Overview
This lesson explains passive transport: the movement of particles across cell membranes from higher to lower concentration without energy. It covers diffusion, osmosis, and facilitated diffusion.
Passive Transport: Core Idea
- Movement along the concentration gradient from higher to lower concentration.
- Requires no cellular energy; process occurs naturally and automatically.
- Occurs across membranes or in open spaces depending on the type.
Key Components of Solutions and Membranes
- Solution: liquid mixture with a dissolved substance present throughout.
- Solvent: liquid part of a solution; depicted as aqua-colored background fluid.
- Solute: dissolved particles within the solvent; shown as yellow particles.
- Semi-permeable membrane: barrier allowing only certain sized particles to pass freely.
Diffusion
- Definition: particle movement from high to low concentration until balanced.
- Does not require energy; likened to a ball rolling downhill.
- Can occur with or without a semi-permeable membrane present.
- Equilibrium: state when both sides have about equal numbers of solute particles.
- Everyday example: air freshener spreads so people farther away smell it over time.
Osmosis
- Definition: diffusion of water molecules across a semi-permeable membrane.
- Membrane allows water through but blocks larger solute particles.
- Focus on water concentration gradient, not solute movement itself.
- Water moves from high to low water concentration until equilibrium occurs.
- Cellular context: water enters or leaves cells until balanced with surroundings.
- Also passive; no energy required for water movement across membranes.
Facilitated Diffusion
- Definition: diffusion through specialized protein channels in membranes.
- Channels act like ports or tunnels enabling specific substances to pass.
- Moves particles from high to low concentration, as indicated by “diffusion.”
- Passive process; no energy input required from the cell.
- Typically transports particles larger than those slipping through lipid layers.
Summary Table: Passive Transport Types
| Type | What Moves | Pathway | Energy Required | Direction | Notes |
|---|
| Diffusion | Solute particles | Directly through membrane or open space | No | High to low concentration | Occurs with or without membranes |
| Osmosis | Water molecules | Through semi-permeable membrane | No | High to low water concentration | Solute may be blocked by membrane |
| Facilitated Diffusion | Specific molecules/ions | Protein channels in cell membrane | No | High to low concentration | For larger or specific particles |
Key Terms & Definitions
- Concentration gradient: difference in particle concentration across a space or membrane.
- Equilibrium: balanced state with similar particle or water proportion on both sides.
- Semi-permeable membrane: allows certain particles to pass based on size.
- Protein channels: membrane proteins forming tunnels for specific molecules to diffuse.
Action Items / Next Steps
- Practice identifying whether a scenario is diffusion, osmosis, or facilitated diffusion.
- Explain equilibrium using solute and water proportions across a membrane.
- Match molecules to likely transport methods based on size and membrane properties.