Membrane Transport and Facilitated Diffusion

Membrane Transport and Facilitated Diffusion

Selective Permeability

The cell membrane is selectively permeable, meaning it allows certain substances to cross more easily than others. This property is essential for maintaining the internal environment of the cell.

• Direct passage across the membrane: Small, non-polar, hydrophobic molecules can cross the membrane without assistance.

  • Examples: Hydrocarbons, CO2, O2, N2

• Difficult passage or protein-assisted passage: Hydrophilic, polar molecules, large molecules, and ions cannot easily cross the membrane and often require transport proteins.

  • Examples: Sugars (such as glucose), water (due to polarity), ions

Types of Membrane Transport

There are two main types of transport across biological membranes: passive transport and active transport.

• Passive transport: Movement of molecules that does not require energy from the cell. Molecules move with their concentration or electrochemical gradient.

  • Involved in import of materials and export of waste.

  • Includes: Diffusion, Osmosis, Facilitated diffusion

• Active transport: Movement of molecules that requires energy (usually ATP) because substances are moved against their concentration gradient.

Passive Transport

Passive transport is a spontaneous process that results from the constant motion of molecules. Substances move from areas of high concentration to areas of low concentration, i.e., down their concentration gradient.

• Diffusion: The net movement of molecules from a region of higher concentration to a region of lower concentration until equilibrium is reached.

• Osmosis: The diffusion of water across a selectively permeable membrane. Water moves from an area of lower solute concentration to an area of higher solute concentration.

• Facilitated diffusion: The movement of molecules across the membrane via specific transport proteins, increasing the rate of diffusion for certain substances (e.g., ions, water, carbohydrates).

Key Points of Passive Transport

• Molecules diffuse directly across the membrane at different rates depending on their properties.

• Even with diffusion, the membrane remains selectively permeable.

• Maintaining water balance via osmosis is crucial for the survival of all organisms.

Examples and Applications

• Diffusion: Oxygen and carbon dioxide gases move across cell membranes by simple diffusion.

• Osmosis: Plant roots absorb water from the soil by osmosis.

• Facilitated diffusion: Glucose enters red blood cells via a specific carrier protein.

Important Definitions

• Concentration gradient: The difference in the concentration of a substance between two areas.

• Electrochemical gradient: The combined effect of a concentration gradient and an electrical gradient across a membrane.

• Selective permeability: The property of a membrane that allows some substances to pass more easily than others.

Osmosis Diagram Explanation

• Water molecules move through a selectively permeable membrane, but solute molecules (e.g., sugar) cannot.

• Water moves to the side with higher solute concentration, balancing solute concentrations on both sides.

Equations

• Fick's Law of Diffusion:

$J = -D \frac{dC}{dx}$

• Where $J$ is the rate of diffusion, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

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