Cell Biology

The plasma membrane is a phospholipid bilayer with embedded proteins. Small nonpolar molecules (O₂, CO₂) diffuse through the bilayer. Ions and most polar molecules need channels or carriers.

Passive transport (simple diffusion, facilitated diffusion through channels or carriers) moves substances down their electrochemical gradient. Active transport moves them against it. Primary active transport uses ATP directly (Na⁺/K⁺-ATPase). Secondary active transport uses an existing ion gradient (the Na⁺/glucose symporter in intestinal cells).

Osmosis is water movement across a selectively permeable membrane toward the side with higher solute concentration (lower water potential). Aquaporins speed water movement but do not change its direction. Only solutes that cannot cross the membrane contribute to tonicity.

Proteins made on free ribosomes stay in the cytosol or are imported into the nucleus, mitochondria, chloroplasts, or peroxisomes by targeting sequences. Proteins with an N-terminal signal peptide are made on ribosomes that dock at the rough ER. These proteins enter the secretory pathway: ER → Golgi → vesicles → plasma membrane, lysosome, or secretion.

Mitochondria and chloroplasts have their own DNA and ribosomes, double membranes, and divide independently. This is evidence for their endosymbiotic origin. Most of their proteins are still encoded in the nucleus and imported.

Microfilaments (actin) handle cell shape, crawling, and cytokinesis in animal cells. Microtubules (tubulin) form the spindle, cilia and flagella, and tracks for kinesin (usually toward the plus end) and dynein (toward the minus end). Intermediate filaments give tensile strength.

Drugs that stabilize or destabilize microtubules (for example taxol or colchicine) block mitosis. Questions often ask you to connect a drug to the stage it arrests.

Hydrophilic signals (peptide hormones, most neurotransmitters) bind cell-surface receptors. Hydrophobic signals (steroids, thyroid hormone) cross the membrane and bind intracellular receptors that often act as transcription factors.

G-protein-coupled receptors activate G proteins, which regulate enzymes such as adenylyl cyclase (cAMP → PKA) or phospholipase C (IP₃ → Ca²⁺ release; DAG → PKC). Receptor tyrosine kinases dimerize and autophosphorylate, often activating the Ras–MAPK pathway. Each step amplifies the signal, and each step can be switched off.

Interphase (G₁, S, G₂) is followed by mitosis and cytokinesis. Cyclin–CDK complexes drive transitions. Checkpoints halt the cycle if DNA is damaged (p53 at G₁/S) or if chromosomes are not attached to the spindle (the spindle assembly checkpoint before anaphase).

Know how DNA content per cell changes: 2C in G₁, 4C after S phase, back to 2C in each daughter cell. Meiosis is covered in Genetics, but the same bookkeeping applies.