From the smallest prokaryote to a complex eukaryotic cell. Organelles and what each one does, the plasma membrane and how things move across it, tonicity and osmosis, and the endosymbiotic origins of mitochondria and chloroplasts. This is the unit where cells become alive.
Eleven topics from the College Board CED, in order.
Topic 2.1
Cell Structure: Subcellular Components
The organelles of eukaryotic cells and what each one does.
Topic 2.2
Cell Structure & Function
How organelles work together; structural differences between cell types.
Topic 2.3
Cell Size
Surface area to volume ratio and why it limits how big cells can get.
Topic 2.4
Plasma Membranes
Fluid mosaic model — phospholipids, proteins, cholesterol, carbohydrates.
Topic 2.5
Membrane Permeability
What can and can't cross the bilayer freely, and why.
Topic 2.6
Membrane Transport
Passive vs. active transport — and the energy that distinguishes them.
Topic 2.7
Facilitated Diffusion
Channel and carrier proteins that move substances down their gradient.
Topic 2.8
Tonicity & Osmoregulation
Hypertonic, hypotonic, isotonic — and how plant and animal cells respond.
Topic 2.9
Mechanisms of Transport
Active transport pumps (Na⁺/K⁺) plus bulk transport (endo- and exocytosis).
Topic 2.10
Cell Compartmentalization
Why dividing the cell into membrane-bound compartments is a winning strategy.
Topic 2.11
Origins of Cell Compartmentalization
Endosymbiotic theory — how mitochondria and chloroplasts joined the cell.
About Unit 2
Unit 2 is where biology starts to feel like biology. After Unit 1's chemistry foundation, you'll learn the parts of the cell — organelles like the nucleus, ER, Golgi, mitochondria, and chloroplasts — and how they cooperate. You'll meet the plasma membrane in detail (fluid mosaic model) and learn the rules of membrane transport: what can cross passively, what needs a protein, and what requires energy.
This unit is 10–13% of the AP Bio exam — heavier than Unit 1 — and takes about 12–14 class periods. The most-tested topics are tonicity (hypertonic/hypotonic), the difference between passive and active transport, and the endosymbiotic theory. You'll also learn why surface area to volume ratio limits cell size and shapes things like the small intestine's villi.
The College Board ties Unit 2 to three of the four Big Ideas:
Big Idea 1
Evolution — endosymbiosis & the origin of eukaryotes
Big Idea 2
Energetics — active transport uses ATP
Big Idea 4
Systems — compartmentalization & selective permeability