AP Biology Unit 2 Cheat Sheet: Cell Structure & Function

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.

• ✅ Download your free Unit 2 Cheat Sheet PDF above

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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