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Unit 2 maps the cell — the organelles, the membrane that bounds them, and the transport that moves material across it. The big quantitative idea is surface-area-to-volume ratio, which explains why cells stay small and why they fold their membranes. It also introduces endosymbiotic theory, which links straight back to Unit 7 evolution.
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The organelles and their jobs: nucleus, ribosomes, rough and smooth ER, Golgi, mitochondria, chloroplasts, lysosomes, vacuoles, and the cytoskeleton. The endomembrane system as a connected manufacturing-and-shipping pathway. Differences between prokaryotic and eukaryotic cells.
Why cells are small: as a cell grows, volume increases faster than surface area, so exchange across the membrane cannot keep up. This drives adaptations like microvilli and the small size of cells, and is a frequent quantitative MCQ topic.
The phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. Selective permeability — why small nonpolar molecules cross freely while ions and large polar molecules need transport proteins. Membrane fluidity and what changes it.
Passive transport (simple diffusion, facilitated diffusion, osmosis) versus active transport that uses ATP to move solutes against their gradient. Tonicity — hypotonic, isotonic, hypertonic — and water potential in plant cells. Bulk transport by endocytosis and exocytosis.
How internal membranes let eukaryotes run incompatible reactions at once. Endosymbiotic theory: mitochondria and chloroplasts arose from engulfed prokaryotes, supported by their double membranes, own circular DNA, and own ribosomes.
Unit 2 MCQs frequently give you a surface-area-to-volume calculation or a tonicity scenario and ask you to predict the direction of water or solute movement. Membrane questions test whether you know which molecules need a transport protein. On FRQs, Unit 2 commonly appears as experimental design with dialysis tubing or potato cores, where you must explain results using water potential — and as justify-with-evidence prompts for endosymbiotic theory.
We make Unit 2 quantitative from day one: students compute surface-area-to-volume ratios and water potential rather than memorizing outcomes, because that is exactly how the exam frames them. We run a live osmosis lab walkthrough so the tonicity vocabulary sticks to a real result, and we connect endosymbiosis forward to Unit 7 so it is reinforced twice. Live online, US time zones.
AP Biology Unit 2 Cell Structure and Function covers eukaryotic and prokaryotic organelles, the fluid mosaic plasma membrane, passive and active transport, tonicity and water potential, surface-area-to-volume ratio, and endosymbiotic theory.
Cells stay small because of the surface-area-to-volume ratio. As a cell grows, its volume increases faster than its surface area, so the membrane cannot exchange materials fast enough to support the larger volume. This is a core quantitative concept in Unit 2.
The College Board CED lists Unit 2 at approximately 10 to 13 percent of the multiple-choice section. This is published as a range, so treat it as approximate.
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We teach this unit live online and grade your practice against the real College Board rubric. Faculty trained at AIIMS (All India Institute of Medical Sciences, India's top medical school).