3 questions, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 3
In a plasma membrane, the phosphate heads of the phospholipids face the cytosol and the extracellular fluid, and the fatty acid tails face each other in the interior of the membrane. Which statement best explains this arrangement?
Answer and reasoning
AThe tails are the polar part of each molecule, and polar groups strongly attract one another. A student who thinks the tails are the polar part of a phospholipid picks this. The tails are nonpolar hydrocarbon chains; the heads are the polar part.
BThe cell places each phospholipid so that its tail is shielded from the surrounding water. A student who thinks the cell places its molecules where they are needed picks this. The arrangement forms on its own because of how the polar heads and nonpolar tails interact with water.
CThe heads are polar and interact with water, while the nonpolar tails are kept away from it.Correct The phosphate heads are polar (hydrophilic) and interact favorably with the water on each side of the membrane. The fatty acid tails are nonpolar (hydrophobic), so they cluster together, facing each other in the interior, away from water.
DThe heads are too large to fit inside the membrane, so the thin tails pack there instead. A student who thinks size alone decides what fits inside the membrane picks this. The heads stay at the surfaces because they are polar and the interior is nonpolar, not because of their size.
A protein is embedded in the plasma membrane and spans it from one side to the other. Which statement correctly describes the side groups of its amino acids?
Answer and reasoning
AHydrophobic in the part within the lipid tails; hydrophilic in the parts exposed to waterCorrect The surface of the protein that lies among the nonpolar fatty acid tails has nonpolar (hydrophobic) side groups, which interact with the tails. The parts exposed to the watery cytosol or extracellular fluid have polar or charged (hydrophilic) side groups, which interact with water.
BHydrophilic in the part within the lipid tails; hydrophilic in the parts exposed to water A student who thinks proteins are water-soluble throughout picks this. Hydrophilic side groups would not interact favorably with the nonpolar fatty acid tails, so the part of the protein among the tails is hydrophobic.
CHydrophobic in the part within the lipid tails; hydrophobic in the parts exposed to water A student who thinks an embedded protein is hydrophobic throughout picks this. Only the region among the fatty acid tails needs to be hydrophobic; the parts in contact with water are hydrophilic.
DHydrophilic in the part within the lipid tails; hydrophobic in the parts exposed to water A student who thinks polar and nonpolar groups attract each other picks this. Polar side groups interact with water, not with nonpolar tails, so the arrangement is the reverse of this.
According to the fluid mosaic model, which statement correctly describes the plasma membrane of a vertebrate animal cell?
Answer and reasoning
AProteins, cholesterol, glycoproteins and glycolipids sit among the phospholipids and move sideways.Correct In the fluid mosaic model the framework is a phospholipid bilayer in which proteins, steroids such as cholesterol (in vertebrate animals), glycoproteins and glycolipids are embedded. All of these components can move within the membrane, sideways among the phospholipids.
BProteins, cholesterol and other components move into and out of the cell, which makes the membrane fluid. A student who thinks membrane fluidity means components moving into and out of the cell picks this. 'Fluid' describes components moving within the membrane, sideways among the phospholipids, not crossing it.
CProteins form two continuous layers that coat the phospholipids, holding the other parts in place. A student who pictures membrane proteins as continuous layers on the two surfaces of the bilayer picks this. Proteins are individual molecules embedded among the phospholipids, and the components are not held in place: they can move within the membrane.
DProteins, cholesterol and glycolipids are embedded in a single layer of phospholipids around the cell. A student who thinks the plasma membrane is a single layer of phospholipids picks this. The framework is a bilayer: two layers of phospholipids whose fatty acid tails face each other in the interior of the membrane.
In preparation: 0 of 3 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
2.3.A.1 Phospholipid Fix
Phospholipid
A lipid built from a polar, hydrophilic phosphate-containing head and two nonpolar, hydrophobic fatty acid tails, so one molecule has both hydrophilic and hydrophobic regions.
Phospholipid bilayer
The double layer of phospholipids that forms the framework of a membrane. The polar phosphate heads face the aqueous fluid on each side (cytosol and external environment); the nonpolar fatty acid tails of the two layers face each other in the membrane's interior.
Hydrophilic and hydrophobic
Hydrophilic ('water-loving') groups are polar or charged and interact favorably with water; hydrophobic ('water-fearing') groups are nonpolar and are excluded by water, so they cluster with other nonpolar groups.
Students often think The fatty acid tails are the polar, water-attracting part of a phospholipid and face the watery fluid, while the heads face the membrane's interior. In fact The polar (hydrophilic) phosphate heads face the watery cytosol and extracellular fluid; the nonpolar (hydrophobic) fatty acid tails face each other in the membrane's interior.
Students often think The two layers of a phospholipid bilayer are separate sheets with a watery space between them. In fact No. The two layers of phospholipids lie tail to tail with no water between them; the interior of the bilayer is the nonpolar region formed by the fatty acid tails.
2.3.A.2 Embedded membrane protein Fix
Embedded membrane protein
A protein that sits within the phospholipid bilayer, often spanning it. Such proteins can be hydrophilic (with charged and polar side groups), hydrophobic (with nonpolar side groups), or both in different regions.
Hydrophilic regions of membrane proteins
Parts of a membrane protein with charged and polar side groups. They lie in the interior of the protein, for example lining a channel through it, or are exposed to the cytosol.
Hydrophobic regions of membrane proteins
Parts of a membrane protein with nonpolar side groups. They form the protein surface that interacts with the fatty acid tails in the interior of the membrane.
Students often think Proteins are water-soluble molecules, so every part of a membrane protein has hydrophilic side groups. In fact No. Embedded proteins can be hydrophilic, hydrophobic, or both. A protein that spans the membrane has hydrophobic (nonpolar) side groups on the surface that contacts the fatty acid tails and hydrophilic side groups in the parts exposed to water.
Students often think A protein embedded in the membrane is hydrophobic throughout, because it sits in a hydrophobic layer. In fact Not necessarily. Embedded proteins can be hydrophilic, hydrophobic, or both. In a protein that spans the membrane, the parts exposed to the cytosol or lining a channel through the protein are hydrophilic; the surface that contacts the fatty acid tails is hydrophobic.
2.3.B.1 Fluid mosaic model Fix
Fluid mosaic model
The model of membrane structure in which a framework of phospholipids is embedded with proteins, steroids, glycoproteins and glycolipids (the 'mosaic'), all of which can move around the surface of the cell within the membrane (the 'fluid').
Steroids in membranes
Lipids with a ring structure, such as cholesterol, which is embedded among the phospholipids of the plasma membranes of vertebrate animals.
Glycoproteins and glycolipids
Membrane proteins and lipids with carbohydrate chains attached; like the other components of the membrane, they can move within the membrane.
Lateral movement in membranes
The movement of phospholipids, proteins and other components sideways within the plane of the membrane, around the surface of the cell. It is faster at higher temperatures, within the range in which cells live.
Students often think At low temperatures molecules stop moving, so nothing in a cold membrane moves at all. In fact No. At lower temperatures membrane molecules move more slowly, but they do not stop: movement within the membrane is reduced, not eliminated, at temperatures such as 15 °C or 0 °C.
Students often think A quantity that increases over time increases by the same amount in each equal time interval. In fact Not necessarily. Many biological processes start fast and then slow down, or follow other curves; the change in each interval has to be read from the data.
7 more questions. Every wrong answer here is a real mistake students make, and you see why it is wrong as soon as you answer.
Question 1 of 7
A plasma membrane separates the watery extracellular fluid from the watery cytosol. In each diagram, the extracellular fluid is above the membrane and the cytosol is below it. Which diagram correctly represents the arrangement of phospholipids in the membrane?
Answer and reasoning
ADiagram 1 A student who thinks the tails are the polar part, so that they face the water, picks this diagram. The tails are nonpolar and are kept away from water; here they point into the extracellular fluid and the cytosol.
BDiagram 2 A student who thinks the membrane is a single layer of phospholipids picks this diagram. With one layer, the nonpolar tails would be exposed to the watery cytosol.
CDiagram 3 A student who thinks a bilayer is two separate sheets with water between them picks this diagram. The tails here face a layer of water; in a membrane the tails of the two layers meet in a nonpolar interior.
DDiagram 4Correct Both faces of the membrane are in contact with watery fluid, so both faces must be formed by the polar heads. Two layers, heads facing the extracellular fluid and the cytosol and nonpolar tails facing each other in the interior, form a bilayer in which no tail is exposed to water.
The diagram represents a channel protein in a plasma membrane. Which of the numbered regions of the protein are most likely to have mainly hydrophilic (polar or charged) side groups?
Answer and reasoning
ARegion 3 alone A student who thinks polar side groups are attracted to the nonpolar tails picks this. Region 3 contacts the fatty acid tails and is hydrophobic; regions 1 and 2 are in contact with water.
BRegions 1 and 2Correct Region 1 extends into the watery cytosol, and region 2 lines the water-filled pore in the interior of the protein, so both have mainly polar or charged side groups. Region 3 contacts the nonpolar fatty acid tails, so it is hydrophobic.
CAll three regions A student who thinks every part of a protein is hydrophilic picks this. Region 3 lies against the nonpolar fatty acid tails, so its side groups are nonpolar.
DNo numbered region A student who thinks an embedded protein is hydrophobic throughout picks this. Regions 1 and 2 are in contact with the cytosol and with the water in the pore, so they are hydrophilic.
In a hypothetical cell, a membrane protein crosses the plasma membrane once. The segment that crosses the membrane is made mainly of amino acids with nonpolar side groups. A mutation replaces several of these amino acids with amino acids that have charged side groups of similar size. Which prediction about the mutant protein is best supported?
Answer and reasoning
AIts altered segment will interact poorly with the fatty acid tails, so it will sit less stably in the membrane.Correct The segment that crosses the membrane is held there because its nonpolar side groups interact with the nonpolar fatty acid tails. Charged side groups interact poorly with the tails, so the altered segment fits less stably in the membrane's interior.
BIts altered segment will be held more firmly by the nonpolar fatty acid tails that surround it. A student who thinks polar or charged groups are attracted to nonpolar groups picks this. Charged side groups do not interact favorably with the nonpolar tails.
CIts position will not change, as the new amino acids are about the same size as the old ones. A student who thinks size alone decides what fits in the membrane's interior picks this. The new side groups are charged, and charge matters: the interior of the membrane is nonpolar.
DIt will work better than before, since mutations arise to improve the proteins a cell needs. A student who thinks mutations arise to improve what the cell needs picks this. Mutations occur without regard to need, and this one disrupts the interaction that holds the protein in the membrane.
In an investigation, the proteins on the surface of one type of cell were labeled with a green fluorescent dye, and those of a second type of cell with a red dye. Pairs of cells were then fused to form single cells, and the fused cells were kept at 37 °C or at 15 °C. The graph shows the percentage of fused cells in which the green and red labels were fully mixed over the whole cell surface. Which statement is supported by the data?
Answer and reasoning
AAt 15 °C, none of the fused cells had mixed labels at any point during the 40 minutes shown. A student who thinks molecules stop moving at low temperatures picks this. At 15 °C the percentage reached 6% by 40 minutes: mixing was slow, not absent.
BAt 37 °C, the percentage with mixed labels rose by the same amount in every 10-minute interval. A student who assumes a steady, straight-line increase picks this. The rises were 25, 30, 25 and 10 percentage points in successive 10-minute intervals.
CAt 37 °C the percentage with mixed labels rose over 40 min, but at 15 °C it stayed low.Correct At 37 °C the percentage of fused cells with mixed labels rose from 0 to 90% in 40 minutes; at 15 °C it rose only to 6%. Membrane proteins moved within the membrane much faster at the higher temperature.
DThe percentage of cells with mixed labels rose at about the same rate at the two temperatures. A student who thinks temperature does not affect how fast membrane components move picks this. After 40 minutes, 90% of cells at 37 °C but only 6% at 15 °C had mixed labels.
A student claims that proteins in the plasma membrane can move sideways within the membrane. Which observation would best support this claim?
Answer and reasoning
AAs the cell grows, the total amount of protein in its plasma membrane increases steadily. A student who treats the addition of new protein as evidence of movement picks this. An increase in total protein shows that protein is being added, not that proteins already in the membrane move sideways.
BWhen fluorescent proteins in one small patch are permanently bleached, the patch glows again within a minute.Correct Bleaching destroys the fluorescence only of the proteins in the patch. If the patch glows again within a minute, too quickly for new proteins to be made and delivered, unbleached labeled proteins must have moved sideways into it from the surrounding membrane.
CMicrographs show proteins scattered among the phospholipids rather than forming a separate layer. A student who takes evidence of a mosaic pattern as evidence of movement picks this. A single image shows where proteins are, not whether they move.
DProteins labeled on the outer surface of the plasma membrane are later found inside the cytosol. A student who thinks membrane fluidity means proteins moving into and out of the cell picks this. Finding proteins in the cytosol shows that they left the membrane, not that they move sideways within it.
Membrane proteins of a hypothetical cell were labeled with a fluorescent dye. A laser bleached the dye in one small patch of membrane, and the time taken for the patch to regain half of its original fluorescence, as unbleached proteins moved in, was measured at three temperatures. The graph shows the mean half-recovery time at each temperature (n = 8 cells), with error bars representing ±2 SE of the mean. Which conclusion is best supported by the data?
Answer and reasoning
ARecovery takes exactly as long at 25 °C as at 37 °C. A student who thinks overlapping error bars show that two means are equal picks this. The bars at 25 °C and 37 °C overlap, so no significant difference is shown, but that does not make the means the same.
BRecovery is significantly faster at 37 °C than it is at 25 °C. A student who treats any difference between means as significant picks this. The mean is lower at 37 °C (17 s) than at 25 °C (22 s), but the ±2 SE bars overlap, so the difference may be due to chance.
CRecovery time does not differ significantly with temperature. A student who thinks temperature does not affect how fast membrane proteins move picks this. The bars at 10 °C and 25 °C do not overlap, so the data show a likely significant difference.
DRecovery is significantly slower at 10 °C than at 25 °C.Correct The ±2 SE bars at 10 °C (42–54 s) and 25 °C (18–26 s) do not overlap, so the difference is likely to be significant: proteins move more slowly at 10 °C.
Working No test statistic is calculated; the decision rests on the ±2 SE error bars. 10 °C: 48 ± 6 s (42–54 s). 25 °C: 22 ± 4 s (18–26 s). 37 °C: 17 ± 4 s (13–21 s). 10 °C vs 25 °C: the bars do not overlap (42 > 26), so the difference is likely significant. 25 °C vs 37 °C: the bars overlap (18–21 s), so a significant difference is not shown, which does not show that the means are equal.
A student plans to test whether temperature affects how quickly membrane proteins move. She will label the membrane proteins of one cell green and those of another cell red, fuse the two cells, and record the percentage of fused cells whose labels have fully mixed. Groups of fused cells will be kept at 15 °C, 25 °C and 37 °C. Which feature of her plan is necessary for a valid comparison among the groups?
Answer and reasoning
AScoring the 15 °C group later than the others, to give it a fair chance to mix A student who thinks a fair test gives a slow group more time picks this. Scoring the groups at different times adds a second variable, so a difference could be caused by time rather than temperature.
BScoring every group at the same time after fusion, so that only temperature differs among themCorrect Time after fusion affects how many cells have mixed labels, so it must be kept the same in every group. Then temperature is the only difference between the groups, and differences in the percentage with mixed labels can be attributed to it.
CAdding a 0 °C group as a control, as no membrane proteins move at that temperature A student who thinks molecules stop moving at low temperatures picks this. Proteins still move slowly at 0 °C, and an extra temperature is not necessary for comparing the planned groups.
DUsing only one fused cell at each temperature, so that each one can be watched closely A student who thinks one careful trial is enough picks this. Individual cells vary, so many cells are needed in each group; that is why the percentage of cells with mixed labels is recorded.
Compiled from the AP Biology Course and Exam Description (effective Fall 2025) and our question bank · Specialist review in progress. How these pages are made · Free, no account
