Membrane Structure and Transport Mechanisms

Membranes

Structure of Biological Membranes

Biological membranes are essential components of cells, providing structural integrity and regulating the movement of substances. They are primarily composed of a phospholipid bilayer with embedded proteins.

• Hydrophobic heads: The phosphate-containing heads of phospholipids are hydrophilic (water-attracting).

• Hydrophobic tails: The fatty acid tails are hydrophobic (water-repelling), creating a barrier to most water-soluble substances.

Permeability

Selective Permeability of Membranes

Cell membranes are selectively permeable, allowing certain molecules to pass while restricting others. This property is crucial for maintaining cellular homeostasis.

• Small uncharged molecules: These can pass through the membrane easily (e.g., O2, CO2).

• Ions and large molecules: These require specific transport mechanisms due to their charge or size.

Passive Transport

Diffusion

Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration, driven by the concentration gradient.

• Solute: The substance being dissolved and transported.

• Passive: No energy input is required; movement occurs naturally.

• Concentration gradient vs. electrical gradient: Diffusion can be influenced by differences in concentration and electrical charge across the membrane.

• Equilibrium: The state where the concentration of solute is equal on both sides of the membrane.

• Rate of diffusion: Depends on temperature, size of molecules, and steepness of the gradient.

• Diffusion with charged particles: Involves both concentration and electrical gradients.

• Electrochemical gradient: The combined effect of concentration and electrical gradients.

• Electrochemical equilibrium: Achieved when the net movement of ions stops due to balanced gradients.

Equation for Rate of Diffusion:

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

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

Consequences of Size and Shape

The rate and efficiency of diffusion are affected by the size and shape of organisms. Larger organisms may require specialized transport systems.

• As organisms get bigger, diffusion alone becomes insufficient for transport across cells.

Solutions

Solutions are mixtures of solutes dissolved in solvents. The concentration of solutes affects osmotic movement across membranes.

• Solute concentration: Determines the direction of water movement.

Osmosis

Osmosis is the diffusion of water across a semi-permeable membrane from an area of lower solute concentration to higher solute concentration.

• Semi-permeable membrane: Allows water to pass but restricts solute movement.

• Polar: Water is a polar molecule, facilitating its movement through membranes.

• Hypertonic: Solution with higher solute concentration compared to another.

• Hypotonic: Solution with lower solute concentration.

• Isotonic: Solutions with equal solute concentrations.

Solution Type	Relative Solute Concentration	Effect on Cell
Hypertonic	Higher outside cell	Cell shrinks (water leaves)
Hypotonic	Lower outside cell	Cell swells (water enters)
Isotonic	Equal	No net water movement

Facilitated Diffusion

Facilitated diffusion is a passive transport process where specific proteins help move substances across the membrane.

• Channel proteins: Form pores for molecules to pass through.

• Pore: Openings in the membrane for passive movement.

• Aquaporins: Specialized channels for water transport.

• Gated channels: Channels that open or close in response to stimuli.

• Carrier proteins: Bind to specific molecules and change shape to transport them.

• Unbound protein: Carrier is ready to bind substrate.

• Glucose binding: Example of carrier-mediated transport.

• Conformational change: Protein changes shape to move substrate.

• Release: Substrate is released on the other side of the membrane.

Active Transport

Secondary Active Transport

Secondary active transport uses the energy from the movement of one molecule down its gradient to drive the transport of another molecule against its gradient.

• Cotransporters: Proteins that move two substances simultaneously.

• Symporter: Moves two substances in the same direction.

• Antiporter: Moves substances in opposite directions.

Pumps

Pumps are proteins that use energy (usually from ATP) to move substances against their concentration gradients.

• Proton pump: Actively transports protons (H+) across membranes, creating electrochemical gradients.

Equation for Active Transport (Example: Sodium-Potassium Pump):

$3 \text{Na}^+_{\text{out}} + 2 \text{K}^+_{\text{in}} + \text{ATP} \rightarrow 3 \text{Na}^+_{\text{in}} + 2 \text{K}^+_{\text{out}} + \text{ADP} + \text{P}_i$

Example: The sodium-potassium pump maintains cellular ion balance and is vital for nerve impulse transmission.