2.4 Membrane Permeability

1. AIt has tiny pores between its phospholipids that let small particles through but not large ones.
   A student who thinks the membrane sorts substances only by size, like a sieve, picks this. Small ions such as Na⁺ are blocked by the hydrophobic interior, while small nonpolar molecules cross it freely.
2. BIts hydrophobic interior lets some substances cross readily but restricts many others. Correct
   The interior of the membrane is formed by the nonpolar fatty acid tails. Small nonpolar molecules pass through it readily, while ions and polar molecules are restricted and cross mainly through embedded proteins, so the membrane is selectively permeable.
3. CIt detects which substances the cell needs and then allows just those to cross into the cell.
   A student who thinks the membrane senses what the cell needs picks this. Whether a substance crosses depends on its properties and on the proteins present, not on the cell's needs.
4. DIts polar heads block nonpolar molecules, so polar molecules cross it most easily.
   A student who thinks the polar heads are the barrier picks this. The barrier is the nonpolar interior, which nonpolar molecules such as O₂ cross most easily.

CED 2.4.A.1 · Read this in Fix

1. AEach substance crosses faster than every substance that is larger than it is.
   A student who thinks the membrane sorts substances only by size picks this. H₂O is smaller than O₂ but crosses about a thousand times more slowly, and Na⁺, a much smaller particle than glucose, crosses far more slowly than glucose.
2. BH₂O crosses at about the same rate as O₂ and CO₂, the other small molecules.
   A student who thinks small molecules cross the bilayer freely whether or not they are polar picks this. H₂O, which is polar, crosses at about 10⁻³ of the rate of O₂ and CO₂.
3. CNa⁺ crosses more slowly than glucose, a molecule with many times the mass of a Na⁺ ion. Correct
   Glucose crosses at about 10⁻⁷ and Na⁺ at about 10⁻¹², roughly 100,000 times more slowly, even though a Na⁺ ion has far less mass than a glucose molecule. Charge, not size alone, decides how readily a substance crosses the hydrophobic interior.
4. DThe polar molecules, H₂O and glucose, cross faster than the nonpolar O₂ and CO₂.
   A student who thinks polar substances cross the membrane more easily than nonpolar ones picks this. O₂ and CO₂, which are nonpolar, cross fastest of all the substances shown.

CED 2.4.A.2 · Read this in Fix

1. ADiagram 1 Correct
   The interior of the bilayer is nonpolar. Small nonpolar O₂ crosses freely (large amount); small polar, uncharged H₂O passes through in small amounts; the charged Na⁺ is prevented from crossing by the nonpolar tails (almost none).
2. BDiagram 2
   A student who thinks small molecules cross the bilayer freely whether or not they are polar picks this diagram. H₂O is polar, so it passes through the lipid bilayer only in small amounts.
3. CDiagram 3
   A student who thinks the membrane sorts substances only by size picks this diagram, in which all three small particles cross freely. Na⁺ is small but charged, and the nonpolar tails prevent it from crossing the bilayer.
4. DDiagram 4
   A student who thinks the polar heads are the barrier, blocking nonpolar molecules, picks this diagram. The barrier is the nonpolar interior, which O₂ crosses freely and which blocks the ion Na⁺.

CED 2.4.A.3 · Read this in Fix

1. AIn bacteria, fungi and plants, the cell wall gives a structural boundary and blocks some substances. Correct
   Cell walls of Bacteria, Archaea, Fungi and plants provide a structural boundary and are a permeability barrier for some substances, although water and many small solutes pass through them; the plasma membrane inside the wall controls which solutes enter.
2. BIn plants, the cell wall rather than the plasma membrane controls which solutes enter the cell.
   A student who thinks the cell wall controls which solutes enter picks this. The plasma membrane, inside the wall, does this; water and many small solutes pass through the wall.
3. CIn plants, the cell wall stops water from entering, so plant cells do not swell in pure water.
   A student who thinks the cell wall keeps water out picks this. Water passes through the wall; a plant cell in pure water swells until its contents press against the wall.
4. DCell walls are found around plant cells but usually not around those of fungi or bacteria.
   A student who thinks cell walls are a feature of plant cells, absent from fungi and bacteria, picks this. The cells of Bacteria, Archaea and Fungi also have cell walls.

CED 2.4.B.1 · Read this in Fix

1. AIts uptake of both glucose and O₂ will stop, since all substances cross through proteins.
   A student who thinks every substance crosses the membrane through proteins picks this. O₂ diffuses directly across the phospholipid bilayer.
2. BIts glucose uptake will not change, since glucose passes freely through the phospholipid bilayer.
   A student who thinks small molecules cross the bilayer freely whether or not they are polar picks this. Glucose is a large polar molecule, which the hydrophobic interior restricts.
3. CIts glucose uptake will rise through other routes to meet the cell's own need for glucose.
   A student who thinks a cell takes in more of a substance when it needs more picks this. The need for glucose does not open new routes across the membrane.
4. DIts glucose uptake will fall sharply, while O₂ will still diffuse in across the bilayer. Correct
   Glucose is a large polar molecule, so it crosses the membrane mainly through transport proteins, and blocking every transport protein cuts its uptake. O₂ is a small nonpolar molecule that crosses the phospholipid bilayer directly, so its entry is unaffected.

CED 2.4.A.2 · Read this in Fix

1. AReject it: the results prove that charge alone controls whether a particle can enter.
   A student who thinks rejecting a null hypothesis proves the alternative picks this. The data support an effect of charge, but they do not prove it, or show that charge is the only factor; polarity also matters, as the difference between Z and X shows.
2. BFail to reject it: just 10 preparations per substance are too few to support any conclusion.
   A student who thinks small samples cannot support any conclusion picks this. With error bars this narrow, 10 preparations are enough to show a likely significant difference.
3. CReject it: the ±2 SE bars for X and Y do not overlap, so the difference is likely real. Correct
   X and Y are of similar size and both hydrophilic, but only Y is charged. X reached 34% (bar 30–38%) and Y 1% (bar 0–2%). The ±2 SE bars do not overlap, so the difference is likely significant, and the null hypothesis that charge has no effect is rejected.
4. DFail to reject it: the data have error bars, so they vary too much to be trusted.
   A student who thinks error bars mean the data are unreliable picks this. Error bars show how precisely each mean is estimated; here they are narrow compared with the difference between X and Y, and they do not overlap.

Working No test statistic is calculated; the decision rests on the ±2 SE error bars. X (polar, uncharged): mean 34%, bar 30–38%. Y (ion): mean 1%, bar 0–2%. The bars do not overlap (30 > 2), so the difference between X and Y is likely significant at about the 95% level; reject the null hypothesis that charge has no effect. Z (nonpolar): mean 96%, bar 93–99%, shows that the vesicles let a small nonpolar molecule in freely.

CED 2.4.A.3 · Read this in Fix

1. ACells in both solutions will lose their walls but stay intact, as the membrane alone resists the water.
   A student who thinks the plasma membrane alone can stop a cell from bursting picks this. Without its wall, a cell in distilled water takes in water until its membrane bursts.
2. BCells in solution 1 will shrink as water leaves them; cells in solution 2 will stay intact.
   A student who thinks water moves toward the lower solute concentration picks this. Water moves into the cells, where the solute concentration is higher than in distilled water.
3. CCells in both solutions will burst, with nothing now to stop water from rushing into them.
   A student who thinks the cell wall is what keeps water out picks this. Water crosses the wall in any case; in solution 2 there is no net entry of water, because the solute concentrations inside and outside are equal.
4. DCells in solution 1 will swell and burst; cells in solution 2 will lose their walls but stay intact. Correct
   In distilled water, water enters the cells by osmosis, and without a wall to resist swelling they burst (osmotic lysis). In solution 2 the solute concentrations inside and outside are equal, so there is no net entry of water, and the cells survive without walls.

CED 2.4.B.1 · Read this in Fix

1. AThe wall blocks the water, so no more water can enter the cell than the cell is able to hold.
   A student who thinks the cell wall keeps water out picks this. The model shows water passing through the wall into the cell.
2. BThe wall pushes back on the swelling cell, so its membrane is not stretched until it bursts. Correct
   In the model, water enters the cell through the wall and membrane, and the swollen contents press against the wall, which exerts an inward force on them. Because the wall resists further expansion, the plasma membrane is not stretched until it breaks, which protects the cell from osmotic lysis.
3. CThe membrane pumps the extra water back out of the cell, using energy that is obtained from ATP.
   A student who thinks cells pump excess water out using energy picks this. Water crosses the membrane by osmosis; the model shows the wall, not a pump, limiting the cell's expansion.
4. DThe plasma membrane is strong enough by itself to withstand the water entering the cell.
   A student who thinks the plasma membrane alone can stop a cell from bursting picks this. The model shows the wall pushing inward on the cell's contents; without that force, the membrane could stretch until it burst.

CED 2.4.B.1 · Read this in Fix

1. AVesicles with the protein in glucose solution and protein-free vesicles in the same glucose solution Correct
   The question is whether the protein is needed, so the protein is the independent variable. The two preparations should differ only in whether the protein is present, with the same glucose solution, time and conditions, so that any difference in glucose entry can be attributed to the protein.
2. BVesicles with the protein in glucose solution and the same vesicles in a solution with no glucose
   A student who thinks a control is the treatment that leaves out the substance being studied picks this. This comparison shows only that glucose must be present to enter; both preparations contain the protein, so it cannot show whether the protein is needed.
3. CVesicles with the protein in glucose solution and protein-free vesicles in a stronger glucose solution
   A student who thinks a fair test gives the disadvantaged group more of the substance picks this. Changing the glucose concentration as well as the protein adds a second variable, so a difference could not be attributed to the protein.
4. DVesicles with the protein in glucose solution, sampled repeatedly to see how fast glucose enters
   A student who thinks one group measured repeatedly can show the effect of a variable picks this. Without protein-free vesicles for comparison, the entry of glucose cannot be attributed to the protein.

CED 2.4.A.2 · Read this in Fix