transport across cell membranes

• Water molecules can pass through the non polar region of a phospholipid bi layer even though they are polar
• water molecules can pass through a membrane because they are really small , and the movement happens at a very slow rate
• cell surface membrane contains a large number of protein molecules and cholesterol
• Cholesterol molecules have a polar hydrophilic group at one end , this group attracts the polar head groups of phospholipds
• One end of cholesterol is hydrophobic and attracts the hydrophobic side of a phospholipid
• The non-polar and polar attraction between cholesterol and phospholipid molecules increases the strength of the cell surface membrane , so becomes less likley to be damaged
• Cholesterol :

  • controls fluidity of the membrane ( reduces sideway movement of molecules)
  • prevents membrane from becoming too fluid in warm conditions and too rigid in cool conditions
  • reduce movement of water soluble chemicals across membrane
• Scientists call the structure of the cell surface membrane the fluid mosaic model
• The cell membrane has two types of proteins :

  
  

  intrinsic or integral

  extrinsic or peripheral proteins
• Intrinsic proteins are fully embedded in the membrane from one side to the other
• Intrinsic proteins have hydrophobic amino acids on their surface as they pass through the bi layer
• Protein channels are Intrinsic proteins
• Protein channels contain a channel running through the centre
• Protein channels are lined inside with hydrophylic amino acids and are filled with water
• Protein channels allow water soluble molecules and ions to diffuse through
• Carrier proteins can change their shape or position to transfer molecules or ions through the membrane
• Extrinsic or peripheral proteins do not span the membrane
• Extrinsic proteins are found on one side of the membrane or the other
• Sometimes extrinsic proteins are attached to intrinsic proteins
• Extrinsic proteins functions :

  • structural
  • act as enzymes
  • receptors for molecules (e.g hormones )
• Gylcoproteins -> membrane protein with a carbohydrate molecule attached
• Glycoproteins functions ->

  • allow cells to attach to eachother to from tissues
  • role in the immune system
  • receptors for hormones
• glycolipids -> carbohydrates attached to phospholipids
• Glycolipids are used when cells come in contact with each other
• Glycolipids on the surface of one cell can be recognised by another cell when the cells come in contact
• Glycolipids can also act as antigens
• diffusion is the movement of particles from a high concentrated area to a low concentrated area
• diffusion is a passive process as it does not require energy
• Facilitated diffusion -> hydrophylic substances diffuse across a cell membrane via protein molecules
• active transport -> transfer of particles from a region of low concentration to high concentration against a concentration gradient
• Active transport requires metabolic energy which is provided by the molecule ATP
• Active transport process

  1. molecule or ion attaches to receptor site of carrier protein
  2. ATP molecule binds to carrier protein
  3. ATP undergoes hydrolysis -> phosphate and ADP
  4. phosphate attaches to carrier protein and causes it to change shape
  5. changed shape causes molecule / ion to other side the it is released.
  6. shape changed back and everything is reversed
• active transport uses alot of ATP
• Carrier proteins used in active transport are very specific