Unit 2: Cell Structure and Function
AP Biology: 159 practice questions with detailed explanations.
Unit Study Guide
Executive Summary
Unit 2 tours the cell's compartments and explains how membranes control what enters and leaves. Compartmentalization is the theme: separate spaces run separate chemistry.
Subcellular components
Eukaryotes hold membrane-bound organelles: nucleus (DNA), rough ER (protein synthesis), smooth ER (lipid synthesis, detox), Golgi (packaging), mitochondria (ATP), chloroplasts (sugars), lysosomes (digestion), vacuoles (storage). Ribosomes — not membrane-bound — assemble proteins in all cells.
Surface area to volume
As cells grow, volume (r³) outpaces surface area (r²), so exchange across the membrane becomes limiting. Small cells and folded membranes (cristae, villi, root hairs) raise SA:V.
Membrane structure
The fluid mosaic model: a phospholipid bilayer with embedded proteins and cholesterol. Hydrophilic heads face water; hydrophobic tails face inward. Small nonpolar molecules cross freely; ions and large polar solutes need channels or carriers.
Transport
Passive transport runs down gradients without energy: diffusion, facilitated diffusion (channel and carrier proteins), osmosis. Active transport pumps solutes against gradients using ATP (Na⁺/K⁺ pump). Endocytosis and exocytosis move large cargo in vesicles.
Tonicity
Water moves toward higher solute concentration. Hypertonic surroundings shrink animal cells (crenation) and plasmolyze plant cells; hypotonic surroundings swell animal cells (lysis) but turgid plant cells are protected by cell walls. Isotonic = no net flow.
Compartmentalization
Separate organelles allow incompatible reactions to coexist (lysosome's acid hydrolases vs cytosol). The endosymbiotic theory explains mitochondria and chloroplasts: double membranes, own DNA, bacterial-like ribosomes.