Membrane Transport

Membrane Transport

Plasma Membrane

• Definition: The outer boundary of the cell; controls the movement of substances in and out of the cell; separates the extracellular fluid from the cytoplasm (the material inside the cell).

Composition of Plasma Membrane

• Primary Components:
  • Phospholipids: The fundamental building blocks of all cell membranes.
  • Proteins: Interspersed among lipid molecules like tiles in a mosaic (Fluid Mosaic Model).
  • Cholesterol: Provides stability to the membrane.
  • Carbohydrates: Often involved in cellular recognition and signaling.
• Fluid Mosaic Model:
  • Describes the membrane's flexibility and the ability of proteins to move laterally within the lipid bilayer, contributing to its fluid quality.

Selective Permeability of Plasma Membrane

• Definition: The plasma membrane is selectively permeable, meaning it regulates what enters and exits the cell.
• Factors Affecting Diffusion Rate:
  • Size and Charge: The rate of diffusion across the membrane is impacted by the size of the molecule and its charge.

Permeability Scale

• High Permeability (cm/sec):
  • Hydrophobic molecules (e.g., O2, CO2, N2): 10°
  • Water (H2O): 10^{-2}
• Small, Uncharged Polar Molecules:
  • H2O: 10^{4}
  • Indole: 10^{-6}
  • Glycerol: 10^{-8}
• Large, Uncharged Polar Molecules:
  • Glucose: 10^{-8}
  • Sucrose: 10^{-10}
• Ions:
  • Cl⁻, K⁺, Na⁺: 10^{-12}

Movement Across the Plasma Membrane

• Molecules typically move down their concentration gradient, from high to low concentration.
• Moving against a concentration gradient requires energy, primarily in the form of ATP.
• Energy Currency of the Cell: ATP (adenosine triphosphate), is produced by the mitochondria during aerobic respiration.

Types of Transport Mechanisms

• Passive Transport: Movement across the membrane that does not require energy. Includes:
  • Simple diffusion
  • Facilitated diffusion
  • Osmosis
• Active Transport: Movement across the membrane that requires energy.

Detailed Transport Mechanisms

1. Diffusion

   • Simple Diffusion:
     • Random movement of a substance from an area of higher concentration to lower concentration, resulting in an equal distribution and the elimination of the concentration gradient.
   • Key Concepts:
     • Concentration gradient: The difference in the concentration of a substance across a space.
     • Equilibrium: The end result of diffusion where the concentration is equal on both sides of the membrane.

2. Facilitated Diffusion:

   • Movement from a higher concentration to a lower concentration, assisted by membrane proteins (carrier proteins).
   • Example: Glucose cannot pass through the lipid bilayer without assistance from carrier proteins.
   • Passive Mechanism: Does not require energy.

3. Osmosis:

   • Definition: The diffusion of water through a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration until equilibrium is reached.
   • Movement of water across a selectively permeable membrane follows the same principles as diffusion (concentration gradient and equilibrium).

4. Tonicity:

   • Definition: Refers to the ability of a solution to cause water movement across the cell membrane.
     • Hypotonic: Lower solute concentration outside the cell; leads to net water movement into the cell, potentially causing swelling or lysis.
     • Isotonic: Equal solute concentration; no net water movement.
     • Hypertonic: Higher solute concentration outside the cell; results in net water movement out of the cell, causing the cell to crenate.

Summary of Passive Processes

• Three Processes: Diffusion, osmosis, and facilitated diffusion share the need for a concentration gradient and continue until equilibrium is reached, all being passive in nature (movement down the concentration gradient at no energy cost to the cell).

1. Active Transport:
   • Movement often from a region of lower concentration to higher concentration, requiring energy (typically from ATP) and the aid of a carrier protein. It can involve both small and large molecules.
   • Example: Sodium-Potassium Pump
     • Function: Maintains the ionic gradient across the membrane by transporting Na⁺ out and K⁺ into the cell.
     • Process:
     1. Cytoplasmic Na⁺ binds to the pump.
     2. Binding stimulates phosphorylation by ATP, causing a shape change in the protein, expelling Na⁺ outside.
     3. Extracellular K⁺ binds, and loss of the phosphate group restores original conformation, releasing K⁺ into the cell.

Endocytosis

• Types: Mechanism by which large molecules or fluid droplets enter cells through engulfing by the plasma membrane, forming vesicles.
  • Three Types:
  • Phagocytosis (cell eating): Envelops large particles or bacteria.
  • Pinocytosis (cell drinking): Absorbs extracellular fluid along with solutes.
  • Receptor-mediated Endocytosis: Specific uptake of molecules that attach to surface receptors.

Phagocytosis and Pinocytosis

• Phagocytosis: Internalizes large particles to form a phagosome (food vacuole).
• Pinocytosis: Primarily used for absorption, taking in portions of extracellular fluid along with all dissolved solutes, non-specific in nature.

Exocytosis

• Definition: Process by which large molecules leave the cell, enclosed in vesicles that travel to the plasma membrane to release contents outside.

Receptor-Mediated Endocytosis

• Mechanism: A vesicle is formed when specific molecules bind to their respective receptors on the cell membrane.
• Examples: Cholesterol and influenza virus utilize this mechanism.

Influenza Virus Example

• Haemagglutinin (HA): The viral protein responsible for both receptor binding and membrane fusion; relevant strains include H5N1 (bird flu) and H1N1 (swine flu).

Summary of Transport Mechanisms