Cell Transport Overview

Overview

This lecture covers how substances move into and out of cells through the plasma membrane, focusing on passive and active transport processes.

The plasma membrane is mainly composed of phospholipids and scattered proteins.
Phospholipids have a hydrophilic (water-loving) head and two hydrophobic (water-fearing) tails.
The phospholipid bilayer has heads facing outward and tails facing inward.
The cytosol is inside the cell; interstitial fluid surrounds the cell.

Passive transport does not require cellular energy.
Types include simple diffusion, facilitated diffusion, and osmosis.
Diffusion is movement from high to low concentration (down a concentration gradient) until equilibrium is reached.
Simple diffusion applies to small, nonpolar molecules (e.g., O₂, CO₂, fatty acids) moving directly through the membrane.
Facilitated diffusion is for small, charged, or polar molecules needing membrane proteins to cross.
Channel-mediated diffusion uses protein channels (leak or gated) for ions.
Carrier-mediated diffusion uses carrier proteins that change shape to transport molecules like glucose or amino acids.
Osmosis is the passive movement of water across a selectively permeable membrane, either between phospholipids or through aquaporins.

Active transport requires energy from the cell.
Active transport moves substances against their concentration gradient (low to high) via protein pumps.
Primary active transport uses ATP directly (e.g., sodium-potassium pump exchanges 3 Na⁺ out, 2 K⁺ in).
Secondary active transport uses energy from the movement of another substance down its gradient (symport: same direction, antiport: opposite directions).

Vesicular transport moves large substances using membrane-bound vesicles.
Exocytosis releases materials from the cell by fusing vesicles with the plasma membrane.
Endocytosis brings substances into the cell by engulfing them in vesicles formed from the plasma membrane.
- Phagocytosis ("cell eating") engulfs large particles.
- Pinocytosis ("cell drinking") takes in fluid and dissolved solutes.
- Receptor-mediated endocytosis uses membrane receptors to capture specific molecules.

Phospholipid bilayer — double-layered structure of the plasma membrane formed by phospholipids.
Passive transport — movement of substances across the membrane without energy.
Diffusion — movement from high to low concentration.
Facilitated diffusion — diffusion with help from membrane proteins.
Osmosis — diffusion of water across a selectively permeable membrane.
Active transport — movement against a concentration gradient, requiring energy.
Vesicular transport — movement of large substances via vesicles.
Endocytosis — intake of substances by membrane folding.
Exocytosis — release of substances by vesicle fusion.