2B.1 - cell membranes

• what is the function of the cell surface membrane?

  • acts as a barrier between the environment.• controls what exits and enters the cell.
• what is the function of organelle membrane?

  surrounds organelles, dividing the cell into components.
• describe the structure of a phospholipid.

  a glycerol backbone with a hydrophilic phosphate head and two hydrophobic fatty acids (one saturated and one unsaturated).
• what does hydrophilic mean?

  it attracts water.
• what does hydrophobic mean?

  it repels water.
• what is the main function of the phospholipid bilayer?

  it allows the membrane to be partially permeable.
• what molecules can pass through the lipid bilayer?

  small and non polar molecules.
• why can some molecules not pass through the phospholipid bilayer?

  large and charged (water soluble) molecules cannot pass through the bilayer as they are repelled by the hydrophobic tails and pass through extremely slowly.
• is the phospholipid bilayer flexible?

  yes.
• what is an extrinsic protein? give an example.

  a protein found on the surface of the cell membrane e.g. receptors.
• what are intrinsic proteins? give an example.

  proteins that span from one side of the membrane to the other e.g. channel proteins.
• what is an ion channel?

  a transport protein with a polar pore through which ions can pass.
• what are transport proteins?

  they transport specific molecules.
• what are glycoproteins?

  polysaccharide chain that extends outwards from the protein.
• what are receptors?

  binding site for hormones.
• what is a glycolipid?

  lipid with polysaccharide chain attached.
• what is cholesterol?

  a type of lipid present in all membranes. (except bacteria)
• what is the function of cholesterol in the membrane?

  fits between phospholipids and maintains stability.
• why is too much cholesterol bad?

  cells need some flexibility e.g. red blood cells to maintain biconcave shape.
• what is the function of cholesterol in animal cells?

  maintains the shape as they have no cell wall.
• whats the function glycolipids and glycoproteins?

  the attached carbohydrate enables them to act as receptor molecules. these recognition sites help cells attach to eachother.
• what model is used to describe the arrangement of molecules in the membrane?

  fluid mosaic model.
• describe the fluid mosaic model.

  the membrane is a fluid structure where the individual phospholipids can move in relation to eachother, and a mosaic of various (sized and shaped) proteins embedded/attached thoughout the bilayer.
• how does temperature affect membrane permeability?

  changes the kinetic energy of the phospholipids in the bilayer, the different speeds they can move affect structure and permeability.
• what is membrane permeability like below zero degrees?

  • little energy so phospholipids don't move much.• they are packed closely so the structure is rigid.• channel proteins are denatured.• permeability is increased.
• what is the membrane permeability like between 0 and 45 degrees?

  • some energy, so the bilayer can move. • not packed as closely. • increased permeability.
• what is the membrane permeability like above 45 degrees?

  • phospholipids break.• proteins denature. • more pressure. • membrane becomes permeable.
• How has the cell membrane been specialised for rapid transport.

  Folded membranes to increase surface area

  more protein channels for facilitated diffusion