Biology Chapter on Membrane Structure and Function

Membrane Function

• Membranes are selectively permeable barriers that regulate the movement of substances into and out of cells.
• Membrane proteins play a crucial role in determining the unique functions of different membranes.

Membrane Structure

• The Fluid Mosaic Model describes the structure of membranes as a dynamic mosaic of lipids and proteins.
• Phospholipids are a primary component of membranes, consisting of a hydrophilic head (glycerol backbone and phosphate group) and a hydrophobic tail (two fatty acids).
• Phospholipids form a bilayer, with the hydrophobic tails facing inwards and hydrophilic heads facing outwards.
• Phospholipids exhibit three types of movements within the membrane:
  • Lateral diffusion: Frequent movement within the plane of the bilayer
  • Rotation: Frequent spinning motion
  • Flip-flop: Rare movement across the bilayer
• Membrane fluidity is influenced by:
  • Temperature: Higher temperature increases fluidity, lower temperature decreases fluidity.
  • Phospholipid composition: Longer fatty acid chains decrease fluidity, shorter chains increase fluidity. More double bonds (C=C) increase fluidity, fewer decrease fluidity.
  • Cholesterol: Acts as a fluidity buffer, decreasing fluidity at high temperatures and increasing fluidity at low temperatures.
• Membrane proteins are embedded within the phospholipid bilayer and can be classified as:
  • Integral membrane proteins: Embedded within the membrane, some span the entire bilayer (transmembrane proteins).
  • Peripheral membrane proteins: Associated with the membrane but not embedded within it.
• Transmembrane proteins perform various functions:
  • Channels: Allow the passage of specific ions or molecules.
  • Carriers: Bind to specific molecules and transport them across the membrane.
  • Receptors: Bind to signaling molecules and initiate cellular responses.
  • Enzymes: Catalyze reactions within the membrane.
• Membrane proteins can be anchored to the cytoskeleton on the inside of the cell or the extracellular matrix on the outside of the cell.
• Cholesterol is also a component of the plasma membrane of mammalian cells.

Membrane Transport

• Membranes act as selectively permeable barriers, allowing some molecules to pass through easily while others require assistance.
• Small, nonpolar molecules (e.g., oxygen, carbon dioxide, fatty acids) can move across the membrane via simple diffusion due to their high lipid solubility.
• Larger, polar molecules and ions are poorly lipid soluble and require membrane proteins for transport.
• Membrane potential refers to the electrical charge difference across the membrane.
  • It is generally negative inside the cell relative to the outside.
• Electrochemical gradient combines the concentration gradient and the membrane potential, influencing the movement of ions across the membrane.

Diffusion

• Diffusion is the net movement of molecules or ions from a region of high concentration to a region of low concentration.
• Diffusion rate is influenced by:
  • Temperature: Higher temperature increases diffusion rate, lower temperature decreases diffusion rate.
  • Concentration gradient: Larger gradient increases diffusion rate, smaller gradient decreases diffusion rate.
  • Molecular weight: Larger molecules diffuse more slowly than smaller molecules.

Simple Diffusion

• Simple diffusion does not require membrane proteins and does not involve energy expenditure.
• Molecules move passively down their concentration gradient.
• The direction of movement (into or out of the cell) is determined by the concentration gradient.
• Examples of molecules that move by simple diffusion: oxygen, carbon dioxide, fatty acids.

Transporting Polar Molecules and Ions

• Polar molecules and ions require the assistance of membrane proteins for transport as they cannot pass through the membrane directly.
• Transmembrane proteins involved in transport are called transport proteins.
• Transport proteins can be grouped into two categories:
  • Channels: Form pores through the membrane that allow specific molecules or ions to pass through passively.
  • Carriers: Bind to specific molecules and change conformation to transport them across the membrane.
  • Some require energy (active transport: moving against concentration gradient) and some do not (facilitated diffusion: moving down concentration gradient)