Membranes and Receptors - Session 2

Membranes and Receptors - Session 2

• Lecture Objectives: Describe the function of membranes as permeability barriers to small, hydrophilic molecules. Explore protein-mediated mechanisms allowing the uptake and extrusion of specific water molecules and ions.

Body Fluids

• Intracellular Fluid: Fluid within cells.
• Extracellular Fluid: Fluid outside of cells.
  • Intravascular fluid: Blood plasma.
  • Interstitial fluid: Fluid surrounding cells.

The Cell Membrane

• Structure: The cell membrane is a thin barrier (8nm thick) that separates the living cell from the aqueous environment. It's primarily composed of a phospholipid bilayer with embedded proteins.

Phospholipids

• Structure: Phospholipids have a hydrophilic head (attracted to water) and hydrophobic tails (repelled by water).
• Arrangement: These arrange themselves as a bilayer, with hydrophobic tails facing inwards and hydrophilic heads facing outwards.

Phospholipid Bilayer

• Boundary: This structure serves as a border defining the cell.
• Impermeability: The bilayer is largely impermeable to polar molecules (e.g., water, sugar, ions) but permeable to nonpolar molecules (e.g., lipids).

Cell Membrane Function

• Separation: The cell membrane separates the interior of the cell from its surroundings.
• Regulation: The cell membrane controls what enters and exits the cell. Hydrophobic (nonpolar) molecules can pass through more readily than hydrophilic (polar) molecules.
• Thickness: The membrane is approximately 8 nanometers thick.

Permeability to Polar Molecules

• Semi-permeable: The membrane becomes semi-permeable due to protein channels.
• Specific Channels: Specific protein channels allow the passage of specific materials across the membrane (e.g., water, ions, amino acids).

Cell Membrane Components

• Transmembrane Proteins: Transmembrane proteins embedded in the bilayer create semi-permeable channels.
• Lipid Bilayer Membrane: The foundation of the cell membrane.

Examples of Membrane Proteins

• Aquaporins: Water channels found in bacteria.
• Porins: Channels in bacterial outer membranes.
• Proton Pump (channels): Found in photosynthetic bacteria, these proteins facilitate movement of ions by changing shape.

Movement Across the Cell Membrane

• Diffusion: Movement of substances from high to low concentration. This can be both simple (through the lipid bilayer) or facilitated (through protein channels). Water moves through osmosis.
• Lipid Diffusion: Fats and other lipids can cross directly without the help of a protein.
• Facilitated Diffusion: Movement of polar or charged molecules through channels that require no energy input.
• Active Transport: Movement of molecules against their concentration gradient, which requires ATP.

Types of Active Transport

• Primary Active Transport: Requires ATP directly for transport, such as the sodium-potassium pump.
• Secondary Active Transport (Cotransport): The energy is derived secondarily from ionic concentration gradients created by primary active transport. In this process, two or more molecules move together.
  • Symport: Two molecules move in the same direction.
  • Antiport: Two molecules move in opposite directions.
    • Examples: Na+-glucose co-transport or Na+-Ca2+ exchange

Uniport, Symport and Antiport

• Uniport: Moves one molecule across the membrane.
• Symport: Moves two molecules in the same direction across the membrane.
• Antiport: Moves two molecules in opposite directions across the membrane.

Additional Notes

• Diffusion: Movement of molecules from high to low concentration. This occurs naturally across a membrane.
• Concentration Gradient: Difference in the concentration of a substance across a membrane.