Cell membranes FD and AT

• How does the phospholipid bilayer affect entry and exit of substances into and out of the cell?

  - forms a barrier that prevents water soluble substances, polar substances and large substances from moving across the membrane

  - but allows the movement of lipid soluble substances, non polar substances and small substances across the membrane

  - therefore the membrane is semi permeable
• Describe the fluid mosaic model of a cell membrane

  - membrane is made from a phospholipid bilayer

  - hydrophilic heads are on the outside and hydrophobic tails are on the inside

  - phospholipids are able to move within a layer giving fluidity

  - there is cholesterol in the membrane that binds the phospholipids together to reduce the fluidity

  - there are intrinsic proteins such as carriers or channels that pass all the way through the bilayer

  - extrinsic proteins are on the surface of the membrane or pass part of the way through it

  - the pattern of the proteins form the "mosaic structure" and this pattern can change

  - there are also glycoproteins and glycolipids in the membrane
• Describe simple diffusion

  - net movement from a higher to lower concentration

  - molecules can pass between phospholipids in the bilayer

  - it is a passive process (no energy is required)
• Describe facilitated diffusion

  - net movement from a higher to lower concentration

  - uses carrier or channel proteins

  - proteins have a specific tertiary structure so they have a complementary shaped receptor site to the molecule

  - it is a passive process (no energy is required)
• Describe active transport

  - movement from a lower to higher concentration

  - uses carrier proteins

  - proteins have a specific tertiary structure so they have a complementary shaped receptor site to the molecule

  - ATP (energy) is required
• Explain how three features of a plasma membrane adapt it for its functions

  - phospholipid bilayer acts as a barrier to water soluble substances and larger molecules however allows lipid soluble or non polar substances to pass through

  - there is cholesterol in the membrane that is a further barrier to water soluble substances. It binds the tails of the phospholipids together and decreases fluidity of the membrane which gives it greater stability.

  - intrinsic proteins pass all the way through the membrane such as channels or carriers. They transport large or charged substances across the membrane. They have a specific tertiary structure so only transport substances that have a complimentary shape to the receptor site. Carriers enable active transport and channels and other carriers enable facilitated diffusion to take place
• What is the similarity between facilitated diffusion and active transport?

  Both involve carrier (intrinsic) proteins
• What is the differences between facilitated diffusion and active transport?

  - AT requires energy / ATP whereas FD is passive

  - AT is from lower to higher concentration whereas FD is from higher to lower concentration
• How are some cells specialised for absorption?

  - folded membrane so larger surface area for absorption

  - large number of carrier/channel proteins

  - large number of mitochondria to release energy for active transport
• What are 2 similarities between diffusion and osmosis?

  - both from high concentration to low concentration

  - both passive processes