2.5 biological membranes

• Phospholipids


  Molecules with two fatty acid molecules bonded to a glycerol molecule, and a phosphate group
• Phospholipids


  • Have a hydrophobic region (fatty acids) and a hydrophilic region (phosphate group)
  • Arrange themselves in a bilayer in water, with the hydrophobic regions in the center and the hydrophilic regions interacting with water
• Phospholipid bilayer


  The basis of all cell membranes, including the cell surface membrane and organelle membranes
• Cell membranes


  • Act as barriers
  • Locations for chemical reactions
  • Involved in cell signaling
• Cell surface membrane


  Contains a hydrophobic center that allows hydrophobic molecules to pass through, but prevents hydrophilic water-soluble molecules from easily passing through
• Cell surface membrane


  • Contains protein molecules, some spanning the membrane and others only on one side
  • Contains cholesterol, which increases the strength and stability of the membrane, and controls its fluidity
• Fluid mosaic model


  The structure of the cell surface membrane, where the phospholipids are fluid and the proteins are arranged in a mosaic-like pattern
• Water molecules can pass through the cell membrane, even though they are polar, because they are extremely small
• Intrinsic or integral proteins


  Fully embedded in the membrane from one side to the other, passing right through the lipid bilayer
• Intrinsic proteins


  • Have hydrophobic amino acids on the outside surface that can interact with the hydrophobic fatty acid tails in the phospholipid bilayer
• Types of intrinsic proteins


  • Protein channels
  • Carrier proteins
• Protein channels


  Intrinsic proteins that contain a channel running through the center, lined with hydrophilic amino acids and filled with water molecules, allowing water-soluble molecules and ions to diffuse through
• Carrier proteins


  Intrinsic proteins that can change their shape or position to transfer molecules or ions from one side of the membrane to the other
• Extrinsic or peripheral proteins


  Do not span the membrane, instead found on one side of the membrane or the other, sometimes attached to intrinsic proteins
• Extrinsic proteins


  • Have a range of functions, including structural roles, acting as enzymes, and acting as receptors for other molecules such as hormones
• Glycoproteins


  Membrane proteins that contain a carbohydrate molecule attached
• Functions of glycoproteins


  • Allow cells to attach to each other to form tissues
  • Play a role in the immune system, presenting antigens to T cells
  • Act as receptors for hormones
• Glycolipids


  Carbohydrates attached to phospholipid molecules in the cell membrane
• Functions of glycolipids


  • Used when cells come into contact with each other, the glycolipids on the surface of one cell can be recognized by another cell
  • Can act as antigens, for example in determining blood group
• Facilitated diffusion


  The process where hydrophilic substances diffuse across the cell membrane via protein molecules
• Diffusion is the net or overall movement of particles from a region of higher concentration to a region of lower concentration, down the concentration gradient
• Diffusion is a passive process that does not require any metabolic energy
• Cell membrane


  • The center of the cell membrane is hydrophobic due to the fatty acid tails of the phospholipid molecules
  • Hydrophobic substances can diffuse rapidly across the membrane
  • Hydrophilic substances cannot diffuse across the membrane
• Water molecules can diffuse across membranes even though they are polar, because they are very small
• Facilitated diffusion


  1. Hydrophilic substances diffuse across the cell membrane via protein molecules

  2. Protein molecules allow the hydrophilic substances to cross the membrane without interacting with the hydrophobic center of the phospholipid bilayer
• Carrier proteins


  • Have a binding site for a specific chemical
  • When the chemical binds, it causes a change in the tertiary structure of the carrier protein, bringing the chemical across the membrane where it is released
• Protein channels


  • Have a central pore lined with hydrophilic amino acids and containing water
  • Allow hydrophilic substances to pass through from one side of the membrane to the other
  • Are selective for the chemicals that can pass through
  • Can be always open or only open in response to a certain trigger (e.g. chemical binding or voltage change)
• Facilitated diffusion is still a type of diffusion, where chemicals move from a region of higher concentration to a region of lower concentration
• Metabolic energy is not required for facilitated diffusion
• Co-transport


  A mechanism of active transport where the movement of one molecule or ion is coupled to the movement of another molecule or ion
• Glucose absorption in the ilium

  1. Glucose moves down concentration gradient into epithelial cells via facilitated diffusion

  2. Sodium-potassium pump actively transports sodium out of and potassium into epithelial cells using ATP

  3. Sodium-glucose co-transporter uses sodium concentration gradient to actively transport glucose into epithelial cells
• Epithelial cells in ilium


  • Contain many mitochondria to provide ATP for sodium-potassium pump
  • Have folded membrane with microvilli to increase surface area for transport proteins
• Glucose absorbed into epithelial cells

  Rapidly carried away by bloodstream, maintaining steep concentration gradient for facilitated diffusion into blood