B 2.1 Membranes and Transport

Plasma Membrane Structure

• All cells have a plasma (or cell) membrane composed of two layers of phospholipids, known as the lipid bilayer.
• Functions:
  • Controls what enters and leaves the cell.
  • Described as semi-permeable or selectively permeable.

Permeability Factors

• Permeability depends on:
  • Size: Large molecules have lower permeability.
  • Charge: Non-permeable to polar or charged molecules due to repulsion by hydrophobic tails.
  • Non-polar and small molecules like oxygen can pass through easily.

Diffusion

• Diffusion: Passive movement from high to low concentration, no energy required.
  • Occurs with or without a membrane.
  • Moves until equilibrium is reached.

Membrane Proteins

• Integral Proteins: Span across the membrane, involved in transport.
• Peripheral Proteins: Attached to the surface of the membrane.
• Functions include:
  • Joining cells (JET RAT pneumonic: Join, Enzymes, Transport, Recognition, Attachment, Transduction).

Transport Mechanisms

• Channel Proteins: Facilitate passive transport for specific molecules.
  • Embedded across the membrane to form a channel for specific ions or molecules.
• Protein Pumps: Facilitate active transport, require ATP to move substances against concentration gradient.

Osmosis

• Special type of diffusion specific to water.
• Aquaporins: Special channel proteins that facilitate the movement of water across the membrane.

Facilitated Diffusion

• Passive transport through a channel protein, no energy required.
• Different channels needed for different molecules (e.g., glucose, ions).

Active Transport

• Moves molecules from low to high concentration (against the gradient), requires energy (ATP).
• Protein pumps used for this purpose.

Glycoproteins and Glycolipids

• Glycoproteins: Proteins with carbohydrate chains, important for cell recognition and joining.
• Glycolipids: Lipids with carbohydrate chains, specific to eukaryotic cells.

Fluid Mosaic Model

• Current model of the cell membrane structure.
• Fluid: Components can move around.
• Mosaic: Made of various parts including the lipid bilayer and proteins.