Membrane Transport

• Fluid - Mosaic Model:

  The model for the 'cell' membrane is called the fluid - mosaic model. It is fluid because, the phospholipids are constantly moving, and a mosaic as the proteins are scattered in amongst the phospholipids like the tiles in a mosaic.
• The phospholipids -

  The phospholipid bilayer acts as a barriers to many substances as on substances which are small, lipid soluble and non-polar can diffuse through.
• The proteins -

  Usually span from one side of the phospholipid bilayer to the other but can also sit on one of the surfaces.
• The carbohydrates -

  Found on the outer surface of all eukaryotic cell membranes, and are attached to the membrane proteins or sometimes to the phospholipids.

  Proteins with carbohydrates attached are called glycoproteins.

  Phospholipids with carbohydrates attached are called glycolipids.
• Proteins can be transporters. Transport proteins must span the membrane ie , carrier or channel proteins.
• Proteins can be receptors. Receptor proteins must be on the outside surface of the cell membranes and have a specific binding site where hormones or other chemicals can bind to form a hormone - receptor complex.
• proteins can be enzymes. Enzyme proteins catalyse reactions in the cytoplasm or outside the cell, such as maltase in the small intestine.
• Proteins can be antigens. Antigen proteins are involved in cell recognition.
• Lipid Diffusion (simple diffusion):
• Lipid Diffusion (simple diffusion):

  • The movement of particles from a high to low concentration until they are evenly distributed (equilibrium)
  • A few substances can diffuse directly through the lipid bilayer part of the membrane.
  • The only substances that can do this are lipid soluble molecules such as steroids, or very small molecules, such as H2O, O2 and CO2.
• Osmosis:

  • Osmosis is the movement of water from a high to low water potential through a partially permeable membrane, until evenly distributed.
  • Pure water has the highest water potential 0kPa. Any solutes dissolved in the water lower its water potential.
• Facilitated Diffusion:

  • Channel Proteins form a water-filled pore or channel in the membrane. Ions like Na+, K+, Ca+ diffuse across membranes through specific ion channels.
  • Carrier Proteins have a binding site for a specific solute and constantly flip between two states. They change shape, on binding.
• Active Transport:

  • The pumping of substances across a membrane, by a protein pump molecules, using energy from hydrolysis of ATP.
  • The carrier protein binds a molecule of the substance to be transported on one side of the membrane, changes the shape, and releases it on the other side.
  • Highly specific
  • It moves substances against the gradient.