood. This simple treatment has lowered infant mortality worldwide. (Fking amazing wow - simple knowledge - profound change in the world).

How do missing or defective transport systems affect individuals with inherited disease.

One example of how missing/defective specific transport systems affect inherited disease is in the case of cystinuria, a human disease characterized by the absence of a carrier protein that transport cysteine and some other amino acids across the membranes of kidney cells. Kidney cells normally reabsorb these amino acids from the urine and return them to the blood, but an individual afflicted with cystinuria develops painful stones from amino acids that that accumulate and crystallize in the kidneys. I wonder if it possible to go into the genome and identify the gene that codes for carrier protein, and check if it is constantly function correctly. Is it possible to complete gene therapy on an individual with with this inherited disease using retroviruses, that will permanently install the "normal" gene?

Give an example of diffusion

One important example is the uptake of oxygen by a cell performing cellular respiration. Dissolved oxygen diffuses into the cell across the plasma membrane. As long as cellular respiration consumes the O2 as it enters, diffusion into the cell will continue because the concentration gradient favors movement in that direction.

Select the correct statement about osmosis.

Osmotic equilibrium cannot be reached unless solute concentrations equalize across the membrane. If a dead cell is placed in a solution hypotonic to the cell contents, osmosis will not occur. *Osmosis is the diffusion of water molecules across a selectively permeable membrane

How does the unicellular protist Paramecium caudatum deal with living in an environment that is hypotonic to itself? (This means that it has the issue of taking into too much water - which possibly means it will be lysed)

P. caudatum has a plasma membrane that is less permeable to water than the membranes of most other cells, but this only slows the uptake of water, which continually enters the cell. ( I believe the P. caudatum must have a decreased amount of aquaporins if any at all) The P.caudatum cell doesn't burst because it is also equipped with a contractile vacuole, an organelle that functions as a bilge pump to force water out of the cell as fast as it enters by osmosis. (Well that is a damn handy mechanism - Amazing how nature has forced life to adapt.)

Where do we find integral proteins?

Penetrated in the the hydrophobic interior of the lipid bilayer (transmembrane proteins/ span the membrane) or extended only partway into the hydrophobic interior. (ex: integrins)

What type of environment are plant cells generally healthiest?

Plant cells are turgid (firm) and generally healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the wall pushing back on the cell. This is considered normal.

_______ determine most of the membrane's functions.

Proteins

____ also have hydrophobic and hydrophilic regions as well.

Proteins within membranes

What is a proton pump?

Proton pumps are electrogenic pumps that store energy by generating voltage (charge separation) across membranes. A proton pump translocates positive charge in the form of hydrogen ions. The voltage and H+ concentration gradient represent a dual energy source that can drive other processes, such as the uptake of nutrients. Most proton pumps are powered by ATP.

Describe what occurs in receptor-mediated endocytosis.

Receptor-mediated endocytosis enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid, to which specific substances (ligands) bind. The receptor proteins then cluster in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins. Next, each coated pit forms a vesicle containing the ligand molecules. Notice that there are relatively more bound molecules (purple) inside the vesicle, but other molecules (green) are also present. After the ingested material is liberated from the vesicle, the emptied receptors are recycled to the plasma membrane by the same vesicle.

Define isotonic

Referring to a solution that, when surrounding a cell, causes no net movement of water into or out of the cell, because there is a constant equilibrium.

Define hypertonic

Referring to a solution that, when surrounding a cell, will cause the cell to lose water. (This solution has more solutes than the cell.)

What form does the energy come in order to begin the process of active transport?

Specific membrane proteins use energy, usually in the form of ATP, to do the work of active transport. The sodium-potassium pump is an example.

What model replaced the Davson-Daniellie sandwich model of the membrane?

The Davson-Danielli sandwich model of the membrane has been replaced by the fluid mosaic model, in which amphipathic (containing both hydrophobic and hydrophilic sections) proteins are embedded in the phospholipid bilayer. Proteins with related functions often cluster in patches.

How does the cell secrete certain biological molecules?

The cell secretes certain biological molecules by the fusion of vesicles with the plasma membrane; this process is called exocytosis. A transport vesicle that has budded from the Golgi apparatus moves along microtubules of the cytoskeleton to the plasma membrane. When the vesicle membrane and plasma membrane come into contact, specific proteins rearrange the lipid molecules of the two bilayers so that the two membranes fuse. The contents of the vesicle then spill to the outside of the cell, and the vesicle membrane becomes part of the plasma membrane. Many secretory cells use exocytosis to export products. For example, the cells in the pancreas that make insulin secrete it into the extracellular fluid by exocytosis. In another example, neurons (nerve cells) use exocytosis to release neurotransmitters that signal other neurons or muscle cells. When plant cells are making walls, exocytosis delivers proteins and carbohydrates from Golgi vesicles to the outside of the cell.

What types of cells are surrounded by walls?

The cells of plants, prokaryotes, fungi, and some protists are surrounded by walls. When such a cell is immersed in a hypotonic solution - bathed in rainwater, for example - the wall helps maintain the cell's water balance.

What are some of the functions of membrane proteins?

The functions of membrane proteins include transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix. Short chains of sugars linked to proteins (in glycoproteins) and lipids (in glycolipids)on the exterior side of the plasma membrane with surface molecules of other cells.

In concept 6.7 you learned that animal cells make an extracellular matrix (ECM). Describe the cellular pathway of synthesis and deposition of an ECM glycoprotein.

The glycoprotein would be synthesized in the ER lumen, move through the Golgi apparatus, and then travel in a vesicle to the plasma membrane, where it would undergo exocytosis and become part of the ECM.

Given the internal environment of a lysosome, what transport protein might you expect to see in its membrane?

The internal environment of a lysosome is acidic, so it has a higher concentration of H+ than does the cytoplasm. Therefore, you might expect the membrane of the lysosome to have a proton pump such as that shown in Figure 7.20 to pump H+ into the lysosome.

How does the membrane potential act like a battery?

The membrane potential acts like a battery, an energy source that affects the traffic of all charged substances across the membrane. Because the inside of the cell is negative compared with the outside, the membrane potential favors the passive transport of cations into the cell and anions out of the cell. Thus, two forces drive the diffusion of ions across a membrane: a chemical force (the ion's concentration gradient) and an electrical force (the effect of the membrane potential on the ion's movement). This combination of forces acting on an ion is called the electrochemical gradient.

What is the function of the contractile vacuole of Paramecium caudatum?

The vacuole collects fluid from a system of canals in the cytoplasm. When full, the vacuole and canals contract, expelling fluid from the cell (LM).

In the supermarket, produce is often sprayed with water. Explain why this makes vegetables look crisp.

The water is hypotonic to the plant cells, so the plant cells take up the water. Thus, the cells of the vegetable remain turgid rather than plasmolyzing (when the plant cell shrivels, and its plasma membrane pulls away from the wall) and the vegetable (for example, lettuce or spinach) remains crisp and not wilted.

What is thermal energy (heat)?

Thermal energy, which is also known as heat, is a type of energy that molecules have, due to their constant motion.

In what ways are membranes crucial to life?

They act as the gate keepers that decide what things are allowed in and out of the cell. A better modern day analogy would be the bouncers in front of clubs. If cells had no membranes, life would not exist. (7.1) Plasma membranes define the cell by separating the cellular components from the external environment. This allows conditions inside cells to be controlled by the membrane proteins, which regulate entry and exit of molecules and even cell function. The processes of life can be carried out inside the controlled environment of the cell, so membranes are crucial. In eukaryotes, membranes also function to subdivide the cytoplasm into different compartments where distinct processes can occur, even under differing conditions such as pH.

How do integral proteins pump substances across the membrane?

They can do this because they can hydrolyze ATP as an energy source.

How do phospholipids and proteins move about within the membrane?

They move laterally (side to side). The unsaturated hydrocarbon tails of some phospholipids keep membranes fluid at lower temperatures, while cholesterol helps membranes resist changes in fluidity caused by temperature changes. Differences in membrane lipid composition, as well as the ability to change lipid composition, are evolutionary adaptations that ensure membrane fluidity.

When does cholesterol make membranes less fluid and how?

This happens at relatively high temperatures--at 37*C, the body temperature of humans. At this temperature cholesterol will make the membrane less fluid by restraining phospholipid movement.

As the cell grows, its plasma membrane expands. Does this involve endocytosis or exocytosis?

This involves exocytosis. When a transport vesicle fuses with the plasma membrane, the vesicle membrane becomes part of the plasma membrane.

What is the sodium-potassium pump? (Fig 7.18)

This transport system pumps ions against steep concentration gradients: Sodium ion concentration [Na+] is high outside the cell and low inside, while potassium ion concentration [K+] is low outside the cell and high inside. The pump oscillates between two shapes in a cycle that moves 3 Na+ out of the cell for every 2 K+ pumped into the cell. The two shapes have different affinities for Na+ and K+. ATP powers the shape change by transferring a phosphate group to the transport protein (phosphorylating the protein).

How does active transport work?

To pump a solute across a membrane against its gradient require work; the cell must expend energy. Therefore, this type of membrane traffic is called active transport. The transport proteins that move solutes against their concentration gradients are all carrier proteins rather than channel proteins. This makes sense because when channel proteins are open, they merely allow solutes to diffuse down their concentration gradients rather than picking them up and transporting them against their gradients. Active transport enables a cell to maintain internal concentrations of small solutes that differ from concentrations in its environment. For example, compared with it surroundings, an animal cell has a much higher concentration of potassium (K+) and a much lower concentration of sodium ions (Na+). The plasma membrane helps maintain these steep gradients by pumping Na+ out of the cell and K+ into the cell.

If the sodium ion concentration outside the cell increases, and the CFTR channel is open, in what direction will chloride ions and water move across the cell membrane? A) Chloride ions will move out of the cell, and water will move into the cell. B) Both chloride ions and water will move out of the cell. C) Chloride ions will move into the cell, and water will move out of the cell. D) Both chloride ions and water will move into the cell. E) The movement of chloride ions and water molecules will not be affected by changes in sodium ion concentration outside the cell.

V

What other functions do vesicles have aside from transporting substances between the cell and its surroundings?

Vesicles not only transport substances between the cell and its surroundings but also provide a mechanism for rejuvenating or remodeling the plasma membrane. Endocytosis and exocytosis occur continually in most eukaryotic cells, yet the amount of plasma membrane in a nongrowing cell remains fairly constant. Apparently, the addition of membrane by one process offsets the loss of membrane by the other.

Why is it important for a membrane to be neither too solid nor too fluid?

When a membrane solidifies its permeability changes and enzymatic proteins in the membrane may become inactive if their activity requires them to move within the membrane. If the membrane is too fluid it cannot support protein function either.

Do cells have voltages across their membranes?

Yes, all cells have voltages across their plasma membranes. Voltage is electrical potential energy - separation of opposite charges. The cytoplasmic side of the membrane is negative in charge relative to the extracellular side because of an unequal distribution of anions and cations on the two sides. The voltage across a membrane, called a membrane potential, ranges from about -50 to -200 millivolts (mV). (The minus sign indicates that the inside of the cell is negative relative to the outside.

What is an aquaporin?

a channel protein in the plasma membrane of a plant, animal, or microorganism cell that specifically facilitates osmosis, the diffusion of free water across the membrane

What are peripheral proteins?

appendages loosely bound to the surface of the membrane, often to exposed parts of integral proteins; not embedded in the lipid bilayer at all

Membranes ____ _____ static sheets of molecules locked rigidly in place.

are NOT

What holds some membrane proteins in place on the cytoplasmic side?

attachment to the cytoskeleton

What other ingredient is also important?

carbohydrates

Where are the hydrophilic parts of the molecule (integral protein)?

exposed to the aqueous solutions on either side of the membrane

A membrane is held together primarily by ____________ interactions, which are much ________ than covalent bonds.

hydrophobic; weaker

Most of the lipids and some of the proteins can shift about ____.

laterally (that is, in the plane of the membrane, like partygoers elbowing their way through a crowded club)

What are the staple ingredients of membranes?

lipids and proteins

Because cholesterol hinders the close packing of phospholipids, it _____ the temperature required for the membrane to solidify. Thus it is can be thought of as a "______ ______" for the membrane, _______ changes in membrane fluidity that can be caused by changes in temperature.

lowers; "fluidity buffer" ; resisting

What does the hydrophobic region of an integral protein consist of?

one or more stretches of nonpolar amino acids usually coiled into alpha helices.

What is the most abundant lipid in most membranes? And why?

phospholipids; because the ability to form membranes is inherent in their molecular structure

For phospholipids to move _____ in a ____-_____ manner is ____.

transversely; flip-flop; rare

How do some proteins allow for the passage of hydrophilic substances?

via a hydrophilic channel through their center

A protein that spans the phospholipid bilayer one or more times is A) a transmembrane protein. B) an integral protein. C) a peripheral protein. D) an integrin. E) a glycoprotein.

A

According to the fluid mosaic model of cell membranes, which of the following is a true statement about membrane phospholipids? A) They can move laterally along the plane of the membrane. B) They frequently flip-flop from one side of the membrane to the other. C) They occur in an uninterrupted bilayer, with membrane proteins restricted to the surface of the membrane. D) They are free to depart from the membrane and dissolve in the surrounding solution. E) They have hydrophilic tails in the interior of the membrane.

A

Mammalian blood contains the equivalent of 0.15 M NaCl. Seawater contains the equivalent of 0.45 M NaCl. What will happen if red blood cells are transferred to seawater? A) Water will leave the cells, causing them to shrivel and collapse. B) NaCl will be exported from the red blood cells by facilitated diffusion. C) The blood cells will take up water, swell, and eventually burst. D) NaCl will passively diffuse into the red blood cells. E) The blood cells will expend ATP for active transport of NaCl into the cytoplasm.

A

Nitrous oxide gas molecules diffusing across a cell's plasma membrane is an example of A) diffusion across the lipid bilayer. B) facilitated diffusion. C) active transport. D) osmosis. E) cotransport.

A

The presence of cholesterol in the plasma membranes of some animals A) enables the membrane to stay fluid more easily when cell temperature drops. B) enables the animal to remove hydrogen atoms from saturated phospholipids. C) enables the animal to add hydrogen atoms to unsaturated phospholipids. D) makes the membrane less flexible, allowing it to sustain greater pressure from within the cell. E) makes the animal more susceptible to circulatory disorders.

A

Which of the following is a reasonable explanation for why unsaturated fatty acids help keep any membrane more fluid at lower temperatures? A) The double bonds form kinks in the fatty acid tails, preventing adjacent lipids from packing tightly. B) Unsaturated fatty acids have a higher cholesterol content and therefore more cholesterol in membranes. C) Unsaturated fatty acids are more polar than saturated fatty acids. D) The double bonds block interaction among the hydrophilic head groups of the lipids. E) The double bonds result in shorter fatty acid tails and thinner membranes.

A

Which of the following is one of the ways that the membranes of winter wheat are able to remain fluid when it is extremely cold? A) by increasing the percentage of unsaturated phospholipids in the membrane B) by increasing the percentage of cholesterol molecules in the membrane C) by decreasing the number of hydrophobic proteins in the membrane D) by cotransport of glucose and hydrogen

A

Which of the following would likely move through the lipid bilayer of a plasma membrane most rapidly? A) CO2 B) an amino acid C) glucose D) K+ E) starch

A

Describe how signal transduction works. (Figure 7.10c)

A membrane protein (receptor) may have a binding site a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signaling molecule) may cause the protein to change shape, allowing it to relay the message to the inside of the cell, usually by binding to a cytoplasmic protein.

Which of the following membrane activities require energy from ATP hydrolysis? A) facilitated diffusion of chloride ions across the membrane through a chloride channel B) movement of water into a cell C) Na+ ions moving out of a mammalian cell bathed in physiological saline D) movement of glucose molecules into a bacterial cell from a medium containing a higher concentration of glucose than inside the cell E) movement of carbon dioxide out of a paramecium

C

How do changes in the temperature affect biological membranes?

A membrane remains fluid as temperature decreases until finally the phospholipids settle into a closely packed arrangement and the membrane solidifies, much as bacon grease forms lard when it cools. The temperature at which a membrane solidifies depends on the types of lipids it is made of. The membrane remains fluid to a lower temperature if it is rich in phospholipids with unsaturated hydrocarbon tails (ah that is right!). This is because of kinks in the tails where the double bonds are located. Unsaturated hydrocarbon tails cannot pack together as closely as saturated hydrocarbon tails, and this makes the membrane more fluid.

Why is a transport protein needed to move water molecules rapidly and in large quantities across a membrane?

A transport protein is needed to move water molecules rapidly and in large quantities across a membrane because since water is a hydrophilic molecule, it will take much longer to permeate the lipid bilayer, relative to hydrophobic molecules such as CO2. Water is a polar molecule, so it cannot pass very rapidly through the hydrophobic region in the middle of a phospholipid bilayer.

What is the research method for freeze-freeze fracture?

Application: A cell membrane can be split into its two layers, revealing the structure of the membrane's interior. Technique: A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. Each membrane protein goes wholly with one of the layers. Results: SEM's show membrane proteins (the "bumps") in the two layers, demonstrating that proteins are embedded in the phospholipid bilayer.

An animal cell lacking oligosaccharides on the external surface of its plasma membrane would likely be impaired in which function? A) transporting ions against an electrochemical gradient B) cell-cell recognition C) maintaining fluidity of the phospholipid bilayer D) attaching to the cytoskeleton E) establishing the diffusion barrier to charged molecules

B

In which of the following would there be the greatest need for osmoregulation? A) an animal connective tissue cell bathed in isotonic body fluid B) cells of a tidepool animal such as an anemone C) a red blood cell surrounded by plasma D) a lymphocyte before it has been taken back into lymph fluid E) a plant being grown hydroponically (in a watery mixture of designated nutrients)

B

Some regions of the plasma membrane, called lipid rafts, have a higher concentration of cholesterol molecules. As a result, these lipid rafts A) are more fluid than the surrounding membrane. B) are more rigid than the surrounding membrane. C) are able to flip from inside to outside. D) detach from the plasma membrane and clog arteries. E) have higher rates of lateral diffusion of lipids and proteins into and out of the lipid rafts.

B

The formulation of a model for a structure or for a process serves which of the following purposes? A) It asks a scientific question. B) It functions as a testable hypothesis. C) It records observations. D) It serves as a data point among results. E) It can only be arrived at after years of experimentation.

B

What kinds of molecules pass through a cell membrane most easily? A) large and hydrophobic B) small and hydrophobic C) large polar D) ionic E) monosaccharides such as glucose

B

Which of the following is a characteristic feature of a carrier protein in a plasma membrane? A) It is a peripheral membrane protein. B) It exhibits a specificity for a particular type of molecule. C) It requires the expenditure of cellular energy to function. D) It works against diffusion. E) It has few, if any, hydrophobic amino acids.

B

What does it mean to accept or reject a model that is proposed?

Because models are hypotheses, replacing one model of membrane structure with another does not imply that the original model was worthless. The acceptance or rejection of a model depends on how well it fits observations and explains experimental results. New findings may make a model obsolete; even then, it may not be totally scrapped, but revised to incorporate the new observations. The fluid mosaic model is continually being refined. For example, group of proteins are often found associated in long-lasting, specialized patches, where they carry out common functions. The lipids themselves appear to form defined regions as well. Also, the membrane may be much more packed with proteins than imagined in the classic fluid mosaic model-compare the updated model with the original model (7.5 vs 7.3).

Celery stalks that are immersed in fresh water for several hours become stiff and hard. Similar stalks left in a 0.15 M salt solution become limp and soft. From this we can deduce that the cells of the celery stalks are A) hypotonic to both fresh water and the salt solution. B) hypertonic to both fresh water and the salt solution. C) hypertonic to fresh water but hypotonic to the salt solution. D) hypotonic to fresh water but hypertonic to the salt solution. E) isotonic with fresh water but hypotonic to the salt solution.

C

Cell membranes are asymmetrical. Which of the following is the most likely explanation? A) The cell membrane forms a border between one cell and another in tightly packed tissues such as epithelium. B) Cell membranes communicate signals from one organism to another. C) The two sides of a cell membrane face different environments and carry out different functions. D) The "innerness" and "outerness" of membrane surfaces are predetermined by genes. E) Proteins can only be associated with the cell membranes on the cytoplasmic side.

C

In order for a protein to be an integral membrane protein it would have to be A) hydrophilic. B) hydrophobic. C) amphipathic, with at least one hydrophobic region. D) completely covered with phospholipids. E) exposed on only one surface of the membrane.

C

The cell membranes of Antarctic ice fish might have which of the following adaptations? A) very long chain fatty acids B) branched isoprenoid lipids C) a high percentage of polyunsaturated fatty acids D) a higher percentage of trans-fatty acids E) no cholesterol

C

Which of the following is true of integral membrane proteins? A) They lack tertiary structure. B) They are loosely bound to the surface of the bilayer. C) They are usually transmembrane proteins. D) They are not mobile within the bilayer. E) They serve only a structural role in membranes.

C

Which of the following statements is correct about diffusion? A) It is very rapid over long distances. B) It requires an expenditure of energy by the cell. C) It is a passive process in which molecules move from a region of higher concentration to a region of lower concentration. D) It is an active process in which molecules move from a region of lower concentration to one of higher concentration. E) It requires integral proteins in the cell membrane.

C

Which of the following types of molecules are the major structural components of the cell membrane? A) phospholipids and cellulose B) nucleic acids and proteins C) phospholipids and proteins D) proteins and cellulose E) glycoproteins and cholesterol

C

Which of these are not embedded in the hydrophobic portion of the lipid bilayer at all? A) transmembrane proteins B) integral proteins C) peripheral proteins D) integrins E) glycoproteins

C

What are carrier proteins and what is their function?

Carrier proteins are transport proteins that hold onto their passengers and change shape in a way that shuttles them across the membrane. A transport protein is specific for the substance it translocates (moves), allowing only a certain substance (or a small group of related substances) to cross the membrane. For example, a specific carrier protein in the plasma membrane of red blood cells transports glucose across the membrane 50,000 times faster than glucose can pass through on its own. (Wow... amazingly fast..). This "glucose transporter" is so selective that it even rejects fructose, a structural isomer of glucose. ( I wonder if we could manipulate this glucose transporter in order to accept fructose. I wonder what the consequences would be.) Would it be possible to engineer the glucose transporter to perhaps transport glucose across the membrane 100,000 or 1,000,000 times faster than glucose can pass on its own, in order to enhance the speed and power of our Olympic athletes.

Are cell membranes permeable to polar molecules and ions?

Cell membranes ARE permeable to specific ions and a variety of polar molecules. These hydrophilic substances can avoid contact with the lipid bilayer by passing through transport proteins that span the membrane.

Give me an example of the steady traffic of small molecules and ions moving across the plasma membrane in both directions.

Consider the chemical exchanges between a muscle cell and the extracellular fluid that bathes it. Sugars, amino acids, and other nutrients enter the cell, and metabolic waste products leave it. The cell takes in O2 for use in cellular respiration and expels CO2. Also, the cell regulates its concentrations of inorganic ions, such as Na+, K+, Ca2+, and CI-, by shuttling them one way or the other across the plasma membrane. In spite of heavy traffic through them, cell membranes are selectively permeable, and substances do not cross the barrier indiscriminately. The cell is able to take up some small molecules and ions and exclude others. Also, substances that move through the membrane do so at different rates.

In the years since the proposal of the fluid mosaic model of the cell membrane, which of the following observations has been added to the model? A) The membrane is only fluid across a very narrow temperature range. B) Proteins rarely move, even though they possibly can do so. C) Unsaturated lipids are excluded from the membranes. D) The concentration of protein molecules is now known to be much higher. E) The proteins are known to be made of only acidic amino acids.

D

Singer and Nicolson's fluid mosaic model of the membrane proposed that A) membranes are a phospholipid bilayer. B) membranes are a phospholipid bilayer between two layers of hydrophilic proteins. C) membranes are a single layer of phospholipids and proteins. D) membranes consist of protein molecules embedded in a fluid bilayer of phospholipids. E) membranes consist of a mosaic of polysaccharides and proteins.

D

When a membrane is freeze-fractured, the bilayer splits down the middle between the two layers of phospholipids. In an electron micrograph of a freeze-fractured membrane, the bumps seen on the fractured surface of the membrane are A) peripheral proteins. B) phospholipids. C) carbohydrates. D) integral proteins. E) cholesterol molecules.

D

Which of the following is true of the evolution of cell membranes? A) Cell membranes have stopped evolving now that they are fluid mosaics. B) Cell membranes cannot evolve if the membrane proteins do not. C) The evolution of cell membranes is driven by the evolution of glycoproteins and glycolipids. D) All components of membranes evolve in response to natural selection. E) An individual organism selects its preferred type of cell membrane for particular functions.

D

Which of the following statements correctly describes the normal tonicity conditions for typical plant and animal cells? A) The animal cell is in a hypotonic solution, and the plant cell is in an isotonic solution. B) The animal cell is in an isotonic solution, and the plant cell is in a hypertonic solution. C) The animal cell is in a hypertonic solution, and the plant cell is in an isotonic solution. D) The animal cell is in an isotonic solution, and the plant cell is in a hypotonic solution. E) The animal cell is in a hypertonic solution, and the plant cell is in a hypotonic solution.

D

Why are lipids and proteins free to move laterally in membranes? A) The interior of the membrane is filled with liquid water. B) Lipids and proteins repulse each other in the membrane. C) Hydrophilic portions of the lipids are in the interior of the membrane. D) There are only weak hydrophobic interactions in the interior of the membrane. E) Molecules such as cellulose can pull them in various directions.

D

(Figure 7.11 - Impact) How can we block HIV entry into cells as a treatment for HIV infections?

Despite multiple exposures to HIV, a small number of people do not develop AIDS and show no evidence of HIV-infected cells. Comparing their genes with genes of infected individuals, researchers discovered that resistant individuals have an unusual form of a gene that codes for an immune cell-surface protein called CCR5. Further work showed that HIV binds to a main protein receptor (CD4) on an immune cell, but most types of HIV also need to bind to CCR5 as a "co-receptor" to actually infect the cell. An absence of a CCR5 on the cells of resistant individuals, due to the gene alteration, prevents the virus from entering the cells.

Glucose diffuses slowly through artificial phospholipid bilayers. The cells lining the small intestine, however, rapidly move large quantities of glucose from the glucose-rich food into their glucose-poor cytoplasm. Using this information, which transport mechanism is most probably functioning in the intestinal cells? A) simple diffusion B) phagocytosis C) active transport pumps D) exocytosis E) facilitated diffusion

E

The phosphate transport system in bacteria imports phosphate into the cell even when the concentration of phosphate outside the cell is much lower than the cytoplasmic phosphate concentration. Phosphate import depends on a pH gradient across the membrane-more acidic outside the cell than inside the cell. Phosphate transport is an example of A) passive diffusion. B) facilitated diffusion. C) active transport. D) osmosis. E) cotransport.

E

The primary function of polysaccharides attached to the glycoproteins and glycolipids of animal cell membranes is A) to facilitate diffusion of molecules down their concentration gradients. B) to actively transport molecules against their concentration gradients. C) to maintain the integrity of a fluid mosaic membrane. D) to maintain membrane fluidity at low temperatures. E) to mediate cell-to-cell recognition.

E

Water passes quickly through cell membranes because A) the bilayer is hydrophilic. B) it moves through hydrophobic channels. C) water movement is tied to ATP hydrolysis. D) it is a small, polar, charged molecule. E) it moves through aquaporins in the membrane.

E

When a plant cell, such as one from a peony stem, is submerged in a very hypotonic solution, what is likely to occur? A) The cell will burst. B) The cell membrane will lyse. C) Plasmolysis will shrink the interior. D) The cell will become flaccid. E) The cell will become turgid.

E

When biological membranes are frozen and then fractured, they tend to break along the middle of the bilayer. The best explanation for this is that A) the integral membrane proteins are not strong enough to hold the bilayer together. B) water that is present in the middle of the bilayer freezes and is easily fractured. C) hydrophilic interactions between the opposite membrane surfaces are destroyed on freezing. D) the carbon-carbon bonds of the phospholipid tails are easily broken. E) the hydrophobic interactions that hold the membrane together are weakest at this point.

E

Who was/were the first to propose that cell membranes are phospholipid bilayers? A) H. Davson and J. Danielli B) I. Langmuir C) C. Overton D) S. Singer and G. Nicolson E) E. Gorter and F. Grendel

E

Figure 7.7 - Inquiry: Do membrane proteins move?

Experiment: Larry Frye and Michael Edidin, at Johns Hopkins University, labeled the plasma membrane proteins of a mouse cell and a human cell with two different markers and fused the cells. Using a microscope, they observed the markers on the hybrid cell. Results: The hybrid cell had mixed proteins after 1 hour - (Is this not cross species genetics - mouses and humans?). Conclusion: The mixing of the mouse and human membrane proteins indicates that at least some membrane proteins move sideways within the plane of the plasma membrane.

What happened when researchers first used electron microscopes to study cells in the 1950's, that revealed two problems with the "sandwich" model that Davson and Danielli proposed?

First inspection of a variety of membranes revealed that membranes with different functions differ in structure and chemical composition. A second, more serious problem became apparent once membrane proteins were better characterized. Unlike proteins dissolved in cytosol, membrane proteins were not very soluble in water because they were amphipathic. If such proteins were layered on the surface of the membrane their hydrophobic parts would be in aqueous surroundings.

If this drug is successful in providing individuals with HIV resistance, what are the social implications? (My thoughts)

First of all it would be amazing for all the individuals in the world, knowing that their partners do not have HIV. I also wonder if HIV will mutate even more and become even more deadly. HIV could learn how to get around the CCR5 disabling and somehow get into the cell through other means. If this drug is successful and HIV is eliminated from the world, I think it would be wonderful for our world, knowing that this deadly virus no longer existed. The discoverers will win a Nobel Prize. Will it make people more promiscuous leading to greater spread of other diseases? I am not sure. I wonder how we can accurately measure how many individuals in the population have a certain disease. I think the only way we can eradicate all these diseases is to tract every single individual at birth with devices that can find their location, the state of the their health, and the total environment. We will need more and more sensors.

Why are proteins on the surface of a cell important in the medical field?

Proteins on the surface of a cell are important in the medical field because some proteins can help outside agents invade the cell. For example, cell-surface proteins help the human immunodeficiency virus (HIV) infect immune system cells, leading to the acquired immune deficiency syndrome (AIDS). Learning about the proteins that HIV binds to on immune cells has been central to developing a treatment for HIV infection.

The soil immediately around hot springs is much warmer than that in neighboring regions. Two closely related species of native grasses are found, one in the warmer region and one in the cooler region. if you analyzed their membrane lipid compositions, what would you expect to find?

I would expect them to have unsaturated fatty acids(double bonds) which allow for kinks - in their membrane lipid compositions. The grasses living in the cooler region would be expected to have more unsaturated fatty acids in their membranes because those fatty acids remain fluid at lower temperatures. The grasses living immediately adjacent to the hotspring would be expected to have more saturated fatty acids, which would allow the fatty acids to "stack" more closely, making the membranes less fluid and therefore helping them to stay intact at higher temperatures. (Cholesterol could not be used to moderate the effects of temperature on membrane fluidity because it is not found within plant cell membranes. - ( Haha, wow such an obvious mistake on my part, only animals contain cholesterol, always learning, excellent.

What did Hugh Davson and James Danielli suggest in 1935 about biological membranes?

In 1935 these 2 men suggested that the membrane might be coated on both sides with hydrophilic proteins. They proposed a sandwich model: A phospholipid bilayer between two layers of proteins.

Why do enzymes sometimes work in teams? (Figure 7.10b - Some functions of membrane proteins)

In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway.

What is cell-cell recognition?

It is a cell's ability to distinguish one type of neighboring cell from another, it is crucial to the functioning of an organism. It is important, for example, in the sorting of cells into tissues and organs in an animal embryo. It is also the basis for the rejection of foreign cells by the immune system, an important line of defense in vertebrate animals. (Pathogenic bateria, Graft vs Host Disease [Transplanting a heart from a donor into an individual needs to match on what criteria?], - It was evolved to work for us, but these mechanisms are now preventing us to use embryonic stem cells and other technologies - Fking interesting]. Cells recognize other cells by binding to molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane.

What is selective permeability?

It is a property of biological membranes that allows them to regulate the passage of substances across them. One of the earliest episodes in the evolution may have been the formation of a membrane that enclosed a solution different from the surrounding solution, while still permitting the uptake of nutrients and elimination of waste products. The ability of a cell to discriminate in its chemical exchanges with its environment is fundamental to life, and it is the plasma membrane and its component molecules that make this selectively possible.

What does amphipathic?

It means having both a hydrophilic region and a hydrophobic region.

What does the term "glyco" refer to?

It refers to the presence of carbohydrates.

Which of the following statements is correct about diffusion?

It requires integral proteins in the cell membrane. It is very rapid over long distances. *It is a passive process in which molecules move from a region of higher concentration to a region of lower concentration. It is an active process in which molecules move from a region of lower concentration to one of higher concentration. It requires an expenditure of energy by the cell.

How long are membrane carbohydrates?

Membrane carbohydrates are usually short, branched chains of fewer than 15 sugar units. Some are covalently bonded to lipids, forming molecules called glycolipids. However, most are covalently bonded to proteins, which are thereby glycoproteins.

What is intercellular joining? (Figure 7.10e)

Membrane proteins of adjacent cells may hook up together in various kinds of junctions such as gap junctions (I remember this) or tight junctions. This type of binding is more long-lasting than that shown in (d).

What are the characteristics of biological membranes?

Membranes are not static sheets of molecules locked rigidly in place. A membrane is held together primarily by hydrophobic interactions, which are much weaker than covalent bonds. Most of the lipids and some of the proteins can shift laterally - that is, in the plane of membrane, like partygoers elbowing their way through a crowded room. It is quite rare, however, for a molecule to flip-flop transversely across the membrane, switching from one phospholipid layer to the other; to do so, the hydrophilic part of the molecule must cross the hydrophobic interior of the membrane.

Why must membranes be fluid in order to work properly?

Membranes are usually about as fluid as salad oil. When a membrane solidifies, its permeability changes, and enzymatic proteins in the membrane may become inactive if their activity requires them to be able to move within the membrane. However membranes that are too fluid cannot support protein function either. Therefore, extreme environments pose a challenge for life, resulting in evolutionary adaptations that include differences in membrane lipid composition.

How do membrane proteins attach to the cytoskeleton and ECM?

Microfilaments or other elements of the cytoskeleton may be noncovalently bound to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that can bind to ECM molecules can coordinate extracellular and intracellular changes.

What type of molecules have an easier time crossing the lipid bilayer, nonpolar or polar molecules?

Nonpolar molecules have an easier time permeating the lipid bilayer because onpolar molecules, such as hydrocarbons, carbon dioxide, and oxygen, are hydrophobic and can therefore dissolve in the lipid bilayer of the membrane and cross it easily, without the aid of membrane proteins.

Two molecules that can cross a lipid bilayer without help from membrane proteins are O2 and CO2. What property allows this to occur?

O2 and CO2 are both nonpolar molecules, therefore they can easily pass through the hydrophobic interior of a membrane. However, the hydrophobic interior of the membrane impedes the direct passage of ions and polar molecules, which are hydrophilic, through the membrane.

How are membrane proteins held in place to the plasma membrane?

On the cytoplasmic side of the plasma membrane, some membrane proteins are held in place by attachment to the cytoskeleton. And on the extracellular side, certain membrane proteins are attached to the fibers of the extracellular matrix (integrins are one type of integral protein). These attachments combine to give animal cells a stronger framework than the plasma membrane alone could provide.

What are peripheral proteins?

Peripheral proteins lie on the membrane of cells. They are not embedded in the lipid bilayer at all; they are appendages loosely bound to the surface of the membrane, often to exposed parts of integral proteins.

How does the hydrophobic interior of the membrane impede the direct passage of ions and polar molecules, which are hydrophilic, through the membrane.

Polar molecules such as glucose and other sugars pass only slowly through a lipid bilayer, and even water, an extremely small polar molecule, does not cross very rapidly. A charged atom or molecule and its surrounding shell of water find the hydrophobic interior of the membrane even more difficult to penetrate. Furthermore, the lipid bilayer is even more difficult to penetrate. Furthermore, the lipid bilayer is only one aspect of the gate-keeper system responsible for the selective permeability of a cell. Proteins built into the membrane play key roles in regulating transport.

Why does knowing about how certain individuals in the population have an unusual form of a gene that codes for an immune cell-surface protein called CCR5, important to us?

Researchers have been searching for drugs to block cell-surface receptors involved in HIV infection. The main receptor protein, CD4, performs many important functions for cells, so interfering with it could cause dangerous side effects. Discovery of the CCR5 co-receptors provided a safer target for development of drugs that mask CCR5 and block HIV entry. One such drug, maraviroc (brand name Selzentry), was approved for treatment of HIV infection in 2007.

When did scientists begin building molecular models of the membrane?

Scientists began building molecular models of the membrane decades before membranes were first seen with the electron microscope (in the 1950's). In 1915, membranes isolated from red blood cells were chemically analyzed and found to be composed of lipids and proteins. Ten years later, two Dutch scientists reasoned that cell membranes must be phospholipid bilayers. Such a double layer of molecules could exist as a stable boundary between two aqueous compartments because the molecular arrangement shelters the hydrophobic tails of the phospholipids from water while exposing the hydrophilic heads to the water.

How do glycoproteins serve as ID tags in cell to cell recognition? (Figure 7.10d)

Some glycoproteins serve as ID tags that are specifically recognized by membrane proteins of other cells. This type of cell to cell binding is usually short-lived relative to what is in Fig 7.10e.

How do the transport proteins called "channel proteins" function?

Some transport proteins, called channel proteins, function by having a hydrophilic channel that certain molecules or atomic ions use as a tunnel through the membrane. For example, the passage of water molecules through the membrane in certain cells is greatly facilitated by channel proteins known as aquaporins.

Where does the word "mosaic" fit into the aspect of the fluid mosaic model?

Somewhat like a tile mosaic, a membrane is a collage of different proteins, often clustered together in groups, embedded in the fluid matrix of the lipid bilayer. More than 50 kinds of proteins have been found so far in the plasma membrane of red blood cells for example. Phospholipids form the main fabric of the membrane, but proteins determine most of the membrane's functions. Different types of cells contain different sets of membrane proteins, and the various membranes within each cell have a unique collection of proteins.

What would you predict about CCR5 that would allow HIV to bind to it? How could a drug molecule interfere with this binding?

The CCR5 acts as a helper, like someone on the inside (a bad guy on the inside) that lets the rest of the bad guys in. If we can disable this bad guy it could save many lives and allow individuals to become HIV resistant. I believe a drug molecule could interfere with this binding by either acting as a competitive inhibitor, by blocking the site of connection for the virus. Or it could act as a noncompetitive inhibitor - which is a substance that reduces the activity of the enzyme by binding to the enzyme's (protein) shape so that the active site can no longer effectively catalyze the conversion of a substrate to a product. Need to do additional research to see if my educated guessing stands true. I wonder if this drug has gotten to clinical trials yet.

Give me some more examples of variations in the cell membrane lipid compositions of species.

The ability to change the lipid composition of cell membranes in response to changing temperatures has evolved in organisms that live where temperatures vary. In many plants that tolerate extreme cold, such as winter wheat, the percentage of unsaturated phospholipids increases in autumn, an adjustment that keeps the membranes from solidifying during the winter. Certain bacteria and archaea can also change the proportion of unsaturated phospholipids in their cell membranes, depending on the temperature at which they are growing. Overall natural selection has apparently favored organisms whose mix of membrane lipids ensures an appropriate level of membrane fluidity for their environment.

How does membrane sidedness arise?

The asymmetrical arrangement of proteins, lipids and their associated carbohydrates in the plasma membrane is determined as the membrane is being built by the ER and Golgi apparatus.

How is the biological membrane an example of a supramolecular structure?

The biological membrane is an exquisite example of a supramolecular structure (many molecules ordered into a higher level of organization) with emergent properties beyond those individual molecules One of the most important of those emergent properties is the ability to regulate transport across cellular boundaries, a function essential to the cell's existence. The fluid mosiac model helps explain how membranes regulate the cell's molecular traffic.

Mammalian blood contains the equivalent of 0.15 M . Seawater contains the equivalent of 0.45 M . What will happen if red blood cells are transferred to seawater?

The blood cells will expend ATP for active transport of into the cytoplasm. NaCl will be exported from the red blood cells by facilitated diffusion. The blood cells will take up water, swell, and eventually burst. NaCl will passively diffuse into the red blood cells. *Water will leave the cells, causing them to shrivel and collapse.

Aquaporins exclude passage of hydronium ions. Recent research on fat metabolism has shown that some aquaporins allow passage of glycerol, a three carbon alcohol, as well as H2O. Since H3O+ is much closer in size to water than is glycerol, what do you suppose is the basis of this selectivity?

The hydronium ion is charged, while glycerol is not. Charge is probably more significant than size as a basis for exclusion by the aquaporin channel. (Interesting how they said probably, it means this is not conclusive. But why would charge be more significant than size? What is so special about charge that makes it more difficult for a hydronium to pass through aquaporins that allow the passage of water?)

How do the molecules switch from one phospholipid layer to the other?

The lateral movement of phospholipids within the membrane is rapid. Adjacent phospholipids switch positions about 10^7 times per second. A phospholipid can travel the length of many bacterial cells in 1 second. Proteins are much larger than lipids and move more slowly, but some membrane proteins do drift. And some membrane proteins seem to move in a highly directed manner, perhaps driven along cytoskeletal fibers by motor proteins connected to the membrane protein's cytoplasmic regions. However, many other membrane proteins seem to be held immobile by their attachment to the cytoskeleton or to the extracellular matrix.

What is the most abundant lipid in most membranes?

The most abundant lipid in most membranes are phospholipids. The ability of phospholipids to form membranes is inherent in their molecular structure. A phospholipid is an amphipathic molecule, meaning that it has both a hydrophilic region and a hydrophobic region.

What does the selective permeability of a membrane depend on?

The selective permeability of a membrane depends on both the discriminating barrier of the lipid bilayer and the specific transport proteins built into the membrane. But what establishes the direction of traffic across a membrane? At a given time, what determines whether a particular substance will enter the cell or leave the cell? And what mechanisms actually drive molecules across membranes. (More questions than answers - awesome.)

Which of the following molecular movements is due to diffusion or osmosis?

The sodium-potassium pump pumps three sodium ions out of a neuron for every two potassium ions it pumps in. Ten minutes after a perfume bottle is dropped in a room, perfume can be smelled in the opposite corner of the room. *When a plant cell is placed in salt water, water moves out of the central vacuole of the cell.

What is the effect of the steroid cholesterol on membranes?

The steroid cholesterol, which is wedged between phospholipid molecules in the plasma membranes of animal cells, has different effects on membrane fluidity at different temperatures. At relatively high temperatures at 37 C, the body temperature of humans, for example, cholesterol makes the membrane less fluid by restraining phospholipid movement. However, because cholesterol also hinders the close packing of phospholipids, it lowers the temperature required for the membrane to solidify. Thus, cholesterol can be thought of as a "fluidity buffer" for the membrane, resisting changes in membrane fluidity that can be caused by changes in temperature.

What happens when to the transport vesicle that transports glycoproteins, glycolipids, and secretory proteins (purple spheres), when it hits the plasma membrane?

The transport vesicle fuses with the plasma membrane. As the vesicle fuses with the plasma membrane, the outside face of the vesicle becomes continuous with the inside (cytoplasmic) face of the plasma membrane. This releases the secretory proteins from the cell, a process called exocytosis, and positions the carbohydrates of membrane glycoproteins and glycolipids on the outside (extracellular) face of the plasma membrane.

What are the differences between the inside and outside faces of membranes?

The two lipid layers may differ in specific lipid composition, and each protein has directional orientation in the membrane.

What did S. J. Singer and G. Nicolson propose in 1972 after the "sandwich" model was shown to be wrong thanks to the advent of electron microscopes?

These two men proposed in 1972 that membrane proteins reside in the phospholipid bilayer with their hydrophilic regions protruding. This molecular arrangement would maximize contact of hydrophilic regions of proteins and phospholipids with water in the cytosol and extracellular fluid, while providing their hydrophobic parts with a non-aqueous environment.

According to the fluid mosaic model of cell membranes, which of the following is a true statement about membrane phospholipids?

They are free to depart from the membrane and dissolve in the surrounding solution. They occur in an uninterrupted bilayer, with membrane proteins restricted to the surface of the membrane. They frequently flip-flop from one side of the membrane to the other. They have hydrophilic tails in the interior of the membrane. *They can move laterally along the plane of the membrane.

What are integral proteins?

They are proteins that exist on membranes. They penetrate the hydrophobic interior of lipid bilayer. The majority are transmembrane proteins, which span the membrane. Other integral proteins extend only partway into the hydrophobic interior. The hydrophobic regions of an integral protein consists of one or more stretches of nonpolar amino acids, usually coiled into alpha helices. The hydrophilic parts of the molecule are exposed to the aqueous solutions on either side of the membrane. Some proteins also have a hydrophilic channel through their center that allows passage of hydrophilic substances (Interesting, I remember this from lecture!)

What is the function of aquaporins?

This channel protein, aquaporin, allows entry of up to 3 billion (3 * 10^9) water molecules per second, passing single file through its central channel which fits ten at a time. (*** **** thats a lot of molecules - I wonder how much that is in cups?) Without aquaporins, only a tiny fraction of these water molecules would pass through the same area of the cell membrane in a second, so the channel protein brings about a tremendous increase in rate. (Engineering human cells to have no aquaporins, so they would die immediately, due to not have nutrients - evil thought - but holy pits if we remove aquaporins in cancer cells or even pathogenic bacterial cells, perhaps we could slowly kill them overtime, because they don't have enough nutrients.) I believe we could perhaps add more aquaporins to cancer and pathogenic cells in order to flood them with water, causing them to explode, or die due too much liquid - I wonder if that is even possible.

What is the fluid mosaic model?

This is the currently accepted model of cell membrane structure, which envisions the membrane as a mosaic of protein molecules drifting laterally in a fluid bilayer of phospholipids.

What is the method for preparing cells for electron microscopy called and how was it important in demonstrating visually that proteins are indeed embedded in the phospholipid bilayer of the membrane?

This method is called freeze-fracture. Freeze fracture splits a membrane along the middle of bilayer, somewhat like pulling apart a chunky peanut butter sandwich. When the membrane layers are viewed in the electron microscope, the interior of the bilayer appears cobble stoned, with protein particles interspersed in a smooth matrix, in agreement with the fluid mosaic model. Some proteins remain attached to one layer or the other, like the peanut chunks in the sandwich.

What are some examples of variations in the cell membrane lipid compositions of species?

Variations in the cell membrane lipid compositions of many species appear to be evolutionary adaptations that maintain the appropriate membrane fluidity under specific environmental conditions. For instance, fishes that live in extreme cold have membranes with a high proportion of unsaturated hydrocarbon tails, enabling their membranes to remain fluid. At the other extreme, some bacteria and archaea thrive at temperatures greater than 90 C (194 F) in thermal hot springs and geysers. Their membranes include unusual lipids that may prevent excessive fluidity at such high temperatures.

What happens when two solutions separated by a selectively permeable membrane reach osmotic equilibrium?

Water molecules no longer move between the solutions. Water molecules continue to move from the hypotonic solution to the hypertonic solution. *Water molecules move between the two solutions, but there is no net movement of water across the membrane.

Do the carbohydrates on the extracellular side of the plasma membrane vary from species to species?

Yes the carbohydrates on the extracellular side of the plasma membrane vary from species to species, among individuals of the same species, and even from one cell type to another in a single individual (Jesus christ). The diversity of the molecules and their location on the cell's surface enable membrane carbohydrates to function as markers that distinguish one cell from another. For example, the four human blood types designated A, B, AB, and O reflect variation in the carbohydrate part of glycoproteins on the surface of red blood cells.

Which of the following types of molecules are the major structural components of the cell membrane?

nucleic acids and proteins *phospholipids and proteins phospholipids and cellulose proteins and cellulose glycoproteins and cholesterol

White blood cells engulf bacteria through what process?

receptor-mediated exocytosis pinocytosis exocytosis osmosis *phagocytosis

A bacterium engulfed by a white blood cell through phagocytosis will be digested by enzymes contained in?

vacuoles. peroxisomes. *lysosomes. secretory vesicles. Golgi vesicles.