Overview
This lecture explains how substances move across the cell membrane, covering both passive and active transport processes, including key types and mechanisms.
Structure of the Plasma Membrane
- The plasma membrane is made of phospholipids with hydrophilic heads and hydrophobic tails forming a bilayer.
- Membrane proteins are scattered throughout and assist in transport.
- The membrane separates the cytosol (inside) from interstitial fluid (outside).
Passive Transport Processes
- Passive transport doesn't require cell energy.
- Simple diffusion allows small, nonpolar molecules (e.g., O₂, CO₂) to move directly through the membrane.
- Facilitated diffusion moves small charged or polar solutes via membrane proteins.
- Channel-mediated diffusion uses protein channels for ions; channels may be leak (always open) or gated (open with a stimulus).
- Carrier-mediated diffusion uses carrier proteins that change shape to transport molecules like glucose and amino acids.
- Osmosis is the movement of water across the membrane through phospholipids or aquaporins, from high to low water concentration.
Active Transport Processes
- Active transport requires cellular energy (ATP).
- Active transport moves solutes against their concentration gradients.
- Primary active transport uses ion pumps (e.g., sodium-potassium pump) powered by ATP.
- Secondary active transport uses energy from another substance moving down its gradient; includes symport (same direction) and antiport (opposite directions).
Vesicular Transport
- Vesicular transport moves large substances via vesicles and requires energy.
- Exocytosis releases substances outside the cell as vesicles fuse with the membrane.
- Endocytosis brings substances into the cell by folding the membrane to form vesicles.
- Phagocytosis ("cell eating"): engulfs large particles.
- Pinocytosis ("cell drinking"): takes in fluid droplets.
- Receptor-mediated endocytosis: uses receptors to bring in specific molecules.
Key Terms & Definitions
- Phospholipid Bilayer — Double-layered membrane with hydrophilic heads and hydrophobic tails.
- Diffusion — Movement of molecules from high to low concentration.
- Osmosis — Passive water movement across a selectively permeable membrane.
- Facilitated Diffusion — Movement of molecules via membrane proteins.
- Active Transport — Movement of substances against their gradient using energy.
- Ion Pump — Protein moving ions across a membrane using ATP.
- Vesicle — Membrane-bound sac for transporting substances.
- Exocytosis — Vesicle fusion with membrane to secrete contents.
- Endocytosis — Process of taking in substances via membrane folding.
Action Items / Next Steps
- Review diagrams of membrane transport types.
- Practice identifying transport methods for specific molecules.