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 into the available space due to their kinetic energy.
Concentration Gradient: Molecules move from an area of higher concentration to an area of lower concentration, moving down their concentration gradient.
Dynamic Equilibrium: When the concentration of molecules is equal throughout a space, molecules continue to move, but there is no net change in concentration.
2. Passive Transport: Diffusion Across a Membrane
Selective Permeability: Cell membranes allow some substances to cross more easily than others.
Substances That Diffuse Easily:
Nonpolar molecules (e.g., O₂, CO₂) can dissolve in the lipid bilayer and cross easily.
Small polar molecules (e.g., water) can also pass through, but more slowly.
No Energy Required: Passive transport relies on the inherent kinetic energy of molecules and does not require cellular energy (ATP).
3. Osmosis: The Diffusion of Water
Definition: The diffusion of water across a selectively permeable membrane.
Direction of Movement: Water moves from an area of lower solute concentration (higher water concentration) to an area of higher solute concentration (lower water concentration).
Importance: Osmosis is crucial for maintaining cell turgor pressure, which is essential for plant structure and function.
4. Tonicity: The Ability of a Solution to Cause a Cell to Gain or Lose Water
Isotonic Solution: The solute concentration is the same inside and outside the cell; no net water movement occurs, and the cell remains stable.
Hypertonic Solution: The solute concentration is higher outside the cell; water moves out of the cell, causing it to shrivel.
Hypotonic Solution: The solute concentration is lower outside the cell; water moves into the cell, causing it to swell and potentially burst.
5. Facilitated Diffusion: Passive Transport Aided by Proteins
Transport Proteins: Specific proteins assist molecules that cannot diffuse freely across the lipid bilayer.
Channel Proteins: Provide hydrophilic pathways for certain molecules or ions to pass through.
Carrier Proteins: Bind to molecules and change shape to shuttle them across the membrane.
No Energy Required: Facilitated diffusion is still a form of passive transport and does not require ATP.