Transport across membranes

• Describe the structure of phospholipids

  Hydrophilic phosphate 'head':

  • polar (has a charge)
  • points to the outside

  Hydrophobic fatty acid 'tails':

  • non-polar (has no charge)
  • points to the centre

  
• Why is the Fluid Mosaic Model of the cell-surface membrane called that?

  Fluid = molecules can change position and move around each other without leaving gaps in the membrane.

  Mosaic = the many different structures within the cell-surface membrane, which are embedded within the phospholipid bilayer
• What is the role of cholesterol molecules within the phospholipid bilayer?

  • It attracts fatty acid tails
  • Mainly hydrophobic means it prevents the loss of water and dissolved ions from the cell
  • Reduces fluidity of the membrane.
  • Supports shape and strength of cell
• When does the phospholipid bilayer occur?

  If there is water on both sides of the bilayer
• Why would molecules not be able to freely diffuse across the cell-surface membrane?

  • They are not lipid-soluble (so they cannot pass through the bilayer)
  • Too large
  • They have the same charge as the charge on the protein channels
  • Electrically-charged (polar) so they have difficulty passing through non-polar fatty acid tails in the bilayer
• What are the 5 examples of intrinsic/integral proteins?

  1. Channel proteins
  2. Carrier proteins
  3. Electron carrier proteins
  4. Enzyme proteins
  5. Receptor proteins
• What is a channel protein (structure + role)?

  A protein that has holes in it so that specific molecules can pass through e.g. ions, sugars and amino acids that, otherwise, could not diffuse through.

  • Has hydrophobic regions and hydrophilic regions
  • It is passive transport
• What is a carrier protein (role and an example)

  A protein that binds to molecules, and then changes shape to transport and release the molecules into the other side. An example would be protein pumps, which force molecules against the concentration gradient:

  • This is active transport, which requires energy from ATP
  • Only specific to some substances
• What are electron carrier proteins and enzyme proteins?

  Electron carrier proteins carry electrons. They are oxidised when losing electrons, and are undergoing reduction when gaining electrons. (Think OILRIG)

  Enzyme proteins catalyse reactions. Their specific 3D structure forms the active site
• What are receptor proteins?

  Hormone-binding proteins.

  • They have an external binding site
• Explain the 2 different types of peripheral/extrinsic proteins (location and role)

  Both types are either free on the membrane surface or bound to an intrinsic protein.

  Extracellular peripheral proteins - receptors for hormones or neurotransmitters. Or involved in cell recognition.

  Cytosolic side (intracellular) peripheral proteins - cell signalling or chemical reactions. These proteins can dissociate (separate) from the membrane and move into the cytoplasm
• What is a glycoprotein?

  A carbohydrate attached to a protein. Used in recognition of other cells, cell adhesion (when cells attach to other cells to make tissues).
• What is a glycolipid?

  A carbohydrate attached to a lipid.

  • Extends from the bilayer to a watery environment outside the cell - acts as a cell-surface receptor for specific molecules.
  • Used in recognition of other cells
  • Used in stability of membrane
  • Used in cell adhesion
• What are the three factors that affect membrane fluidity (and how?)

  1. Cholesterol - (already mentioned in another flashcard)
  2. Temperature - (kinetic energy of molecules increases if temp increases -> more movement of molecules -> more fluidity)
  3. Unsaturated fatty acids - (Saturated fatty acids have closer phospholipids, and so less fluidity. Whereas, unsaturated fatty acids, the phospholipids are not as close so more fluidity)
• Define simple diffusion

  The net movement of a substance from a region of higher concentration to a region of lower concentration. This is a result of the random movement of molecules, and it is a passive transport (which means it does not require additional energy).
• What molecules can and cannot be transported by simple diffusion?

  Can: small, non-polar molecules rapidly diffuse across the membrane e.g. oxygen and carbon dioxide. Small, polar molecules e.g. water and urea diffuse across much more slowly

  Cannot: charged particles (ions) cannot diffuse across a membrane because they will be attracted or repelled
• Define facilitated diffusion

  When specialised proteins e.g. protein channels/carriers, allow certain molecules to cross the phospholipid bilayer. It is a passive transport (does not require metabolic energy from ATP)
• What are the 3 factors that affect diffusion and how to remember them?

  • Concentration gradient (rate of diffusion is directly proportional)
  • Diffusion pathway (rate of diffusion is indirectly proportional)
  • Surface area (rate of diffusion is directly proportional)

  To remember, use Fick's Law: rate of diffusion is directly proportional to (surface area x concentration gradient) / diffusion pathway
• Define osmosis

  The net movement of water molecules from a region of higher water potential (e.g. -75kPa) to a region of lower water potential (e.g. -189 kPa) across a selectively permeable membrane.
• Pure water has the highest possible water potential of 0kPa. If we add solutes to water, this decreases the water potential. This means solutions have a negative water potential.
• High water potential = more water, less solutes e.g. -75kPa

  Low water potential = less water, more solutes e.g. -189kPa
• Explain the 3 types of solutions.

  1. Isotonic (iso-osmotic) - water potential of the solution is the same as cells, so no osmosis occurs. Same concentration of solutes and water
  2. Hypertonic (hyper-osmotic) - water potential of the solution is lower than the cells, so osmosis moves water out of cells. Higher concentration of solutes
  3. Hypotonic (hypo-osmotic) - water potential of the solution is higher than the cells, so osmosis moves water into cells. Lower concentration of solutes
• What is active transport?

  The movement of molecules or ions into or out of a cell against the concentration gradient using metabolic energy from ATP
• What helps allow larger molecules to cross the phospholipid bilayer?


  Specialised and specific carrier proteins in the cell-surface membrane
• What type of transport is active transport?

  Active - it requires energy in the form of ATP to 'pump' the molecules into the cell
• How does active transport work?

  1. Molecule or ion binds to the carrier protein, on their receptor sites
  2. ATP binds to intracellular (inside cell) side of protein and is hydrolysed into ADP and a phosphate group. The protein undergoes a conformational change (changes shape)
  3. A molecule or ion is released on the other side of the cell-surface membrane.
  4. Phosphate group is released from the protein and the protein reverts to the original shape
• How are epithelial cells lining the ileum specialised for diffusion?

  Possesses microvilli, which increases the surface area
• What is co-transport?

  The movement of a substance against its concentration gradient, coupled with the movement of another substance down its concentration gradient
• Explain the co-transport of glucose, amino acids and sodium ions in epithelial cells.

  1. Na+ ions are actively transported out of cell (lower conc) -> blood plasma (higher conc).
  2. This maintains low Na+ conc in cell
  3. Faciliated diffusion means Na+ ions move from the lumen of the ileum into the cytoplasm of the cells (down the conc gradient). Using co-transport proteins - carrying another molecule e.g. glucose/amino acids.
  4. Facilitated diffusion of glucose/amino acids from cytoplasm -> blood (down its conc gradient)