Cell Membranes and Transport Mechanisms

Cell Membranes: Structure and Selective Permeability

Introduction to Cell Membranes

The cell membrane, also known as the plasma membrane, is a selectively permeable barrier that surrounds all cells. It regulates the movement of substances into and out of the cell, maintaining the internal environment distinct from the external surroundings.

• Selective permeability means that the membrane allows some substances to pass more easily than others.

• The membrane is primarily composed of a phospholipid bilayer with embedded proteins.

Types of Molecules and Membrane Permeability

• Small molecules (e.g., O2, CO2, H2O, ethanol) can often pass through the membrane more easily than larger molecules.

• Non-polar molecules (e.g., O2, CO2) diffuse readily through the lipid bilayer, while polar molecules (e.g., H2O, urea) have more difficulty.

• Charged molecules (ions such as Na+, K+, Mg2+, Ca2+, Cl-) and large polar molecules (e.g., glucose, amino acids, proteins, nucleic acids) generally require specific transport proteins to cross the membrane.

Example: Oxygen (O2) and carbon dioxide (CO2) diffuse freely across the membrane, while ions like Na+ and K+ need channels or pumps.

Cellular Gradients and Homeostasis

Maintaining Gradients Across Membranes

Cells maintain an intracellular environment that is different from the extracellular environment. This difference is essential for cellular function and is achieved by controlling the movement of solutes across the membrane.

• Gradients refer to differences in concentration of solutes across the membrane.

• Types of gradients:

  • Chemical gradient: Difference in concentration of a specific molecule (e.g., glucose, Na+).

  • Electrochemical gradient: Combination of chemical gradient and electrical potential (voltage) across the membrane.

Example: The sodium-potassium gradient is crucial for nerve impulse transmission.

Transport Across Cell Membranes

Overview of Transport Mechanisms

There are three general types of transport across cell membranes:

• Passive Transport (does not require energy input):

  • Simple diffusion

  • Facilitated diffusion

  • Osmosis

• Active Transport (requires energy input, usually from ATP):

  • Primary active transport

  • Secondary active transport (coupled transport)

Passive Transport

• Simple Diffusion: Movement of molecules directly across the membrane from high to low concentration until equilibrium is reached.

  • No energy required.

  • Example: O2 and CO2 diffusion in lungs.

• Facilitated Diffusion: Movement of molecules down their concentration gradient with the help of transport proteins (channels or carriers).

  • No energy required.

  • Transport proteins provide specificity and regulation.

  • Example: Glucose transport into red blood cells via GLUT transporters.

• Osmosis: Diffusion of water across a selectively permeable membrane.

  • Water moves from areas of low solute concentration to high solute concentration.

  • Specialized channels called aquaporins facilitate rapid water movement.

Osmosis and Water Balance

• Isotonic solution: Solute concentration is equal inside and outside the cell; no net water movement.

• Hypotonic solution: Lower solute concentration outside the cell; water enters the cell, which may swell or burst (lysis in animal cells, turgor pressure in plant cells).

• Hypertonic solution: Higher solute concentration outside the cell; water leaves the cell, causing it to shrink (crenation in animal cells, plasmolysis in plant cells).

Example: Red blood cells placed in pure water (hypotonic) will swell and burst.

Active Transport

• Active transport moves molecules against their concentration gradient (from low to high concentration) and requires energy, usually from ATP hydrolysis.

• Essential for maintaining concentration differences of ions and other substances across the membrane.

• Transport proteins involved include pumps (e.g., sodium-potassium pump).

Equation for ATP hydrolysis:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Types of Active Transport Proteins

• Uniporter: Transports a single type of molecule in one direction.

• Symporter (Cotransporter): Transports two different molecules in the same direction across the membrane.

• Antiporter (Exchanger): Transports two different molecules in opposite directions.

Transport Protein	Direction of Transport	Example
Uniporter	One molecule, one direction	Glucose transporter
Symporter	Two molecules, same direction	Sodium-glucose cotransporter
Antiporter	Two molecules, opposite directions	Sodium-potassium pump

Coupled Transport (Secondary Active Transport)

• Uses the energy from the movement of one molecule down its gradient to drive the movement of another molecule against its gradient.

• Example: The sodium-glucose symporter uses the sodium gradient (established by the sodium-potassium pump) to import glucose into the cell.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction (relative to gradient)	Example
Simple Diffusion	No	Down	O2, CO2
Facilitated Diffusion	No	Down	Glucose via GLUT
Osmosis	No	Down (water potential)	Water via aquaporins
Active Transport	Yes (ATP)	Up	Na+/K+ pump
Coupled Transport	Indirect (uses gradient)	Up (for one molecule)	Sodium-glucose symporter

Key Terms

• Selective permeability: The property of membranes that allows some substances to cross more easily than others.

• Gradient: A difference in concentration or electrical charge across a membrane.

• Osmosis: The diffusion of water across a selectively permeable membrane.

• Active transport: The movement of substances against their concentration gradient, requiring energy.

• Facilitated diffusion: Passive movement of molecules across the membrane via transport proteins.