Transport Across Cell Membranes

• Outline role of phospholipids in cell-surface membrane structure.

  Their hydrophilic/hydrophobic interactions lead to the formation of a phospholipid bilayer.

  
  

  1. Allow lipid-soluble substances to enter/exit cell.

  
  

  2. Prevent water-soluble substances entering and leaving cell.

  
  

  3. Make the membrane flexible and self-sealing.
• Outline role of proteins in cell-surface membrane structure.

  Some in surface of bilayer:

  
  

  1. Act to provide mechanical support to membrane.

  
  

  2. Along with glycolipids, act as cell receptors for molecules such as hormones.

  
  

  Some span the entire membrane:

  
  

  3. Protein channels - water filled tubes allowing water-soluble ions to diffuse across the membrane.

  
  

  4. Protein carriers - bind to ions or molecules like glucose/amino acids - then change shape to move these molecules across the membrane.

  
  

  5. Help cells adhere together.

  
  

  6. Form cell-surface receptors for identifying cells.
• Outline role of cholesterol in cell-surface membrane structure.

  Within the phospholipid bilayer of cell-surface membranes:

  
  

  1. Reduce lateral movement of other molecules (including phospholipids) - pulls together the fatty acid tails, limiting movement without making the membrane too rigid.

  
  

  2. Make the membrane less fluid at high temperatures.

  
  

  3. Prevent leakage of water and dissolved ions from the cell, as cholesterol molecules are very hydrophobic.
• Outline role of glycolipids in cell-surface membrane structure.

  Carbohydrate covalently bonded to a lipid in the membrane:

  
  

  1. Carbohydrate portion extends from the phospholipid bilayer into the watery environment outside the cell - there acts as a cell-surface receptor for specific chemicals (ABO blood system)

  
  

  2. Help maintain the stability of the membrane.

  
  

  3. Help cells to attach to one another and so form tissues.
• Outline role of glycoproteins in cell-surface membrane structure.

  Carbohydrate chains attached to many proteins on the outer surface of the cell-surface membrane.

  
  

  1. Cell-surface receptors for hormones and neurotransmitters.

  
  

  2. Help cells to attach to one another and so form tissues.

  
  

  3. Allow cells to recognise one another - lymphocytes can recognise an organism's own cells.
• Comment on the permeability of the cell-surface membrane.

  Controls the movement of substances into/out of the cell.

  
  

  Most molecules don't freely diffuse across it because many are:

  
  

  1. Not lipid-soluble

  2. Too large to pass through

  3. Same charge as protein channel charges - repelled even if small

  4. Charged/polar - can't pass through the non-polar hydrophobic tails in the phospholipid bilayer.
• Explain the "fluid-mosaic" model of the cell-surface membrane structure.


  = arrangement of all the various molecules combined into the structure.

  
  

  Fluid = membrane is flexible and can constantly change in shape as individual phospholipid molecules can move relative to one another.

  
  

  Mosaic = Proteins embedded in the phospholipid bilayer vary in shape, size + pattern like tiles in a mosaic.
• Define diffusion.

  
  

  Give the equation for Fick's Law.

  = The net movement of molecules/ions/particles from a region of higher concentration to a region of lower concentration.

  
  

  Fick's Law:

  
  

  diffusion rate = (SA x gradient x permeability) / (membrane thickness).
• Define facilitated diffusion.

  
  

  Comment on the roles of the proteins involved in facilitated diffusion.


  Movement of larger/charged/polar species made easier by protein channels/carriers that span the membrane.

  
  

  NB=Passive process, only difference is that fd occurs at specific points on the plasma membrane where there are protein molecules.

  
  

  Channel Proteins = Water-filled hydrophilic channels allowing specific water-soluble ions to pass through - selective channels - only open when specific ion binds to protein causing it to change shape -> open and closed on different sides of the membrane.

  
  

  Carrier Proteins = Molecule specific to the protein binds, changes shape of protein -> molecule released to the inside of the membrane.
• Define osmosis.


  The net passage of water molecules from a region of higher water potential to a region of lower water potential across a selectively permeable membrane.
• Comment on osmosis in plant cells when water moves in/out.

  Protoplast (cell parts within cell wall) pushes on cell wall when swells - turgid.

  
  

  If water leaves by osmosis, point is reached where protoplast no longer presses on cellulose cell wall - incipient plasmolysis.

  
  

  Further loss of water by osmosis -> cell contents shrink further and protoplast pulls away from cell wall - cell is said to be plasmolysed.
• Define active transport.


  Movement of particles into/out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins.
• Outline how active transport takes place.

  1. Molecules bind to carrier protein and ATP attaches to the membrane protein on the inside of cell/organelle.

  
  

  2. Binding of phosphate ion to protein changes shape, allowing access for molecules to inside of the membrane but closed to the outside.
• Comment on the relationship between Fick's Law and the adaptations of specialised cells.

  Specialised cells will have different surface areas, numbers of channel/carrier proteins, differences in concentration gradients/ wp ---- all to increase or decrease the rate of movement across cell membranes.

  
  

  --> think how permeability affected by numbers of proteins in a given area etc. - relate to fick's law.
• Outline the process of Co-transport in the ileum.

  1. Na+ actively transported out of epithelial cells into blood via Na+/K+ pumps (carrier molecules in epithelial cell-surface membrane).

  
  

  2.Increased Na+ conc maintained in lumen of intestine than in epithelial cells.

  
  

  3. Na+ diffuse into epithelial cells down Na+ gradient through a co-transport protein ---> carrying glucose (/amino acids) with them.

  
  

  4. Glucose/amino acids pass into the blood plasma by facilitated diffusion using another type of carrier, against their conc gradient.

  
  

  => Indirect active transport - as [Na+] gradient provides energy for process, not ATP - but ATP needed for 1.