Overview
This lecture covers passive transport, focusing on diffusion, facilitated diffusion, and osmosis as ways substances move across cell membranes without using energy.
Passive Transport Basics
- Passive transport is the movement of molecules across membranes without energy input (no ATP required).
- Molecules move from areas of high concentration to low concentration, down the concentration gradient.
- The process continues until equilibrium (equal concentration on both sides) is reached.
Types of Passive Transport
- There are three main types: simple diffusion, facilitated diffusion, and osmosis.
Simple Diffusion
- Movement of small, nonpolar molecules directly through the phospholipid bilayer (e.g., Oâ‚‚, COâ‚‚).
- No proteins, pumps, or channels are involved, just movement through the membrane.
- Stops when equilibrium is reached; movement continues but in equal rates both ways.
Facilitated Diffusion
- Movement of larger or polar molecules (e.g., ions) across membranes with the help of channel or carrier proteins.
- Still moves from high to low concentration, but uses specific proteins to allow passage.
- Examples include glucose and ion transport.
Osmosis
- Passive transport of water molecules across a selectively permeable membrane.
- Water moves from an area of low solute concentration (more water) to high solute concentration (less water).
- Utilizes protein channels called aquaporins since water is polar and cannot pass freely through the nonpolar membrane interior.
Concentration, Solutions, and Tonicity
- Solute: the substance dissolved (e.g., salt); Solvent: the liquid that does the dissolving (e.g., water).
- Water moves toward higher solute concentrations to dilute them.
Tonicity Terms
- Hypotonic solution: Lower solute concentration outside the cell; water moves into the cell, causing it to swell.
- Hypertonic solution: Higher solute concentration outside the cell; water moves out, causing the cell to shrink.
- Isotonic solution: Equal solute concentration; water moves in and out evenly, cell size stays the same.
Key Terms & Definitions
- Passive Transport — Movement of substances across membranes without energy input.
- Diffusion — Movement of molecules from high to low concentration.
- Simple Diffusion — Direct movement of small, nonpolar molecules through the membrane.
- Facilitated Diffusion — Assisted movement of molecules via protein channels.
- Osmosis — Movement of water from low to high solute concentration.
- Concentration Gradient — Difference in concentration of a substance across a space.
- Equilibrium — State where concentrations are equal on both sides of the membrane.
- Phospholipid Bilayer — Main structural component of cell membranes, nonpolar interior.
- Tonicity — Describes relative solute concentrations (hypotonic, hypertonic, isotonic).
- Aquaporin — Protein channel for water transport across cell membranes.
Action Items / Next Steps
- Review examples of passive transport and be able to distinguish between simple diffusion, facilitated diffusion, and osmosis.
- Learn to identify solution tonicity and predict water movement in hypotonic, hypertonic, and isotonic environments.
- Prepare for the next lesson on active transport.