Transport Across Membranes

• The fluid mosaic model describes the surface appearance of the cell membrane due to its intrinsic and extrinsic proteins and fluid phospholipid bilayer
• The membrane of a vacuole is called the tonoplast
• Membranes control the diffusion of substances as well as play a role in cell recognition
• Simple lipid diffusion can occur through the phospholipid bilayer if molecules are small and uncharged or lipid soluble
• The main components of a cell membrane are channel and carrier proteins, phospholipids and cholesterol
• The role of cholesterol in a cell membrane is to keep phospholipids from drifting too close together and drifting apart when warm
• Phospholipid tails point inwards as they are non polar and hydrophobic, phospholipid heads point outwards as they are polar and hydrophillic
• Intrinsic proteins span the whole plasma membrane, extrinsic proteins exist on the membrane surface
• Intrinsic protein examples are carrier and channel proteins
• Extrinsic protein examples are glycolipids and glycoproteins which act as cell identifiers
• Active transport is the transport of molecules against a concentration gradient using ATP
• Facilitated diffusion is movement of molecules via a protein carrier or channel without the use of ATP down a concentration gradient
• Hypotonic is where the outside solution has a higher water potential than the inside
• Hypertonic is where the outer solution has a lower water potential than the inside
• Isotonic is where water potentials are equal, there may be dynamic equilibrium
• Osmosis is the passive transport of water down a water potential gradient across a selectively permeable membrane
• In channel proteins, ions dissolved in water are transported across the membrane, the channel protein is lined with hydrophilic groups
• A solution with a higher water potential has more free water molecules (not associated with an ion)
• In incipient plasmolysis, half of the plant cells are plasmolysed so the solutions are now isotonic
• Red blood cells become haemolysed when they shrivel
• Animal cells have no cell wall so are susceptible to osmotic lysis
• Rate of diffusion is proportional to ( surface area x difference in concentration ) divided by thickness of exchange surface
• Carrier proteins have a complimentary shape to a specific molecule which it will bind to and then undergo a conformational change of shape to transport it across the membrane
• There is a limit to the rate of facilitated diffusion as carrier proteins can become saturated
• In co transport, the sodium potassium antiport pumps sodium ions out of the cell and potassium ions into the cell
• When sodium ion concentration inside the columnar cell is low it creates a concentration gradient between the columnar epithelium and the lumen
• Sodium ions in the lumen diffuse across the cell membrane into the columnar epithelial cell bringing a glucose or amino acid with them
• The glucose concentration in the columnar cell increases creating a concentration gradient, glucose diffuses by facilitated diffusion into the bloodstream
• Cholesterol is a steroid