Overview
This lecture covers the movement of water and solutes across cell membranes, focusing on different types of transport mechanisms, osmotic effects on cells, and key ion concentrations.
Osmosis and Tonicity
- Osmosis is the movement of water across a semipermeable membrane toward higher solute concentration.
- Hypotonic solution: lower solute concentration outside the cell, water enters, cell swells or may burst (lysis).
- Hypertonic solution: higher solute concentration outside, water leaves the cell, cell shrinks (crenation).
- Isotonic solution: equal solute concentration inside and outside the cell; no net water movement.
Passive and Facilitated Transport
- Filtration moves substances across membranes using pressure from high to low (e.g., kidneys, coffee filter).
- Simple diffusion moves molecules from high to low concentration without assistance.
- Facilitated diffusion uses carrier proteins to move substances from high to low concentration; no energy needed.
- Carrier proteins are selective, can experience competition, and become saturated if too many molecules need transport.
Active Transport Mechanisms
- Primary active transport moves substances against the concentration gradient (low to high) using ATP.
- Sodium-potassium pump (Na+/K+ ATPase) moves 3 sodium ions out and 2 potassium ions into the cell; requires ATP.
- Inside the cell: more potassium (K+); Outside: more sodium (Na+) and chloride (Cl-).
- Use the "salty banana" mnemonic: potassium (K+) inside like banana, sodium (Na+) outside like salt.
Secondary Active Transport
- Secondary active transport uses energy from another substance's gradient, not direct ATP, to move substances.
- Co-transport (symport): substances move in the same direction as the driving ion (e.g., both enter cell together).
- Counter-transport (antiport): substances move in opposite directions (one enters, one exits).
Miscellaneous Transport Mechanisms
- Endocytosis: cell engulfs material using membrane, requires ATP; pinocytosis for liquids, phagocytosis for solids.
- Exocytosis: cell expels materials by fusing vesicles with the membrane, requires ATP.
Key Terms & Definitions
- Osmosis — movement of water toward higher solute concentration across a membrane.
- Hypotonic — solution with lower solute concentration than the cell.
- Hypertonic — solution with higher solute concentration than the cell.
- Isotonic — equal solute concentration inside and outside the cell.
- Facilitated diffusion — passive movement via carrier proteins.
- Active transport — movement against gradient using ATP.
- Sodium-potassium pump — protein using ATP to exchange 3 Na+ out, 2 K+ in.
- Co-transport (Symport) — two substances move in same direction.
- Counter-transport (Antiport) — substances move in opposite directions.
- Endocytosis — cell takes in material via membrane engulfing.
- Exocytosis — cell expels material via vesicle fusion.
Action Items / Next Steps
- Review the functions and differences of passive, facilitated, and active transport.
- Memorize the sodium-potassium pump mechanism and related ion concentrations.
- Study types of endocytosis and exocytosis.