Unit 2 · Topic 2.3 end-of-topic test
The drawing shows a single animal cell floating in a drop of liquid, with three labels: X, Y and Z.
What are X, Y and Z?
A red blood cell drifts in blood. A student says: "The plasma membrane is a film of lipid, so the cell's surface must be oily on the outside, with water only on the inside."
Which statement about the two faces of the membrane is correct?
Three students drew a cross-section of a plasma membrane, with the extracellular fluid above and the cytosol below. Each phospholipid is drawn as a circle (the head) with two wavy tails.
Which drawing is correct, and why?
On a labeled cross-section of a plasma membrane, a student writes "oily interior" across the middle band, where the two layers of phospholipids meet.
Why is that middle band oily?
A student's drawing of a plasma membrane shows a single layer of phospholipids, with every head facing the extracellular fluid and every tail pointing into the cytosol.
What is wrong with the drawing?
The figure shows a membrane protein drawn as a chain of amino acids with three regions. Region 1 has charged R groups, region 2 has only nonpolar R groups, and region 3 has polar and charged R groups.
Where does each region sit when the protein is in a plasma membrane?
A student says a protein can sit in a membrane only if the whole protein is hydrophobic, because the membrane is oily.
Which statement corrects the student?
A frozen plasma membrane is split apart between its two layers, exposing the middle of the membrane. Under the microscope, many bumps stick up from the newly exposed inner faces.
What are the bumps most likely to be?
The model shows a plasma membrane with the extracellular fluid above and the cytosol below. Five parts are numbered.
Which numbered part is a glycolipid?
A fragment of a red blood cell's plasma membrane is examined. Carbohydrate chains are found attached to proteins and lipids on one face of the fragment only.
Which face is it, and what does that tell you?
Cholesterol is a steroid with four carbon rings and no charged groups. In an animal cell's plasma membrane it is present in large amounts.
Where in the membrane does cholesterol sit?
A mouse cell and a human cell are fused into one cell. Before fusing, the mouse cell's membrane proteins were tagged to glow red and the human cell's to glow green. At 0 minutes, red is on one half of the fused cell and green on the other half. At 40 minutes at 37 °C, red and green are mixed evenly over the whole surface. The total brightness of each color is the same as at 0 minutes.
What does the result show about the membrane?
A textbook calls the plasma membrane a fluid mosaic. A student reads "mosaic" as saying the membrane is a rigid wall of tiles cemented in place.
What does each word describe?
In a membrane, a phospholipid drifts sideways past its neighbors many times a second.
As it drifts, how does the phospholipid stay oriented?
The model shows a plasma membrane with five numbered parts. The two watery sides are labeled side A and side B, and the model leaves it to you to work out which side is the cytosol.
Which side is the extracellular fluid, and what tells you?
A student describes the job of each part of a plasma membrane in keeping the cell's inside separate from the outside. Four of the descriptions are listed.
Which description is wrong?
A student's model of a plasma membrane shows its pieces moving over an hour. One of the movements shown would drag a polar part through the hydrophobic interior, which a real membrane rules out.
Which movement is that?
(a) Describe the model by naming parts 1 to 5. (1 point)
A full-credit answer: 1 is a phospholipid, one of the two layers of the bilayer; 2 is a membrane protein; 3 is cholesterol; 4 is a glycoprotein, a protein with a carbohydrate chain attached; 5 is a glycolipid, a lipid with a carbohydrate chain attached.
Check the box for each point your answer earns
Accept: "lipid bilayer" or "phospholipid bilayer" for 1; "protein that spans the membrane" for 2. Do not award the point for naming only "phospholipids" and "proteins" without telling parts 2, 4 and 5 apart.
Common slip: Naming only phospholipids and proteins. Parts 2, 4 and 5 are told apart by their carbohydrate chains and by whether the piece is a protein or a lipid.
(b) Explain why part 2 sits with its middle stretch inside the membrane and its two ends in the water. (1 point)
A full-credit answer: Water’s partial charges pull on the charged and polar R groups at the two ends, so those ends stay in the cytosol and the extracellular fluid. The nonpolar R groups in the middle give water nothing to hold, so that stretch is pushed out of the water and lies against the hydrocarbon tails in the hydrophobic interior.
Check the box for each point your answer earns
Accept: "the nonpolar middle is hydrophobic so it hides among the tails; the charged ends are hydrophilic so they stay in the water" provided the answer says why (water pulls on charged or polar groups and not on nonpolar ones). Do not award the point for "like sits with like" with no mention of charges or water.
Common slip: Saying like sits with like, with no mention of water or charges. The point needs what water does: it pulls on charged and polar groups and has nothing to hold on nonpolar ones.
(c) The model shows one instant. On paper, draw an arrow on the model showing how part 2 moves over the next hour, and state one thing about part 2 that stays the same as it moves. (1 point)
A full-credit answer: The arrow runs sideways along the membrane, within the layer, because the protein drifts. What stays the same: it still spans the membrane the same way up, its nonpolar middle in the interior and its charged ends in the water on the same sides as before.
Check the box for each point your answer earns
Accept: a described movement in words in place of a drawn arrow, provided it says sideways or along the membrane. Do not award the point for an arrow that carries the protein out into the cytosol or the extracellular fluid, or for "nothing changes" without naming the unchanged feature.
Common slip: Drawing the arrow out into the cytosol or the extracellular fluid, or saying nothing changes without naming the unchanged feature.
(d) Explain how the arrangement shown in the model lets the membrane keep the cytosol and the extracellular fluid as two separate solutions. (1 point)
A full-credit answer: The two layers of hydrocarbon tails form a continuous oily band, the hydrophobic interior, between the two watery solutions, and water holds every piece in that arrangement: heads and polar parts pulled into the water, tails and nonpolar parts pushed into the interior. So the barrier holds together even while its pieces drift, and the salt solution inside stays separate from the solution outside.
Check the box for each point your answer earns
Accept: "the oily middle has no water in it, so the two watery solutions do not mix through it" together with a statement that water holds the pieces in place. Do not award the point for "the proteins hold the membrane together" or for "nothing can cross the membrane".
Common slip: Saying the proteins hold the membrane together, or that nothing can cross. Water holds the pieces in place, and the oily middle keeps the two watery solutions from mixing through it.
(a) Describe the two solutions the plasma membrane sits between and what the interior of the membrane is made of. (1 point)
A full-credit answer: The cytosol inside and the blood, the extracellular fluid, outside are both watery solutions, so the membrane has water against both faces. Its interior is the hydrocarbon tails of the two phospholipid layers, meeting tail to tail, with no charges or partial charges: the hydrophobic interior.
Check the box for each point your answer earns
Accept: "oily middle made of the phospholipid tails" for the interior. Both the two watery solutions and the tail interior are needed for the point.
Common slip: Describing only the solutions or only the interior. Both are needed: two watery solutions, and an oily middle made of the tails.
(b) Explain why the normal protein sits spanning the membrane, with one end in the cytosol and the other in the blood. (1 point)
A full-credit answer: Water’s partial charges pull on the charged and polar R groups at the two ends, so those ends stay in the water on each side. The nonpolar middle run gives water nothing to hold, so it is pushed out of the water into the hydrocarbon tails, and the protein settles with that run across the interior and an end in the water on each side.
Check the box for each point your answer earns
Accept: "the middle is hydrophobic and the ends are hydrophilic" provided the answer says what water does to each. Do not award the point for "the protein is made in the membrane" or "the protein is glued in by the phospholipids".
Common slip: Saying the protein is glued in by the phospholipids or made in the membrane. Water’s pull on the ends and push on the middle is what sets where it sits.
(c) Predict what happens to the position of the variant protein. (1 point)
A full-credit answer: The variant no longer sits stably across the membrane. Its middle run now carries charges, which water pulls toward itself, so the protein fails to settle in the membrane and stays in the water, or is pulled out into the cytosol or the blood.
Check the box for each point your answer earns
Accept either prediction: 'it never settles into the membrane and stays in the water (the cytosol)' or 'it is pulled out of the membrane into the water'; also 'it cannot span the membrane'. Do not award the point for 'nothing changes' or for 'the membrane opens a hole to let the charged part through'.
Common slip: Saying nothing changes, or that the membrane opens a hole for the charged part. Charged groups in the middle run change where water puts the protein.
(d) Justify your prediction using what holds each part of a protein where it sits. (1 point)
A full-credit answer: Where each part of a membrane protein sits is decided by which parts water holds on to. The charged R groups now in the middle run are pulled on by water’s partial charges and have nothing to hold them among the uncharged tails, so the run that used to anchor the protein in the hydrophobic interior now pulls toward the water, and the protein loses the nonpolar stretch that kept it spanning.
Check the box for each point your answer earns
Accept: a comparison with the normal protein, whose all-nonpolar middle is excluded from water and so stays in the tails. Do not award the point for "charged things cannot enter a membrane" with no reference to water holding them or to the uncharged tails.
Common slip: Saying charged things cannot enter a membrane, with no reference to water holding them or to the uncharged tails.