Unit 2 Quick Review

Cell Theory

• All living things = cells
• Cell = basic unit of life
• All cells from pre-existing cells (Virchow)

Prokaryote vs. Eukaryote

• Prokaryote: no nucleus, no membrane-bound organelles, circular DNA, 70S ribosomes, 1–10 μm
• Eukaryote: nucleus, membrane-bound organelles, linear DNA, 80S ribosomes, 10–100 μm
• Both have ribosomes, DNA, plasma membrane, cytoplasm

SA:V Ratio

• As size ↑, volume grows faster than surface area → SA/V ↓
• Small cells exchange materials more efficiently
• Adaptations: microvilli, root hairs, folded membranes (cristae, thylakoids)
• Formula for a cube: $SA/V = 6/s$

Organelles

• Nucleus — DNA storage, transcription; nucleolus makes rRNA
• Rough ER — protein synthesis (ribosomes attached)
• Smooth ER — lipid synthesis, detox, Ca²⁺ storage
• Golgi — modifies, sorts, packages proteins (cis → trans)
• Lysosomes — hydrolytic enzymes, autophagy
• Mitochondria — ATP via cellular respiration; double membrane, cristae, own DNA
• Chloroplasts — photosynthesis; thylakoids in grana, stroma
• Ribosomes — protein synthesis (free = cytoplasmic, bound = secreted)
• Vacuole — large central one in plants = turgor pressure
• Cell wall — cellulose (plants), chitin (fungi), peptidoglycan (bacteria)

Endomembrane Pathway

Rough ER → vesicle → Golgi → vesicle → plasma membrane (exocytosis)

Endosymbiotic Theory (Lynn Margulis)

Mitochondria and chloroplasts were once free-living prokaryotes. Evidence:

• Double membrane
• Own circular DNA
• 70S ribosomes
• Binary fission
• Bacterial size

Fluid Mosaic Model (Singer & Nicolson, 1972)

• Phospholipid bilayer + embedded proteins
• Amphipathic phospholipids: polar head, nonpolar tails
• Cholesterol = fluidity buffer (fluid at cold, rigid at hot)
• Unsaturated fatty acids = more fluid (kinks)
• Glycoproteins/glycolipids = cell recognition

Membrane Permeability

• Cross freely: small nonpolar (O₂, CO₂)
• Blocked: large polar, ions, charged molecules
• Water uses aquaporins

Transport

• Passive (no ATP): simple diffusion, facilitated diffusion, osmosis — down gradient
• Active (ATP): against gradient — e.g., Na⁺/K⁺ pump (3 Na⁺ out, 2 K⁺ in)
• Secondary active: uses gradient set up by primary (Na⁺/glucose cotransporter)
• Endocytosis: phago- (solid), pino- (liquid), receptor-mediated
• Exocytosis: vesicle fuses with plasma membrane

Tonicity

Environment	Water moves	Animal cell	Plant cell
Hypotonic	In	Lyses	Turgid ✓
Isotonic	None	Normal ✓	Flaccid
Hypertonic	Out	Shrivels	Plasmolysis

Water Potential

$\Psi = \Psi_S + \Psi_P$ $\Psi_S = -iCRT$

• Water moves from high Ψ → low Ψ
• Pure water: Ψ = 0
• Adding solutes makes Ψ more negative
• $i$ = ion constant (sucrose = 1, NaCl = 2)
• $R$ = 0.0831 L·bar/(mol·K), $T$ in Kelvin

💡 Exam Tip: On FRQs, always show your water potential work step by step. Partial credit is generous if you set up the formula correctly, even with arithmetic errors.

💡 Exam Tip: When asked "why are cells small," never just say "SA/V ratio." Explain that higher SA/V means faster exchange of nutrients and wastes relative to metabolic demand.

Key Terms

• Amphipathic — molecule with polar and nonpolar regions
• Aquaporin — water channel protein
• Autophagy — lysosome digesting cell's own components
• Crenation — animal cell shriveling in hypertonic solution
• Cristae — inner mitochondrial membrane folds
• Endosymbiosis — one cell living inside another
• Exocytosis — vesicle release out of cell
• Glycoprotein — carb + protein, for cell recognition
• Grana — stacks of thylakoids
• Osmosis — diffusion of water
• Plasmolysis — plant cell membrane pulling from wall in hypertonic
• Tonicity — solute concentration of solution relative to cell
• Turgor pressure — pressure of plant cell contents against wall

Must-Know for the Exam ✓

• 3+ differences between prokaryotes and eukaryotes
• Why small cells are more efficient (SA/V with consequence)
• Protein secretion pathway in order
• 3+ pieces of evidence for endosymbiotic theory
• Role of cholesterol in membrane fluidity
• Difference between simple, facilitated, and active transport
• Tonicity effects on animal vs. plant cells
• Water potential calculation with units
• Na⁺/K⁺ pump stoichiometry (3 out, 2 in, 1 ATP)
• At least 2 examples of SA/V adaptations