A one-page visual summary of Cell Structure & Function — every organelle, transport mechanism, and key concept you need to know for the exam, on a single screen.
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.
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.