Overview
This lecture explains how substances move across the plasma membrane via passive and active transport processes, detailing the key mechanisms and types involved.
Plasma Membrane Structure
- The plasma membrane is mainly composed of a phospholipid bilayer with embedded proteins.
- Phospholipids have hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.
- The bilayer separates the cytosol inside the cell from the interstitial fluid outside.
Passive Transport Processes
- Passive transport does not require cellular energy.
- Types include simple diffusion, facilitated diffusion, and osmosis.
- Simple diffusion allows small, nonpolar molecules (e.g., O2 and CO2) to pass through the membrane.
- Facilitated diffusion helps small, polar, or charged molecules cross the membrane with protein assistance.
- Channel mediated diffusion uses protein channels for ions; channels can be leak (always open) or gated (open by stimulus).
- Carrier mediated diffusion uses carrier proteins that change shape to move substances like glucose and amino acids.
- Osmosis is the passive movement of water across a selectively permeable membrane, either through phospholipids or aquaporins.
Active Transport Processes
- Active transport requires cellular energy (usually ATP).
- Active transport moves substances against their concentration gradients.
- Primary active transport uses protein "pumps" (e.g., sodium-potassium pump) powered directly by ATP.
- Secondary active transport uses the energy from one substance moving down its gradient to move another up its gradient.
- Symport moves two substances in the same direction; antiport moves them in opposite directions.
Vesicular Transport
- Vesicular transport uses membrane-bound sacs (vesicles) to move large substances.
- Exocytosis releases materials from the cell by fusing vesicles with the plasma membrane.
- Endocytosis brings substances into the cell via new vesicles; includes phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis.
- Both endocytosis and exocytosis require energy.
Key Terms & Definitions
- Phospholipid Bilayer — Double layer of phospholipids forming the cell membrane.
- Diffusion — Movement of substances from high to low concentration.
- Osmosis — Passive movement of water across a selectively permeable membrane.
- Facilitated Diffusion — Passive transport using membrane proteins for polar or charged molecules.
- Active Transport — Energy-requiring movement of substances against a concentration gradient.
- Vesicle — Membrane-bound sac used for substance transport.
- Symport/Antiport — Types of secondary active transport moving substances in the same/opposite directions.
Action Items / Next Steps
- Review the differences between passive and active transport mechanisms.
- Study examples of each transport type and membrane structure.