AP Biology, AP exams, 10–13%

Unit 2: Cell Structure and Function

Unit 2 covers subcellular components and their jobs, the surface-area-to-volume constraint on cell size, and everything that crosses the plasma membrane. Membrane transport and tonicity are among the most reliably tested ideas in the course. The unit closes with the origins of compartmentalization, including endosymbiotic theory.

Position: 2 of 8 units
Published weighting: 10–13% in the official framework
Topics covered: 6 topics in this guide

What is covered

Topics inside this unit.

These topics describe the scope students most often review for this part of AP Biology. Treat them as a study checklist rather than an exhaustive list of what a single exam form can sample.

Subcellular components: Organelle structure and function in prokaryotic and eukaryotic cells, with an emphasis on what each compartment enables.
Cell size and surface area to volume: Why exchange constraints limit cell size, and how shape and folding increase the effective exchange surface.
Plasma membranes and permeability: The fluid mosaic model and which molecules cross a phospholipid bilayer freely, slowly, or not at all.
Passive and active transport: Diffusion, facilitated diffusion, and the pumps, cotransporters, and vesicular routes that require energy.
Tonicity and osmoregulation: Water potential, hypertonic and hypotonic environments, and how organisms manage the resulting water movement.
Compartmentalization and endosymbiotic theory: How internal membranes create distinct chemical environments, and the evidence that mitochondria and chloroplasts were once free-living.

Study tips

How to practice Cell Structure.

Section-specific guidance that follows from how this material is usually tested.

Work water-potential calculations until the sign conventions are automatic; the arithmetic is easy but the direction of flow is where points are lost.
Build one table contrasting passive and active transport by energy source, direction relative to the gradient, and example.
Rehearse the evidence for endosymbiotic theory as a short list you can produce in a free-response answer.

Where this fits

Unit 2: Cell Structure and Function is 2 of 8 units in the AP Biology guide, and it carries a published weighting of 10–13% in the official framework. Use the full guide to balance it against everything else you have to review.

Back to the AP Biology guide Official exam information

Sample Cell Structure questions

Practice questions, not yet expert-reviewed

Original practice questions written to the published framework for this section. No expert has reviewed them, and they are not taken from any official exam. Try each one before you open the answer.

Question 1Foundational
Which organelle modifies, sorts, and packages proteins received from the endoplasmic reticulum before they are shipped to their destinations?
1. ARough endoplasmic reticulum
2. BGolgi apparatus
3. CLysosome
4. DSmooth endoplasmic reticulum
Show answer
Answer: B. Golgi apparatus
The Golgi apparatus receives transport vesicles from the endoplasmic reticulum, chemically modifies their cargo, and tags it for delivery. The rough endoplasmic reticulum is the tempting distractor because it is where secreted proteins are first synthesized and folded, but sorting and shipping happen after the protein leaves it. Lysosomes digest material rather than package it.
Key insight: Rough endoplasmic reticulum makes and folds; Golgi modifies, sorts, and ships.
Question 2Foundational
What term describes a solution whose solute concentration is lower than that inside a cell placed in it?
1. AHypertonic
2. BIsotonic
3. CHydrophilic
4. DHypotonic
Show answer
Answer: D. Hypotonic
A hypotonic solution has fewer dissolved solutes than the cell, so water moves into the cell by osmosis and the cell swells. Hypertonic is the reverse case, in which water leaves the cell and it shrivels. Isotonic solutions produce no net water movement.
Key insight: Water follows solutes: it moves toward the side with the higher solute concentration.
Question 3Intermediate
A cube-shaped cell measures 2 micrometers along each edge. What is the ratio of its surface area to its volume?
1. A1 to 3
2. B2 to 1
3. C3 to 1
4. D6 to 1
Show answer
Answer: C. 3 to 1
Surface area is 6 times the area of one face, or 6 times 4 square micrometers, which equals 24 square micrometers, and volume is 2 cubed, or 8 cubic micrometers. Dividing gives 24 divided by 8, a ratio of 3 to 1. Choosing 6 to 1 comes from stopping at the six faces without dividing by the 2 micrometer edge length.
Key insight: Surface area grows with the square of length while volume grows with the cube, so bigger cells exchange materials less efficiently.
Question 4Intermediate
What is the direct energy source used by the sodium-potassium pump to move both ions against their concentration gradients?
1. AATP hydrolysis
2. BThe sodium concentration gradient
3. CSimple diffusion
4. DAbsorption of light energy
Show answer
Answer: A. ATP hydrolysis
The sodium-potassium pump is a primary active transporter: it hydrolyzes ATP, and the released energy drives a conformational change that moves ions uphill. The sodium gradient is the tempting distractor because it does power transport, but only for secondary active transporters such as the sodium-glucose cotransporter, and that gradient is itself built by the pump. Simple diffusion moves solutes down a gradient and requires no energy at all.
Key insight: Primary active transport spends ATP directly; secondary active transport spends the gradient that ATP paid for.
Question 5Advanced
A 0.15 molar sucrose solution sits in an open beaker at 300 kelvin. Using the solute potential equation with an ionization constant of 1, a pressure constant of 0.0831 liter bars per mole per kelvin, and no applied pressure, what is the water potential of the solution in bars?
1. A-24.9 bars
2. B-3.74 bars
3. C-0.37 bars
4. D3.74 bars
Show answer
Answer: B. -3.74 bars
In an open container the pressure potential is 0, so water potential equals solute potential. Solute potential is the negative of 1 times 0.15 times 0.0831 times 300, which equals -3.74 bars. The value -24.9 bars comes from leaving the 0.15 molar concentration out of the product, and a positive answer ignores the negative sign built into the solute potential equation.
Key insight: Adding solute always lowers water potential, so the sign of a solute potential is never positive.

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Unit 2: Cell Structure and Function