Unit 2 · Topic 2.3 end-of-topic test
Suggested time: about 46 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.
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 the labeled cross-section of a plasma membrane shown, a student has written “oily interior” beside the middle band, where the two layers of phospholipids touch.
Why is that middle band oily?
The drawing shows a student's model of a plasma membrane: 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?
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?
The figure shows a membrane protein drawn as a chain of amino acids with three regions, marked 1, 2 and 3. The R groups in 1 are charged, the R groups in 2 are all nonpolar, and the R groups in 3 are polar and charged.
Where does each region sit when the protein is in a plasma membrane?
The drawing shows a plasma membrane split open between its two layers. Many bumps stand out from the two exposed faces.
What could the bumps be?
The model shows a plasma membrane with the extracellular fluid above and the cytosol below. Five parts are numbered.
Which number marks a glycolipid?
A red blood cell drifts in blood. The blood is the cell’s extracellular fluid, outside the cell; the cytosol is inside the cell. A fragment of the cell’s plasma membrane is examined: one face of the fragment touched the blood and the other face touched the cytosol. Carbohydrate chains are found attached to proteins and lipids on one face of the fragment only.
Which face carries the chains, and why?
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 sheet 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.
Which side is the extracellular fluid, and what tells you?
A student describes the location of each part of a plasma membrane. Four of the descriptions are listed.
Which description is wrong?
Over an hour, the pieces of a plasma membrane move about.
Which of the following movements is impossible in a living plasma membrane?
(a) Identify the pieces numbered 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 the membrane protein, the glycoprotein and the glycolipid apart.
Common slip: Naming only phospholipids and proteins. The glycoprotein and the glycolipid each carry a carbohydrate chain; the plain membrane protein does not. The glycoprotein is a protein; the glycolipid is a lipid.
(b) The part marked 2 spans both layers. Explain, using how water treats each of its parts, why it sits with its middle stretch inside the membrane and its two ends in the water. (1 point)
A full-credit answer: The extracellular fluid and the cytosol both contain water.
The two ends of the membrane protein have charged and polar R groups, which water’s partial charges pull on, so one end is held in the extracellular fluid and the other in the cytosol.
The middle stretch of the protein has nonpolar R groups, and water is not attracted to it, so the water molecules stay with one another and exclude that stretch.
It 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 is attracted to charged and polar groups and is not attracted to nonpolar ones.
(c) The model shows one instant. Describe how the part marked 2 moves over the next hour, and state one thing about it that stays the same as it moves. (1 point)
A full-credit answer: Over the hour, the protein drifts sideways along the membrane, staying within the two layers.
It moves because the membrane is fluid: every piece drifts sideways within its layer.
What stays the same is the way the protein sits.
It still spans the membrane the same way up, with its nonpolar middle in the hydrophobic interior and its charged ends in the water on the same sides as before.
Check the box for each point your answer earns
Accept: any wording that has the protein move along the membrane and stay the same way up. Do not award the point for a movement that carries the protein out into the cytosol or the extracellular fluid, or for “nothing changes” without naming the unchanged feature.
Common slip: Moving the protein out into the cytosol or the extracellular fluid, or saying nothing changes without naming the unchanged feature. The protein moves along the membrane, and it keeps spanning it the same way up.
(d) Explain how the arrangement shown in the model, with heads to the water on both faces and the tails touching in the middle, lets the membrane keep the cytosol and the extracellular fluid as two separate solutions. (1 point)
A full-credit answer: The extracellular fluid and the cytosol both contain water.
Water pulls the phospholipid heads and the polar parts of the proteins into the water on each face, and pushes the hydrocarbon tails and the nonpolar parts into the middle.
So the tails of the two layers lie together in a continuous oily band, the hydrophobic interior.
The polar and charged substances dissolved in each solution are attracted by water, and nothing in the oily band can hold them, so they stay on their own side.
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 of what water does to the pieces (pulls the polar parts into the water, pushes the nonpolar parts into the interior). 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 pulls each polar part into the water and pushes each nonpolar part into the interior, so the oily band stays continuous. That oily band keeps the dissolved polar substances of 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 membrane sits between the cytosol on one side and the blood, this cell’s extracellular fluid, on the other.
The cytosol and the blood are both watery solutions, so the membrane has water against both faces.
Its interior is the hydrocarbon tails of the two phospholipid layers, lying tail to tail.
The tails carry no charges or partial charges, so this interior is 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. The answer needs both: 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: The cytosol and the blood both contain water.
Water’s partial charges pull on the charged and polar R groups at the two ends of the protein, so each end is attracted into the water on its side.
Water is not attracted to the nonpolar middle stretch, so water pushes it out of the water and into the hydrocarbon tails.
So the protein settles with that stretch across the interior and one 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) Make a claim about what happens to the position of the variant protein. (1 point)
A full-credit answer: The variant is no longer held spanning the membrane.
Its middle stretch now carries charged R groups.
Water pulls on charged R groups, so the middle stretch is pulled toward the water.
Therefore the protein does not settle across the membrane: it stays in the water, or it is pulled out into the cytosol or the blood, or it sits against one face without crossing it.
Check the box for each point your answer earns
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 stretch change where water puts the protein.
(d) Support your claim 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 is attracted to.
In the normal protein the middle stretch is all nonpolar, so it stays among the uncharged tails and anchors the protein in the hydrophobic interior.
In the variant the middle stretch carries charged R groups.
Water’s partial charges pull on them, and the tails cannot hold them.
Therefore that stretch pulls toward the water, and the protein loses its anchor.
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.