Bio 329- smartwork 9,10,11

Which of the following are consistent with the Na+-K+ pump and the illustration? A.Na+ ions are pumped against their chemical gradient. B.K+ ions are pumped against their electrical gradient. C.Na+ ions are pumped against their electrical gradient. D.K+ ions are pumped against their chemical gradient.

A,C,D - in the activity of the Na+ -K+ pump, Na+ ions are pumped against their chemical gradient and against their electrical gradient, meaning that sodium is pumped against its electrochemical gradient - K+ ions, are only pumped against their chemical gradient and not against their electrical gradient

Which of the following is supported by the information in the figure A.Glucose enters the cell by simple diffusion. B.Nucleotides enter the cell by facilitated diffusion. C.Sodium and potassium are involved in co-transport.

B and C - nucleotides enter the cell through a membrane transport protein - facilitated diffusion- protein mediated transport - sodium ions are pumped from the cell and potassium ions pumped into the cell- co-transport - glucose does not enter the cell by simple diffusion through membrane lipids, but enters like nucleotides do, by facilitated diffusion - glucose transporter in the plasma membrane - the protein, which consists of a polypeptide chain that crosses the membrane at least 12 times, can adopt several conformations- and it switches reversibly and randomly between them - in one conformation, the transporter exposes binding sites for glucose to the exterior of the cell; in another, it exposes the sites to the cell interior

Shown is a schematic diagram of a membrane phospholipid. Which segment will always carry a negative charge? A- choline B- phosphate C- glycerol D- saturated fatty acid (hydrocarbon tail) E- unsaturated fatty acid (hydrocarbon tail)

B- phosphate

How do transporters and channels select which solutes they help move across the membrane? A. Both channels and transporters discriminate between solutes mainly on the basis of size and electric charge. B. Channels discriminate between solutes mainly on the basis of size and electric charge; transporters bind their solutes with great specificity in the same way an enzyme binds its substrate. C. Transporters discriminate between solutes mainly on the basis of size and electric charge; channels bind their solutes with great specificity in the same way an enzyme binds its substrate. D. Channels will allow the passage of any solute as long as it has an electrical charge; transporters bind their solutes with great specificity in the same way an enzyme binds its substrate. E. Channels allow the passage of solutes that are electrically charged; transporters facilitate the passage of molecules that are uncharged.

B. Channels discriminate between solutes mainly on the basis of size and electric charge; transporters bind their solutes with great specificity in the same way an enzyme binds its substrate. - channels discriminate mainly on the basis of size and electric charge: when the channel is open, only ions of an appropriate size and charge can pass through - a transporter transfers only those molecules or ions that fit into specific binding sites on the protein - transporters bind their solutes with great specificity, in the same way an enzyme binds its substrate, and it is this requirement for specific binding that gives transporters their selectively

Which of the following mechanisms prevents osmotic swelling in plant cells? A. tough cell walls B. the collection of water in contractile vacuoles C. the expulsion of water from contractile vacuoles D. the activity of Na+ pumpsE. turgor pressure

A. tough cell walls - cells use different tactics to avoid osmotic swelling - plant cells are prevented from swelling by their cell walls, allowing them to tolerate a large osmotic difference across their plasma membrane - this results in turgor pressure when the pressure from swelling presses against the cell wall and is utilized by plants to help them achieve structural stability

Which type of membrane transport protein can perform either passive or active transport? A. transporters B. Neither type of membrane transport protein can perform both passive and active transport. C. both channels and transporters D. channels

A. transporters - passive transport, which is the movement of solutes across a membrane by following their concentration gradient, and active transport, which uses the input of energy to move solutes across a membrane and can move them against a concentration gradient, can both be accomplished by transporter proteins - a key aspect of active transport is the specificity of the molecule to be transported for its transporter protein - transporters that accomplish active transport are called pumps

What is true of the inside of a cell? It is slightly more negative than the outside of a cell. It has the same charge as the outside of the cell. It is slightly more positive than the outside of a cell.

It is slightly more negative than the outside of a cell. - the inside of a cell is slightly more negative than the outside of a cell - this uneven charge distribution tends to pull positively charged solutes into the cell and drive negatively charged ones out - in animal cells, for example, the resting membrane potential can be anywhere between -20 and -200 millivolts mV, depending on the organism and cell type - the value is expressed as a negative number because the interior of the cell is more negatively charged than the exterior

In most animal cells, which ion can move through "leak" channels? H+ Ca2+ K+ Na+ Cl-

K+ - in most animal cells, K+ ions can move through "leak" channels - when leak channels are open, they allow K+ to move freely out of the cell - in a resting cell, these are the main ion channels open in the plasma membrane, rendering the membrane much more permeable to K+ than to other ions - when K+ flows out of the cell- down the concentration gradient generated by the ceaseless operation of the Na+ pump- the loss of positive charge inside the ell creates a voltage difference, or membrane potential

What function is served by the carbohydrates attached to cell-surface proteins? a. allow cells to establish and maintain their shape b. protect the cell from mechanical and chemical damage c. establish a distinctive identity for cell-cell recognition d. promote cell-cell adhesion e. lubricate cells to keep them from sticking together

b, c, d, e - the carbohydrate layer on the surface of cells in a multicellular organism provides several advantages - first, it serves to protect cells from mechanical and chemical damage - next it serves as a kind of distinctive uniform; the glycocalyx is a characteristic of each cell type and is recognized by other cell types that interact with it - specific oligosaccharides in this carbohydrate layer are involved, for example, in the recognition of an egg by sperm - similarly, in the early stages of a bacterial infection, carbohydrates on the surface of white blood cells called neutrophils are recognized by sugar-binding proteins on the cells lining the blood vessels at the site of the infection; this recognition causes the neutrophils to adhere to the blood vessel wall and the migrate from the bloodstream into the infected tissue, where they help destroy the invading bacteria - further, because the oligosaccharides and polysaccharides attract water molecules, they also give the cell a slimy surface, which helps motile cells such as white blood cells squeeze through narrow spaces and prevents blood cells from sticking to one another or the walls of blood vessels - thus, the glycocalyx allows cells to recognize and adhere to one another, yet avoid becoming permanently stuck together

Which is a mechanism for restricting the movement of proteins in the plasma membrane? A. coating proteins with carbohydrates B. using barriers such as tight junctions C. tethering proteins to the extracellular matrix D. forming a covalent linkage with membrane lipids E. tethering proteins to the surface of another cell F. tethering proteins to the cell cortex

b,c,e,f - because a membrane is a two dimensional fluid, many of its proteins, like its lipids. can move freely within the plane of the bilayer - to restrict their movement, proteins in the plasma membrane can be tethered to structures outside the cell- for example, to molecules in the extracellular matrix or on an adjacent cell - they can also be tethered to relatively immobile structures inside the cell, especially to the cell cortex - in addition, membrane proteins can be corralled within barriers that form when cells closely associate - such as a barrier is formed along the line where one epithelial cell is sealed to an adjacent epithelial cell by a tight junction - at this side, specialized junctional proteins form a continuous belt around the cell where the cell contacts its neighbors, creating a seal between adjacent plasma membrane - membrane proteins are unable to diffuse past the junction - however, glycoproteins are as free to diffuse within the plane of the membrane as other proteins whose movement is not restricted by mechanisms mentioned - the attachment of carbohydrates does not, on its own, restrict the movement of the proteins to which sugars are attached - similarly, proteins that are attached to the membrane via a covalently linked lipid behave much the same way as many other membrane protein - the lipids to which they are attached are free to diffuse within the plane of the membrane

Margarine is produced from vegetable oil by a process that does which of the following? a. removes carbons, which decreases the length of their phospholipid tails b. adds carbons, which increases the length of their phospholipid tails c. adds hydrogen, which removes the double bonds from their phospholipid tails d. freezes the phospholipids, which removes their double bonds e. removes hydrogen, which increases the number of double bonds in their phospholipid tails

c. adds hydrogen, which removes the double bonds from their phospholipid tails -the fats produced by plants are generally unsaturated and therefore liquid at room temperature - animal fats, such as butter or lard, are generally saturated and therefore solid at room temperature - margarine is made of vegetable oils that have had the double bonds removed from their tails by the addition of hydrogen, a process called hydrogenation - this treatment renders margarine more solid and butterlike at room temperature

How does the inclusion of cholesterol affect animal cell membranes? a. It makes the lipid bilayer more permeable. b. It has little effect on the properties of the lipid bilayer. c. It tends to make the lipid bilayer less fluid. d. It makes the lipid bilayer wider. e. It tends to make the lipid bilayer more fluid.

c. It tends to make the lipid bilayer less fluid. - in animal cells, membrane fluidity is part by the inclusion of cholesterol, which constitutes up to 20% (by weight) of the lipids in the plasma membrane - because cholesterol molecules are short and rigid, they fill the spaces between neighboring phospholipid molecules left by the kinks in their unsaturated tails - the intercalation of cholesterol within the membrane phospholipids makes the bilayer stiffer and less flexible, as well as permeable - it does not, however, alter the thickness of the membrane

In a typical animal cell, proteins constitute what percentage of the mass of the plasma membrane? a. 25% b. 98% c. 75% d. 50% e. 2%

d. 50% - although the lipid bilayer provides the basic structure of all cell membranes and serves as permeability barrier to the hydrophilic molecules on either side of it, most membrane functions are carried out by membrane proteins - in animals, proteins constitute about 50% of the mass of most plasma membranes, the remainder being lipid plus the relatively small amounts of carbohydrate found on some of the lipids (glycolipids) and many of the proteins (glycoproteins)

What is true of human red blood cells? They possess a nuclear membrane, but no other internal membranes. They possess no internal membranes. They possess no membranes. They possess internal membranes, but no plasma membrane.

they posses no internal membranes - mature human red blood cells lack a nucleus and other intracellular organelles - thus, red blood cells have no internal membranes, only a plasma membrane - this lack of internal membranes makes them an attractive system for studying the structure and function of the plasma membrane

Determine whether the following statement is true or false: The glucose-Na+ symporter in epithelial cells uses the electrochemical gradient of Na+ to draw glucose into the cell.

true - the glucose-Na+ symporter in epithelial cells uses the electrochemical gradient of Na+ to draw glucose into the cell - because the electrochemical for Na+ is so steep, as Na+ enters the cell, glucose is brought into the cell with it, even when glucose concentrations in the cell are high - for the glucose uniport, conformational changes in a transporter mediate the passive transport of a solute such as a glucose - however, if glucose concentrations in the cell are high, then the uniport would not be sufficient mechanism to keep glucose entering the cell - the glucose-Na+ symporter solves this problem because it will create a constant flow of glucose into the intestinal epithelial cells

Lipid bilayers are highly impermeable to which molecule(s)? a. water b. carbon dioxide c. oxygen d. steroid hormones e. Na+ and Cl-

e. Na+ and Cl- - lipid bilayers are highly impermeable to many charged ions, and Na+ and Cl- are common examples of ions that are excluded from the hydrophobic interior of a lipid bilayer - for ion transport, cell membranes contain channel proteins that permit passage of ions - in contrast, small, nonpolar molecules, such as CO2 and O2, and hydrophobic steroid hormones easily pass through the lipid bilayer - water, even though it is polar, is small enough that it does enter the membrane at a measurable rate - however, water moves much more quickly through membrane proteins call aquaporins

Glucose enters the cell by which process? osmosis active transport simple diffusion facilitated diffusion

facilitated diffusion - glucose enters the cell by facilitated diffusion - when there is a concentration gradient, glucose passively diffuses into the cell through the glucose transporter - protein-mediated passive diffusion is called facilitated diffusion - conformational changes in a transporter mediate the passive transport of glucose - the glucose transporter: - in the outward-open state, the binding sites for solute are exposed on the outside - the inward-open state, the sites are exposed on the inside of the bilayer - in the occluded state, the sites are not accessible from either sides

In passive transport, the net movement of a charged solute across the membrane is determined by which of the following? the membrane potential its osmotic gradient alone its electrochemical gradient its concentration gradient

its electrochemical gradient - in passive transport, the net movement of a charged solute across the membrane is determined by its electrochemical gradient, which is a composite of two forces: one due to the concentration gradient and the other due to the membrane potential

What is a functionally specialized region of a cell membrane, typically characterized by the presence of specific proteins, called? sphingomyelin domain cell cortex carbohydrate layer glycocalyx membrane domain

membrane domain - cells have ways of confining particular proteins to localized areas within the bilayer, thereby creating functionally specialized regions on the surface of the cell or organelle - these subregions are called membrane domains - a tight junction creates two separate membrane domains: the apical membrane and the combined baso-lateral domain - proteins within these membrane regions can diffuse freely throughout their specific domains, but are prevented from entering the other domain by the tight junction

What is the voltage difference across a membrane of a cell called? electrical current potential balance gradient establishment membrane potential

membrane potential - in animal cells, for example, th4e resting membrane potential can be anywhere between -20 and -200 millivolts (mV), depending on the organism and cell type - the value is expressed as a negative number because the interior of the cell is more negatively charged than the exterior

The movement of an ion down its concentration gradient is called what? active transport passive transport osmosis pumping

passive transport - the movement of an ion down its concentration gradient is called passive transport - for example, if a solute is present at a higher concentration outside the cell than inside, and an appropriate channel or transporter is present in the plasma membrane, the solute will move into the cell by passive transport, without expenditure of energy by the membrane transport protein

Intracellular condensates are non-membrane bound biochemical subcompartments that form due to phase separation among networks of weakly interacting molecules. Sabari et al., 2018, proposed that the transcriptional coactivator BRD4 helps form intracellular condensates containing other transcriptional proteins. A prediction of this proposal is that BRD4 should behave as a liquid within the condensate with rapid movement. Which procedure could be used to analyze movement of BRD2 in living cells? A. fluorescence recovery after photobleaching (FRAP) B. solubilization with detergents C. fusion of mouse and human cells D. any of the listed techniques

- FRAP can track rates of diffusion of fluorescently tagged molecules - a portion of the fluorescent molecules are bleached with a laser, and movement of unbleached molecules into the bleached area is tracked over time - in 2018 Sabari found that after BRD4-fused GFP was photobleached, the fluorescence recovered quickly, suggesting liquid-like rates of movement consistent with an intracellular condensate

active transport

- active transport is the movement of an ion against its concent5ration gradient and it is carried out by special types of transporters called pumps - to allow for the accumulation of an excess of an ion on one side of a membrane, energy must be used, as the ions will naturally attempt to reach equilibrium

Which of the following can be a component of cell membranes? Choose one or more: sugar lipid DNA cholesterol protein

- all cell membrane consist of a two-ply sheet of lipids into which proteins have been inserted - this lipid bilayer serves as a permeability barrier to most water-soluble molecules, while the nutrient embedded within it carry out the other function of the membrane and give different membranes their individual characteristics - some lipids and proteins have sugar molecules attached to them - these sugars, which face the cell exterior, form a layer that protects cells and aids in cell recognition - cholesterol is a lipid found in the membrane of animal cells, where it regulates membrane fluidity - no cell membrane contain their own genetic material

Which term correctly describes the entire phospholipid molecule? Choose one: apathetic hydrophilic hydrophobic amphipathic hydropathic

- amphipathic - phospholipids contain both a hydrophilic head group and a pair of hydrophobic tails; they are therefore amphipathic - having both hydrophilic and hydrophobic parts is crucial in driving lipid molecules to assemble into bilayers in an aqueous environment - apathetic is a term that refers to an indifference - hydropathic refers to any type of medical treatment that involves the use of water

carbohydrates on the surface of leukocytes play an important role in responding to infection or inflammation. the following steps of the response are in order: a. cytokines are released at sites of infection or inflammation and stimulate endothelial cells of b. endothelial cells express selectins on their plasma membrane c. selectins bind to carbohydrates on the surface of leukocytes, causing them to stick d. leukocytes roll along vessel walls e. leukocytes crawl out of vessel into adjacent tissue

- at sites of infection, cytokines are released that stimulate local endothelial cells of blood vessels to express selectin proteins on their plasma membrane - selectin proteins bind specific carbohydrates on the surface of cells, including leukocytes (white blood cells) - when selectin binds to carbohydrate on a leukocyte, the binding is relatively weak, so contacts are made and broken - this leads to a slow rolling of the leukocyte along the blood vessel wall - at certain points, the leukocyte can adhere tightly and squeezer between the endothelial cells to pass pit the vessel and into the affected tissue where they help respond to the infection of inflammation

The plasma membrane is involved in which activities? Choose one or more - cell recognition - DNA replication and repair - RNA interference - cell growth and motility - cell signaling - import and export of nutrients and wastes

- cell recognition - cell growth and motility - cell signaling - import and export of nutrients and wastes - the plasma membrane is like a living skin that surrounds the exterior of the cell - it grows as the cell gows and contains proteins that allow the cell to move through its environment, import nutrients, export wast4es, and assess its surrounding, including neighboring cells - DNA replication and repair do not involve the plasma membrane; in eukaryotic cells, replication and repair take place in the nucleus - RNA interference is a laboratory method for silencing the expression of genes

which portion of a membrane phospholipid faces the outside of the membrane? Choose one: head tail fatty acids amphipathic portion none, because phospholipids are confined to the interior of the membrane

- head - amphipathic molecules, such as membrane lipids, contain both hydrophobic and hydrophilic components - these components are subject to two conflicting forces: the hydrophilic head is attracted to water, while the hydrophobic tails shun water and seek to aggerate with other hydrophobic molecules - this conflict is resolved by the formation of a lipid bilayer- an arrangement that satisfies all parties and is energetically most favorable

Which characteristic describes the tails of phospholipids? Choose one: amphipathic hydrophilic stiff hydrophobic coated with sugars

- hydrophobic - the phospholipids found in cell membranes combine two very different properties in single molecule: each lipid has a hydrophilic (water loving) head linked to a pair flexible, hydrophobic (water fearing) tails - this combination makes them amphipathic

T/F Given enough time, virtually any molecule will diffuse across a lipid bilayer.

- lipid bilayers are selectively permeable, meaning that they more easily allow some molecules to cross than others - for example, small, nonpolar molecules like gases can easily pass while ions, because of their charge, are largely prevented from crossing the membrane - however, with enough time, essentially all molecules will eventually diffuse across the lipid bilayer, even if the diffusion is inefficient - transmembrane transporter proteins increase the efficiency of movement of molecules that otherwise wouldn't easily cross the membrane, while still providing selectivity

integral vs peripheral proteins

- many membrane proteins extend through the bilayer, with part of their mass on either side - like their lipid neighbors, these transmembrane proteins are amphipathic, having both hydrophobic and hydrophilic regions - their hydrophobic regions lie in the interior of the bilayer, nestled against the hydrophobic tails of the lipid molecules - the hydrophilic regions are exposed to the aqueous environment on either side of the membrane - other membrane proteins are located almost entirely in the cytosol and are associated with the cytosolic half of the lipid bilayer by an amphipathic alpha helix exposed on the surface of the protein - yet other proteins lie entirely on the outside of the bilayer, on one side of the other, anchored to the membrane by one or more covalently attached lipid groups - all of these proteins, which are directly attached to the lipid bilayer and can be removed only by disrupting the bilayer with detergents, are considered integral membrane proteins - detergents are small, amphipathic, lipidlike molecules that have a hydrophilic head and a single hydrophobic tail - when mixed in great excess with membranes, the hydrophobic ends of detergent molecules interact with the membrane-spanning hydrophobic regions of the transmembrane proteins, as well as with the hydrophobic tails of the phospholipid molecules, thereby disrupting the lipid bilayer and separating the proteins from most of the phospholipids - because the other end of the detergent molecule is hydrophilic, these interaction draw the membrane proteins into the aqueous solution as protein-detergent complexes - another class of membrane proteins are bound to one face of the membrane or the other indirectly and are held in place only by their interactions with other membrane proteins - such proteins can be released from the membrane by more gentle extraction procedures that interfere with protein-protein interactions but leave the lipid bilayer intact (peripheral)

Why do phospholipids form bilayers in water? Choose one: The hydrophobic head is attracted to water, while the hydrophilic tail shuns water. The hydrophobic tail is attracted to water, while the hydrophilic head shuns water. The hydrophilic head is attracted to water, while the hydrophobic tail shuns water. The hydrophobic head shuns water, while the hydrophilic tail is attracted to water. The hydrophilic head is insoluble in water.

- the hydrophilic head is attracted to water. while the hydrophobic tail shuns water - phospholipids are amphipathic molecules: their hydrophilic heads are water soluble, whereas their hydrophobic tails are repelled by water - in an aqueous environment, the insoluble, hydrophobic tails will shield themselves from water by interacting with one another in the interior bilayer - the hydrophilic heads, meanwhile will form electrostatic interactions and hydrogen bonds with water molecules on either side of the bilayer

In a lipid bilayer, where do lipids rapidly diffuse? - in and out of the bilayer - back and forth from one monolayer to the other in the bilayer - within the plane of one monolayer and back and forth between the monolayers - not at all, because they remain in place within the bilayer - within the plane of their own monolayer

- within the plane of their own monolayer - the lipid bilayer acts like a two dimensional fluid - once phospholipids are inserted into the bilayer by the enzymes that synthesize them, they diffuse rapidly and continuously within the plane of the monolayer into which they were added - phospholipids very rarely tumble from one monolayer to the other - the cell contains transporters that can move phospholipids from one membrane monolayer to the other as needed - phospholipids also do not spontaneously pop in and out of the bilayer; their hydrophobic tails make interaction with the aqueous environment around the membrane unfavorable - phospholipids can be added to or removed from the bilayer as part of lipid vesicles

Most sports drinks contain both carbohydrates and salts. The carbohydrates replace glucose burned during exercise and the salts replace salts lost in sweat. The salt also helps the small intestine absorb glucose. Pick the answer that accurately describes which salt is most beneficial for glucose absorption. A. NaCl, because Na+ is needed for glucose entry. B. HCl, because H+ is needed for glucose entry. C. KCl, because K+ is needed for glucose entry. D. KCl, because Cl- is needed for glucose entry.

A. NaCl, because Na+ is needed for glucose entry. - many different transmembrane proteins contribute to glucose absorption across the intestinal epithelium - the Na+ -K+ ATPase, located in the basal membrane, keeps intracellular sodium low - glucose is brought into the epithelial cells from the gut lumen, against its concentration gradient, by the action of a glucose-Na+ symport protein

Which of the following statements is not true regarding active transport by transmembrane pumps? A. Some solutes are transported across the membrane in tandem with other molecules, both moving from lower concentration to higher concentration. B. Some solutes are moved against their concentration gradients, from one side of a membrane to the other, using energy from ATP hydrolysis. C. Some solutes are moved across a membrane against their concentration gradient using energy from sunlight.

A. Some solutes are transported across the membrane in tandem with other molecules, both moving from lower concentration to higher concentration. - it is true that some solutes are transported across the membrane in tandem with other molecules, but both do not move from lower concentration to higher concentration - coupled pumps can link the uphill transport of one solute across a membrane to the downhill transport of another - in this manner, the concentration gradient for one solute is used to drive the mechanism of the pump that moves another solute against its concentration gradient - in terms of other energy sources that drive membrane pumps, some solutes are moved across a membrane against their concentration gradient using energy from sunlight; such movement is facilitated by light-driven pumps - additionally, some solutes are moved against their concentration gradients, from one side of a membrane to the other, using energy from ATP hydrolysis; such movement is facilitated by ATP-driven pumps

In the technique called optogenetics, light-gated Na+ channels are introduced into the brains of living animals. Activation of these channels by light can depolarize the membranes of neurons that contain them, selectively activating these target cells. Since its inception, optogenetics has been expanded to include other types of light-gated channels, such as a channel that is selective for Cl- instead of Na+. If this light-gated Cl- channel were introduced into neurons in a region of the brain that stimulates feeding, what might you expect to see? A. The animals would avoid eating, even when they are hungry—but only when the channels are activated by light. B. The animals would avoid eating, even when they are hungry. C. In response to light activation, the animals would overeat, even when they are full. D. The channels would have no effect on behavior because the animal's normal Na+ channels would allow normal depolarization of neurons that regulate feeding. E. The animals would avoid eating, but only during the day.

A. The animals would avoid eating, even when they are hungry—but only when the channels are activated by light. - for this experiment, the animals would avoid eating, even when they are hungry- but only when the channels are activated by light - because the cells are deep in the animal's brain, normal daylight will not reach them, so the activating light is supplied by a special fiber-optic cable implanted in the animal's brain - the reverse experiment, when light gated Na+ channels are introduced into these neurons instead of he Cl- channels, activation by light causes animals to overeat- even when they have recently fed

Which of the following would produce the most fluid lipid bilayer: A. phospholipids with tails of 18 carbon atoms and two double bonds B. phospholipids with fully saturated tails of 18 carbon atoms C. phospholipids with fully saturated tails of 20 carbon atoms D. large amounts of cholesterol E. phospholipids with tails of 20 carbon atoms and two double bonds

A. phospholipids with tails of 18 carbon atoms and two double bonds - the fluidity of a membrane- the ease with its lipid molecules move within the plane of the bilayer- depends on the nature of the lipids' hydrocarbon tails; the closer and more regular the packing of the tails, the more viscous and less fluid the membrane will be - a shorter chain length and the presence of double bonds both reduce the tendency of the phospholipid tails to interact with one another and pack tightly, thereby increasing the fluidity of the membrane

A toxin present in scorpion venom prolongs the duration of action potentials in nerve cells. Which of these actions would best explain how this toxin exerts its effect? A. It prolongs the inactivation of voltage-gated Na+ channels. B. It slows the inactivation of voltage-gated Na+ channels. C. It accelerates the opening of voltage-gated K+ channels. D. It inhibits the opening of voltage-gated Na+ channels. E. It slows the inactivation of voltage-gated K+ channels.

B. It slows the inactivation of voltage-gated Na+ channels. - the toxin in scorpion venom exerts its effect by slowing the inactivation of voltage-gated Na+ channels, causing them to be stuck in the open conformation and prolonging the action potential - in contrast, prolonged inactivation of voltage-gated Na+ channels would delay subsequent action potentials and inhibition of voltage-gated Na+ channels would prevent depolarization of the membrane

Which of the following requires an input of energy to occur? A. the movement of a solute from a region of higher concentration on one side of a membrane to a region of lower concentration on the other side B. The movement of a solute from a region of lower concentration on one side of a membrane to a region of higher concentration on the other side C. Both of these options require energy investment because diffusion is a change in a system, and any change requires energy.

B. The movement of a solute from a region of lower concentration on one side of a membrane to a region of higher concentration on the other side - active transport occurs when a solute from a region of lower concentration on one side of a membrane moves to a region of higher concentration on the other side of the membrane - this action requires an input of energy to occur - passive transport describes the movement of a solute from a region of higher concentration on one side of a membrane to a region of lower concentration on the other side, and does not require an energy investment

Which of the following describes the resting membrane potential of a neuron? A. a voltage difference of 0 millivolts (mV) across the membrane B. a state in which the flow of positive and negative ions across the plasma membrane is precisely balanced C. a voltage difference across the plasma membrane when the neuron has been stimulated D. a voltage difference across the plasma membrane, with more positive membrane potential inside E. a voltage difference that is chiefly a reflection of the electrochemical Na+ gradient across the plasma membrane

B. a state in which the flow of positive and negative ions across the plasma membrane is precisely balanced - the resting membrane potential is the voltage difference across a membrane of an unstimulated cell, and for most animal cells, the resting membrane potential is negative, between -20 and -200 mV - the resting membrane potential is a state in which the movement of positive and negative ions across the membrane is precisely balanced; in this state, no further differences in charge will accumulate and there are not necessarily an equal number of positive and negative charges on both sides of the membrane - rathe, the resting membrane potential in animal cells is chiefly a reflection on the electrochemical K+ gradient across the plasma membrane

Which organelle is important for controlling the concentration of calcium ions in the cytosol? A. nucleus B. endoplasmic reticulum C. lysosome D. Golgi apparatus

B. endoplasmic reticulum - the endoplasmic reticulum is important for controlling the concentration of calcium ions in the cytosol - Ca2+ pumps in the endoplasmic reticulum membrane, as well as the plasma membrane, keep cytosolic Ca2+ concentrations low - eukaryotic cells in general maintain a very low concentration of free Ca2+ in their cytosol (about 10^-4 mM) compared to the much higher concentration of Ca2+ outside of the cell (typically 1-2 mM) - this huge concentration differences is achieved mainly by means of ATP-driven Ca2+ pumps in both the plasma membrane and the endoplasmic reticulum membrane, which actively remove Ca2+ from the membrane cytosol

Which term describes a coupled transporter that moves both solutes in the same direction across a membrane? A. uniport B. symport C. antiport

B. symport - by paying attention to the prefixes of these terms, you can deduce the nature of the movement of molecules through them - the prefix sym means same while anti means opposite - therefore, a symport will move two different molecules across a membrane in the same direction, while an antiport will move the different molecules in opposite directions across the membrane - the prefix uni means one and hence uniport will only move one type of molecule across a membrane

In bacteria, the transport of many nutrients, including sugars and amino acids, is driven by the electrochemical H+ gradient across the plasma membrane. In E. coli, for example, an H+-lactose symporter mediates the active transport of the sugar lactose into the cell. Given what you know about coupled transport, which is likely true of the H+-lactose symporter? A. If the H+ gradient were reversed, the transporter could serve as an H+-lactose antiport. B. To undergo the conformational change that releases lactose into the cell, the transporter hydrolyzes ATP. C. The transporter oscillates randomly between states in which it is open to either the extracellular space or the cytosol. D. Lactose and H+ ions bind to two different conformations of the transporter. E. The transporter goes through an intermediate state in which the lactose-binding site is open to both sides of the membrane.

C. The transporter oscillates randomly between states in which it is open to either the extracellular space or the cytosol. - for a transporter to move solutes in the same direction across a membrane, the solutes must bind to the same conformation of the transporter - the H+ -lactose symporter oscillates randomly between states in which it is open to either the extracellular space or the cytosol - in one state, the transporter is open to either the extracellular space; in the other, it is open to the cytosol - to transition from one state to the other, the transporter must pass through an "occluded" state in which the transporter is either empty or both solutes are bound - coupled transporters use the flow of one solute down its electrochemical gradient to drive the transport of a second solute against its electrochemical gradient; therefore, no ATP hydrolysis is needed- the energy source is, in this case, the ion gradient

Why do cells lack membrane transport proteins that are specific for the movement of O2? A. because transport of oxygen across cell membranes is energetically unfavorable B. because oxygen is transported in and out of the cell by special oxygen-binding proteins such as hemoglobin C. because oxygen dissolves readily in lipid bilayers D. because oxygen, dissolved in water, can enter cells via aquaporins E. because oxygen concentrations must be kept low inside cells to avoid creating reactive superoxide radicals that can damage DNA and proteins

C. because oxygen dissolves readily in lipid bilayers - cells lack membrane transport proteins that are specific for the movement of O2 because oxygen dissolves readily in lipid bilayers - this small , nonpolar molecule can diffuse across the cell membrane without the need for a membrane transport protein - the channels of aquaporins are lined with amino acids that provide an environment for the formation to transient hydrogen bonds that facilitate the passage of water molecules, which line up in single file - the channels of aquaporins exclude ions and most other molecules, including O2

Sodium ions, oxygen (O2), and glucose pass directly through lipid bilayers at dramatically different rates. Which of the following choices presents the correct order, from fastest to slowest? A. glucose, sodium ions, oxygen B. oxygen, sodium ions, glucose C. oxygen, glucose, sodium ions D. sodium ions, oxygen, glucose E. glucose, oxygen, sodium ions

C. oxygen, glucose, sodium ions - various molecules pass directly through lipid bilayers at dramatically different rates - oxygen is small and nonpolar - glucose is large and polar, but it has no charge - sodium ions least readily diffused

A group of researchers wanted to sort different white blood cell types (monocytes, lymphocytes, and granulocytes) apart from each other based on size differences and to remove unwanted contaminating red blood cells. After a particular manipulation, the red blood cells lysed. The remaining white blood cells increased in size and, more importantly, the size differences among cells increased, allowing for size-based sorting (which requires minimum size differences among cells). What manipulation did the researchers use to increase cell size? A. patch-clamp recording to monitor ion channel activity B. placing cells in an environment with lower temperatures than the cells were previously exposed to C. placing cells in an environment with a lower solute concentration than that in the cells D. placing cells in an environment with a higher solute concentration than that in the cells

C. placing cells in an environment with a lower solute concentration than in the cell - a difference in solute concentration on either side of a membrane leads to osmosis, the passive movement of water across a membrane from a region of low solute concentration (where water concentration is high) to a region of high solute concentration (where the water concentration is low) - the researchers suggest that after exposure to deionized water, different cell populations swell at different rates due to the relative abundance of aquaporins

When glucose moves across a phospholipid bilayer by passive transport, which factor determines the direction of its transport? A. the amount of energy available to fuel the transport process B. the charge difference across the membrane C. the concentrations of glucose on either side of the membrane D. whether the cell is metabolically active or not

C. the concentrations of glucose on either side of the membrane - when glucose moves across a phospholipid bilayer by passive transport, the concentrations of glucose on either side of the membrane determines the direction of its transport - unlike ions, which move across membranes according to their concentration and membrane potential, glucose is uncharged, so the direction it moves is determined by its concentration gradient alone

Which of the following statements is true? A. Inside the cell, the quantity of positively charged ions is much less than the quantity of negatively charged ions. B. Inside the cell, the quantity of positively charged ions is much greater than the quantity of negatively charged ions. C. Inside the cell, there are no negatively charged ions. D. Inside the cell, the quantity of positively charged ions is almost equal to the quantity of negatively charged ions. E. Inside the cell, there are no positively charged ions.

D. Inside the cell, the quantity of positively charged ions is almost equal to the quantity of negatively charged ions. - inside the cell, the quantity of positively charged ions is almost equal to the quantity of negatively charged ions - for the cytoplasm of a cell to avoid being significantly disrupted by electrical forces, the quantity of positively charged ions must be balanced by an almost exactly equal quantity of negatively charged inorganic and organic anions, including nucleic acids, proteins, and many cell metabolites - although the electrical charges inside and outside of the cell are generally kept in balance, tiny excesses of positive or negative charge, concentrated in the neighborhood of the plasma membrane, do occur - such electrical imbalances generate a voltage difference across the membrane - the resting membrane potential is integral to many activities that occur across the plasma membrane

When a neuron is activated by a stimulus, its plasma membrane will change until it reaches a membrane potential of about +40 mV. What is special about this value? A. It is approximately the membrane potential at which the electrochemical gradient for K+ is zero. B. It is the opposite of the resting membrane potential C. It is the threshold potential that opens voltage-gated Na+ channels. D. It is approximately the membrane potential at which the electrochemical gradient for Na+ is zero E. It is the threshold potential at which voltage-gated Na+ channels close.

D. It is approximately the membrane potential at which the electrochemical gradient for Na+ is zero - when a neuron is activated by stimulus, its plasma membrane will change until it reaches a membrane potential of about +40 mV - this value is special because it is approximately the membrane potential at which the electrochemical gradient for Na+ is zero - around +40 mV, Na+ ions have no further tendency to enter or leave the cell - in other words, they are near their theoretical equilibrium potential - according to the nernst equation the electrochemical gradient for K+ is near zero when a cell is at its resting potential, around -20 to -200 mV

Which of the following accurately describes the role of the Na+-K+ pump? A. It maintains a higher K+ concentration outside the cell. B. It maintains a lower Na+ concentration outside the cell. C. It equilibrates the concentrations of Na+ and K+ across the plasma membrane. D. It maintains a higher Na+ concentration outside the cell.

D. It maintains a higher Na+ concentration outside the cell. - the Na+ -K+ pump uses energy from the hydrolysis of ATP to establish a strong electrochemical gradient of Na+ and K+ ions, with Na+ ions pumped to the extracellular space and K+ ions to the cytosol - establishing these electrochemical gradients is such a fundamental process in cells that about one-third of the ATP consumed by the cell is used by this pump - in addition to ATP hydrolysis, the function of the pump requires a carefully orchestrated set of conformational changes to operate and ensures that only Na+ and K+ ions are moved across the membrane by the pump

When the glucose-Na+ symport protein is in its outward-open state, which is more likely to occur? A. The transporter releases Na+ into the cell. B. The transporter releases glucose into the cell. C. Glucose binds to its binding site. D. Na+ binds to its binding site. E. Both solutes bind simultaneously.

D. Na+ binds to its binding site. - a glucose-Na+ symport uses electrochemical Na+ gradient to drive the active import of glucose - when the glucose-Na+ symport protein is in its outward-open state, Na+ binding to its binding site is more likely to occur - because Na+ concentrations are high outside the cell, Na+ readily binds to the transporter in its outwards-open state - the transporter must then wait for a rare glucose molecule to bind - in the outward-open state, the transporter binds to solutes in the extracellular space - the pump is open to the extracellular space; in another state (inward-open), it is open to the cytosol

Which membrane would show a more rapid recovery of fluorescence in a FRAP study? A. a membrane containing equal amounts of saturated and unsaturated fatty acids B. a membrane containing a larger proportion of saturated fatty acids C. The saturation of fatty acids in a cell membrane does not affect the speed of fluorescence recovery in a FRAP study. D. a membrane containing a larger proportion of unsaturated fatty acids E. a membrane containing a large amount of cholesterol

D. a membrane containing a larger proportion of unsaturated fatty acids - FRAP (fluorescence recovery after photobleaching) is a method used to measure the fluidity of a cell membrane - the more fluid a membrane is the more rapid its recovery in FRAP - the fluidity of a cell membrane- the ease with which its lipid molecules move within the plane of the bilayer- depends on its phospholipid composition and, in particular, on the nature of the hydrocarbon tails: the closer and more regular the packing of the tails, the more viscous and less fluid the bilayer will be - hydrocarbons: the length and the number of double bonds they contain affect how tightly they pack in the bilayer - a membrane containing a larger portion of unsaturated fatty acids would be the most fluid and hence would show the fastest recovery in a FRAP study

Mutation in the hemoglobin gene can cause sickle-cell anemia. The defective protein found in sickle-cell anemia causes red blood cells to "sickle"—become a misshapen C shape. These misshapen cells abnormally stick to each other and can become trapped by leukocytes (white blood cells) that are rolling or paused on the endothelial cells lining the vessel. This causes blockages of small blood vessels, causing severe pain and strokes called vaso-occlusive crisis. A new drug that binds and blocks selectin proteins is in phase III clinical trials to test for improvement in patients' symptoms. Why might this be an effective treatment for vaso-occlusive crisis? A. Blocking selectins would block the ability of selectin to bind carbohydrates on the surface of red blood cells, preventing the blockage. B. Blocking selectins on red blood cells would prevent the red blood cells from binding to the blood vessel endothelial cells, preventing the blockage of red blood cells. C. Blocking selectins would reduce activation of pain sensors in the blood vessels. D. Blocking selectins would block the ability of selectin to bind leukocytes, so leukocytes would be less likely to move slowly along the vessel wall and cause a blockage of red blood cells.

D. blocking selectins would block the ability of selectins to bind leukocytes, so leukocytes would be less likely to move slowly along the vessel wall and cause a blockage of red blood cells - selectins are expressed by the endothelial cells lining veins - the selectins bind to carbohydrates on the surface of leukocytes (white blood cells) to slow the movement of the leukocytes through the vein - the leukocytes roll along the vessel wall before squeezing between endothelial cells into the surrounding tissue - a drug that can bind and block selectin proteins would lessen the number of leukocytes bound to the vessel wall - there would then be fewer leukocytes to trap the deformed red blood cells and the red blood cells should continue to move through the blood vessels - fewer blockages would lead to less pain and a reduced risk of strokes that occur in vaso-occlusive crisis

Which of the following form tiny hydrophilic pores in the membrane through which solutes can pass by diffusion? A. liposomes B. pumps C. anions D. channels E. transporters

D. channels - membrane channels form tiny hydrophilic pores in the membrane through which solutes can pass through diffusion - solutes that are small enough to pass through the channel will diffuse through, while those that are too large will not - most channels only permit passage of ions and are therefore referred to as ion channels - because ions are electrically charged, their movements can create powerful electric force- or voltage- across the membrane

Which of the following correctly describes osmosis? A. the movement of water from an area of low solvent concentration to an area of high solvent concentration B. the movement of water from an area of low water concentration to an area of high water concentration C. the movement of water from an area of high solute concentration to an area of low solute concentration D. the movement of water from an area of low solute concentration to an area of high solute concentration

D. the movement of water from an area of low solute concentration to an area of high solute concentration - osmosis is the movement of water from an area of low solute concentration to an area of high solute concentration - when the solute concentration is high, the water concentration is low; water will then naturally move toward an area of high solute concentration, diluting the solute - another way to consider this is that like any molecule, water tends to move down its concentration gradient, from an area where its concentration is high to one which its concentration is lower

In one experiment, investigators create a liposome—a vesicle made of phospholipids—that contains a solution of 1 mM glucose and 1 mM sodium chloride. If this vesicle were placed in a beaker of distilled water, what would happen the fastest? A. Na+ would diffuse out. B. Cl- would diffuse out. C. NaCl would diffuse out. D. Glucose would diffuse out. E. H2O would diffuse in.

E. H2O would diffuse in. - in this experiment, the possible molecules that can move across the membrane are water, glucose, and ionized sodium chloride - Glucose requires a transporter to move across a lipid membrane because of its relatively large size and the experimental design does not include these transporter proteins in the liposome - glucose will not leave the liposome - Sodium chloride will ionize into Na+ and Cl- when dissolved into solution - While small in size, the charge of these molecules likewise means that they cannot diffuse across the nonpolar liposome membrane without a channel protein - On the other hand, water is small enough, as well as noncharged, meaning that it can cross the membrane - Distilled water outside of the cell lacks dissolved solutes (i.e., high water concentration), whereas the interior of the liposome has a relatively high concentration of two different solutes (i.e., low water concentration) - Water will follow its concentration gradient and move into the liposome.

When Na+ channels are opened in an animal cell, what happens to the membrane potential? A. It becomes more negative inside the cell. B. It stays the same. C. It disappears, and membrane potential stabilizes at 0 mV. D. It rapidly reaches the resting membrane potential. E. It becomes less negative inside the cell.

E. It becomes less negative inside the cell. - Na+ channels of cells are usually opened in response to stimulation; the resting membrane potential is associated with an unstimulated cell - when stimulated, Na+ channels open in an animal cell and the membrane potential charges; it becomes less negative inside the cell compared to the resting membrane potential - this is because when Na+ channels are opened, Na+ rushes into the cell - this rapid entry of positive ions makes the membrane potential less negative inside - if the depolarization is sufficiently large, it will cause voltage-gated Na+ channels in the membrane to open transiently at the site

The epithelial cells that line the gut have glucose-Na+ symport proteins that actively take up glucose from the lumen of the gut after a meal, creating a high glucose concentration in the cytosol. How do these cells release that glucose for use by other tissues in the body? A. The cells have glucose channels in their plasma membrane. B. Glucose diffuses down its concentration gradient through the lipid bilayer of the plasma membrane. C. The cells have a glucose pump that expels the glucose needed by other tissues. D. The cells run the glucose-Na+ symport proteins in reverse. E. The cells have glucose uniports in their plasma membrane.

E. The cells have glucose uniports in their plasma membrane. - the two types of glucose transporters enable gut epithelial cells to transfer glucose across the epithelial lining of the gut - epithelial cells that have absorbed intestinal glucose release that glucose for use by other tissues in the body through glucose uniports in their plasma membrane - these passive glucose uniports allow glucose to move down its concentration gradient, out of the cell - the glucose uniport is only found on the basal and lateral regions of the plasma membrane, ensuring it doesn't flow back into the gut lumen

Which of the following statements is true? K+ and Na+ are both maintained at high concentrations inside the cell compared to out. Na+ is the most plentiful positively charged ion outside the cell, while K+ is the most plentiful inside. K+ and Na+ are present in the same concentration on both sides of the plasma membrane. K+ and Na+ are both excluded from cells. K+ is the most plentiful positively charged ion outside the cell, while Na+ is the most plentiful inside.

Na+ is the most plentiful positively charged ion outside the cell, while K+ is the most plentiful inside. - because lipid bilayers are impermeable to inorganic ions, inorganic ions, living cells are able to maintain internal ion concentrations that are very different from the concentrations of ions in the medium that surrounds them - Na+ is the most plentiful positively charged ion outside the cell, while K+ is the most plentiful inside - cells expend a great deal of energy to maintain this chemical imbalance, and such electrical imbalances generate a voltage differences across the membrane called the membrane potential

The Na+ pump in the plasma membrane of animal cells uses energy from ATP hydrolysis to pump sodium and potassium ions against their electrochemical gradients. In which direction are the ions pumped across the membrane? Na+ and K+ both out Na+ in and K+ out Na+ and K+ both in Na+ out and K+ in

Na+ out and K+ in - the Na+ pump in the plasma membrane of animal cells uses energy from ATP hydrolysis to pump sodium and potassium ions against their electrochemical gradients; specifically, NA+ is pumped out of the cell and K+ is pumped into the cell - Na+ is at a higher concentration outside the cell and K+ is at a higher concentration inside the cell - because Na+ is maintained at a much higher concentration outside the cell, it can enter cells passively if given an opportunity - furthermore, this Na+ gradient can be used to drive the import of other substances into the cell by membrane pumps that exhibit a coupled transport mechanism

Which statements are true about the differences between phospholipids and detergents? a. Detergents are shaped like cones, whereas phospholipids are more cylindrical. b. Phospholipids have two hydrocarbon tails, whereas detergents have just one. c. Phospholipids form bilayers in water, whereas detergents tend to form micelles. d. Phospholipids are amphipathic, whereas detergents are hydrophobic. e. Phospholipids are hydrophobic, whereas detergents are amphipathic.

a,b,c - detergents are small, lipidlike molecules that differ from membrane phospholipids in that they have only a single hydrophobic tail - because they have one tail, detergent molecules are shapes like cones, whereas phospholipids-with their two hydrophobic are more cylindrical in shape - these shapes affect how these molecules behave in an aqueous environment - phospholipids, which are amphipathic, form lipid bilayers in which the hydrophobic tails are shielded from water on the interior of the bilayer - detergents are also amphipathic - but instead of forming a bilayer, the conical molecules pack together into small clusters called micelles, in which their hydrophobic tails are shielded in the micelle interior and their hydrophilic heads are exposed to the water

Which of the following is a function of proteins in the plasma membrane? a. serve as anchors to attach the cell to the extracellular matrix b. transmit extracellular signals to the cell interior c. allow specific ions to cross the plasma membrane, thereby controlling its electrical properties d. transport molecules across the membrane e. generate the energy required for lipids to diffuse within the membrane

a,b,c,d - each type of cell membrane contains a different set of proteins, reflecting the specialized functions of the particular membrane - some membrane proteins transport particular nutrients and metabolites across the lipid bilayer - others anchor the membrane the to macromolecules on either sides, allowing cells to establish connection with neighboring cells or with the extracellular matrix to form tissues - still other other proteins function as receptors that detect chemical signals in the cell's environment and relay them into the cell interior, or work as enzymes to catalyze specific reactions at the membrane - ion channels establish charge imbalances that help drive membrane transport and allow communication by electrically excitable cells

Why are the oils found in plant seeds and the fat droplets found in the fat (or adipose) cells of animals similar? a. Both are hydrophobic. b. Both are unsaturated. c. Both are saturated. d. Both are amphipathic. e. Both form a lipid bilayer in water.

a. both are hydrophobic - triacylglycerols, which are the main constituents of animal fats and plant oils, have three fatty acid tails and no hydrophilic head - these fats are thus entirely hydrophobic - fats that are strictly hydrophobic coalesce into a single large droplet when dispersed in water

What effect do double bonds have on phospholipid hydrocarbon tails and on the fluidity of the membrane? a. Double bonds decrease the ability of hydrocarbon tails to pack together, which makes the bilayer more fluid. b. Double bonds increase the ability of hydrocarbon tails to pack together into a rigid mass, which makes the bilayer more fluid. c. Double bonds have little effect on membrane fluidity. d. Double bonds increase the ability of hydrocarbon tails to pack together into a rigid mass, which makes the bilayer less fluid. e. Double bonds decrease the ability of hydrocarbon tails to pack together into a rigid mass, which makes the bilayer less fluid.

a. double bonds decrease the ability of hydrocarbon tails to pack together, which makes the bilayer more fluid - the fluidity of a cell membrane depends on the nature of lipids' hydrocarbon tails: the closer and more regular the packing of the tails, the more viscous and less fluid the bilayer will be - double bonds create small kinks in the hydrocarbon tails, making it more difficult for them to pack together - for this reason, lipid bilayers that contain a large proportion of unsaturated hydrocarbon tails are more fluid that those with smaller proportions

When scientists were first studying the fluidity of membranes, they did an experiment using hybrid cells. Certain membrane proteins in a human cell and a mouse cell were labeled using antibodies coupled with differently colored fluorescent tags. The two cells were then coaxed into fusing, resulting in the formation of a single, double-sized hybrid cell. Using fluorescence microscopy, the scientists then tracked the distribution of the labeled proteins in the hybrid cell. Which best describes the results they saw and what they ultimately concluded? a. Initially, the mouse and human proteins were confined to their own halves of the newly formed hybrid cell, but over time, the two sets of proteins became evenly intermixed over the entire cell surface. This suggests that proteins, like lipids, can move freely within the plane of the bilayer. b. Initially, the mouse and human proteins were confined to their own halves of the newly formed hybrid cell, but over time, the two sets of proteins recombined such that they all fluoresced with a single, intermediate color. c. At first, the mouse and human proteins were confined to their own halves of the newly formed hybrid cell, but over time, the two sets of proteins became divided such that half faced the cytosol and half faced the hybrid cell exterior. This suggests that flippases are activated by cell fusion. d. The mouse and human proteins remained confined to the portion of the plasma membrane that derived from their original cell type. This suggests that cells can restrict the movement of their membrane proteins to establish cell-specific functional domains. The mouse and human proteins began to intermix and spread across the surface of the hybrid cell, but over time, one set of proteins became dominant and the other set was lost. This suggests that cells can ingest and destroy foreign proteins. e. Initially, the mouse and human proteins intermixed, but over time, they were able to resegregate into distinct membrane domains. This suggests that cells can restrict the movement of membrane proteins.

a. initially, the mouse and human proteins were confined to their own halves of the newly formed hybrid cell, but over time, the two sets of proteins became evenly intermixed over the entire cell surface. this suggests that proteins, like lipids, can move freely within the plane of the bilayer - because a membrane is a two-dimensional fluid, many of its proteins, like its lipids, can move freely within the plane of the bilayer - this lateral diffusion was initially demonstrated by experimentally fusing a mouse cell with a human cell to form a large, hybrid cell and then monitoring the distribution of certain mouse and human plasma membrane proteins - at first, the mouse and human proteins are confined to their own halves of the newly formed hybrid cell, but within half an hour or so, the two sets of proteins became evenly mixed over the entire cell surface - to monitor the movement of the selected proteins, the cells were labeled with antibodies that bind to either human or mouse proteins; these antibodies are coupled to two different fluorescent tags- shown in red and blue- so that the proteins to which the antibodies can be distinguished in a fluorescence microscope

When a vesicle fuses with the plasma membrane, which way will the monolayer that was exposed to the interior of the vesicle face? a. the cell exterior b. the endomembrane system c. The direction the monolayer will face will be established randomly. d. It depends on where, along the plasma membrane, the vesicle fuses. e. the cell cytoplasm

a. the cell exterior - most cell membranes are asymmetric and have distinct "inside" and "outside" faces: the cytosolic monolayer always faces the cytosol, while the noncytosolic monolayer is exposed to either cell exterior- in the case of the plasma membrane- or the interior space (lumen) of an organelle - this asymmetry is preserved as membranes, in the form of vesicles, which bud from one organelle and fuse with another or with the plasma membrane

why must all living cells carefully regulate the fluidity of their membranes? a. to permit membrane lipids and proteins to diffuse from their site of synthesis to other regions of the cell b. to allow membranes, under appropriate conditions, to fuse with one another and mix their molecules c. to ensure that membrane molecules are distributed evenly between daughter cells when a cell divides d. to constrain and confine the movement of proteins within the membrane bilayer e. to allow cells to function at a broad range of temperatures

a. to permit membrane lipids and proteins to diffuse from their site of synthesis to other regions of the cell b. to allow membranes, under appropriate conditions, to fuse with one another and mix their molecules c. to ensure that membrane molecules are distributed evenly between daughter cells when a cell divides - the fluidity of a membrane enables many membrane proteins to diffuse rapidly in the plane of the bilayer and to interact with one another, for example in cell signaling. - because a membrane is a two dimensional fluid, many of its proteins, like its lipids, can move freely within the plane of the bilayer - membrane fluidity does not promote the restriction of protein movement - a fluid membrane permits newly synthesized lipids and proteins to diffuse from sites where they are inserted into the bilayer to other regions of the cell - not all cells are exposed to a broad range of environmental temperatures. mammalian cells for example, are generally maintained within a narrow range of temperatures. these cells, therefore, do not regulate their membrane fluidity in response to fluctuating temperature

Each of these mechanisms of transport, as illustrated above, is best categorized by which term? A. simple diffusion B. osmosis C. active transport D. facilitated diffusion

active transport - a solute moving up its concentration gradient - this means that there is an energetic investment that is being used to pump a solute- the definition of active transport - first, gradient-driven pumps link the uphill transport of one solute across a membrane to the downhill transport of another - second, ATP-driven pumps use the energy released by the hydrolysis of ATP to drive uphill transport - third, light-driven pumps, which are found mainly in bacterial cells, use energy derived from sunlight to drive uphill transport

The drug scopolamine is used to treat dizziness, motion sickness, and smooth muscle spasms. When isolated muscle cells are incubated with scopolamine, addition of acetylcholine no longer depolarizes the muscle cell membrane or stimulates muscle cell contraction. Which would best explain how scopolamine exerts its muscle-relaxing effects? A. It inhibits the opening of Ca2+ channels in the sarcoplasmic reticulum. B. It inhibits the transporters that pump Na+ into the muscle cell cytosol during an action potential. C. It inhibits the opening of voltage-gated K+ channels. D. It inhibits the opening of acetylcholine-gated Na+ channels in the muscle cell membrane. E. It inhibits the transporters that pump Ca2+ into the muscle cell cytosol during an action potential.

b. It inhibits the opening of acetylcholine-gated Na+ channels in the muscle cell membrane. - scopolamine exerts its muscle-relaxing effects by inhibiting the opening of acetylcholine-gated NA+ channels in the muscle cell membrane - normally, acetylcholine triggers muscle contraction by opening a ligand-gated Na+ channel, which leads to membrane depolarization and contraction of the muscle cell because during an action potential, Na+ enters the cytosol through voltage-gated Na+ channels - however, in scopolamine-treated cells, the ligand-gated Na+ channel will not open and the action potential will not generate

The following graphs show the number of adherent leukocytes found on the blood vessel wall in control conditions and after adding a selectin inhibitor, which blocks the function of selectin. Which of the following graphs correctly shows the effect of a selectin inhibitor on adherence of leukocytes to the vessel wall? a. graph a- control is lower that drug added b. graph b- control is higher than drug added c. graph c- control and drug added are pretty close in levels

b. drug b - a drug that inhibits selectin function on the endothelial cells will inhibit the ability of the selectin to bind to carbohydrates on the surface of leukocytes - graph b shows fewer leukocytes adhering to endothelial cells because selectin is blocked - the leukocytes still express the carbohydrates on their surface, but the selectin on the endothelial cells is blocked from binding - this leads to fewer leukocytes being bound and can lead to fewer red blood cells trapped in sickle-cell anemia

In an electron transport chain, electrons are passed from one transmembrane electron carrier to another, driving proton movement across a membrane (see image below). The protons then flow through ATP synthase (not shown) to generate ATP. researchers probed how membrane fluidity affects electron transport chain activity and ATP production in E. coli by manipulating membrane fluidity and measuring respiration. how could researchers have increased membrane fluidity? a. increase the amount of cholesterol present in the bacterial membranes b. increase the proportion of phospholipids with unsaturated fatty acids c. decreases the temperature of the media the E. coli were grown in d. increase the length of the fatty acid tails in phospholipids

b. increase the proportion of phospholipids with unsaturated fatty acids - the fluidity of a bilayer depends on the composition of the bilayer, with shorter chain lengths and unsaturated fatty acids decreasing interaction between adjacent phospholipids and thereby increasing membrane fluidity - the researchers found that more fluid membranes increased cellular respiration due to increased diffusion of the electron transport chain carriers - higher temperatures also increase membrane fluidity, so cells produce longer chains with fewer double bonds (less unsaturated) at higher temperatures to maintain In an electron transport chain, electrons are passed from one transmembrane electron carrier to another, driving proton movement across a membrane (see image below). The protons then flow through ATP synthase (not shown) to generate ATP proper fluidity

Fluorescence recovery after photobleaching (FRAP) is used to monitor the movement of fluorescently labeled molecules within the plane of a cell membrane. The molecules labeled are often proteins, but lipids can be labeled too. How would the curve that represents FRAP for labeled proteins compare to the curve representing labeled lipids? a. The FRAP curve for proteins would show a much more rapid recovery to initial levels of fluorescence. b. The FRAP curve for lipids would show a much more rapid recovery to initial levels of fluorescence. c. The FRAP curve for lipids would show a much more rapid recovery but only reach about 50% of the initial levels of fluorescence. d. The curves would be identical. e. The FRAP curve for proteins would show a much more rapid recovery but only reach about 50% of the initial levels of fluorescence.

b. the FRAP curve for lipids would show a much more rapid recovery to initial levels of fluorescence - in the FRAP study, the components of the cell membrane- its lipids or, more often, its proteins- are labeled with some sort of fluorescent marker - labeling membrane proteins can be accomplished by incubating cells with a fluorescent antibody or by covalently attaching a fluorescent protein such as green fluorescent protein GFP to a membrane protein - observing the fluorescence recovery of labeled proteins yields insights into the movement of membrane proteins, but does little to elucidate the dynamics of lipid diffusion - because lipids are so much smaller than proteins, they diffuse throughout the plane of the membrane much more rapidly - the fluorescence recovery for labeled should be much more rapid than that of labeled proteins and should reach the initial level of fluorescence fairly quickly

In eukaryotic cells, phospholipids are synthesized by enzymes bound to which of the following? a. the cytosolic face of the Golgi apparatus b. the cytosolic face of the endoplasmic reticulum c. the cytosolic face of the plasma membrane d. both monolayers of the endoplasmic reticulum e. the inside of the endoplasmic reticulum

b. the cytosolic face of the endoplasmic reticulum - in eukaryotic cells, new phospholipids are manufactured in the endoplasmic reticulum- the same organelle that is responsible for producing membrane-bound proteins - the enzymes that synthesize phospholipids are bound to the cytosolic face of the ER, where they have access to the free fatty acids that serve as substrates for the reaction - the enzymes deposit the newly made phospholipids exclusively in the cytosolic half of the bilayer

On what side of the plasma membrane are the carbohydrate chains of glycoproteins, proteoglycans, and glycolipids located? a. the underside b. the extracellular side c. both sides d. the cytosolic side e. the inside

b. the extracellular side - like some of the lipids in the outer layer of the plasma membrane, most of the proteins in the plasma membrane have sugars covalently attached to them - the great majority of these proteins have short chains of sugars, called oligosaccharides, linked to them; they are called glycoproteins - other membrane proteins, the proteoglycans, contain one or more long polysaccharide chains - all of the carbohydrates on the glycoproteins, proteoglycan, and glycolipids are located on the outside of the plasma membrane, where they form a sugar coating called the carbohydrate layer or glycocalyx

In 1925, scientists exploring how lipids are arranged within cell membranes performed a key experiment using red blood cells. Using benzene, they extracted the lipids from a purified sample of red blood cells. Because these cells have no nucleus and no internal membranes, any lipids they obtained were guaranteed to come from the plasma membrane alone. The extracted lipids were floated on the surface of a trough filled with water, where they formed a thin film. Using a movable barrier, the researchers then pushed the lipids together until the lipids formed a continuous sheet only one molecule thick. The researchers then made an observation that led them to conclude that the plasma membrane is a lipid bilayer. \Which of the following would have allowed the scientists to come to this conclusion? a. When pushed together, the extracted lipids dissolved in water. b. The extracted lipids covered twice the surface area of the intact red blood cells. c. The extracted lipids covered half the surface area of the intact red blood cells. d. The extracted lipids covered the same surface area as the intact red blood cells.

b. the extracted lipids covered twice the surface area of the intact red blood cells - when the extracted lipids were pushed together into one continuous monolayer, the researchers found that they occupied twice the area of the original, intact cell - additional experiments showed that lipids can spontaneously form bilayers when mixed with water - together, these observations suggest that in an intact cell membrane, the lipid molecules are doubled up to form a bilayer- an arrangement that has a profound influence on cell bio - lipid molecules are not very soluble in water because part of the molecule is hydrophobic. nudging them closer together with a movable barrier (imagine the edge of a ruler) would not change solubility

In this figure, what do the areas shown in red represent? a. the amphipathic side chains of the transmembrane α helices b. the hydrophilic side chains of the transmembrane α helices c. the hydrophilic side chains of the transmembrane β barrel d. the hydrophobic lipid tails of the bilayer e. the hydrophobic side chains of the transmembrane β barrel f. the hydrophobic side chains of the transmembrane α helices

b. the hydrophilic side chains of the transmembrane alpha helices - multiple amphipathic alpha helices can come together to form a pore in the membrane - they hydrophobic parts of the helix will interact with the hydrophobic hydrocarbon tails of the phospholipids within the lipid bilayer - the parts of the protein shown in red , which line the water-filled pore, are the hydrophilic parts of the alpha helices - a polypeptide chain can also cross the lipid bilayer not as an alpha helix, but as a beta sheet - in this case the beta sheets will be rolled into a cylinder, forming a keglike structure called a beta barrel - as with an alpha helix, the amino acid side chains that face the inside of a beta barrel, and therefore line the aqueous channel, would be mostly hydrophilic, while those on the outside of the barrel, which contact the hydrophobic core of the lipid bilayer, would be exclusively hydrophobic

In the α helices of transmembrane proteins, the hydrophobic side chains face which direction? a. inside of the membrane-spanning helix b. the outside of the membrane-spanning helix c. the cytosolic side of the membrane d. the external or lumenal side of the membrane

b. the outside of the membrane-spanning helix - the membrane-spanning segments of a transmembrane protein, which run through the hydrophobic environment of the interior of the lipid bilayer, are composed largely of amino acids with hydrophobic side chains - because these side chains cannot from favorable interactions with water molecules, they prefer to interact with the hydrophobic tails of the lipid molecules, where no water is present - in contrast to the hydrophobic side chains, the peptide bonds that join the successive amino acids in a protein are normally polar, making the polypeptide backbone itself hydrophilic - because water is absent from the interior of the bilayer, atoms that are part of the polypeptide backbone are thus driven to form hydrogen bonds with one another - in a membrane-spanning alpha helix, the hydrophobic side chains are exposed on the outside of the helix, where they contact the hydrophilic lipid tails, while the atoms of the hydrophilic polypeptide backbone from hydrogen bonds with one another with helix

What is typically true of ion channels? A. They are nonselective. B. They are gated C. They operate by active transport. D. They are open all the time. E. They hydrolyze ATP.

b. they are gated - selective ion channels are not open continuously; they open briefly and then close again - this is referred to as "gated" channel because the flow of ions can happen only when the channel is in proper conformation - for most of these ion channels, a specific stimulus triggers them to open - when open, these ion channels facilitate passive diffusion, allowing solutes to move down their electrochemical gradient - this process does not require an input of energy

Which contains the largest number of carbon-carbon double bonds? a. butter b. vegetable oil c. saturated fats d. margarine e. lard

b. vegetable oil - fats that have hydrocarbon tails with no double bonds are said to be fully saturated - animal fats such as butter and lard are generally saturated, which makes them solid at room temperature - the fats produced by plants are generally unsaturated, meaning their hydrocarbons have one or more double bonds (liquid at room temp) - to produce margarine, vegetable oils are hydrogenated: the addition of hydrogen removes their double bonds, making the oils more solid and butterlike at room temp

The shape of a cell and the mechanical properties of its plasma membrane are determined by a meshwork of fibrous proteins called what a. glycocalyx b. basal lamina c. cell cortex d. lamellipodium e. tight junction

c. cell cortex - the plasma membrane of animal cells is stabilized by a meshwork of filamentous proteins, called cell cortex, that is attached to the underside of the membrane - the cortex has been extensively studied in red blood cells, where it helps the cells maintain their distinctive biconcave shape as they squeeze through narrow blood vessels - the main component of the cell cortex in red blood cells is flexible protein lattice made from long, thin fibers of sceptrin - animals or humans that produce a structurally abnormal form of sceptrin tend to be anemic- they have fewer and more fragile red blood cells

To study the structure of a particular membrane protein, the target protein is usually removed from the membrane and separated from other membrane proteins. Shown below are three different proteins associated with the cell membrane. Treatment with high salt would release which protein or proteins from the bilayer? a. integral proteins b. integral protein c. peripheral proteins

c. peripheral proteins - treatment with high salts would disrupt protein-protein interactions, which often involve multiple electrostatic attractions between charged amino acid side chains on the proteins surfaces - protein c is held to the membrane solely by its interaction with another protein - removing integral proteins from the membrane requires the distribution of the lipid bilayer with detergents

Which of the following inhibits inorganic ions, such as Na+ and Cl-, from passing through a lipid bilayer? A. the hydrophilic exterior of the lipid bilayer B. the watery environment on either side of the lipid bilayer C. the hydrophobic interior of the lipid bilayer D. the carbohydrate layer on the surface of the lipid bilayer E. the ions' large size

c. the hydrophobic interior of the lipid bilayer - the hydrophobic interior of a lipid bilayer inhibits the passage of all ions (including Na+ and Cl-) because ions are charged and the interior of the membrane is very nonpolar - the watery environment on either side of the lipid bilayer is ideal for the solubility of ions because ions will dissolve well in water due to its polar nature - in contrast, ions are repelled by the nonpolar, hydrophobic hydrocarbon tails of the phospholipids that compose the interior of the plasma membrane

Animals exploit the phospholipid asymmetry of their plasma membrane to distinguish between live cells and dead ones. When animal cells undergo a form of programmed cell death called apoptosis, phosphatidylserine—a phospholipid that is normally confined to the cytosolic monolayer of the plasma membrane—rapidly translocates to the extracellular, outer monolayer. The presence of phosphatidylserine on the cell surface serves as a signal that helps direct the rapid removal of the dead cell. How might a cell actively engineer this phospholipid redistribution? a. by boosting the activity of a flippase in the plasma membrane b. by inverting the existing plasma membrane c. by inactivating a scramblase in the plasma membrane d. by activating a scramblase and inactivating a flippase in the plasma membrane e. by inactivating both a flippase and a scramblase in the plasma membrane

d. by activating a scramblase and inactivating a flippase in the plasma membrane - when a cell is no longer needed or damaged beyond repair, they activate a form of programmed cell death (apoptosis) - a cell actively destroys itself from within, digesting proteins and degrading its DNA - it also displays signals that direct circulating phagocytic cells to engulf remains - one of these signals involves the relocation of phosphatidylserine - an apoptotic cell displays phosphatidylserine- normally confined to the cytosolic side monolayer of the plasma membrane- on its surface - the scramblase that transfers random phospholipids from one monolayer of the plasma membrane to the other must be activated - the flippase that would normally transfer phosphatidylserine from the extracellular monolayer to the cytosolic monolayer must be inactivated - these actions cause phosphatidylserine to rapidly accumulate at the cell surface - boosting the activity of flippases causes phosphatidylserine to be selectively transferred to the cytosolic half of the membrane (healthy cell) - when flipases are inactivated, any phosphatidylserine that had already made it to the extracellular side of the plasma membrane (through the random action of scramblases) would, indeed, remain there - if scramblase is to be inactivated, any newly synthesized phosphatidylserines would remain trapped in the cytosolic half of the bilayer

Porin proteins—which form large, water-filled pores in mitochondrial and bacterial outer membranes—fold into β-barrel structures. The amino acids that face the outside of the barrel have what kind of side chains? a. amphipathic hydrophilic b. polar c. charged d. hydrophobic

d. hydrophobic - although the alpha helix is by far the most common form in which a polypeptide chain crosses a lipid bilayer, the polypeptide chain of some transmembrane proteins (such as porins) crosses the lipid bilayer as a beta sheet that is rolled into a cylinder, forming keglike structure called a beta barrel - as expected, the amino acid side chains that face the inside of the barrel and therefore line the aqueous channel, are mostly hydrophilic, while those on the outside of the barrel, which contact the hydrophobic core of the lipid bilayer, are exclusively hydrophobic

When grown at higher temperatures, bacteria and yeast maintain an optimal membrane fluidity by doing which of the following? a. producing membrane lipids with tails that are shorter and contain fewer double bonds b. producing membrane lipids with tails that are shorter and contain more double bonds c. producing membrane lipids with tails that are longer and contain more double bonds d. producing membrane lipids with tails that are longer and contain fewer double bonds e. adding cholesterol to their membranes

d. producing membrane lipids with tails that are longer and contain fewer double bonds - how fluid a lipid bilayer is at a given temperature depends on its phospholipid composition- particularly the nature of the hydrocarbon tails - the closer and more regular the packing of the tails, the more viscous and less fluid the bilayer will be - in bacterial and yeast cells, which have to adapt to varying temperatures, both the lengths degree of saturation of the hydrocarbon tails in the bilayer are adjusted constantly to maintain a membrane with a relatively consistent fluidity - at higher temperatures, for example, the cell makes membrane lipids with tails that are longer and that contain fewer double bonds - this allows the membrane lipids to maximize their interactions and thus to pack more tightly, which keeps the membrane from becoming too fluid - although animal cells do not generally have to cope with large ranges of temperature, they can modulate membrane fluidity by the inclusion of the sterol cholesterol - this option is not available to bacteria and yeast, which do not produce cholesterol

What type of protein moves randomly selected phospholipids from one monolayer of a lipid bilayer to the other? a. flippase b. phospholipase c. none; such movement occurs spontaneously and relatively quickly d. scramblase e. none; phospholipids cannot move from one monolayer to another

d. scramblase - although newly synthesized phospholipids are deposited into the cytosolic half of the ER bilayer, the membrane manages to grow evenly - the movement of lipids from one monolayer of the membrane to the other rarely occurs spontaneously - instead, lipids are relocated by scramblases, which remove randomly selected phospholipids from one half of the bilayer and insert them into the other - as a result of this scrambling, newly made phospholipids are redistributed equally between each monolayer of the ER membrane - although flippases also transport membranes form one side of the bilayer to the other, these transporters remove SPECIFIC phospholipids from the side of the bilayer facing the side of the bilayer facing the exterior space and flip them into the monolayer that faces the cytosol - the action of flippases thereby promotes membrane asymmetry

In an artificial lipid bilayer, how far can a phospholipid potentially diffuse in one second? a. 200 microns (halfway across a typical amoeba) b. 2 meters (the length of some of the longest nerve cells in the body) c. 20 nanometers (the width of a typical ribosome) d. 2 nanometers (the width of a DNA double helix) e. 2 microns (the length of a large bacterial cell)

e. 2 microns (the length of a large bacterial cell) - random thermal motions allow membrane phospholipids to continuously exchange places with their neighbors within the same monolayer - with no proteins to hamper its movement, a lipid in an artificial bilayer may diffuse a length equal to that of an entire bacterial cell about 2 microns in about one second - membrane phospholipids also rotate rapidly around their long axis, some reaching speeds of 500 revolutions per second

Organisms that live in cold climates adapt to low temperatures by doing which of the following? a. increasing the amounts of saturated fatty acids in their membranes to help decrease the fluidity of their membranes b. increasing the amounts of unsaturated fatty acids in their membranes to help decrease the fluidity of their membranes c. increasing the amounts of saturated fatty acids in their membranes to help keep their membranes fluid d. decreasing the amounts of unsaturated fatty acids in their membranes to help keep their membranes fluid e. increasing the amounts of unsaturated fatty acids in their membranes to help keep their membranes fluid

e. increasing the amounts of unsaturated fatty acids in their membranes to help keep their membranes fluid - cold temperatures slow all molecular motion - as such, they tend to make cell membranes more rigid - thus, organisms that live in cold climates compensate by incorporating membrane lipids that increase the fluidity of their membranes - this would include increasing the amounts of unsaturated fatty acids, which do not pack together as tightly as saturated fatty acids - Antarctic fishes have an unusually huge percentage of unsaturated phospholipids in their membranes - this composition helps keep their membranes fluid at very low temperatures

When the transport vesicle shown below fuses with the plasma membrane, which monolayer will face the cell cytosol? a. It depends on the cargo the vesicle is carrying. b. It depends on whether the vesicle is coming from the endoplasmic reticulum or the Golgi apparatus. c. The blue monolayer will face the cytosol. d. Half the time the orange monolayer will face the cytosol, and half the time the blue monolayer will face the cytosol. e. The orange monolayer will face the cytosol.

e. the orange monolayer will face the cytosol - most cell membranes are asymmetric, as the two halves of the bilayer often include strikingly different sets of phospholipids - the asymmetry is preserved as membranes bud from one organelle and fuse with another, or with the plasma membrane - cell membranes have distinct "inside" and "outside" faces: the cytosolic monolayer always faces the cytosol, while the noncytosolic monolayer is exposed to either the cell exterior - in this case the cell membrane- or the interior space (lumen) of an organelle - important because it helps preserve the asymmetric distribution of phospholipids and glycolipids, which may be confined 5o one or another monolayer to carry out their physiological function

In a patch of animal cell membrane about 10 μm in area, which will be true? a. Because the lipid bilayer acts as a two-dimensional fluid, there is no way to predict the relative numbers of proteins and lipids in any patch of cell membrane. b. There will be about an equal number of proteins and lipids. c. There will be more proteins than lipids. d. There will be more carbohydrates than lipids. e. There will be more lipids than proteins.

e. there will be more lipids than proteins - proteins constitute about half of the mass of an animal cell membrane - therefore, in terms of mass, proteins and lipids provide an equal share - however, lipids are much smaller than proteins, so a cell membrane typically contains 50 times more lipid molecules than protein molecules - carbohydrates are only present on a subset of proteins (glycoproteins) and lipids (glycolipids). - they contribute a relatively small amount to the mass of a cell membrane

Determine whether the following statement is true or false: The glucose-Na+ symport protein uses the electrochemical Na+ gradient to drive the active transport of glucose into the cell. Once this transporter has bound both Na+ and glucose, it preferentially opens toward the cytosol, where it releases both solutes.

false - a glucose-Na+ symporter uses the electrochemical Na+ gradient to drive the active import of glucose - the pump oscillates randomly between alternate states - because conformational changes are reversible, once the transporter has bound both solutes, two things can happen 1. it can flip into the inward-open state, allowing both solutes to enter the cytosol 2. it can also remain in the outward-open state. in this case, the solutes would dissociate into the extracellular space and nothing would be gained - even though Na+ and glucose can each bind to the pump in either of theses "open" states, the pump can transition between them only through an "occluded" state in which both glucose and Na+ are bound ("occluded-occupied") or neither is bound ("occluded- empty")

Determine whether the following statement is true or false: A symport protein would function as an antiport protein if its orientation in the membrane were reversed.

false - a symport protein would not function as an antiport protein if its orientation in the membrane were reversed - a symport protein binds two different solutes on the same side of the membrane; an antiport protein binds two different solutes binds to two different solutes on opposite sides of the membrane - thus, if a symport were "upside down" in the membrane, then both solutes likely would be moved in the opposite direction compared to normal - just inverting a protein does not alter the mechanics of the protein from symport to antiport or vise versa

t/f: Water passes through the cell membrane only through specialized channels called aquaporins.

false - the ability to regulate water balance across the plasma membrane is critically important for the normal function of cells - water molecules are uncharged and relatively small, meaning they can move across the membrane on their own, albeit at a somewhat slow rate - the presence of aquaporins in the membrane allows for even more efficient transit of water molecules through the membrane, giving the cell the ability to quickly and precisely regulate its water balance

The electrochemical Na+ gradient established by the Na+ pumps in the plasma membrane allows animal cells to do what? propagate electrical signals control their pH import nutrients, such as sugars and amino acids stimulate muscle cell contraction

propagate electrical signals control their pH import nutrients, such as sugars and amino acids stimulate muscle cell contraction - the electrochemical Na+ gradient drives the coupled transport of many substances into the cell, including nutrients such as sugars and amino acids - the Na+ -H+ exchanger in the plasma membranes of many animal cells uses the downhill influx of Na+ to pump H+ out of the cell - The flow of Na+ ions through voltage-gated Na+ channels depolarizes cell membranes and fuels the electrical excitability of nerve cells - And although voltage-gated Ca2+ channels are directly responsible for triggering muscle cell contraction, these channels are opened by transmitter-gated cation channels (such as the acetylcholine receptor) that depolarize the muscle cell membrane. - The fundamental importance of the electrochemical Na+ gradient could explain why animal cells spend 30% or more of their energy operating the ATP-driven Na+ pump.