plasma membrane

• protein channels


  these proteins form water filled hydrophilic channels across the membrane. they allow specific water soluble ions to pass through.the channels are selective , each opening in the presence of a specific ion.

  if a particular ion is not present, the channel remains closed.there is control over the entry and exit of ions. the ions bind with the protein causing it to change shape in a way that closes it to one side of the membrane and opens it to the other side.
• Phospholipids are amphipathic molecules. This means that they have a hydrophilic, polar phosphate head and two hydrophobic fatty acid tails. These components of the phospholipids cause them to orientate themselves.
• describe what is meant by membrane fluidity


  membrane fluidity allows proteins to diffuse laterally in to the membrane
• what is meant by ampithatic ?


  having both hydrophilic and hydrophobic parts.
• why is the fluid mosaic model called a fluid mosaic ?

  fluid - individual phospholipids can move , giving flexibility to the structure and is constantly changing shape

  mosaic- because proteins are embedded in the phospholipid bilayer vary in shape, size, pattern in the same way as the stones or tiles of mosaic
• Fluid Mosaic Model


  A model of cell membranes where phospholipids are in a dynamic, fluid state and proteins are embedded in the bilayer, creating a complex, mosaic-like structure.
• Fluid


  Ability of individual phospholipids to move and change shape within the cell membrane, allowing for flexibility and dynamic structure.
• what is the function of the phospholipid bilayer?


  allow diffusion of lipid soluble substances . prevents movement of water soluble substances.
• What are two functions of cholesterol embedded in the cell membrane?


  Reduce fluidity and permeability of the cell membrane.Increase rigidity of the cell membrane.
• co-transport


  uses special carrier proteins called co-transporters. binds two molecules at a time , one moves down the concentration gradient and this is used to move the other molecule across the membrane against its concentration gradient . it can also be said that one molecule allows the other molecule to move and it can be the same direction of travel.
• name and describe 5 ways substances can move across the cell surface membrane into a cell

  simple diffusion of small non polar molecules down the concentration gradient

  facilitated diffusion down a concentration gradient via channel proteins and carrier proteins

  osmosis of water down a water potential gradient

  active transport which goes against the concentration gradient via carrier protein and ATP

  co-transport of two different substances using carrier proteins
• movement of Na+ out of the cell allows the absorption of glucose into the cell lining the ileum


  a lower concentration of Na+ inside the cell compared with outside the cell . generates a concentration/diffusion gradient for Na+ from ileum into cells . Na+ moving (in) by co-transport, brings glucose with it
• describe and explain two features you would expect to find in a cell specialised for absorption

  1. Folded membrane/microvilli so large surface area (for absorption);

  2. Large number of co-transport/carrier/channel proteins so fast rate of absorption
• What role do carrier proteins play in cell membranes?


  They allow active transport of substances.
• How does membrane structure affect the movement of substances across cell membranes?


  It determines which substances can pass through and how easily they can do so.
• What type of substances can diffuse through the phospholipid bilayer?


  Nonpolar or lipid-soluble substances
• What is the function of channel proteins in cell membranes?


  They allow facilitated diffusion and co-transport of substances.
• What does the phospholipid bilayer prevent from crossing the membrane?

  Polar or hydrophillic substances
• What do aquaporins facilitate in cell membranes?


  Osmosis
• How do the shape and charge of channel or carrier proteins affect substance movement?


  They determine which substances can move through the membrane.
• What does the number of channels or carriers in a membrane indicate?


  It determines how much movement of substances will occur.
• How does membrane surface area influence diffusion?


  A larger surface area allows for more diffusion to occur.
• What effect does cholesterol have on cell membranes?


  It affects fluidity, rigidity, and permeability of the membrane.
• facilitated diffusion is where particles diffuse through transport proteins in the membrane.
• Describe the steps involved in the active transport of one particle through the membrane. Include a description of how ATP is used.


  The desirable particle and a molecule of ATP bind to their respective binding sites on the transport protein in the membrane.ATP is hydrolysed to ADP and a phosphate ion, transferring energy to the transport protein and triggering it to change shape. This forces the particle through the protein and across the membrane.Finally, the phosphate ion is released, causing the protein to revert to its original shape.
• why is glucose absorbed by facilitated diffusion ?


  glucose is polar, hydrophilic (water soluble), lipophobic (not lipid soluble) and large.
• Describe how the transport protein in the cell-surface membrane transfers a particle from the inside of a cell to the outside via diffusion.


  The transport protein is triggered to open by e.g. a chemical messenger.A particle diffuses down the concentration gradient, from an area of high concentration to an area of low concentration.This is a passive process called “facilitated diffusion”.
• What is the process of co-transport across a membrane?


  It involves the transport of two particles against their concentration gradients.
• What does the transport protein bind to during co-transport?


  The transport protein binds the first particle and a molecule of ATP.
• What happens to ATP during the co-transport process?


  ATP is hydrolyzed into one molecule of ADP and one phosphate ion.
• What effect does the hydrolysis of ATP have on the transport protein?


  It causes the transport protein to change shape, forcing the first particle across the membrane.
• What occurs after the first particle is transported across the membrane?

  The shape change reveals a second binding site in the protein for the second particle.
• What triggers the release of the phosphate ion during co-transport?


  The binding of the second particle triggers the release of the phosphate ion.
• What happens to the transport protein after the phosphate ion is released?


  The protein reverts to its original shape, forcing the second particle across the membrane in the opposite direction.
• What are the main steps in the co-transport of two particles across a membrane?


  1. Transport protein binds the first particle and ATP.
  2. ATP is hydrolyzed to ADP and a phosphate ion.
  3. Transport protein changes shape, moving the first particle across the membrane.
  4. Second binding site is revealed for the second particle.
  5. Phosphate ion is released, reverting the protein to its original shape.
  6. Second particle is forced across the membrane in the opposite direction.
• transport proteins in passive transport


  Passive transport via facilitated diffusion uses both channel and carrier proteins.
• transport proteins in active transport


  Active transport only uses carrier proteins.
• What is the structure of the cell membrane primarily composed of?

  Phospholipid bilayer
• How does the nonpolar core of the phospholipid bilayer affect substance movement?


  It permits small, non-polar, uncharged, and hydrophobic particles to cross the membrane by simple diffusion
• What types of particles are blocked from diffusing through the phospholipid bilayer?


  Large, polar, charged, and hydrophilic particles