Passive Membrane Transport

Simple & Facilitated Passive Transport

Passive membrane transport is a fundamental process in biochemistry, allowing molecules to move across biological membranes without the input of cellular energy. There are two main types of passive transport: simple diffusion and facilitated diffusion.

• Simple Diffusion: Movement of molecules directly through the phospholipid bilayer, driven by the concentration gradient.

• Facilitated Diffusion: Movement of molecules across the membrane via specific membrane proteins (channels or carriers), also driven by the concentration gradient.

Key Differences

• Simple Diffusion: No protein required; small, nonpolar molecules (e.g., O2, CO2) diffuse freely.

• Facilitated Diffusion: Requires a membrane protein; allows polar or charged molecules (e.g., glucose, ions) to cross the membrane.

Example:

• Oxygen diffuses into cells by simple diffusion.

• Glucose enters cells via facilitated diffusion using a glucose transporter.

Practice Question:

• The difference between simple and facilitated diffusion is that facilitated diffusion requires a protein transporter.

Kinetics of Passive Transport

The rate of passive transport depends on the concentration gradient across the membrane. The greater the gradient, the faster the rate of diffusion. However, the kinetics differ between simple and facilitated diffusion.

• Simple Diffusion: Rate increases linearly with concentration gradient.

• Facilitated Diffusion: Rate increases with concentration but eventually plateaus (saturates) due to the limited number of transport proteins.

Mathematical Representation

• Facilitated diffusion follows a hyperbolic curve similar to Michaelis-Menten kinetics:

• Simple diffusion follows a linear relationship:

Practice Question:

• If the rate of movement of a substance across a membrane is measured at different concentrations and the resulting graph is hyperbolic, the transport is facilitated diffusion.

Summary Table: Simple vs. Facilitated Diffusion

Additional info:

• Facilitated diffusion is specific: only certain molecules are transported by a given protein.

• Both types of passive transport move substances down their concentration gradient (from high to low concentration).