AP Biology Unit 2: Cell Structure & Function
Study cell organelles, membranes, transport, compartmentalization with exam-format practice questions and rubric-based scoring.
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Inside This Unit: The Full Breakdown
Cell Structure and Function explores how prokaryotic and eukaryotic cells are organized, how membranes regulate what enters and exits, and how compartmentalization allows cells to carry out specialized functions simultaneously.
Why it matters
Cell biology questions appear in every section of the AP exam. Understanding membrane transport, organelle function, and the differences between cell types is essential for explaining processes from photosynthesis to immune response.
Key concepts
- Prokaryotic cells lack membrane-bound organelles; eukaryotic cells have compartmentalized organelles that allow specialized functions.
- The fluid mosaic model describes membranes as dynamic structures of phospholipids, proteins, and carbohydrates that regulate transport.
- Passive transport (diffusion, osmosis, facilitated diffusion) requires no energy; active transport and bulk transport (endocytosis, exocytosis) require ATP.
- Cell size is limited by the surface-area-to-volume ratio — as cells grow, their volume increases faster than their surface area, reducing efficiency.
Cell Types and Organelles
All cells share a plasma membrane, DNA, and ribosomes, but eukaryotic cells are far more complex than prokaryotic cells. Eukaryotes have a nucleus that houses DNA, an endomembrane system (endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles) that processes and transports proteins, and mitochondria that generate ATP. Plant cells additionally have chloroplasts for photosynthesis, a central vacuole for storage and turgor pressure, and a rigid cell wall. Prokaryotes (bacteria and archaea) lack these organelles but are highly successful due to their rapid reproduction and metabolic diversity. The endosymbiotic theory explains how mitochondria and chloroplasts likely originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.
Membrane Structure and Transport
Cell membranes are described by the fluid mosaic model: a bilayer of phospholipids with embedded proteins that can move laterally. The hydrophobic interior of the membrane blocks most polar molecules and ions, making the membrane selectively permeable. Small nonpolar molecules like O₂ and CO₂ cross freely by simple diffusion. Water moves by osmosis through aquaporins. Larger polar molecules and ions require transport proteins — channel proteins or carrier proteins — for facilitated diffusion. All passive transport moves substances down their concentration gradient without energy input. Active transport uses ATP to move substances against their gradient, as seen in the sodium-potassium pump.
Cell Size and Compartmentalization
Cells must remain small because of the surface-area-to-volume ratio. As a cell grows, its volume increases as the cube of its radius, but its surface area increases only as the square. Eventually a large cell cannot import nutrients or export wastes fast enough across its membrane. This constraint explains why most cells are microscopic and why large organisms are multicellular rather than single giant cells. Compartmentalization in eukaryotic cells solves a related problem: different reactions require different conditions. The lysosome maintains an acidic pH for digestion, the ER provides a separate space for protein folding, and the mitochondrial inner membrane creates a proton gradient for ATP synthesis.
AP exam tip
AP free-response questions love to ask you to predict what happens to a cell in a hypotonic, hypertonic, or isotonic solution. Always specify whether the cell is an animal cell (which may lyse or crenate) or a plant cell (which has a cell wall preventing lysis).
Connections to other units
- Unit 3 (Cellular Energetics): Mitochondria and chloroplast structure directly enables chemiosmosis and the light reactions.
- Unit 4 (Cell Communication): Membrane receptors on the cell surface initiate signal transduction pathways.
- Unit 7 (Natural Selection): Endosymbiotic theory connects cell evolution to the origin of eukaryotes.
