Membrane Transport and Tonicity

Core Review

The Cell Membrane

The plasma membrane is a phospholipid bilayer with embedded proteins. Phospholipids are amphipathic — hydrophilic phosphate heads face outward (toward aqueous environments) and hydrophobic fatty acid tails face inward. This arrangement forms a selectively permeable barrier. Small, nonpolar molecules (O₂, CO₂) and small uncharged polar molecules (H₂O, ethanol) can cross directly. Large polar molecules and ions require protein channels or carriers. Cholesterol (in animal cells) maintains membrane fluidity at varying temperatures.

Passive Transport (No ATP Required)

Simple Diffusion: Molecules move directly through the phospholipid bilayer from high to low concentration. Rate depends on concentration gradient, temperature, molecule size, and lipid solubility. Oxygen and carbon dioxide exchange in alveoli relies on simple diffusion.

Facilitated Diffusion: Molecules move down their gradient through transmembrane protein channels or carriers. Two types:

• Channel proteins: Form hydrophilic pores. Ion channels (Na⁺, K⁺, Ca²⁺, Cl⁻) are often gated — they open/close in response to voltage changes, ligand binding, or mechanical stress. Aquaporins are water-specific channels.
• Carrier proteins: Bind specific solutes and undergo a conformational change to transport them. Glucose enters cells via GLUT transporters through facilitated diffusion. Carriers exhibit saturation kinetics — at high substrate concentrations, the transport rate plateaus when all binding sites are occupied.

Osmosis: The diffusion of water across a selectively permeable membrane from a region of lower solute concentration (more free water) to a region of higher solute concentration (less free water). Water moves to dilute the side with more solute.

Active Transport (ATP Required)

Primary Active Transport — Na⁺/K⁺ ATPase: This pump is present in the plasma membrane of all animal cells. For each ATP hydrolyzed, it transports 3 Na⁺ out of the cell and 2 K⁺ into the cell — both against their concentration gradients. This creates the resting membrane potential (inside negative), provides the gradient for secondary active transport, and maintains cell volume. The pump is electrogenic (3 positive charges out, 2 in → net movement of 1 positive charge out per cycle).

Secondary Active Transport: Uses the electrochemical gradient established by primary active transport. In cotransport (symport), the "downhill" movement of Na⁺ drives the "uphill" movement of another molecule (e.g., Na⁺/glucose cotransporter in the small intestine). In countertransport (antiport), Na⁺ moving in drives another ion out (e.g., Na⁺/Ca²⁺ exchanger in cardiac muscle).

Bulk Transport

Endocytosis: The plasma membrane engulfs extracellular material, forming a vesicle.

• Phagocytosis ("cell eating"): large particles, pseudopodia extension.
• Pinocytosis ("cell drinking"): fluid and dissolved solutes.
• Receptor-mediated endocytosis: specific ligands bind receptors clustered in clathrin-coated pits; LDL cholesterol uptake is a classic example.

Exocytosis: Intracellular vesicles fuse with the plasma membrane and release contents. This is how neurotransmitters are released at synapses and how secretory proteins exit the cell.

Tonicity

Tonicity describes the effect of extracellular solute concentration on cell volume.

Red blood cells (RBCs) are the classic animal cell example. In a hypotonic solution, RBCs swell and burst (hemolysis). In a hypertonic solution, they shrivel (crenation). Isotonic saline (0.9% NaCl) maintains normal RBC shape.

Common Traps

• "Passive = no protein needed": This is wrong. Facilitated diffusion IS passive (no ATP) but requires transport proteins. Only simple diffusion requires no protein.
• Arteries always carry oxygenated blood? A related circulation trap: the pulmonary artery carries deoxygenated blood. Similarly, not all "water follows solute" questions are straightforward — think about which compartment has more dissolved particles.
• Hypertonic means more water? No — hypertonic means more solute (less free water). Students often reverse this.