Cell membranes

• Why are cell membranes described as having a 'fluid mosaic structure'?

  fluid - the phospholipids are constantly moving

  mosaic- there are many different molecules within the bilayer adding structure
• what is the role of the phospholipid bilayer in a cell membrane?

  due to its hydrophilic head and hydrophobic tails the phospholipids form a bilayer with the heads extracellular.

  this creates a hydrophobic core that does not allow water soluble molecules to pass through acting as a barrier to these molecules.

  it allows lipid soluble substances tp enter and leave the cell
• What is a glycoprotein and what is its function?

  1) An extrinsic protein which has a carbohydrate chain attached to it

  2) Acts as a recognition site for other cells; also helps cells to attach to one another and form tissues
• What is a glycolipid and what is its function?


  1) A lipid which has a carbohydrate chain attached to it

  2) Acts as a recognition site; helps cells to attach to one another and form tissues; helps maintain membrane stability
• what is the function of a channel protein?

  to move large molecules across a cell membrane by allowing the molecule to attach itself to it then the tertiary structure of the protein changes so the protein changes shape and pushed the molecule across the cell membrane.
• What is the function of a carrier protein?


  allows charged particles (e.g water soluble ions) to pass through a cell membrane by acting as a pore for ions that are specific to the protein
• What is the function of cholesterol in a cell membrane?

  - cholesterol molecules fit in-between and bind to the phospholipid tails causing them to pack more tightly together - this restricts the movement of the bilayer adding RIGIDITY

  -prevents leakage of water and ions from the cell

  - helps to maintain he shape of animal cells - important for cells that aren't supported
• Describe simple diffusion


  -Passive process

  -the net movement of chemical down its concentration gradient

  - can happen as long as the substance can move freely across a cell membrane
• Describe facilitated diffusion


  passive process

  carrrier proteins an channel proteins facilitate the movement of large and charged molecules down their concentration gradient.
• describe osmosis


  passive process

  - the diffusion of water molecules from an area of higher water potential to an area of lower water potential
• What is water potential?

  Water potential is the likelihood of water molecules to diffuse out of or into a solution.
• what substance has the highest water potential

  pure water
• how do you describe to solutions with the same water potential

  isotonic
• Describe active transport


  very similar to faccilitated diffusion

  -molecules move across cell membranes via channel and carrier proteins but from a low concentration to a high concentration

  - requires energy
• what are the 2 main differences between facilitated diffusion and active transport

  -AT moves substances AGAINST the concentration gradient, FD moves substances WITH the concentration gradient

  -AT requires energy, FD is passive
• What is the source of energy for active transport

  ATP produced during respiration undergoes hydrolysis to produce ADP and a phosphate ion, this releases the energy used for active transport