What you'll learn
Unit 2 examines how cell structure enables function: organelles, membranes, transport, and why cell size is limited. ~10–13% of the AP Biology exam.
Prokaryotic vs eukaryotic cells
- Prokaryotes (bacteria): no nucleus, no membrane-bound organelles, smaller.
- Eukaryotes (plants, animals, fungi, protists): nucleus + membrane-bound organelles.
Key organelles (structure → function)
- Nucleus — stores DNA, directs transcription.
- Ribosomes — protein synthesis (free or on rough ER).
- Rough ER — makes/modifies proteins; Smooth ER — lipids, detox.
- Golgi apparatus — modifies, packages, ships proteins.
- Mitochondria — ATP via cellular respiration (double membrane).
- Chloroplasts (plants) — photosynthesis.
- Lysosomes — digestion/recycling.
- Vacuole — storage; large central vacuole in plant cells. The endosymbiotic theory explains mitochondria/chloroplasts (own DNA, double membranes, ribosomes).
The cell membrane
A phospholipid bilayer (fluid mosaic) with embedded proteins. Amphipathic phospholipids form the bilayer; cholesterol modulates fluidity. Selectively permeable.
Transport across membranes
- Passive (no ATP): diffusion, osmosis (water), facilitated diffusion (channels/carriers) — down the gradient.
- Active (ATP): pumps move substances against the gradient (e.g. sodium-potassium pump).
- Bulk: endocytosis / exocytosis. Tonicity: cells shrink in hypertonic, swell/burst in hypotonic, stable in isotonic solutions.
Surface-area-to-volume ratio
As a cell grows, volume increases faster than surface area, limiting exchange — this caps cell size and explains adaptations (microvilli, folds) that increase surface area.
Exam tips
- Tie each organelle to its function in a process (e.g. protein from ribosome → rough ER → Golgi → vesicle).
- Predict water movement using tonicity and water potential.
- Use SA:V to explain why cells stay small or fold their membranes.
Common mistakes
- Saying osmosis moves solutes (it moves water).
- Forgetting active transport needs ATP and goes against the gradient.
- Mixing up hypertonic/hypotonic effects on the cell.