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Unit 2 • Cell Structure & Membranes

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Surface area/volume: smaller cells diffuse faster; connect to villi/microvilli and prokaryote efficiency.

Membrane dynamics: lateral movement, flip-flop rarity, cholesterol buffering fluidity.

Transport question traps: identify which transports can saturate (carriers) vs cannot (simple diffusion).

Signal transduction amplification math—one GPCR → many cAMP → PKAs → many targets.

Eukaryote vs Prokaryote

Eukaryotes: membrane-bound organelles, linear chromosomes, mitosis/meiosis, larger (10–100 µm). Advantage: compartmentalization → efficiency + regulation.

Prokaryotes: no nucleus, circular DNA in nucleoid, plasmids, smaller (0.1–5 µm), binary fission. Advantage: fast division + high SA/V.

Key Organelles

Nucleus

Houses DNA; nucleolus builds rRNA and ribosomal subunits; double membrane with pores.

Ribosome

Protein synthesis; free → cytosolic proteins, bound (RER) → secretion/membranes.

Mitochondria

Aerobic respiration; cristae hold ETC/ATP synthase, matrix hosts link + Krebs.

Lysosome

Acidic hydrolases for recycling + apoptosis; optimized at low pH.

Rough ER

Protein folding, glycosylation; membrane + secreted proteins.

Smooth ER

Lipid synthesis, detox, Ca²⁺ storage (muscle SR).

Golgi

Protein modification/sorting; cis → trans stack; ships vesicles.

Cytoskeleton

Microtubules (tracks/mitosis), microfilaments (shape, movement), IFs (tension).

Fluid Mosaic

Dynamic bilayer with proteins, cholesterol; unsaturated tails increase fluidity.

Membrane Fluidity & Transport

Simple diffusion

Small/nonpolar down gradient; no protein, no energy.

Facilitated diffusion

Polar/charged down gradient via channel or carrier; no ATP.

Active transport

Against gradient; needs ATP or coupled gradient (pumps, symporters, antiporters).

Hypertonic

Higher solute outside; cell loses water → shrivels/crenates (plants: plasmolysis).

Hypotonic

Lower solute outside; cell gains water → swells (plants: turgid, animals risk lysis).

Isotonic

Equal solute; no net water change, best for animal cells.

Fluid Mosaic extras

Cis unsaturated tails ↑ fluidity (cold adaptation). Saturated tails ↓ fluidity (heat stability). Cholesterol buffers both extremes. Carriers saturate; channels generally do not.

Signaling Snapshot

Ligand

Signal molecule that binds a receptor (peptide, steroid, ion).

Reception

Ligand binds receptor (GPCR, RTK, ion channel); shape change starts cascade.

Transduction

Multi-step relay (kinase cascades, phosphorylation, second messengers).

GPCR

Activates G-protein → GTP; triggers effectors (adenylyl cyclase, ion channels).

Second messenger

Small, fast signals (cAMP, IP3, Ca²⁺); amplify extracellular message.

Kinase / phosphatase

Kinase adds phosphate to activate/inactivate; phosphatase removes to reset.