Membrane Transport

Constituents of the Cell Membrane

The cell membrane, also known as the plasma membrane, is a selectively permeable barrier that surrounds the cell. Its structure is primarily composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails, forming a bilayer that provides fluidity and selective permeability.

• Proteins: Integral and peripheral proteins serve as channels, carriers, receptors, and enzymes.

• Cholesterol: Stabilizes membrane fluidity and integrity.

• Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids), involved in cell recognition and signaling.

Key Feature: The amphipathic nature of phospholipids allows the membrane to be semi-permeable, enabling selective passage of substances.

Types of Membrane Transport

Membrane transport refers to the movement of substances across the cell membrane. It can be classified as passive or active transport.

• Passive Transport: Movement of molecules down their concentration gradient without energy input.

• Active Transport: Movement of molecules against their concentration gradient, requiring energy (usually ATP).

Examples of molecules:

• O2 and CO2 cross by simple diffusion.

• Glucose and ions (Na+, K+) require transport proteins.

Characteristics of Transport Proteins

Transport proteins facilitate the movement of specific molecules across the membrane. They are highly selective and can be classified as channels or carriers.

• Channel Proteins: Form hydrophilic pores for specific ions or water (e.g., aquaporins, ion channels).

• Carrier Proteins: Bind to specific molecules and undergo conformational changes to transport them (e.g., glucose transporter, Na+/K+ pump).

Specific Examples: Sodium-potassium pump (Na+/K+ ATPase), glucose transporter (GLUT), and sodium-glucose symporter.

Diffusion

Diffusion is the passive movement of molecules from an area of higher concentration to an area of lower concentration.

• Simple Diffusion: Direct movement through the lipid bilayer (e.g., gases, small nonpolar molecules).

• Facilitated Diffusion: Movement via transport proteins (e.g., ions, glucose).

Factors Affecting the Rate of Diffusion

• Concentration gradient

• Temperature

• Surface area of the membrane

• Size and polarity of molecules

• Presence of transport proteins

Fick's Law of Diffusion:

Where is the rate of diffusion, is the diffusion coefficient, and is the concentration gradient.

Concentration Gradient

A concentration gradient exists when there is a difference in the concentration of a substance across a space or membrane. Molecules tend to move down their concentration gradient until equilibrium is reached.

Coupled Transport

Coupled transport involves the simultaneous movement of two substances across the membrane, often using the energy from one molecule moving down its gradient to drive another against its gradient.

• Symport: Both substances move in the same direction.

• Antiport: Substances move in opposite directions.

Example: Sodium-glucose symporter uses the Na+ gradient to transport glucose into the cell.

Active Coupled Transport

Active coupled transport (secondary active transport) uses the energy stored in the gradient of one molecule (often Na+) to drive the transport of another molecule against its gradient.

• Example: Na+/glucose symporter in intestinal epithelial cells.

Uniports, Symports, and Antiports

Transport proteins can be classified based on the direction and number of substances they move:

• Uniport: Transports a single type of molecule in one direction (e.g., GLUT for glucose).

• Symport: Transports two different molecules in the same direction (e.g., Na+/glucose symporter).

• Antiport: Transports two different molecules in opposite directions (e.g., Na+/K+ pump).

Na+/K+ Pump and Na+/Glucose Co-Transporter

The Na+/K+ pump is an example of primary active transport, moving Na+ out and K+ into the cell using ATP.

The Na+/glucose co-transporter uses the Na+ gradient to transport glucose into the cell against its concentration gradient (secondary active transport).

Endocytosis and Exocytosis

These are bulk transport mechanisms for moving large particles or volumes across the membrane.

• Endocytosis: The process by which cells engulf substances into a pouch which becomes a vesicle inside the cell.

• Exocytosis: The process by which vesicles fuse with the plasma membrane to release their contents outside the cell.

Types of Endocytosis: Phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis.

Entry of Viruses via Endocytosis

Many viruses enter host cells by binding to specific receptors and being internalized through receptor-mediated endocytosis. Once inside, the virus can release its genetic material to initiate infection.

Virotherapy

Virotherapy is a therapeutic approach that uses viruses to treat diseases, most commonly cancer. Modified viruses can selectively infect and destroy cancer cells or deliver therapeutic genes.

• Example: Oncolytic viruses are engineered to target and kill tumor cells while sparing normal cells.