biological membranes

• Biological membranes surround all cells and organelles, composed of a sea of phospholipids with protein molecules between them
• Main function of the membrane is controlling the movement of substances in and out of the cell/organelle
• Membrane contains receptors for molecules like hormones and enables adjacent cells to stick together
• Fluid mosaic model: the structure of the membrane is named after the fluidity of the membrane and the mosaic arrangement of proteins
• Cholesterol is an important lipid that helps maintain fluidity and stability of the membrane.
• • Active transport: transports molecules through carrier proteins from low to high concentration, requiring ATP energy
  • Exocytosis and endocytosis: transport large particles and bulk transport through vesicles made from the cell surface membrane
• Types of movement through cell membranes:

  • Diffusion: passive movement of small, non-polar lipid soluble molecules from high to low concentration through the phospholipid bilayer
  • Facilitated diffusion: requires channel proteins to transport polar, charged, and water-soluble molecules
  • Osmosis: diffusion of water molecules from low solute concentration to high solute concentration through a partially permeable membrane
  • Osmosis: diffusion of water molecules from high water potential to low water potential through a partially permeable membrane
• Factors affecting the rate of gas exchange by diffusion:

  • Surface area of the surface increases
  • Diffusion distance decreases
  • Diffusion gradient becomes steeper
• Phospholipids are amphiphilic molecules with hydrophobic tails and polar heads.
• The hydrophobic tails are embedded within the bilayer while the polar heads face towards the outside of the membrane.