The basics
What it covers: The structure of eukaryotic cells, the plasma membrane and how things move across it, tonicity, and the endosymbiotic origins of mitochondria and chloroplasts.
Exam weight: About 10–13% of the AP Biology exam.
The big question: How does the structure of a cell — its membrane, its organelles, and its compartments — make it possible for the cell to do everything it needs to do?
Big Ideas covered: Evolution (BI 1), Energetics (BI 2), Systems Interactions (BI 4).
Key topics at a glance
Prokaryote vs. Eukaryote
Prokaryotes (bacteria, archaea) — no nucleus, no membrane-bound organelles. Eukaryotes — true nucleus and many membrane-bound organelles.
Key Organelles
Nucleus (DNA), ribosomes (proteins), rough ER (secreted proteins), smooth ER (lipids), Golgi (sorting/shipping), mitochondria (ATP), chloroplasts (photosynthesis), lysosomes (digestion), vacuoles (storage).
Surface Area to Volume
Volume grows faster than surface area as cells get bigger. Eventually transport across the membrane can't keep up — limiting cell size. Folded shapes (microvilli) maximize SA:V.
Plasma Membrane (Fluid Mosaic)
Phospholipid bilayer + embedded proteins + cholesterol (regulates fluidity) + carbohydrates on the outside for cell recognition.
Selective Permeability
Small nonpolar molecules (O₂, CO₂) and small uncharged polar molecules (H₂O) cross freely. Large or charged molecules need transport proteins.
Passive vs. Active Transport
Passive: down the gradient, no energy (diffusion, osmosis, facilitated diffusion). Active: against the gradient, requires ATP (Na⁺/K⁺ pump, endo-/exocytosis).
Tonicity
Hypertonic outside → water leaves, cell shrinks. Hypotonic outside → water enters, cell swells (or becomes turgid in plants). Isotonic → no net movement.
Endosymbiotic Theory
Mitochondria & chloroplasts were once free-living prokaryotes engulfed by an early eukaryote. Evidence: own DNA (circular), own ribosomes, double membranes, binary fission.
The key terms you must know
- Prokaryote vs. eukaryote — defining structural difference is the presence (eukaryote) or absence (prokaryote) of a membrane-bound nucleus and organelles.
- Endomembrane system — the connected network of nuclear envelope, ER, Golgi, lysosomes, and vesicles that work together to make and ship proteins.
- Endosymbiotic theory — mitochondria and chloroplasts originated as engulfed prokaryotes; explains their double membranes, own DNA, and ribosomes.
- Fluid mosaic model — phospholipid bilayer + embedded proteins + cholesterol; "fluid" because the molecules can move laterally.
- Selective permeability — only certain substances cross the membrane freely; the basis of cellular control over its internal environment.
- Diffusion / Osmosis — passive movement down a concentration gradient; osmosis is specifically the diffusion of water across a selective membrane.
- Facilitated diffusion — passive transport through channel or carrier proteins; no ATP, moves down the gradient.
- Active transport — moves substances against the gradient using ATP; example is the Na⁺/K⁺ pump.
- Endocytosis / Exocytosis — bulk transport in (engulfing) and out (vesicle fusing with membrane).
- Tonicity — relative solute concentration of two solutions; determines direction of water movement (hypertonic, hypotonic, isotonic).
- Surface area to volume ratio — the geometric limit on cell size; smaller cells (or folded shapes) have a higher SA:V.
Key themes to remember
- Compartmentalization is the eukaryotic advantage. Membrane-bound organelles let incompatible reactions happen at the same time in different parts of the cell.
- The plasma membrane is the gatekeeper. Almost every Unit 2 concept comes back to selective permeability and transport.
- Energy distinguishes active from passive transport. If a substance moves against its gradient, the cell must spend ATP. Down the gradient = free.
- Water follows solutes. In osmosis, water moves toward the more concentrated solute side.
- Surface area to volume ratio limits cell size. Cells stay small or evolve folded surfaces to keep transport efficient.
- Endosymbiosis is the link to Unit 7. Mitochondria and chloroplasts are evolutionary evidence — engulfed prokaryotes living on inside larger cells.
Common exam traps
- Hypotonic isn't always bad. For a plant cell, a hypotonic environment makes it turgid — that's the healthy state. For an animal cell, the same environment causes lysis.
- Water moves TOWARD the higher solute concentration. Easy to flip mid-question. Solute high = water low = water enters that side.
- Facilitated diffusion is still passive. It uses proteins but doesn't use ATP. Don't confuse the use of proteins with active transport.
- Active transport always uses energy. Even if the substance is small and crossing slowly, going against the gradient costs ATP.
- Mitochondria and chloroplasts are NOT just organelles. They were once free-living prokaryotes — that's the basis of endosymbiosis, and it's a frequent FRQ topic.
- Surface area to volume drives a LOT of biology. Microvilli, root hairs, lung alveoli, gills — all increase SA without much extra volume.
- "Selectively permeable" ≠ "impermeable." Many things cross freely. Selective just means some need help (proteins) and some don't.