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AP®BiologyCollege BoardExam QuestionsUnit 2: Cell Structure & FunctionCell Membranes & Transport

Cell Membranes & Transport (College Board AP® Biology): Exam Questions

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Cell Membranes & Transport

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1
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Researchers studying membrane dynamics use fluorescent markers to track protein movement within the plasma membrane. They observe that some proteins move freely, while others remain localized.

Which of the following statements best describes the fluid mosaic model of the cell membrane?

  • Phospholipids and proteins move laterally within the bilayer.

  • Membrane proteins move between the two phospholipid layers of the bilayer.

  • Phospholipids are covalently bonded to each other for stability.

  • Cholesterol prevents any movement within the membrane.

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2
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A patient receives an IV solution, containing glucose and electrolytes, to maintain hydration and nutrient balance. Once glucose and electrolytes enter the blood stream they are present in the blood plasma at a high concentration.

Which transport process allows glucose to enter red blood cells from the IV fluid?

  • Exocytosis

  • Facilitated diffusion

  • Active transport

  • Endocytosis

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3
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The processes of exocytosis and endocytosis allow the transport of molecules across the plasma membrane.

Which of the following best describes the processes of exocytosis and endocytosis?

  • Exocytosis is the process by which large macromolecules are transported into cells, while endocytosis is the process by which cells remove macromolecules and particulate matter.

  • Exocytosis is the process by which small molecules are transported out of cells, while endocytosis is the process by which cells take in micromolecules and particulate matter.

  • Exocytosis is the process by which small molecules are transported into cells, while endocytosis is the process by which cells remove micromolecules and particulate matter.

  • Exocytosis is the process by which large macromolecules are transported out of cells, while endocytosis is the process by which cells take in macromolecules and particulate matter.

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4
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A scientist places red blood cells into three different solutions and observes changes to cell shape.

Which of the following correctly describes a red blood cell placed in a hypotonic solution?

  • The cell remains the same size.

  • The cell shrinks.

  • The cell swells and may burst.

  • The cell loses water and the vacuole shrinks.

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1
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Cell membranes consist of a structural framework of different molecules that can flow around the surface of the cell within the membrane.

Diagram of a plasma membrane with various structures labelled 1 to 5
Figure 1. Diagram of a cell membrane

Which of the following predicts the effect on the membrane of increasing the proportion of molecule 1?

  • The fluidity of the membrane would decrease.

  • The fluidity of the membrane would increase.

  • The fluidity of the membrane would remain the same.

  • The membrane would rupture.

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21 mark

Proteins in the cell surface membranes of mouse cells and human cells were labelled with coloured dyes, with a different colour for human and mouse proteins.

When a cell from each species is fused together, the different-coloured labels are first found in different regions of the hybrid cell’s cell surface membrane. After 40 minutes, the labels are evenly distributed around the entire cell surface membrane. This is shown in the diagram below; the viewpoint is looking down onto the surface of the membrane.

Diagram showing human and mouse membrane sections with proteins, illustrating fusion into a single mixed membrane with phospholipid heads.

  • Groups of protein and phospholipid molecules in the cell surface membrane are attached to each other and move together.

  • Only protein molecules in the outer layer of the cell surface membrane can move freely between phospholipid molecules

  • All protein molecules in the cell surface membrane are fixed to structures within the cell, but phospholipid molecules move freely between them

  • Protein molecules in the outer layer of the cell surface membrane and those which span the bilayer can move freely between phospholipid molecules

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3
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The structure of a molecule affects its ability to pass through the plasma membrane. Membrane proteins are required for facilitated diffusion across the plasma membrane.

Which of the following is not true of membrane proteins?

  • Charged ions, including Na+ and K+, require channel proteins to move through the membrane.

  • Membrane proteins are necessary for active transport.

  • Large quantities of water pass through aquaporins.

  • Membrane proteins are in a fixed position within the plasma membrane.

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41 mark

Students were investigating osmosis in the plant cells of carrots.

Step 1: The student cut five cylinders of carrot using a cork borer to the same length

Step 2: The student placed the cylinders in five increasing concentrations of sucrose solution

Step 3: The student recorded the masses of the carrot cylinders after 24 hours

The student's results are shown in the graph in Figure 1.

Graph showing percentage change in mass against sucrose concentration in molarity. The line curves downwards from 5% at 0M to -7% at 1M.

Which of the following conclusions is supported by the results in the graph?

  • The solution is isotonic to the carrot at 0 (M) because there is a net movement of water into the carrot by osmosis.

  • The solution is isotonic to the carrot at 0.22 (M) because there is no net movement of water into the carrot by osmosis.

  • The solution is isotonic to the carrot at 1 (M) because there is a net movement of water out of the carrot by osmosis.

  • The solution is not isotonic to the carrot during the student's experiment.

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5
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Osmoregulation maintains water balance and allows organisms to control their internal solute composition/water potential. Protists maintain osmoregulation by pumping water out of the cell using an organelle called contractile vacuoles (CVs)

Scientists investigated protists living in a body of water 200m inland from a coastal region. The table gives the data collected by the scientists to calculate the solute potential.

i

C

R

T

2

0.12 M

0.00831 kJ mol-1 K-1

12 oC

Which of the following is the solute potential of the body of water?

  • 0.568 (MPa)

  • -0.568 (MPa)

  • 0.024 (MPa)

  • -0.024 (MPa)

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1
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Scientists place a plant cell in a solution with a water potential of -0.5 MPa. The plant’s cytoplasm has a water potential of -0.8 MPa.

Which direction will water move, and what is the effect on the plant cell?

  • Water moves into the cell, making it turgid.

  • Water moves out of the cell, causing plasmolysis.

  • No water movement occurs.

  • The plant cell bursts.

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21 mark

Scientists compare the membrane fluidity of two different bacterial species—one living in the Arctic and one in a hot spring. They find that the Arctic bacteria have more unsaturated fatty acids, while the hot spring bacteria have higher cholesterol levels in their membranes.

Which of the following best explains any aspect of these findings?

  • Cholesterol reduces membrane fluidity at high temperatures, preventing excessive permeability.

  • Cholesterol increases membrane permeability at high temperatures, allowing large molecules to enter and exit cells between the phospholipids.

  • Unsaturated fatty acids increase the rigidity of membranes, keeping membranes intact at low temperatures.

  • Saturated fatty acids in Arctic bacteria work to stabilize membranes.

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3
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Vesicles release neurotransmitters at synapses in the nervous system.

Figure 1 shows the events leading up to and after neurotransmitter release.

Diagram of synaptic transmission showing steps from action potential at the axon terminal to neurotransmitter binding and signal in postsynaptic cell.
Figure 1. The events leading up to and after neurotransmitter release.

Which of the following best describes the events occurring at point 6 in Figure 1?

  • Vesicles fuse with the presynaptic membrane and neurotransmitters are released via facilitated diffusion.

  • Vesicles fuse with the presynaptic membrane and neurotransmitters are released via exocytosis.

  • Vesicles fuse with the presynaptic membrane and neurotransmitters are released via active transport.

  • Vesicles fuse with the presynaptic membrane and neurotransmitters are released via endocytosis.

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4
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Scientists investigated how kidney cells regulate water balance under different conditions. They genetically modified a group of cells to remove aquaporins; a type of membrane protein that facilitates water movement. Normal kidney cells and modified kidney cells were exposed to a hypertonic solution. The results of the investigation are shown in Figure 1.

Line graph showing percentage of water loss over time in kidney cells. Blue line for normal cells rises faster than the slower rising red line for modified cells.
Figure 1. Water loss in normal and modified kidney cells.

Which of the following best explains the results shown in Figure 1?

  • Modified kidney cells lose less water due to their lack of aquaporins, which prevents water from leaving the cell by osmosis.

  • The hypertonic solution inhibits the movement of water molecules in both cell types.

  • Water loss in normal kidney cells is increased as the high solute concentration in the hypertonic solution damages aquaporins and water loss is not controlled.

  • The absence of aquaporins in modified cells increases membrane permeability to solutes, therefore more water is lost via osmosis.

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