Download Summary for Chapter 7 – Metabolism: Transformations

Summary for Chapter 7 – Metabolism: Transformations and Interactions
During digestion the energy-yielding nutrients—carbohydrates, lipids, and proteins—are broken down to glucose (and
other monosaccharides), glycerol, fatty acids, and amino acids. Aided by enzymes and coenzymes, the cells use
these products of digestion to build more complex compounds (anabolism) or break them down further to release
energy (catabolism). High-energy compounds such as ATP may capture the energy released during catabolism.
The breakdown of glucose to energy begins with glycolysis, a pathway that produces pyruvate. Keep in mind that
glucose can be synthesized only from pyruvate or compounds earlier in the pathway. Pyruvate may be converted to
lactate anaerobically or to acetyl CoA aerobically. Once the commitment to acetyl CoA is made, glucose is not
retrievable; acetyl CoA cannot go back to glucose. Figure 7-9 summarizes the breakdown of glucose.
The body can convert the small glycerol portion of a triglyceride to either pyruvate (and then glucose) or acetyl CoA.
The fatty acids of a triglyceride, on the other hand, cannot make glucose, but they can provide abundant acetyl CoA.
Acetyl CoA may then enter the TCA cycle to release energy or combine with other molecules of acetyl CoA to make
body fat.
The body can use some amino acids to produce glucose, whereas others can be used either to generate energy or to
make fat. Before an amino acid enters any of these metabolic pathways, its nitrogen-containing amino group must be
removed through deamination. Deamination, which produces ammonia (NH 3), may be used to make nonessential
amino acids and other nitrogen-containing compounds; the rest is cleared from the body via urea synthesis in the
liver and excretion via the kidneys.
After a balanced meal, the body handles the nutrients as follows. The digestion of carbohydrate yields glucose (and
other monosaccharides); some is stored as glycogen, and some is broken down to pyruvate and acetyl CoA to
provide energy. The acetyl CoA can then enter the TCA cycle and electron transport chain to provide more energy.
The digestion of fat yields glycerol and fatty acids; some are reassembled and stored as fat, and others are broken
down to acetyl CoA, which can enter the TCA cycle and electron transport chain to provide energy. The digestion of
protein yields amino acids, most of which are used to build body protein or other nitrogen-containing compounds, but
some amino acids may be broken down through the same pathways as glucose to provide energy. Other amino acids
enter directly into the TCA cycle, and these, too, can be broken down to yield energy.
If energy intake exceeds the body’s energy needs, the result will be weight gain—regardless of whether the excess
intake is from protein, carbohydrate, or fat. The difference is that the body is much more efficient at storing energy
when the excess derives from dietary fat.
When fasting, the body makes a number of adaptations: increasing the breakdown of fat to provide energy for most of
the cells, using glycerol and amino acids to make glucose for the red blood cells and central nervous system,
producing ketones to fuel the brain, suppressing the appetite, and slowing metabolism. All of these measures
conserve energy and minimize losses.