Rucete ✏ Campbell Biology In a Nutshell
Unit 2 THE CELL — Concept 7.3 Passive Transport Is Diffusion of a Substance Across a Membrane with No Energy Investment
Cells maintain homeostasis by regulating the movement of substances across their membranes. One key mechanism is passive transport, which allows molecules to move without the cell expending energy.
1. Diffusion: The Movement Toward Equilibrium
- Definition: The tendency of molecules to spread out evenly due to kinetic energy.
- Concentration Gradient: Molecules move from high to low concentration.
- Dynamic Equilibrium: Molecules continue moving, but net concentration remains balanced.
2. Passive Transport: Diffusion Across a Membrane
- Selective Permeability: Some substances cross the membrane more easily than others.
- Substances That Diffuse Easily:
- Nonpolar molecules (O₂, CO₂) cross freely.
- Small polar molecules (e.g., water) cross slowly.
- No Energy Required: Molecules move via their own kinetic energy, without ATP.
3. Osmosis: The Diffusion of Water
- Definition: Water diffuses across a selectively permeable membrane.
- Direction of Movement: Water moves from low solute to high solute concentration.
- Importance: Osmosis maintains turgor pressure in plant cells.
4. Tonicity: The Ability of a Solution to Cause a Cell to Gain or Lose Water
- Isotonic Solution: Equal solute concentration inside and outside the cell; no net water movement.
- Hypertonic Solution: Higher solute outside; water leaves the cell → shrivels.
- Hypotonic Solution: Lower solute outside; water enters the cell → swells or bursts.
5. Facilitated Diffusion: Passive Transport Aided by Proteins
- Transport Proteins: Help molecules that can't freely cross the membrane.
- Channel Proteins: Form hydrophilic passageways for specific ions/molecules.
- Carrier Proteins: Bind and change shape to transport their cargo.
- No Energy Required: Facilitated diffusion does not require ATP.