AP Biology Cell Structure and Function — Worked Answer Explanations

Unit 2 · 8 questions explained

Below is a complete answer key for our AP Biology Cell Structure and Function practice questions. For each question you'll find the correct choice, a full written explanation of how to get there, and — for every wrong answer — a short note on exactly why it's tempting and where it goes wrong. Reading these straight through is one of the fastest ways to find the gaps in a unit before exam day.

Prefer to test yourself first? Take the timed Cell Structure and Function practice test and come back here to review, or head back to the Cell Structure and Function unit overview.

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Question 1 · Easy
Which structural feature distinguishes prokaryotic cells from eukaryotic cells?
- A
  Prokaryotes lack a membrane-bound nucleus.Correct
- B
  Prokaryotes lack ribosomes.
  Why not B: Both have ribosomes, although prokaryotic ribosomes are smaller (70S vs 80S).
- C
  Prokaryotes lack DNA.
  Why not C: Both have DNA.
- D
  Prokaryotes lack a cell membrane.
  Why not D: All cells have a cell membrane.
Explanation
Prokaryotes (bacteria, archaea) lack a true membrane-bound nucleus and other membrane-bound organelles. Their DNA is in a nucleoid region. Eukaryotes have nuclei and various membrane-bound organelles.

Key takeaway
The defining difference: eukaryotes have a true nucleus, prokaryotes do not.
Question 2 · Easy
What is the primary role of the cytoskeleton in a eukaryotic cell?
- A
  Producing ATP for cellular work.
  Why not A: Mitochondria produce most ATP.
- B
  Providing structural support, organelle positioning, and intracellular transport.Correct
- C
  Storing genetic information.
  Why not C: DNA in the nucleus does this.
- D
  Synthesizing proteins.
  Why not D: Ribosomes synthesize proteins.
Explanation
The cytoskeleton (microfilaments, intermediate filaments, microtubules) provides shape, anchors organelles, enables motility, and serves as tracks for vesicle and organelle movement (e.g., kinesin/dynein along microtubules).

Key takeaway
Cytoskeleton: structure, organization, transport, and motility.
Question 3 · Easy
The mitochondrion has its own DNA and double membrane. This evidence best supports which hypothesis?
- A
  Mitochondria evolved from invaginations of the plasma membrane.
  Why not A: Doesn't explain the presence of unique DNA.
- B
  Mitochondria originated as engulfed prokaryotes (endosymbiotic theory).Correct
- C
  Mitochondria evolved from the endoplasmic reticulum.
  Why not C: ER doesn't have its own DNA.
- D
  Mitochondria are vestigial structures with no current function.
  Why not D: Mitochondria are essential for ATP production.
Explanation
Endosymbiotic theory: an ancestral eukaryotic cell engulfed an aerobic prokaryote, which became the mitochondrion. The double membrane (host membrane + endosymbiont membrane), circular DNA, and prokaryote-sized ribosomes all support this.

Key takeaway
Mitochondria and chloroplasts originated as engulfed prokaryotes — the endosymbiotic theory.
Question 4 · Easy
A cell with a high rate of protein secretion would be expected to have an abundance of:
- A
  Lysosomes.
  Why not A: Lysosomes degrade material — not directly involved in secretion.
- B
  Smooth ER.
  Why not B: Smooth ER handles lipid synthesis, not protein secretion.
- C
  Rough ER and Golgi apparatus.Correct
- D
  Peroxisomes.
  Why not D: Peroxisomes detoxify; not part of secretion pathway.
Explanation
Secretory proteins are synthesized at the rough ER (ribosomes on its surface), modified, and trafficked to the Golgi for further modification and packaging into vesicles for export.

Key takeaway
Secretory pathway: ribosomes on RER → ER → Golgi → vesicles → plasma membrane.
Question 5 · Easy
If a freshwater plant is placed in a hypertonic salt solution, what will happen to its cells?
- A
  Cells will burst (lyse).
  Why not A: Bursting happens in hypotonic, not hypertonic, environments.
- B
  Cells will swell with water.
  Why not B: Hypertonic environment causes water to leave cells.
- C
  Cells will plasmolyze, with the membrane pulling away from the wall.Correct
- D
  Cells will remain unchanged due to the cell wall.
  Why not D: Cell wall doesn't prevent water loss.
Explanation
In a hypertonic solution, water leaves the cell by osmosis. The membrane shrinks and pulls away from the rigid cell wall — this is plasmolysis. The cell wall remains intact but the cell is no longer turgid.

Key takeaway
Hypertonic + plant cell → plasmolysis (membrane separates from wall).
Question 6 · Easy
Which transport mechanism requires both a protein channel and ATP?
- A
  Simple diffusion.
  Why not A: No protein, no ATP.
- B
  Facilitated diffusion.
  Why not B: Uses protein, but no ATP.
- C
  Active transport.Correct
- D
  Osmosis.
  Why not D: Passive movement of water; no ATP needed.
Explanation
Active transport (e.g., the sodium-potassium pump) uses ATP hydrolysis to move solutes against their gradient through a membrane protein. Facilitated diffusion is also protein-mediated but is passive (down the gradient, no ATP).

Key takeaway
Active transport: protein + ATP, against gradient. Facilitated diffusion: protein only, with gradient.
Question 7 · Easy
Cilia and flagella in eukaryotes share which structural feature?
- A
  Both are made of actin microfilaments.
  Why not A: Made of microtubules, not microfilaments.
- B
  Both have a 9+2 arrangement of microtubules.Correct
- C
  Both are extensions of the cell wall.
  Why not C: Cell walls don't extend into cilia/flagella.
- D
  Both rotate using a proton motive force.
  Why not D: That describes prokaryotic flagella.
Explanation
Eukaryotic cilia and flagella share the 9+2 arrangement: 9 outer microtubule doublets surrounding a central pair. They beat by ATP-driven sliding of microtubules via dynein motor proteins.

Key takeaway
Eukaryotic cilia and flagella both use the 9+2 microtubule structure powered by dynein.
Question 8 · Medium
Why does the surface area to volume ratio limit cell size?
- A
  Larger cells have insufficient genetic material.
  Why not A: Cells make as much DNA as they need.
- B
  As volume grows faster than surface area, exchange with the environment becomes inadequate.Correct
- C
  Gravity collapses larger cells.
  Why not C: Gravity is not the limiting factor at cellular scales.
- D
  Larger cells run out of DNA.
  Why not D: DNA is not consumed.
Explanation
Surface area scales as $r^{2}$ while volume scales as $r^{3}$ . As cells grow, the ratio SA/V decreases, eventually preventing adequate diffusion of nutrients in and waste out.

Key takeaway
Cells stay small to maintain a high surface-area-to-volume ratio for efficient exchange.