1. Sphingomyelin and phosphatidyl choline are both membrane lipids, and have the amphipathic nature this entails. Both have a polar head and two hydrophobic tails; however, in sphingomyelin, part of the head group and one of the tails are derived from the same molecule (sphingosine), whereas in phosphatidyl choline, the head piece is partly derived from glycerol and both tails are fatty acids (in sphingomyelin only one of the tails is a fatty acid). The polarity in both molecules is given by an phosphocholine group esterified to an alcohol residue. In phosphatidyl choline the tails are also joined to the head piece by ester linkages, whereas the 'separate' tail of sphingomyelin is linked by an amide group.
  2. Proteins can be and phospholipids are amphipathic (parts of a single molecule are hydrophobic and hydrophilic). This allows them to auto-assemble into micelles, monolayers, bilayers and vesicles.
  3. The fluid mosaic explains
    • The composition of membranes.
    • How the membrane appears under EM.
    • How membranes can have pores.
    • The migration of lipids in the bilayer.
    • Asymmetrical distribution of lipids across the membrane.
    • Fluidity of the membrane and lateral diffusion.
    • How the membrane interacts with the ECM and cytoskeleton.
  4. The major biochemical differences between the membrane lipids of the three domains are
    • Eukarya: phosphatidyl choline, cholesterol, sphingolipids (including glycolipids).
    • Bacteria: phosphatidyl ethanolamine.
    • Archaea: phytanyl ethers, diphytanyl ethers.
  5. They are all poikilothermic, i.e. they are cold-'blooded', and have to maintain membrane fluidity using unsaturated lipids. Mammals have high internal body temperatures, so have the opposite problem – of keeping their membranes less fluid, which is why they are full of saturated fat and cholesterol.
  6. The erythrocyte cell membrane stimulates an immune response because erythrocyte membranes contain glycolipids with large oligosaccharide head groups. A and B antigens stimulate an immune response, whereas the H antigen (present in O) does not.
  7. Answers to table:

    Transporter

    Uni-, sym- or antiport?

    Carrier or channel; any gating?

    Form of energy required?

    Glucose/Na+ pump

    Symport

    Carrier

    Na+ gradient

    AChE receptor

    [Uniport] (really just a pore, as there is no active pumping)

    Ligand gated ion channel

    None

    Fo/F1 proton ATPase

    Uniport

    Carrier

    H+ gradient or ATP

    Na+/K+ ATPase

    Antiport or multifunctional uniport depending how you look at it, and how you define an antiport

    Carrier

    ATP