Lipids are structural parts of cell membranes and various complex functional compounds (such as hormones). They are stored as droplets of triglycerides (fats) in adipose tissues. These triglycerides are a valuable source of energy. They are particularly important sources for various internal organs such as the heart. They are used for prolonged activity in skeletal muscle as, for example, in migrating birds. Fats are also key energy sources for hibernating mammals. These are animals that go into a state of deep sleep during an entire season, or part of one.
As in carbohydrate (sugar) oxidation, the final oxidation of fats takes place in the mitochondria. But first, the fat must be mobilized. It is broken down by enzymes in the adipose tissue to release free fatty acids. Adipose tissue is connective tissue containing animal fat. The fatty acids are then transferred by the blood to the cells of tissue that require energy. In the cells, before entering the mitochondria, the fatty acids are linked with coenzyme A (CoA). The resulting compound then enters the mitochondria via a complex process. This involves the intervention of another molecule-earn itine.
The conversion of fatty acids to CoA derivatives requires the input of energy as ATP. But this investment is soon recovered. The reaction takes place in a repetition of a four-step sequence. This is a process called beta-oxidation, or the fatty acid spiral. The net result of each sequence in the spiral is removal of the two end carbon atoms on the fatty acid. This leaves the fatty acid two carbons shorter, ready for another sequence in the spiral. The fatty acid then joins the citric acid cycle. The process is repeated until the final fragment left is acetyl-CoA. In each sequence of the fatty acid spiral, four hydrogen atoms are removed. Oxidation of the fatty acid yields five molecules of ATP.
The process is varied slightly if the acid contains an odd number of carbon atoms or is partly unsaturated (containing carbon-carbon double bonds). But in all cases, it is a very good energy source. Not only are five ATP molecules produced for each two-carbon fragment, but each fragment is itself oxidized to form 12 more ATP's. For example, in the breakdown of the 16-carbon palmitic acid, a total of 129 ATP molecules are produced.
The adaptability of the oxidation pathways described earlier is especially important in the breakdown of the third energy source, protein. The structures of the amino acids making up the protein vary to the extent that the preliminary pathway for each amino acid is different. Some are converted into a compound called acetyl-CoA. Others are converted into intermediates that join at later stages of the citric acid cycle. In each case, the amino group
first part of photosynthesis. The principal stages of these reactions are illustrated in the diagram below (the details are described in the main text). Overall, however, sunlight is trapped by photosynthetic pigments (chiefly chlorophylls). This trapped sunlight is used as the energy source to power a sequence of reactions. In these reactions, water is split into electrons, hydrogen ions, and oxygen. ATP is produced and nicotinamide adenine diphosphate is reduced. The ATP and the reduced phosphate are then utilized in the dark reaction.
on the amino acid has to be removed first. In normal circumstances, protein plays a minor role in total energy production. Except during starvation, it provides (mainly after conversion to glucose) only 10 per cent of the body's needs.