BackBiological Membrane Transport: Diffusion and Selective Permeability
Study Guide - Smart Notes
Tailored notes based on your materials, expanded with key definitions, examples, and context.
Biological Membrane Transport
Concept: Biological Membrane Transport
Biological membranes regulate the movement of molecules across the lipid bilayer, maintaining cellular homeostasis. Molecules tend to diffuse from regions of high concentration to low concentration, a process driven by the concentration gradient.
Diffusion: The passive movement of molecules from an area of higher concentration to lower concentration.
Selective Permeability: Biological membranes are selectively permeable, allowing certain molecules to pass while restricting others.
Permeable Membranes: Only specific molecules can diffuse freely across the membrane, depending on their properties.
Diffusion & Selectively Permeable Membranes
Selective permeability is a key feature of biological membranes, which are composed of a phospholipid bilayer. This structure allows small, nonpolar molecules to diffuse freely, while larger or charged molecules require assistance.
Phospholipid Bilayer: The fundamental structure of cell membranes, consisting of hydrophilic heads and hydrophobic tails.
Example: Oxygen (O2) and carbon dioxide (CO2) can diffuse freely, while ions and large polar molecules cannot.
Which Molecules Freely Cross Membranes?
The ability of a molecule to diffuse across a membrane depends on its size, charge, and polarity.
Freely Diffuse Without Facilitation | Cannot Freely Diffuse |
|---|---|
Small | Large |
Uncharged | Charged |
Nonpolar/Hydrophobic | Polar/Hydrophilic |
Example: Water (H2O) can diffuse slowly; ions like Na+ and Cl- require channels or transporters.
Diffusion Across a Membrane
Diffusion is illustrated by the movement of molecules from the extracellular space into the cytosol, following the concentration gradient.
Key Equation: Fick's Law of Diffusion: where J is the flux, D is the diffusion coefficient, and is the concentration gradient.
Example: Oxygen diffuses into cells where its concentration is lower.
Practice: Membrane Permeability Ranking
Students are asked to rank molecules by their ability to diffuse across a biological membrane without facilitation.
Example Question: Which molecule most easily diffuses across a biological membrane: H2O, Cl-, HPO42-, urea, or acetamide?
Answer: Acetamide (small, uncharged, and less polar) diffuses most easily.
Practice: Ranking Molecules by Permeability
Given a set of molecules (acetamide, urea, butyramide), students rank their permeability across the membrane.
Factors Affecting Permeability: Size, polarity, and charge.
Example: Acetamide > Urea > Butyramide (from most to least permeable).
Map of Biological Membrane Transport
Molecular transport across biological membranes can occur via a variety of mechanisms, including passive and active transport.
Passive Transport: Includes simple diffusion and facilitated diffusion (via channels or carriers).
Active Transport: Requires energy input (e.g., ATP) to move molecules against their concentration gradient.
Example: Sodium-potassium pump (Na+/K+ ATPase) is a classic example of active transport.
Transport Type | Energy Requirement | Example |
|---|---|---|
Simple Diffusion | No | O2, CO2 |
Facilitated Diffusion | No | Glucose via GLUT transporter |
Active Transport | Yes | Na+/K+ ATPase |
Additional info: The map of membrane transport mechanisms helps students visualize the relationships and differences between various transport processes.
