Figure 2–50 Phosphate bonds have different energies. examples of different types of phosphate bonds with their sites of hydrolysis are shown in the molecules depicted on the left. Those starting with a gray carbon atom show only part of a molecule. examples of molecules containing such bonds are given on the right, with the standard free-energy change for hydrolysis in kilojoules. The transfer of a phosphate group from one molecule to another is energetically favorable if the free-energy change (ΔG) for hydrolysis of the phosphate bond of the first molecule is more negative than that for hydrolysis of the phosphate bond in the second. Thus, under standard conditions, a phosphate group is readily transferred from 1,3-bisphosphoglycerate to aDp to form aTp. (standard conditions often do not pertain to living cells, where the relative concentrations of reactants and products will influence the actual change in free energy.) The hydrolysis reaction can be viewed as the transfer of the phosphate group to water.
subunits in the large branched polysaccharide glycogen, which is present as small granules in the cytoplasm of many cells, including liver and muscle. The synthesis and degradation of glycogen are rapidly regulated according to need. When cells need more ATP than they can generate from the food molecules taken in from the bloodstream, they break down glycogen in a reaction that produces glucose 1-phosphate, which is rapidly converted to glucose 6-phosphate for glycolysis (Figure 2–52).
Quantitatively, fat is far more important than glycogen as an energy store for animals, presumably because it provides for more efficient storage. The oxidation of a gram of fat releases about twice as much energy as the oxidation of a gram of glycogen. Moreover, glycogen differs from fat in binding a great deal of water, producing a sixfold difference in the actual mass of glycogen required to store the same amount of energy as fat. An average adult human stores enough glycogen
Figure 2–51 The storage of sugars and fats in animal and plant cells. (a) The structures of starch and glycogen, the storage form of sugars in plants and animals, respectively. Both are storage polymers of the sugar glucose and differ only in the frequency of branch points. There are many more branches in glycogen than in starch. (B) an electron micrograph of glycogen granules in the cytoplasm of a liver cell. (C) a thin section of a chloroplast from a plant cell, showing the starch granules and lipid (fat droplets) that have accumulated as a result of the biosyntheses occurring there. (D) fat droplets (stained red) beginning to accumulate in developing fat cells of an animal. (B, courtesy of Robert fletterick and Daniel s. friend; C, courtesy of K. plaskitt; D, courtesy of Ronald m. evans and peter Totonoz.) for only about a day of normal activities, but enough fat to last for nearly a month. If our main fuel reservoir had to be carried as glycogen instead of fat, body weight would increase by an average of about 60 pounds.
The sugar and ATP needed by plant cells are largely produced in separate organelles: sugars in chloroplasts (the organelles specialized for photosynthesis), 2 Figure 2–52 How sugars are produced from glycogen. Glucose subunits are released from glycogen by the enzyme glycogen phosphorylase. This produces glucose 1-phosphate, which is rapidly converted to glucose 6-phosphate for OH glycolysis.
Figure 2–53 Some plant seeds that serve as important foods for humans.