AP Biology Unit 2: Cells MCQs – 30 Practice Questions with Answers (College Board CED)
AP-level practice for Unit 2 of the College Board CED (Fall 2025): cell structure and function, cell size, the plasma membrane, membrane permeability and transport, tonicity and osmoregulation, compartmentalization, and the origins of cell compartmentalization. Questions emphasise data interpretation and experimental reasoning.
Cell Structure and Function
Q1. A researcher finds membrane-bound organelles and a nucleus in a newly discovered single-celled organism. What is the best classification of this organism?
- (A) A bacterium, because all single-celled organisms are prokaryotes
- (B) A virus, because viruses contain a nucleus
- (C) A eukaryote, because a membrane-bound nucleus and organelles define eukaryotic cells
- (D) An archaeon without a cell wall, because archaea always have nuclei and membrane-bound organelles
Show Answer
Answer: C – The presence of a membrane-bound nucleus and other membrane-bound organelles is the defining feature of eukaryotic cells.
Q2. A cell is packed with rough endoplasmic reticulum and Golgi apparatus, and it secretes large amounts of protein. Which function best fits this cell?
- (A) Synthesis and export of proteins for use outside the cell
- (B) Storage of starch for later energy use, in granules packed through the cytoplasm
- (C) Photosynthesis in the absence of light
- (D) Contraction of muscle fibres
Show Answer
Answer: A – Rough endoplasmic reticulum synthesises proteins, and the Golgi apparatus modifies and packages them for secretion, so a cell rich in both is specialised for protein export.
Q3. A liver cell is exposed to a drug that damages its smooth endoplasmic reticulum. Which cellular task is most likely to suffer?
- (A) Production of ribosomal RNA in the nucleolus
- (B) Synthesis of proteins destined for secretion
- (C) Replication of the cell’s DNA before division, which occurs in the nucleus during S phase
- (D) Lipid synthesis and detoxification of harmful compounds
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Answer: D – The smooth endoplasmic reticulum synthesises lipids and helps detoxify drugs and poisons, while protein secretion is the work of the rough endoplasmic reticulum.
Q4. Mitochondria are abundant in a muscle cell but rare in a mature red blood cell of a mammal. What does this difference best reflect?
- (A) Red blood cells are prokaryotic and muscle cells are eukaryotic, explaining their different organelles
- (B) Cells contain organelles in proportions matched to their energy demands and functions
- (C) Mitochondria are only needed during cell division
- (D) Muscle cells photosynthesise and red blood cells do not
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Answer: B – Organelle abundance reflects function: contracting muscle needs much ATP from mitochondria, while a mammalian red blood cell is specialised for carrying oxygen and lacks mitochondria.
Q5. A plant cell is placed in a solution and its central vacuole shrinks, yet the cell does not burst or collapse completely. Which structure explains why the cell keeps its overall shape?
- (A) The rigid cell wall, which resists changes in cell volume
- (B) The nucleus, which pumps water back into the cell
- (C) The chloroplasts, which produce water by photosynthesis
- (D) The plasma membrane, which becomes rigid in dilute solutions
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Answer: A – The plant cell wall provides structural support, so the cell retains its shape even when the vacuole loses water and turgor pressure falls.
Q6. A scientist treats cells with a chemical that disrupts the cytoskeleton. Which process is most immediately affected?
- (A) DNA replication inside the nucleus
- (B) Production of ATP in the mitochondria
- (C) Intracellular transport of vesicles and maintenance of cell shape
- (D) Diffusion of oxygen across the plasma membrane, which depends only on concentration gradients
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Answer: C – The cytoskeleton provides structural support and tracks along which vesicles and organelles move, so disrupting it hits transport and shape first.
Q7. Two cells are examined: one has a nucleus, mitochondria, and ribosomes bound to membranes, while the other has free ribosomes and a single circular chromosome in the cytoplasm. What distinguishes them?
- (A) The first is a virus and the second is a plant cell
- (B) Both are eukaryotes, differing only in size
- (C) The first is a plant cell and the second is a fungal cell
- (D) The first is eukaryotic and the second is prokaryotic
Show Answer
Answer: D – Membrane-bound organelles mark the eukaryotic cell, while a circular chromosome free in the cytoplasm with unbound ribosomes marks the prokaryotic cell.
Q8. A student observes that ribosomes are present in both prokaryotic and eukaryotic cells. What is the best conclusion?
- (A) Ribosomes are found only in cells that photosynthesise
- (B) Protein synthesis by ribosomes is a shared, fundamental feature of all cells
- (C) Prokaryotic cells evolved from eukaryotic cells by losing their nucleus and membrane-bound organelles
- (D) Ribosomes prove that both cell types have mitochondria
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Answer: B – Ribosomes occur in every cell type because all cells must synthesise proteins, making them a universal feature of cellular life.
Cell Size and the Plasma Membrane
Q9. A spherical cell doubles in radius. How does its surface-area-to-volume ratio change, and what does this imply?
- (A) The ratio halves, so exchange across the surface becomes less adequate for the larger volume
- (B) The ratio doubles, so exchange becomes more efficient
- (C) The ratio stays the same, because surface area and volume grow equally as the cell enlarges in every case
- (D) The ratio falls to zero, so the cell can no longer exchange materials
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Answer: A – Surface area grows with the square of the radius while volume grows with the cube, so the ratio falls as a cell enlarges, limiting exchange relative to metabolic demand.
Q10. A microscopy student measures an image of a cell as 40 mm across and knows the actual cell is 0.04 mm across. The student divides image size by actual size to get a value of 1000. How should this calculation be described?
- (A) As a CED equation for cell size
- (B) As a formula from the CED equations appendix
- (C) As a measurement of the cell’s surface-area-to-volume ratio, calculated from its dimensions
- (D) As a general microscopy relationship between image size and actual size, not a CED equation
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Answer: D – Dividing image size by actual size is a general microscopy relationship used to work out how much an image is enlarged; it is not presented as a CED equation.
Q11. A cell is flattened into a thin sheet rather than remaining spherical, keeping the same volume. What advantage does this shape give?
- (A) It increases the cell’s DNA content
- (B) It prevents any material from leaving the cell, sealing the cytoplasm off from its surroundings
- (C) It increases the surface area available for exchange relative to volume
- (D) It removes the need for a plasma membrane
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Answer: C – Flattening or folding increases surface area without increasing volume, improving the rate at which materials can be exchanged across the membrane.
Q12. The fluid mosaic model describes the plasma membrane. Which statement best matches this model?
- (A) The membrane is a rigid wall of cellulose with fixed proteins
- (B) Phospholipids and proteins move laterally within a flexible bilayer, with proteins embedded in it
- (C) The membrane is a single layer of lipids with no proteins at all
- (D) Proteins form a solid outer coat and lipids sit only inside the cell, with no movement of either component at all
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Answer: B – In the fluid mosaic model, the bilayer is fluid, its components move sideways, and diverse proteins are embedded within or associated with it.
Q13. A membrane protein has a channel through its centre that allows water molecules to pass rapidly. What is this protein most likely to be?
- (A) An aquaporin, a channel protein specialised for water transport
- (B) A carrier protein that uses ATP to pump sodium ions across the membrane against their gradient
- (C) A receptor protein that binds a hormone
- (D) A structural protein of the cell wall
Show Answer
Answer: A – Aquaporins are channel proteins that greatly increase the rate at which water crosses membranes by facilitated diffusion.
Q14. Small nonpolar molecules cross a membrane quickly, while large polar molecules cross very slowly. What property of the membrane explains this pattern?
- (A) The membrane is freely permeable to everything
- (B) The membrane is made entirely of protein channels
- (C) The membrane actively pumps all molecules in both directions, using ATP for every crossing, regardless of concentration
- (D) The membrane is selectively permeable, with a hydrophobic interior that blocks large polar molecules
Show Answer
Answer: D – Selective permeability arises from the phospholipid bilayer: its hydrophobic core lets small nonpolar molecules through but impedes large or charged polar molecules.
Q15. An experiment compares water movement across two membranes, one with aquaporins and one without. Water crosses the aquaporin membrane far faster. What is the best interpretation?
- (A) Aquaporins actively pump water using ATP
- (B) Aquaporins destroy the lipid bilayer so water leaks through gaps in the membrane, bypassing the bilayer entirely
- (C) Aquaporins provide facilitated diffusion channels that speed water movement down its gradient
- (D) Aquaporins convert water into ions that cross more easily
Show Answer
Answer: C – Aquaporins do not use energy; they are channels that let water diffuse rapidly in the direction of its concentration gradient.
Membrane Transport, Tonicity, and Osmoregulation
Q16. A dye is added to one side of a membrane and spreads until its concentration is equal on both sides, with no energy input. Which process is this?
- (A) Active transport against a gradient
- (B) Simple diffusion down a concentration gradient
- (C) Endocytosis of the dye particles
- (D) Osmosis of the dye through aquaporins, moving with water across the membrane
Show Answer
Answer: B – Net movement from higher to lower concentration until equilibrium, without energy input, is simple diffusion.
Q17. Glucose enters a red blood cell through a carrier protein, moving from higher to lower glucose concentration, and the rate levels off at high glucose concentrations. What does the levelling off indicate?
- (A) The carrier proteins become saturated, limiting the rate of facilitated diffusion
- (B) The cell runs out of ATP for active transport
- (C) Glucose stops diffusing because the membrane becomes impermeable to glucose at high concentrations
- (D) The cell begins to synthesise its own glucose instead
Show Answer
Answer: A – Facilitated diffusion depends on a finite number of carrier proteins, so the rate plateaus when all carriers are occupied, unlike simple diffusion.
Q18. A cell uses ATP to move sodium ions out of the cell even though sodium is already more concentrated outside. Which statement is true of this transport?
- (A) It is osmosis, because water follows the ions
- (B) It is facilitated diffusion, because a channel is involved
- (C) It is simple diffusion, because ions move through the bilayer freely without any help from proteins
- (D) It is active transport, because energy is used to move ions against their gradient
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Answer: D – Movement against a concentration gradient powered by ATP is the definition of active transport.
Q19. Red blood cells are placed in distilled water. They swell and eventually burst. Which term describes the distilled water relative to the cells?
- (A) Hypertonic, because it has a higher solute concentration than the cytoplasm inside the cells
- (B) Isotonic, because solute concentrations are equal
- (C) Hypotonic, because it has a lower solute concentration, so water enters the cells by osmosis
- (D) Saturated, because no more water can enter the cells
Show Answer
Answer: C – Distilled water is hypotonic to the cells, so water moves inward by osmosis, swelling and then bursting cells that lack a wall.
Q20. A freshwater single-celled organism lives in water that is hypotonic to its cytoplasm. It has a contractile vacuole that regularly expels water. What problem does this structure solve?
- (A) The cell would otherwise lose all its water to the surroundings, shrinking until it collapses completely
- (B) The cell would otherwise gain excess water by osmosis and risk bursting
- (C) The cell cannot absorb nutrients without a vacuole
- (D) The vacuole produces ATP for active transport
Show Answer
Answer: B – In a hypotonic environment water constantly enters by osmosis, so the contractile vacuole expels the excess and prevents the cell from swelling and bursting.
Q21. A white blood cell engulfs a bacterium by folding its membrane around it and bringing it inside in a vesicle. Which mechanism is this?
- (A) Phagocytosis, a form of endocytosis for large particles
- (B) Exocytosis, because material leaves the cell
- (C) Simple diffusion of the bacterium through the bilayer, passing directly into the cytoplasm
- (D) Facilitated diffusion through a carrier protein
Show Answer
Answer: A – Engulfing a large particle in membrane to form an internal vesicle is phagocytosis, a type of endocytosis.
Q22. A pancreatic cell releases insulin by fusing secretory vesicles with its plasma membrane. What is this process, and what does it cost the cell?
- (A) Endocytosis, which requires no energy
- (B) Osmosis, which moves water only
- (C) Diffusion, which needs a concentration gradient to move vesicles out of the cell
- (D) Exocytosis, which requires energy to move and fuse vesicles
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Answer: D – Releasing material by vesicle fusion is exocytosis, an energy-requiring bulk transport process.
Compartmentalization and Its Origins
Q23. Why does compartmentalization into membrane-bound organelles benefit a eukaryotic cell?
- (A) It removes the need for enzymes in metabolic pathways
- (B) It allows the cell to survive without a plasma membrane
- (C) It lets incompatible reactions run in separate spaces with locally optimised conditions
- (D) It makes all reactions occur at the same pH throughout the cell, regardless of the compartment
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Answer: C – Separate compartments keep competing or incompatible processes apart and let each maintain the pH, enzymes, and reactants it needs.
Q24. Lysosomes contain hydrolytic enzymes that would damage the cell if released freely. How does compartmentalization protect the cell here?
- (A) The enzymes are kept inactive until they leave the cell entirely
- (B) The enzymes are enclosed within a membrane-bound compartment, separating them from the cytoplasm
- (C) The enzymes are converted into lipids that cannot harm proteins
- (D) The enzymes are stored in the nucleus where reactions cannot occur because no substrates enter the nucleus from the cytoplasm
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Answer: B – Enclosing dangerous enzymes in a lysosome confines their activity to material brought into that compartment.
Q25. Which observation provides the strongest evidence for the endosymbiotic origin of mitochondria?
- (A) Mitochondria have their own circular DNA and double membrane, resembling free-living bacteria
- (B) Mitochondria are the largest organelles in every cell, dwarfing the nucleus and all other structures
- (C) Mitochondria are found only in animal cells
- (D) Mitochondria produce oxygen during photosynthesis
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Answer: A – Their own circular DNA, bacterial-type ribosomes, and double membrane all support descent from engulfed bacteria.
Q26. Chloroplasts, like mitochondria, have a double membrane and their own DNA. What does this similarity best support?
- (A) Chloroplasts evolved from mitochondria by losing their DNA
- (B) Chloroplasts and mitochondria are identical organelles with different names for the same structure
- (C) Plant cells evolved before any prokaryotic cells existed
- (D) Chloroplasts also originated as engulfed prokaryotes that became endosymbionts
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Answer: D – The same lines of evidence that support a bacterial origin for mitochondria support an engulfed photosynthetic prokaryote as the origin of chloroplasts.
Q27. An experiment shows that protein synthesis in mitochondria is blocked by an antibiotic that also blocks bacterial protein synthesis, but cytoplasmic protein synthesis continues. What does this suggest?
- (A) Mitochondria and bacteria share no features at all, having evolved entirely independently of each other
- (B) The antibiotic destroys the cell’s nucleus
- (C) Mitochondrial ribosomes resemble bacterial ribosomes, consistent with an endosymbiotic origin
- (D) Mitochondria are not involved in energy production
Show Answer
Answer: C – Sensitivity to the same antibiotic points to similar ribosome structure, as expected if mitochondria descend from bacteria.
Q28. A eukaryotic cell keeps its DNA in the nucleus, its ATP production in mitochondria, and its protein packaging in the Golgi apparatus. Which principle does this illustrate?
- (A) All organelles perform the same function
- (B) Division of labour through compartmentalization increases cellular efficiency
- (C) Organelles compete for the same substrates at all times
- (D) The nucleus controls every reaction directly without intermediates, issuing instructions to each organelle
Show Answer
Answer: B – Assigning different tasks to specialised compartments is a division of labour that makes the cell more efficient.
Q29. A mutation disables the contractile vacuole of a freshwater protist. What is the most likely outcome when the protist remains in fresh water?
- (A) Water entering by osmosis accumulates until the cell swells and may burst
- (B) The cell stops absorbing nutrients completely, because it can no longer transport solutes inward
- (C) The cell begins to photosynthesise instead
- (D) The cell wall becomes thicker to compensate
Show Answer
Answer: A – Without the vacuole to expel incoming water, osmotic gain continues unchecked in a hypotonic environment, risking lysis.
Q30. Two adjacent plant cells share materials through narrow channels crossing their walls. What are these channels, and why do they matter?
- (A) Tight junctions, which prevent all movement between cells by sealing their membranes together
- (B) Gap junctions, which occur only in animal cells
- (C) Desmosomes, which anchor cells but allow no transport
- (D) Plasmodesmata, which let water, solutes, and signals pass directly between plant cells
Show Answer
Answer: D – Plasmodesmata are cytoplasmic channels through plant cell walls that connect neighbouring cells for direct exchange.
Revisit any questions you missed, then check the CED topic list for Unit 2 to confirm which areas need more revision before moving on to Unit 3.