Cell Membrane Transport Overview

Overview

This lecture reviews cell membrane transport mechanisms, focusing on passive and active transport processes essential for cellular function.

Passive transport moves substances across membranes without energy input (ATP).
Types of passive transport: simple diffusion, facilitated diffusion, and osmosis.
All passive transport moves molecules down their concentration gradients (high to low).
Diffusion rate depends on molecule size, temperature, and distance.
Simple diffusion allows small, nonpolar, lipid-soluble molecules (e.g., O₂, CO₂, fat-soluble vitamins) to cross the membrane unaided.
Facilitated diffusion uses carrier or channel proteins for larger or charged molecules (e.g., glucose, amino acids, ions).
Osmosis is the passive movement of water across the membrane, sometimes via aquaporins.
Osmosis moves water from areas of low solute concentration (low osmolarity) to high solute concentration (high osmolarity).
Selectively permeable membranes allow water passage but prevent solute movement.
Tonicity types: isotonic (equal solute), hypertonic (cell shrinks), hypotonic (cell swells/bursts).

Active transport moves substances against their concentration gradient, requiring ATP.
Primary active transport uses carrier proteins (pumps), such as the sodium-potassium pump, moving Na⁺ out and K⁺ into cells.
The sodium-potassium pump maintains resting membrane potential (voltage difference) essential for nerve and muscle cell function.

Vesicular transport moves large particles or fluids using vesicles and requires ATP.
Endocytosis brings substances into the cell, with three types: phagocytosis (large particles), pinocytosis (fluids/small solutes), and receptor-mediated endocytosis (specific substances).
Exocytosis releases substances from the cell; steps: vesicle moves to membrane, fuses, forms a pore, and releases contents outside.

Passive Transport — movement of substances across membranes without energy input.
Active Transport — movement of substances against their concentration gradient, requiring ATP.
Simple Diffusion — passive movement of small, nonpolar molecules through the membrane.
Facilitated Diffusion — passive movement via protein carriers or channels.
Osmosis — diffusion of water across a membrane from low to high solute concentration.
Osmolarity — concentration of solute in solution (moles/liter).
Tonicity — effect of surrounding solution on cell volume (isotonic, hypertonic, hypotonic).
Sodium-Potassium Pump — active transporter moving Na⁺ out and K⁺ into cells.
Vesicular Transport — ATP-dependent transport using vesicles (includes endocytosis and exocytosis).