Memory tricks for how cells actually work

Passive transport moves substances DOWN their concentration gradient — no ATP required. Includes simple diffusion (gases, lipids), facilitated diffusion (glucose via GLUT transporters), and osmosis (water via aquaporins). Active transport moves substances AGAINST their gradient — requires ATP. Primary active transport uses ATP directly (Na+/K+ ATPase). Secondary active transport uses the gradient created by primary transport (sodium-glucose cotransporter). Endocytosis brings large particles IN; exocytosis sends them OUT.

The Na+/K+ ATPase pump is the most important active transport protein in the body. For every ATP used: 3 sodium ions pumped OUT of the cell, 2 potassium ions pumped IN. This creates the electrochemical gradients essential for nerve impulses, muscle contraction, and nutrient absorption. The pump uses approximately 30% of the body's total ATP at rest. Cardiac glycosides (digoxin) inhibit this pump — increasing intracellular sodium → more calcium → stronger cardiac contractions.

Osmosis is the movement of water across a semipermeable membrane from low solute to high solute concentration. Isotonic solution (same concentration as cell): no net water movement — cell stays normal. Hypotonic solution (less solute than cell): water moves INTO cell — cell swells, may lyse. Hypertonic solution (more solute than cell): water moves OUT of cell — cell shrinks (crenation). Clinical: IV fluids must be chosen carefully — 0.9% NaCl is isotonic, D5W becomes hypotonic after glucose is metabolized.

Resting membrane potential: -70 mV (more negative inside — K+ leaks out, Na+ pumped out). Depolarization: stimulus opens voltage-gated Na+ channels → Na+ rushes IN → membrane potential rises to +30 mV. Repolarization: Na+ channels inactivate, K+ channels open → K+ rushes OUT → returns to -70 mV. After-hyperpolarization: K+ channels slow to close → briefly more negative than resting (-80 mV). Absolute refractory period: cannot fire again regardless of stimulus strength.

Water-soluble hormones cannot cross the lipid membrane — they bind surface receptors and use second messengers. GPCR pathway: ligand → G-protein coupled receptor → adenylyl cyclase → cAMP → protein kinase A → cellular response. RTK pathway: growth factors → receptor tyrosine kinase → RAS → MAPK cascade → gene expression. Lipid-soluble hormones (steroids, thyroid hormones) cross the membrane directly and bind intracellular receptors → bind DNA → change gene expression directly.

Fick's Law states that diffusion rate is proportional to surface area × concentration gradient × diffusion coefficient, and inversely proportional to membrane thickness. Surface area: larger area = faster diffusion (alveoli have huge surface area for this reason). Membrane thickness: thicker membrane = slower diffusion. Concentration gradient: steeper gradient = faster diffusion. Molecular weight: smaller molecules diffuse faster. Lipid solubility: lipid-soluble molecules diffuse faster through membranes. CO2 diffuses 20× faster than O2 across the alveolar membrane.

The resting membrane potential of -70 mV in neurons exists because: K+ channels are open at rest and K+ leaks OUT (following its concentration gradient), making the inside more negative. Na+/K+ ATPase continuously pumps Na+ out and K+ in. Large negatively charged proteins are trapped inside cells. K+ equilibrium potential is -90 mV, Na+ equilibrium potential is +60 mV — the resting potential of -70 mV sits between these, dominated by K+ permeability. Nernst equation calculates the equilibrium potential for a single ion.

Mitochondria: powerhouse — ATP production via oxidative phosphorylation. Contains its own DNA. Endoplasmic reticulum: rough ER (ribosomes, protein synthesis), smooth ER (lipid synthesis, detoxification, Ca2+ storage). Ribosomes: protein synthesis — free ribosomes make cytoplasmic proteins, bound ribosomes make secretory and membrane proteins. Golgi apparatus: protein processing, sorting, and packaging. Lysosomes: contain hydrolytic enzymes — digest cellular debris and pathogens (pH 4.5).

Cellular respiration produces ~38 ATP per glucose molecule. Glycolysis (cytoplasm, no O2 needed): glucose → 2 pyruvate + 2 ATP + 2 NADH. Krebs cycle (mitochondrial matrix): 2 acetyl-CoA → 2 ATP + NADH + FADH2 carriers. Electron transport chain (inner mitochondrial membrane, requires O2): NADH and FADH2 → ~34 ATP via oxidative phosphorylation. Without oxygen: anaerobic glycolysis only produces 2 ATP + lactic acid. Cyanide and carbon monoxide poison the ETC by blocking Complex IV.