Biological Membranes Study Notes

Biological Membranes: Structure, Function, and Transport

Fluid Mosaic Model

• Concept: The biological membrane is a semi-fluid structure consisting of a phospholipid bilayer with various other molecules embedded within it.
• Phospholipid Bilayer: Composed of two layers of phospholipids.
  • Hydrophobic Tails: Located on the inside of the bilayer.
  • Polar Heads: Located on the outside of the bilayer.
• Movement: Components of the membrane, such as phospholipids and embedded proteins, are able to move around, contributing to its fluid nature.
• Embedded Molecules: Other molecules are integrated into this membrane and play diverse roles.

General Structure and Function of the Membrane

• Cell Outline: The membrane defines the boundaries of the cell.
• Regulation of Transport: It regulates what substances enter and exit the cell.
• Associated Carbohydrates:
  • Signaling Molecules: Some carbohydrates act as signals for cellular communication.
  • Coating/Repulsion: Others coat the membrane with polar groups, helping to repel unwanted substances.

Phospholipids

• Structure (Glycerophospholipids):
  • Glycerol Backbone: A central 3-carbon alcohol molecule.
  • Ester Bonds: Link fatty acid tails to the glycerol backbone.
  • Phosphate Group: Attached to one of the glycerol carbons, forming the polar head.
• Amphipathic Nature: Phospholipids have both hydrophilic (polar phosphate heads) and hydrophobic (nonpolar fatty acid tails) regions.
• Membrane Fluidity:
  • Saturated Fatty Acids: Have straight tails, allowing for tighter packing and reduced fluidity.
  • Unsaturated Fatty Acids: Contain cis double bonds, creating kinks that prevent tight packing, increasing membrane fluidity.

Cholesterol

• Role in Membrane:
  • Stabilizes membrane fluidity across temperature ranges.
  • At high temperatures, it decreases fluidity by restricting phospholipid movement.
  • At low temperatures, it increases fluidity by preventing phospholipids from packing too tightly.

Membrane Proteins

• Types of Proteins:
  • Integral Proteins:
  • Span the entire membrane (transmembrane proteins) or are embedded within one leaflet.
  • Have hydrophobic regions interacting with the lipid tails and hydrophilic regions exposed to aqueous environments.
  • Functions: channels, carriers, receptors, enzymes.
  • Peripheral Proteins:
  • Loosely associated with the surface of the membrane.
  • Attached to integral proteins or phospholipid heads via non-covalent interactions.
  • Functions: cell signaling, structural support.

Membrane Transport

• Selectively Permeable:
  • Only certain substances can cross the membrane unaided.
• Passive Transport:
  • Movement of substances down their concentration gradient (from high to low concentration).
  • Requires no energy input.
  • Simple Diffusion: Small, nonpolar molecules (e.g., ${O<em>2}$, ${CO</em>2}$) pass directly through the lipid bilayer.
  • Facilitated Diffusion:
  • Requires transport proteins (channels or carriers).
  • For larger or polar molecules (e.g., glucose, ions).
• Active Transport:
  • Movement of substances against their concentration gradient (from low to high concentration).
  • Requires energy input (e.g., ATP hydrolysis).
  • Primary Active Transport: Directly uses ATP to pump ions/molecules (e.g., ${Na^+ / K^+}$ pump).
  • Secondary Active Transport: Uses energy from an electrochemical gradient created by primary active transport (e.g., cotransport).