2.1.5 biological membranes

• overview of membranes


  All cells and organelle membranes are composed of a phospholipid bilayer

  Cell membranes provide a partially permeable membrane are the site of chemical reactions and have a role in cell communication
• fluid-mosaic model

  • mixture and movement of phospholipids, proteins, glycoproteins and glycolipids the membrane is made of
  • The phospholipids align as a bilayer due to the hydrophilic head being attracted to water and the hydrophobic tails being repelled by water
  • Proteins within the cell surface membrane can be extrinsic and intrinsic
• function of extrinsic proteins


  provide mechanical support or make glycoproteins and glycolipids function in cell recognition as receptors
• function of intrinsic proteins

  carrier or channel proteins involved in the transport of molecules across the membrane
• what are protein channels

  Protein channels form tubes that fill with water-soluble ions to diffuse whereas the carrier proteins will bind with other ions and larger molecules such as glucose and amino acids and change shape to transport them to the other side of the membrane
• what is cholesterol


  present in some membranes and this restricts the lateral movement of other molecules on the membrane. This is useful as it makes the membrane less fluid at high temperatures and prevents water and dissolved ions from leaking out of the cell
• factors affecting membrane structure and permeability

  1. temperature 

  2. solvents
• temperature affecting membrane structure and permeability

  • high temperatures increase the kinetic energy of the phospholipids so that they move even more. This increases the fluidity of the membrane increasing the permeability and the structure can start to break. This makes it easier for particles to cross the membrane
  • High temperatures also denature carrier and channel proteins in the membrane
• solvents affecting membrane structure and permeability

  organic solvents like alcohol dissolve the phospholipid bilayer in membranes. This damage causes the fluidity of the membrane to increase and become more permeable
• movements across the membrane (6)

  1. Simple diffusion

  2. Facilitated diffusion

  3. Osmosis

  4. Active transport

  5. Endocytosis

  6. Exocytosis
• simple diffusion

  • The net movement of molecules from an area of higher concentration to an area of lower concentration until equilibrium is reached. This does not require ATP
  • For molecules to diffuse across the membrane they must be lipid soluble and small
• facilitated diffusion


  A passive process down the concentration gradient through proteins

  The movement of ions and polar molecules which cannot simply diffuse can be transported across membranes by facilitated diffusion using protein channels and carrier proteins
• osmosis


  The movement of water from an area of higher water potential to an area of lower water potential (more negative) across a partially permeable membrane
• hypertonic solutions


  Hypertonic is when the water potential of a solution is more negative than the cell so water will move out of the cell causing the cell to crenate/plasmolyse
• isotonic solutions


  Isotonic is when the water potential of the solution is the same in the solution and the cell within the solution so no water movement will occur
• hypotonic solutions


  Hypotonic is when the water potential of a solution is more positive than the cell therefore water will move into the cell causing the cell to turgid/burst
• active transport

  • The movement of molecules and ions from an area of lower concentration to an area of higher concentration against the concentration gradient using ATP and carrier proteins
  • only certain molecules can bind to the receptor on carrier proteins. ATP will bind to the protein on the inside of the membrane and is hydrolysed into ADP and Pi
  • This causes the protein to change shape and open towards the inside of the membrane
  • causes the molecule to be released on the other side of the membrane
  • The Pi molecule is then released and this results in the protein reverting to its original shape
• endocytosis

  This is a type of active transport that is the bulk transport of molecules into a cell

  • The cell surface membrane bends inwards around the molecule surrounding It to form a vesicle
  • The vesicle pinches off and moves within the cytoplasm
  • Endocytosis can be classified as either phagocytosis when it is a solid particle being taken in or pinocytosis when it is a liquid
  • This requires energy from ATP for the cell to engulf and change shape around the material
• exocytosis


  This is the bulk transport of molecules out of a cell

  • Vesicles move toward the cell-surface membrane and fuse with the membrane and the content of the vesicle is released outside of the cell
  • This process requires energy because ATP is needed to move the vesicle along the cytoskeleton