Chapter 6

What do fats, steroids, and waxes have in common?

(b) Low solubility in water. Low water solubility is the trait that defines lipids.

What two conditions must be present for osmosis to occur?

A membrane that allows only water to pass, but not the solute. Concentration gradient on the two sides of a membrane.

Which of these can RAPIDLY pass directly through the phospholipids of the plasma membrane, without the help of a transport protein?

A only Small, nonpolar, lipid-soluble molecules can pass rapidly through the phospholipids of the plasma membrane. Ions, such as hydrogen ions, and hydrophilic molecules, such as water and glucose, cannot rapidly pass directly through the phospholipids of the membrane. To move rapidly through the membrane, they must pass through membrane transport proteins.

In the illustration, which membrane transport mechanism is best represented by the letter "C"?

Active transport

How does the sodium-potassium pump work to transport sodium and potassium against their concentration gradients?

Adenosine triphosphate (ATP) hydrolysis provides the energy to transport ions against their concentration gradients.

What type of chemical interactions do phospholipids have with their environment?

Amphipathic

Which of the following is true of cell membranes?

Amphipathic proteins can span the membrane.

Which of the following is true of cell membranes?

Amphipathic proteins can span the membrane. The cell membrane consists of a phospholipid bilayer interspersed with transmembrane proteins.

What relationship did researchers find between the electrical gradient and the activity of the sodium ion channel?

As the electrical gradient is reduced, the channels open and sodium ions pass through the membrane along their electrochemical gradient. As the electrical gradient is reduced during depolarization, the sodium channels open and sodium ions pass through the membrane along their electrochemical gradient.

Which statement most accurately describes the relationship between the permeability and fluidity properties of phospholipid bilayers and their structure?

Bilayers are most permeable and most fluid when they consist of shorter, unsaturated hydrocarbon tails. Hydrophobic interactions are weaker for shorter, unsaturated hydrocarbon tails, resulting in a bilayer whose interior is held together less tightly.

Triglycerides vary with respect to the number of ...

C atoms in the tails. double bonds in the tails. Tail lengths of 16 to 22 are common. There may be from 0 to 6 double bonds

Is lipid structure in foods you eat linked to heart disease? The media is full of advice on what you should and should not eat, but these recommendations often change from year to year. You may have heard that polyunsaturated fats are heart-healthy and saturated fats are not, but is it really that simple? What physical property is often associated with these chemical differences?

C=C bonds normally result in kinks that increase the spacing between fats and thus make unsaturated fats more fluid than saturated fats.

In terms of structure, how do channel proteins differ from carrier proteins? Carrier proteins

Change shape to shuttle molecules or ions across the membrane. Undergo conformational changes caused by the binding of the specific molecules.

A "mystery molecule" was isolated in a laboratory and scientists found that the molecule readily crossed artificial membranes. Which of the following molecules can be ruled out as the "mystery molecule"?

Chloride ion. Charged molecules do not cross membranes on their own because it is energetically unfavorable for them to break their interactions with water and enter the hydrophobic interior of the membrane. Thus, a chloride ion cannot be the mystery molecule that easily crossed membranes.

Which of the following lipids is classified as a steroid?

Cholesterol

In the illustration, which membrane transport mechanism is best represented by the letter "A"?

Diffusion

Which statement best describes the results of an experiment designed to test whether the protein cystic fibrosis transmembrane conductance regulator (CFTR)cis a chloride channel?

Electric current flowed only in the presence of CFTR, including the movement of chloride ions.

In the illustration, which membrane transport mechanism is best represented by the letter "B"?

Facilitated diffusion

The model is called a bilayer because there are two main types of molecules present, lipids and proteins.

False

This model shows membrane transport

False

When a vesicle containing 0.1 MNa+MNa+ is placed in a 0.1 MCl−MCl− solution, water molecules will no longer move across the lipid bilayer.

False

When a vesicle containing 0.1 MNa+MNa+ is placed in water, the sodium ions will diffuse out and the vesicle will shrink.

False

The many black lines represent amino acid tails.

False Represents phospholipid tails

The blue ovals (heads) are hydrophobic.

False The heads are hydrophilic

In an experiment, you create two groups of liposomes in a solution containing 0.1MNaCl−0.1MNaCl− one made from red blood cell membranes and the other from frog egg cell membranes. When the liposomes are placed in water, those with red blood cell membranes burst more rapidly than those made from egg membranes. Evaluate each of the following statements and identify those that could explain these results. The red blood cell liposomes are more hypertonic relative to water than the frog egg liposomes.

False, because the interior solution in both sets of liposomes is the same.

The red blood cell liposomes are more hypotonic relative to water than the frog egg liposomes.

False, because the interior solution in both sets of liposomes is the same.

Which statement most accurately describes how the structures of fats, steroids, and phospholipids compare?

Fats and phospholipids contain glycerol whereas steroids do not. Correct. These are defining structural features of fats, phospholipids and steroids.

In terms of structure, how do channel proteins differ from carrier proteins? Channel proteins

Form pores in the membrane. Are open or close in response to a signal. Have different functional groups in the interior of the pore that function as a molecular filter.

Which of the following was evidence for the fluid-mosaic model of membrane structure?

Freeze fracture followed by scanning electron microscopy showed pits and mounds.

Part complete Which of the following was evidence for the fluid mosaic model of membrane structure?

Freeze fracture followed by scanning electron microscopy showed pits and mounds. During the freeze fracture transmembrane proteins remained with either the upper layer or the lower layer of the bilayer. Scientists observed either pits (where proteins had left) or mounds where proteins now protruded above the single lipid layer.

Is lipid structure in foods you eat linked to heart disease?The media is full of advice on what you should and should not eat, but these recommendations often change from year to year. You may have heard that polyunsaturated fats are heart healthy and saturated fats are not, but is it really that simple? An industrial process called hydrogenation is used to convert vegetable oil into a semisolid compound called margarine. Explain why hydrogenation is an appropriate term for this process.

Hydrogenation is used because the process is likely to involve converting C=C bonds into C-C bonds by adding hydrogens. Fats with only C-C bonds (i.e., saturated) tend to be semisolid at room temperature.

Consider the structure of the TTX secreted by the newt. Which of the following functional groups is most common on this molecule?

Hydroxyl The greatest number of functional groups are hydroxyl groups relative to the other functional groups.

For the study shown in the figure, what is the most appropriate control to show that the observed pits and mounds are not simply irregularities in the lipid bilayer caused by the freeze-fracture process?

Instead of a cell, use an artificial membrane composed only of pure phospholipids. The artificial membrane contains no proteins, so pits and mounds should not be observed if they are caused by membrane proteins and not the freeze-fracture process itself.

If a solution surrounding a cell is hypertonic relative to the inside of the cell, how will water move?

It will move out of the cell via osmosis.

Which of the following models provides the best representation of facilitated diffusion via a "channel"?

Large green channel. In the context of membrane models, "channels" refer to channel proteins that permit a particular type of ion or small molecule to pass through it.

Which type of molecule moves across a cell membrane most easily?

Lipid-soluble, nonpolar molecules, such as oxygen and carbon dioxide

Describe, on a molecular level, how you would expect these lipids to behave in water.

Lipids would float on the surface of the water, or collect in droplets suspended in water, reducing their interaction with water to a minimum.

Cooking oil lipids consist of long, unsaturated hydrocarbon chains. Would you expect these molecules to form membranes spontaneously? Why or why not?

No, because they have no polar end to interact with water.

A liposome has a selectively permeable phospholipid bilayer. For a particular solute, can the phospholipid bilayer be selectively permeable in only one direction?

No, selective permeability of a phospholipid bilayer is the same in both directions.

Is lipid structure in foods you eat linked to heart disease?The media is full of advice on what you should and should not eat, but these recommendations often change from year to year.You may have heard that polyunsaturated fats are heart healthy and saturated fats are not, but is it really that simple?To study the effect of food on heart disease, researchers fed a variety of diets to mice and then examined them for atherosclerosis −− the narrowing of arteries that is a leading cause of heart attacks. Data observed from mice fed with trans fats are provided below. Do these data identify the cause of atherosclerosis in the mice? Explain (Note that "causation" means that one event is responsible for the occurrence of the other, while "correlation" means that the two events appear to occur together).

No. The data show that there is a correlation between blood levels of trans fats and atherosclerosis, but these data do not show the cause.

How do fats differ from proteins, nucleic acids, and polysaccharides?

Only fats do not form from the polymerization of monomers. Lipids interact through hydrophobic interactions. The other macromolecules form via the polymerization of monomers: amino acids for proteins, nucleotides for nucleic acids, and monosaccharides for polysaccharides.

Which of these is a phospholipid?

Phospholipids are composed of a phosphate group, a glycerol, and fatty acids.

How does a pump differ from a channel or a carrier?

Pumps require energy for transport, but channels and carriers do not use energy.

Which of these is NOT a lipid? RNA, cholesterol, wax steroids, phospholipid

RNA is a nucleic acid

Four students have done a nice job drawing membrane models. The models are drawn at different scales to serve different purposes.

Rekik's model is the most enlarged, showing detailed structure of the membrane. Thomas's model shows less detail, and Zubia's even less. Elsa's model uses a single line to represent a cell membrane because she shows a whole cell at a less magnified scale.

Which student's model does the best job of showing membrane structure?

Rekik's model shows a detailed view of membrane structure, including lipids, proteins, and carbohydrates.

Which of the following substances is an unsaturated lipid?

Safflower oil

Which of the following is true regarding saturated fats?

Saturated fats contain the maximum number of hydrogen atoms that can attach to the carbon skeleton.

Is lipid structure in foods you eat linked to heart disease? The media is full of advice on what you should and should not eat, but these recommendations often change from year to year. You may have heard that polyunsaturated fats are heart-healthy and saturated fats are not, but is it really that simple? How is the chemical structure of saturated fats different from that of unsaturated fats?

Saturated fats have fatty acid tails with only C−C bonds while unsaturated fats have one or more C=C bonds in the tails.

Describe the functional role of sodium ion channels. How do these channels differ from carrier proteins and active transport pumps?

Sodium ion channels transport sodium ions through a pore across the membrane in either direction, but the net movement would be along the electrochemical gradient. Carriers also transport substances based on the gradient, but they do so by binding to the substance and undergoing a conformational change to move it across the membrane. Pumps differ from channels and carriers by using an energy source outside the gradient, such as ATP, to bind and transport substances across the membrane in one direction. Sodium ion channels transport sodium ions through a pore across the membrane in either direction, with the net movement occurring down the electrochemical gradient. Carriers transport down a concentration gradient, while pumps require an energy source

Which of the following correctly matches the lipid with its description?

Steroids have a distinctive four-ring structure with variable side groups attached. The variable side groups attached to the four-ring structure in steroids determine the functionality of that steroid.

If the poisoning resulted from drinking contaminated water alone, what does this tell you about the solubility and stability of the TTX present on the newt's skin?

TTX would need to be hydrophilic so that it is soluble in water. Since it is still active in the boiled water, it must also be heat resistant.

Cells use the sodium electrical gradient to perform many activities. What membrane protein is used to establish the sodium electrochemical gradient and how does it work?

The Na+/K+-ATPase establishes the gradient by using ATP to undergo conformational changes that result in moving three sodium ions out of the cell and two potassium ions into the cell. The Na+/K+-ATPase pump requires ATP, which causes the pump to undergo conformational changes that result in moving three sodium ions out of the cell and two potassium ions into the cell.

After a sodium gradient is established, which side of the plasma membrane would you expect to be more positively charged?

The extracellular side of the membrane will be more positively charged than the inside due to the energy-requiring activity of the Na+/K+-ATPase pump, and also the greater abundance of K+ leak channels than Na+ leak channels. After a sodium gradient is established, you would expect the extracellular side of the plasma membrane to be more positively charged.

What type of amino acids would you expect to be included in the regulatory alpha helix based on its change in position?

The helix would contain basic (positively charged) amino acids, such as lysine, arginine, and histidine. Yes, one would expect that the helix would contain basic (positively charged) amino acids, such as lysine, arginine, and histidine.

Proteins are amphipathic molecules that contain nonpolar (hydrophobic) amino acids and polar (hydrophilic) amino acids. Where would you expect to find the hydrophobic and hydrophilic amino acid residues of a transmembrane protein?

The hydrophobic amino acids would come into contact with the hydrocarbon tails of the phospholipid bilayer, whereas the hydrophilic amino acids would interact with the intracellular and extracellular environment. The bilayer interior is hydrophobic and nonpolar, so the hydrophobic amino acid residues would come into contact with this area. Hydrophilic proteins would be oriented toward intracellular fluid or toward extracellular fluid, or toward the interior of the protein.

Proteins are amphipathic molecules that contain nonpolar (hydrophobic) amino acids and polar (hydrophilic) amino acids. Where would the hydrophobic and hydrophilic amino acid residues of a transmembrane protein be found?

The hydrophobic amino acids would come into contact with the hydrocarbon tails of the phospholipid bilayer, whereas the hydrophilic amino acids would interact with the polar phospholipid heads and the intracellular and extracellular environments.

Explain how the abundant functional group contributed to the solubility of TTX in the water surrounding the boiled newt.

The hydroxyl is a polar group that forms hydrogen bonds with water molecules. The most abundant functional group is the hydroxyl group, which is a polar group that forms hydrogen bonds with water molecules.

The degree of saturation of phospholipids in a membrane will affect which of the following functions of phospholipids?

The permeability of the membrane

Which of the following statements best describes the fluid-mosaic model of membrane structure?

The phospholipid bilayer contains diverse proteins, including some embedded amphipathic proteins that span the bilayer.

Which of the following statements best describes the fluid-mosaic model of membrane structure?

The phospholipid bilayer contains diverse proteins, including some embedded amphipathic proteins that span the bilayer. This statement best explains the fluid-mosaic model. First described in 1972 by Singer and Nicholson, this model is based on freeze-fracture preparations. It replaced the Davson-Danielli model.

When solutes are diffusing through a membrane from high concentration on one side to low concentration on the other side, what happens when equilibrium is reached?

The rate of diffusion in one direction is the same as the rate of diffusion in the opposite direction.

Is lipid structure in foods you eat linked to heart disease?The media is full of advice on what you should and should not eat, but these recommendations often change from year to year. You may have heard that polyunsaturated fats are heart healthy and saturated fats are not, but is it really that simple?How you prepare food can affect the amount of saturated fats that you eat. For example, if you cook meat on a grill, the product will have less saturated fat than if you were to eat the same meat raw. What occurs during the process of cooking the meat on a grill that would explain this result?

The saturated fat in meat undergoes a change in its physical state as it is heated −− from a semisolid consistency to a liquid. When meat is grilled, the liquefied fat drips off, resulting in less saturated fat in the final product.

What would occur if you added NaClNaCl to the outside solution in this container? Inside and outside solutions isotonic.

The sodium and chloride ions would interact with water molecules outside the vesicle, resulting in water moving to the outside. Since the ions do not readily pass through the membrane, they only interact with water molecules outside the vesicle. This interaction causes water to be drawn from the vesicle to the outside.

Which statement describes what happens once equilibrium is reached between the interior of a vesicle and the surrounding solution?

There is no directional movement of solute or water across the membrane. At equilibrium, the solute or water will continue to move across the membrane, but the rate of movement to the inside of the vesicle will be the same as the rate of movement to the outside.

What do DNA, proteins, and fats have in common?

They contain carbonyl groups.

What happens when phospholipids are placed into water and agitated?

They form liposomes.

What happens when phospholipids are placed into water?

They form liposomes. Liposomes are water-filled vesicles, with phospholipids forming a bilayer that encloses the vesicle. Lipid bilayers are composed of two aligned layers of phospholipids with the tails pointing to the inside.

How do the phospholipids in archaea differ from those in other cells?

They have isoprenoid tails instead of fatty acid tails.

Which student's model does the best job of showing facilitated diffusion of a solute?

Thomas's model shows the diffusion of a solute from an area of higher concentration through a channel protein to an area of lower concentration.

Is lipid structure in foods you eat linked to heart disease? The media is full of advice on what you should and should not eat, but these recommendations often change from year to year. You may have heard that polyunsaturated fats are heart-healthy and saturated fats are not, but is it really that simple? An industrial process called hydrogenation is used to convert vegetable oil into a semisolid compound called margarine.A side effect of hydrogenation is that a small percentage of oil lipids retain their double bonds, but instead of causing kinks, they straighten out the hydrocarbon chain. These are called trans fats based on the configuration of the double bond (cis bonds generate kinks (see the figure below), but trans bonds do not). How do trans fats complicate the previous description of saturated and unsaturated fats?

Trans fats are unsaturated because they have one or more double bonds, but they have physical characteristics similar to saturated fats due to the trans double bond that straightens the hydrocarbon chain.

The blu ovals represent phospholipid heads.

True

The strings of little red dots represent carbohydrates.

True

When a vesicle containing 0.5 MNa+MNa+ is placed in a 0.3 MNa+MNa+ solution, water is imported and the vesicle will swell.

True

The red blood cell liposomes contain more aquaporins than the frog egg liposomes.

True, because aquaporins allow for an increased rate of water transport.

The frog egg liposomes contain ion channels, which are not present in the red blood cell liposomes.

True, because if the NaNa or ClCl ions can cross the membrane in frog eggs, then osmosis may be reduced or even be prevented from occurring.

Which of the following statements accurately describe how solutes affect a vesicle? When a vesicle is placed in a solution that is hypertonic to the inside, the vesicle will shrink.

True.

Starting at the 5' end, what is the base sequence of the RNA strand shown in Figure 4.3?

UAGC

Region A has a low-solute concentration. Region B has a high-solute concentration. They are separated by a selectively permeable membrane that is impermeable to the solute but allows the passage of water. Which of the following is true?

Water moves from region A to region B via osmosis.

Dr. Haxton told one of his students, "To move in the bloodstream, fats need the help of phospholipids." What would a good student say?

Yes. Nonpolar molecules aren't compatible with water. Water rejects nonpolar molecules such as fats, so fats travel inside particles that are coated with polar parts of phospholipids and proteins.

The three types of lipids found in cells (fats, steroids, and phospholipids) are insoluble in water because they all possess __________.

a significant hydrocarbon component

A function of cholesterol that does not harm health is its role _____.

as a component of animal cell membranes. Cholesterol is an important component of animal cell membranes.

Lipid structures vary widely; however, phospholipids and fats are similar in that they both _________.

contain glycerol

Detergents are useful for isolating membrane proteins because __________.

detergents are amphipathic

Detergents are useful for isolating membrane proteins because____.

detergents are amphipathic Detergents are amphipathic molecules that form micelles in water. Detergents break apart membranes by coating the hydrophobic tails of the phospholipids and dissolving them in water. This action is similar to how dish and laundry detergents break up grease on dishes and clothes.

Molecules that move down a concentration gradient and require transport proteins to cross a membrane use __________.

facilitated diffusion

The highlighted part of this molecule (HC-O) is derived from ...

glycerol. The other parts of the molecule are derived from fatty acids.

In fat synthesis,________and fatty acids combine to make fats plus________.

glycerol; water Water is a by-product of the reaction that makes ester links between glycerol and fatty acids.

In the reaction that builds a fat,________ groups react with ________ groups.

hydroxyl; carboxyl Fatty acids supply the carboxyl groups; the hydroxyls come from glycerol.

Lipid vesicles are formed containing pure water. If these vesicles are transferred to a solution that contains a rather high concentration of solutes, the solution outside the vesicle is said to be_____.

hypertonic A hypertonic solution has more solutes than a second solution to which it is being compared.

This cell is in a(n) _____ solution.

hypertonic There is a greater concentration of solute outside the cell.

You know that this cell is in a(n) _____ solution because it _____.

hypertonic solution ... lost water. A cell will lose water when placed in a hypertonic solution.

Unlike animal cells, bacteria have a porous cell wall surrounding their plasma membrane. The cell wall may help bacteria survive in a(n) __________ environment.

hypotonic

You know that this cell is in a(n) _____ solution because the cell _____.

hypotonic ... swelled A cell will gain water when placed in a hypotonic solution.

A food company hydrogenated a barrel of fat. The treatment ...

made the fat less fluid. Hydrogenation removes double bonds, so the tails can straighten.

If a solution surrounding a cell is hypotonic relative to the inside of the cell, the water will _______.

move into the cell via osmosis

Which of these is rich in unsaturated fats? a fat that is solid at room temperature, olive oil, beef fat, lard, butter

olive oil Olive oil is a plant oil, and most plant oils are rich in unsaturated fats.

Diffusion of water through a selectively permeable membrane is known as _________.

osmosis

What name is given to the process by which water crosses a selectively permeable membrane?

osmosis Osmosis is the passive transport of water.

The presence of many C-C and C-H bonds causes fats to be ...

rich in energy. insoluble in water. The many C-C and C-H bonds make fats nonpolar and insoluble in water. They can also be oxidized, releasing much energy.

This figure is an example of a(n) _____.

saturated fat The fatty acid tails lack double bonds.

Lipid bilayer

tails together, heads facing out

The most unsaturated fats have ...

the most double bonds. Every double bond is a place where hydrogen could be added.

Lipids are not considered polymers because _________.

they are not composed of monomer subunits.

Structure A in the figure is a(n) _____.

transport protein The protein is allowing solute molecules to enter the cell.