Highlights Lipids II
1. Trans fats arise by partial hydrogenation of polyunsaturated fatty acids in fats. Note that oils and fats are both triacylglycerols. They differ only in that oils are liquid at room temperature and fats are solid. Oils therefore have more unsaturated fatty acids than fats. Trans fats are associated with numerous health problems.
2. Carbohydrates (sugar compounds) have links to components in a lipid bilayer in an orientation-specific direction – all carbohydrates are located on the portion of the lipid bilayer that is on the outside of the cell. This is because carbohydrates serve as identity markers for cells. These identity markers function in the body’s rejection of transplanted organs.
3. Proteins are essential for movement of materials across the lipiid bilayer into and out of cells, as the only compounds that freely move through the lipid bilayer are water, carbon dioxide, carbon monoxide, and oxygen.
4. Membrane proteins facilitate the movement of materials across the lipid bilayer. There are four main kinds of membrane proteins we will consider here. They are a) Integral Membrane Proteins (cross both sides of the lipid bilayer); b) Associated membrane proteins (found near membranes, but not attached to them); c) Peripheral membrane proteins (embedded in a portion of the lipid bilayer); and d) Anchored membrane proteins (linked to something embedded in the lipid bilayer).
5. Integral membrane proteins are involved in transport of molecules across the bilayer, though that is not the only characteristic of these proteins.
6. Materials have to cross the cellular membrane in order for cells to live. Nutrients must be brought into the cell and waste products must be exported. Crossing the lipid bilayer is a problem for most molecules.
7. Mechanisms for moving materials across the lipid bilayer are Passive (= diffusion – no external energy required) or Active (energy required). The driving force for a passive mechanism is concentration (movement from high concentration to low).
8. Active transport mechanisms require energy to move compounds across a membrane because they are taking materials against concentration gradients (low to high). There are a variety of energy sources, but ATP is a common one.
9. The sodium-potassium pump is an active transport system that transports sodium from inside the cell to outside of it and potassium in the opposite direction. .Since both of these movements involve taking a substance from a lower concentration and moving it to a higher concentration, energy is required and this mechanism is an active transport mechanism. It uses ATP.
10. The sodium and potassium gradients are used in nerve transmission.
11. A secondary transport system is an active transport system that uses a high concentration of a molecule outside the cell to “carry” a molecule with it across a membrane and against a concentration gradient. An example is the lactose transport system which uses a proton gradient to carry lactose against a concentration gradient. Proton gradients like this are also used by cells to make ATP, so there is energy that can be used when an electrocheical gradient is formed.
12. The proton gradient can be created by photosynthetic bacteria using light as an energy source. These bacteria contain a membrane protein that contains vitamin A. Vitamin A is light sensitive and isomerizes from cis to trans in the presence of light. This shape change is used by bacterial to “kick” protons out of the cell, creating a proton gradient. Note that protons cannot freely cross the lipid bilayer and must enter through a specialized transport system, such as described in the point above.
13. Proteins on the cell surface can also act as ‘receptors’. An example of a receptor that internalizes a compound is the LDL receptor, which binds to LDLs (cholesterol-containing complexes in the blood) and brings them inside of the cell. The process is called Receptor-Mediated Endocytosis.
14. Liposomes are artificial membranes that allow researchers to move materials across biological lipid bilayers.
15. Vitamin D is involved in calcium and phosphate metabolism in the body. It is derived from cholesterol and its activation requires UV light.
16. Vitamin E acts as a cellular antioxidant and probably protects membrane lipids from oxidation by reactive oxygen species.
17. Vitamin K is involved in the blood coagulation process.
18. Vitamin A is involved in vision thanks to its light sensitivity and a form of vitamin A is also involved in cellular differentiation.
19. Arachidonic acid is a 20 carbon fatty acid (eicosanoid) with 4 double bonds. It is a precursor of the prostaglandins. Prostaglandins are molecules that are involved in pain and swelling, among other phenomena in the body. Conversion of arachidonic acid to prostaglandins requires action of an enzyme called PGH Synthase, Prostaglandin Synthase, or Cyclooxygenase. Inhibition of cyclooxygenase is a strategy for a pain reliever. An example is aspirin and another is ibuprofen. Both inhibit the action of cyclooxygenase. Some compounds, such as Vioxx inhibited cyclooxygenase, but were found to have side effects that were undesirable.