Corn, nuts, and peas all contain rich stores of starch and fat that provide the young plant embryo in the seed with energy and building blocks for biosynthesis. (Courtesy of the John innes foundation.) and ATP in mitochondria. Although plants produce abundant amounts of both ATP and NADPH in their chloroplasts, this organelle is isolated from the rest of its plant cell by a membrane that is impermeable to both types of activated carrier molecules. Moreover, the plant contains many cells—such as those in the roots— that lack chloroplasts and therefore cannot produce their own sugars. Thus, sugars are exported from chloroplasts to the mitochondria present in all cells of the plant. Most of the ATP needed for general plant cell metabolism is synthesized in these mitochondria, using exactly the same pathways for the oxidative breakdown of sugars as in nonphotosynthetic organisms; this ATP is then passed to the rest of the cell (see Figure 14–42).
During periods of excess photosynthetic capacity during the day, chloroplasts convert some of the sugars that they make into fats and into starch, a polymer of glucose analogous to the glycogen of animals. The fats in plants are triacyl-glycerols (triglycerides), just like the fats in animals, and differ only in the types of fatty acids that predominate. Fat and starch are both stored inside the chloroplast until needed for energy-yielding oxidation during periods of darkness (see Figure 2–51C).
The embryos inside plant seeds must live on stored sources of energy for a prolonged period, until they germinate and produce leaves that can harvest the energy in sunlight. For this reason plant seeds often contain especially large amounts of fats and starch—which makes them a major food source for animals, including ourselves (Figure 2–53).
After a meal, most of the energy that an animal needs is derived from sugars obtained from food. Excess sugars, if any, are used to replenish depleted glycogen stores, or to synthesize fats as a food store. But soon the fat stored in adipose tissue is called into play, and by the morning after an overnight fast, fatty acid oxidation generates most of the ATP we need.
Low glucose levels in the blood trigger the breakdown of fats for energy production. As illustrated in Figure 2–54, the triacylglycerols stored in fat droplets in adipocytes are hydrolyzed to produce fatty acids and glycerol, and the fatty acids released are transferred to cells in the body through the bloodstream. While animals readily convert sugars to fats, they cannot convert fatty acids to sugars. Instead, the fatty acids are oxidized directly.
sugars and fats are Both Degraded to acetyl Coa in mitochondria
In aerobic metabolism, the pyruvate that was produced by glycolysis from sugars in the cytosol is transported into the mitochondria of eukaryotic cells. There, it is rapidly decarboxylated by a giant complex of three enzymes, called the pyruvate dehydrogenase complex. The products of pyruvate decarboxylation are a molecule of CO2 (a waste product), a molecule of NADH, and acetyl CoA (see Panel 2–9).
The fatty acids imported from the bloodstream are moved into mitochondria, where all of their oxidation takes place (Figure 2–55). Each molecule of fatty acid (as the activated molecule fatty acyl CoA) is broken down completely by a cycle of reactions that trims two carbons at a time from its carboxyl end, generating one molecule of acetyl CoA for each turn of the cycle. A molecule of NADH and a molecule of FADH2 are also produced in this process (Figure 2–56).