Overview
This lecture explains how molecules move across the plasma membrane, covering types of membrane transport—passive, active, and vesicle-mediated—to maintain cellular homeostasis.
Structure and Properties of the Plasma Membrane
- The plasma membrane is a bilayer (two layers) of lipids, mainly made of phospholipids with polar heads and nonpolar tails.
- Due to its structure, the plasma membrane is selectively permeable, allowing some molecules to pass while blocking others.
- Non-polar and small molecules pass easily; large or polar molecules cannot cross without assistance.
Passive Transport Mechanisms
- Passive transport moves molecules down their concentration gradient (from high to low concentration) without energy input.
- Simple diffusion allows non-polar molecules to cross the membrane directly.
- Polar or large molecules use facilitated diffusion, requiring specific membrane proteins.
- Channel proteins form open pores for ions; carrier proteins change shape to transport substances like glucose and amino acids.
Active Transport Mechanisms
- Active transport moves molecules against their concentration gradient (low to high), requiring energy (usually from ATP).
- Primary active transport uses ATP directly, e.g., the sodium-potassium pump exchanges Na⁺ out and K⁺ into the cell.
- The sodium-potassium pump cycle involves ATP hydrolysis and conformational changes to move ions.
- Secondary active transport uses the energy from one molecule moving down its gradient to transport another against its gradient.
- Symport moves two molecules in the same direction (e.g., Na⁺ and glucose into the cell); antiport moves them in opposite directions (e.g., Na⁺ in, Ca²⁺ out).
Vesicular Transport
- Vesicular transport moves very large molecules or many molecules at once using vesicles (membrane-bound sacs).
- Exocytosis exports substances from inside to outside the cell by vesicle fusion with the plasma membrane.
- Endocytosis imports substances by engulfing them with the membrane, forming vesicles.
- Phagocytosis: "cell eating" of large particles (e.g., bacteria), done by specialized cells like macrophages.
- Pinocytosis: "cell drinking" of fluids and dissolved particles.
- Receptor-mediated endocytosis: specific molecules bind to cell-surface receptors before being internalized.
Key Terms & Definitions
- Plasma Membrane — a double-layered lipid membrane controlling entry/exit of substances in the cell.
- Selective Permeability — ability to allow some molecules to pass and restrict others.
- Passive Transport — movement of molecules down a concentration gradient without energy.
- Simple Diffusion — direct movement of non-polar molecules through the membrane.
- Facilitated Diffusion — movement of polar/large molecules with the help of proteins.
- Active Transport — movement of molecules against their gradient using energy.
- Sodium-Potassium Pump — protein that uses ATP to exchange Na⁺ and K⁺ ions across the membrane.
- Vesicle — small, membrane-bound sac for transporting substances.
- Exocytosis — vesicle-mediated export of substances from a cell.
- Endocytosis — vesicle-mediated import of substances into a cell.
Action Items / Next Steps
- Review examples of each membrane transport type.
- Study the sodium-potassium pump and its ATP-dependent cycle.
- Prepare to define and compare all types of endocytosis.