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Unit 2 · Cell Structure & Function Flashcards Cheat Sheet Essentials Visual Review MC Practice FRQ Practice

AP Biology Unit 2 Visual Review

A topic-by-topic visual walkthrough of Cells — organelles, cell size, the plasma membrane, every transport mechanism, tonicity, and cell compartmentalization.

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TOPIC 2.1 Cell Structure & Function KEY EUKARYOTIC ORGANELLES Nucleusstores DNA, directs the cell Ribosomesbuild proteins (translation) Rough ERprotein synthesis & transport Smooth ERlipid synthesis, detox Golgi apparatusmodifies, sorts, ships proteins Mitochondriacellular respiration → ATP Chloroplastphotosynthesis (plants only) Lysosomedigestion & recycling (enzymes) Vacuolestorage; turgor in plant cells Cytoskeletonshape, support, movement Cell wallrigid support (plants, fungi, bacteria) Prokaryote vs. eukaryote Prokaryotes (bacteria, archaea): NO nucleus and no membrane-bound organelles; DNA floats in cytoplasm. Eukaryotes (plants, animals, fungi, protists): DNA in a nucleus, with many membrane-bound organelles. Both share: plasma membrane, cytoplasm, ribosomes, DNA. The endomembrane system Nuclear envelope → rough ER → Golgi → vesicles → plasma membrane work together to make, modify, and export proteins along one connected pathway. Structure fits function: cells that secrete a lot have abundant rough ER and Golgi. Each organelle has a specialized function — and its structure is built to carry out that job. The Review Hub · AP Biology Unit 2 TOPIC 2.2 Cell Size & Surface Area-to-Volume Small cell high SA:V ratio ✓ efficient exchange Large cell low SA:V ratio ✗ can't keep up As a cell grows, volume grows faster than surface area. Why cells stay small A cell must exchange nutrients, gases, and wastes across its surface. If the volume it must supply grows too large for its surface, exchange can't keep the cell alive. The math (cube of side s) Surface area = 6s² · Volume = s³ · ratio = 6/s Double the side → surface area ×4 but volume ×8, so the SA:V ratio is cut in half. Smaller = higher ratio. Adaptations for more surface area Cells that need lots of exchange fold their membranes: microvilli in the intestine, or a flat/elongated shape, boost surface area without adding much volume. A high surface area-to-volume ratio lets a cell exchange materials fast enough to survive. The Review Hub · AP Biology Unit 2 TOPIC 2.3 The Plasma Membrane — Fluid Mosaic EXTRACELLULAR FLUID channel glycoprotein CYTOPLASM (inside) "Fluid" — the bilayer moves Phospholipids drift side to side, so the membrane is flexible, self-sealing, and dynamic — not a rigid wall. Cholesterol buffers fluidity: it keeps the membrane from getting too fluid when warm or too stiff when cold. Unsaturated fatty-acid tails also increase fluidity. "Mosaic" — proteins scattered throughout Transport proteins move materials; receptor proteins receive signals; recognition proteins (with attached carbohydrates = glycoproteins) act as cell "ID tags." The hydrophilic phosphate heads face the watery inside and outside; hydrophobic tails hide in the middle. The membrane is a fluid phospholipid bilayer with a mosaic of embedded proteins. The Review Hub · AP Biology Unit 2 TOPIC 2.4 Membrane Permeability ✓ Crosses the bilayer easily • Small nonpolar molecules — O₂, CO₂ • Small uncharged polar molecules — H₂O (slowly) • Lipid-soluble (hydrophobic) substances They dissolve right through the hydrophobic tail core — no protein needed. This is simple diffusion. ✗ Blocked — needs a transport protein • Large molecules — glucose, amino acids • Ions — Na⁺, K⁺, Cl⁻, Ca²⁺ (charged) • Any large polar / hydrophilic molecule The hydrophobic core repels charged and large polar species, so they require channels or carriers (Topic 2.6). Selective permeability The membrane is selectively permeable — it lets some substances through freely while controlling others. Two factors decide how easily something crosses: its size (small = easier) and its polarity/charge (nonpolar = easier). This selectivity is what lets a cell maintain a stable internal environment (homeostasis). Membrane permeability increases with temperature as the bilayer becomes more fluid. Small & nonpolar slips through; large & charged needs help — that's selective permeability. The Review Hub · AP Biology Unit 2 TOPIC 2.5 Membrane Transport — the Big Picture PASSIVE transport no energy · DOWN the gradient • Simple diffusion — small nonpolar molecules • Osmosis — diffusion of water across the membrane • Facilitated diffusion — via proteins (Topic 2.6) Molecules move from HIGH to LOW concentration until they reach equilibrium — powered by their own random motion, so the cell spends no ATP. ACTIVE transport requires energy · AGAINST the gradient • Protein pumps use ATP (e.g., Na⁺/K⁺ pump) • Bulk transport — endocytosis & exocytosis • Builds & maintains concentration gradients Molecules are pushed from LOW to HIGH concentration. Because this works against diffusion, it costs energy — usually ATP. (Details in Topic 2.8.) The concentration gradient is the key idea A gradient is a difference in concentration across the membrane. Passive transport follows it "downhill"; active transport pushes "uphill" against it. Passive = down the gradient, free · Active = up the gradient, costs energy. The Review Hub · AP Biology Unit 2 TOPIC 2.6 Facilitated Diffusion TWO KINDS OF TRANSPORT PROTEIN channel — open pore carrier — flips Channel proteins Form a hydrophilic tunnel for specific ions or water to flow through. Aquaporins are water channels that speed osmosis dramatically. Carrier proteins Bind a specific molecule (like glucose), then change shape to shuttle it across. Each carrier is specific to the molecule it transports. Still PASSIVE — no energy required Facilitated diffusion moves large or charged molecules that can't cross the bilayer alone — but it still runs DOWN the concentration gradient, so the cell spends no ATP. The protein just provides a passageway. Transport rate levels off when all proteins are busy (saturation) — unlike simple diffusion. Channels & carriers move molecules down the gradient without ATP — passive, but protein-assisted. The Review Hub · AP Biology Unit 2 TOPIC 2.7 Tonicity & Osmoregulation HYPOTONIC solution water IN → cell swells lower solute outside · may lyse (burst) ISOTONIC solution water ⇄ no net change equal solute · dynamic equilibrium HYPERTONIC solution ← water OUT cell shrinks higher solute outside · shrivels (crenates) Water follows solute Osmosis moves water toward the side with HIGHER solute concentration (lower water potential). "Tonicity" describes the solution RELATIVE to the cell. Hyper = more solute; hypo = less; iso = equal. Osmoregulation Organisms actively control their water & solute balance. Plant cells rely on a rigid cell wall to resist bursting (turgor pressure); paramecia pump water out with a contractile vacuole. Water moves toward higher solute: hypotonic swells, hypertonic shrinks, isotonic is balanced. The Review Hub · AP Biology Unit 2 TOPIC 2.8 Active Transport & Bulk Transport Active transport (protein pumps) Uses ATP to move molecules AGAINST their gradient. Sodium–potassium (Na⁺/K⁺) pump 3 Na⁺ OUT and 2 K⁺ IN per ATP — builds the gradients nerve and muscle cells need to fire. Secondary (cotransport) uses a gradient built by a pump. Bulk transport (vesicles) Endocytosis — bringing material IN Phagocytosis (solids), pinocytosis (liquids), and receptor-mediated endocytosis (specific molecules). Exocytosis — sending material OUT Vesicles fuse with the membrane to secrete their cargo. Active vs. passive — the core difference Passive transport is "downhill" and free; active transport is "uphill" and costs energy. Cells use active transport to concentrate nutrients, expel wastes, and maintain the ion gradients that store energy. All bulk transport (endo- and exocytosis) requires energy because it moves large amounts of material and rearranges the membrane itself — it's a form of active transport. Poisons that stop ATP production (e.g., cyanide) shut down active transport but not simple diffusion. Active transport spends ATP to move materials against the gradient or in bulk via vesicles. The Review Hub · AP Biology Unit 2 TOPIC 2.9 Cell Compartmentalization Membranes create separate internal environments Membrane-bound organelles let a eukaryotic cell run many different — even opposing — chemical reactions at the same time, each in its own optimized compartment with its own pH, enzymes, and conditions. This "division of labor" makes eukaryotic cells far more efficient than they'd be as one open mixture. Isolates reactions Lysosomes keep digestive enzymes contained so they don't destroy the whole cell. Harmful steps stay sealed off. Concentrates molecules Reactants and enzymes are gathered together, so reactions run faster. Higher local concentration. Increases surface area Internal folded membranes (cristae in mitochondria, thylakoids in chloroplasts) maximize reaction space. Example: the mitochondrion Its inner membrane is folded into cristae (surface area) and encloses the matrix — separating the reactions of cellular respiration. Internal membranes give organelles their own specialized micro-environments — a division of labor. The Review Hub · AP Biology Unit 2 TOPIC 2.10 Origins of Compartmentalization — Endosymbiosis A large ancient cell engulfed a smaller prokaryote — which survived inside as an organelle. → mitochondrion → chloroplast Evidence for the endosymbiotic theory Mitochondria & chloroplasts have their OWN circular DNA They have their own ribosomes (like a bacterium's) They reproduce by binary fission, independent of the cell They have a double membrane (from being engulfed) They are about the size of a typical prokaryotic cell Each of these traits is exactly what you'd expect if these organelles were once free-living bacteria. A mutually beneficial partnership The engulfed prokaryote provided the host with energy (ATP from respiration) or food (sugar from photosynthesis), while the host provided protection and nutrients. Over time they became inseparable. The internal membrane system (ER, nuclear envelope) likely arose from infoldings of the plasma membrane. Endosymbiotic theory: mitochondria & chloroplasts descend from engulfed free-living prokaryotes. The Review Hub · AP Biology Unit 2
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This is great for review the night before the exam — fast, visual, and covers everything you need to remember about Unit 2's cell biology.