AP Biology Unit 2 Cheat Sheet: Cell Structure & Function
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TLDR
Eukaryotes have specialized organelles; prokaryotes do not.
The cell membrane is a fluid mosaic: phospholipid bilayer with proteins and carbohydrates.
Transport can be passive, active, or bulk (endocytosis/exocytosis).
Surface area-to-volume ratio (SA:V) limits cell size — smaller cells exchange materials more efficiently.
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Why Unit 2 Matters
Unit 2 is where AP Biology shifts from molecules to actual cells. The exam expects you to connect structure to function: Why does a mitochondrion have folds? Why are small cells more efficient? If you can explain these connections clearly, you’ll do well in both multiple-choice and free-response questions.
Key Topics in Unit 2
Organelles and Their Functions
Nucleus: DNA storage, transcription.
Ribosomes: protein synthesis (free = cytoplasmic proteins; bound = exported proteins).
Rough ER: folds and modifies proteins.
Smooth ER: lipid synthesis, detox.
Golgi apparatus: modifies and packages proteins (FedEx of the cell).
Mitochondria: ATP production; inner folds (cristae) increase surface area.
Chloroplasts: photosynthesis; thylakoids stacked into grana (pancakes that trap sunlight).
Lysosomes: digestion and recycling.
Vacuoles: storage; central vacuole maintains plant turgor pressure.
Cytoskeleton: structure, transport, and motility.
Prokaryotes vs Eukaryotes
Prokaryotes: no nucleus, no membrane-bound organelles, smaller.
Eukaryotes: nucleus, organelles, larger.
Shared: plasma membrane, cytoplasm, DNA, ribosomes.
Membrane Structure
Fluid Mosaic Model: phospholipid bilayer with proteins and carbohydrates.
Hydrophilic heads face outward; hydrophobic tails hide inward.
Cholesterol: regulates fluidity.
Proteins: channels, carriers, receptors, enzymes.
Carbohydrates: cell-to-cell recognition.
Mnemonic: “Heads like water, tails hide.”
Transport Across Membranes
Passive transport: diffusion, facilitated diffusion, osmosis.
Active transport: requires ATP, pumps molecules against gradient.
Bulk transport:
Endocytosis → phagocytosis, pinocytosis, receptor-mediated.
Exocytosis → release of vesicles.
Surface Area-to-Volume Ratio (SA:V)
Small cells have higher SA:V → faster exchange of materials.
As cell size increases, SA:V decreases → diffusion becomes inefficient.
Adaptations: microvilli, villi, folded membranes.
Mnemonic: “Small cells stay efficient.”
👉 Tutor Tip: Whenever the exam mentions an organelle, immediately ask: What is its structure, and how does that structure enable its function?
Example: Mitochondria → folded inner membrane → increased surface area → more ATP production.
Mini Formula Box
For a cube:
Surface Area = 6s²
Volume = s³
SA:V = 6/s
As s increases, SA:V decreases.
Visual Mnemonics
Golgi: stack of pancakes + vesicles = “FedEx of the cell.”
Mitochondria: inner folds (cristae) = “Folded membranes = more energy.”
Chloroplasts: stacked thylakoids = “Stacks of pancakes = sunlight traps.”
SA:V: small cube vs large cube = “Small cells stay efficient.”
Common Exam Pitfalls
Confusing free vs bound ribosomes.
Forgetting mitochondria and chloroplasts have their own DNA (endosymbiotic theory).
Assuming all transport requires energy (only active transport does).
Misapplying SA:V — large multicellular organisms solve this with specialized organs.
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