AP Biology / Unit 2

AP Biology Unit 2: Cell Structure and Function

Unit 2 is where biochemistry becomes biology. Molecules from Unit 1 are now inside cells, organized and doing work. It's 10–13% of the exam and heavily tested in both MCQ and FRQ.

Aligned to the 2025–26 College Board CED (Topics 2.1–2.11).

Prokaryotic vs. eukaryotic cells

Feature	Prokaryotic	Eukaryotic
Nucleus	None — DNA in nucleoid region	Membrane-bound nucleus
Organelles	No membrane-bound organelles	Extensive membrane-bound organelles
Ribosomes	70S	80S in cytoplasm; 70S in mitochondria/chloroplasts
Size	1–10 µm	10–100 µm
Examples	Bacteria, archaea	Animals, plants, fungi, protists

Point-saver: “Mitochondria is the powerhouse of the cell” is explicitly listed by the College Board as not acceptable. You must describe the double membrane, cristae (folds that increase surface area), and how that supports ATP synthesis via oxidative phosphorylation.

The endomembrane system

A network that synthesizes, modifies, packages, and transports proteins and lipids. It does not include mitochondria, chloroplasts, or peroxisomes.

Protein secretion path: nucleus → mRNA → rough ER ribosome → polypeptide enters ER lumen → vesicle → Golgi (modify and sort) → secretory vesicle → plasma membrane → exocytosis.

Component	Function
Nucleus / envelope	Houses DNA; nuclear pores regulate transport
Rough ER	Synthesizes and begins modifying secreted/membrane proteins
Smooth ER	Lipid synthesis, steroid hormones, detox, Ca²⁺ storage
Golgi apparatus	Further modifies, sorts, and packages into vesicles
Lysosomes	Hydrolytic enzymes — digestion, autophagy, apoptosis trigger
Vacuoles	Storage; plant central vacuole maintains turgor pressure

Surface area-to-volume ratio

As cells grow, volume scales with radius³ while surface area scales with radius². A smaller SA:V ratio means the cell cannot exchange materials fast enough — this is what limits cell size and motivates folded membranes (cristae, microvilli).

The plasma membrane and transport

A phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. Cholesterol moderates fluidity at temperature extremes.

Passes freely	Needs transport protein	Needs ATP (active)
Small nonpolar (O₂, CO₂)	Glucose via GLUT carriers	Na⁺/K⁺ pump
Small uncharged polar (water, ethanol)	Ions via channels	Proton pump (H⁺)
—	Water via aquaporins	Ca²⁺ pumps

Bulk transport

Endocytosis: cell engulfs material in a vesicle (phagocytosis = solids; pinocytosis = liquids).
Exocytosis: vesicle fuses with the membrane and releases contents.

Tonicity and osmosis

Water moves from higher water potential (lower solute) to lower water potential (higher solute). Ψ = Ψs + Ψp is on the formula sheet.

Environment	Animal cell	Plant cell
Hypotonic	Swells, may lyse	Turgid (healthy)
Isotonic	Normal	Flaccid
Hypertonic	Shrivels (crenation)	Plasmolysis

Endosymbiotic theory

Evidence that mitochondria and chloroplasts began as engulfed prokaryotes:

Double membranes (inner = original prokaryote; outer = host vesicle).
Their own circular DNA and 70S ribosomes — like bacteria.
Reproduce by binary fission within the cell.

Why students lose points here

Saying water moves “toward higher solute” instead of toward lower water potential — same direction, weaker reasoning.
Confusing the Golgi cis face (receives from ER) with the trans face (sends to membrane).
Calling osmosis “drinking water” — it's passive diffusion, not an active process.
Including mitochondria or chloroplasts in the endomembrane system — they're excluded.
Stopping at “powerhouse” instead of describing cristae, matrix, and the inner membrane's role in oxidative phosphorylation.

FRQ patterns

Cell diagram: identify an organelle and explain how structure supports function — connect them explicitly.
Membrane transport: given a cell in a solution, predict direction of water movement and name the mechanism.
Experimental design: design a test of membrane permeability — IV, DV, control, measurement.

How Unit 2 connects

Unit 3: Inner-membrane compartments set up the electron transport chain.
Unit 4: Signal transduction starts at the plasma membrane.
Unit 6: Nuclear pores control mRNA export; rough ER translates proteins.

FAQ

What is the most important thing to know in Unit 2?

Structure-to-function relationships. The exam doesn't just ask what organelles do — it asks how their structure enables their function. Cristae increase surface area for ATP synthase. The nuclear envelope regulates mRNA export. Rough ER has ribosomes because it produces proteins. Every structure has a mechanistic reason.

What is the fluid mosaic model?

The accepted model of the plasma membrane: a phospholipid bilayer (hydrophilic heads out, hydrophobic tails in) with proteins, cholesterol, and carbohydrates embedded throughout. Both lipids and proteins move laterally — that's the 'fluid' part. The 'mosaic' refers to the varied protein types interspersed in the bilayer.

What's the difference between passive and active transport?

Passive transport requires no energy and moves molecules down their concentration gradient (diffusion, facilitated diffusion, osmosis). Active transport requires ATP and moves molecules against their gradient — like the Na⁺/K⁺ pump or proton pumps.

What is plasmolysis?

Plasmolysis is what happens to a plant cell in a hypertonic solution. Water leaves the cell by osmosis and the plasma membrane pulls away from the rigid cell wall. The cell becomes flaccid because it has lost turgor pressure.

Why do mitochondria have two membranes?

Endosymbiotic theory: the inner membrane is the original membrane of the engulfed prokaryote; the outer membrane came from the host cell's engulfment vesicle. The two membranes create the intermembrane space, which is critical for the proton gradient that drives ATP synthesis.

Keep going

Turn this content into points.

Cramapple grades AP Biology MCQs and FRQs at the criterion level, then tells you the next highest-value point to chase.

Start AP Biology prep How it works

One-time purchase. No subscription.