Download Chapter 8 Summary

Chapter 8 Summary
Metabolic pathways consist of many interrelated, enzyme-catalyzed chemical reactions. These pathways
can be categorized as either catabolic or anabolic. Anabolic pathways promote the synthesis of new
compounds and energy storage, whereas catabolic pathways promote the mobilization of stored energy
and the breakdown of energy-yielding nutrients. Chemical reactions are catalyzed by enzymes, some
requiring cofactors or coenzymes to function. A cofactor is an inorganic substance that is part of the
enzyme’s structure. Coenzymes are organic molecules that assist enzymes in redox reactions by
transferring hydrogen ions (H+) and electrons (e-) to and from molecules. Coenzymes have oxidized
(NAD+, FAD, and NADP+) and reduced (NADH + H+, FADH2, and NADPH + H+) forms. Enzymes
involved in metabolic pathways are regulated primarily by hormones. The hormone insulin promotes
energy storage, whereas the hormone glucagon promotes energy mobilization. The hormones cortisol
and epinephrine also promote catabolic pathways, being released in response to stress.
Cells rely on the energy contained in the chemical bonds of ATP. Some ATP is generated by substrate
phosphorylation, a process that adds a phosphate group (P i) directly to ADP. However, most ATP is
synthesized by oxidative phosphorylation, which involves a series of chemical reactions that make up the
electron transport chain. When NADH + H+ and FADH2 enter the electron transport chain, their electrons
and hydrogen ions are removed. The electrons are passed along protein complexes, and the energy
released is used to pump the hydrogen ions (H+) out of the mitochondrial matrix. The movement of
hydrogen ions back into the mitochondrial matrix releases energy that is used by the enzyme ATP
synthase to attach a phosphate group to ADP, generating ATP. Last, iron-containing protein complexes
called cytochromes reunite the electrons (e-) and hydrogen ions (H+), which in turn combine with oxygen
to form water.
Liver and muscle cells break down glycogen into glucose by a process called glycogenolysis. Glucose
catabolism begins with glycolysis, an anaerobic pathway that converts glucose to pyruvate. Oxygen
availability determines if pyruvate is converted to acetyl-CoA or lactate. If oxygen is available, acetylCoA is formed and enters the citric acid cycle, resulting in the formation of NADH + H + and FADH2.
These coenzymes can enter the electron transport chain and drive ATP formation via oxidative
phosphorylation. Protein is broken down to amino acids by proteolysis. For amino acids to be used as an
energy source, the nitrogen-containing amino group is removed via transamination and deamination.
The remaining structure (-ketoacid) can enter the citric acid cycle, ultimately generating ATP via the
electron transport chain. Lipid catabolism begins with lipolysis, releasing fatty acids and glycerol. Fatty
acids are oxidized via -oxidation and the citric acid cycle to form NADH + H+ and FADH2, which then
enter the electron transport chain to generate additional ATP.
Anabolic pathways play important roles in storing excess energy and in synthesizing energy-yielding
molecules when glucose availability is limited. The hormone insulin stimulates liver and muscle tissues
to store excess glucose as glycogen (glycogenesis). Insulin also promotes the conversion of glucose and
amino acids to fatty acids and the subsequent production of triglyceride in adipose and liver tissues
(lipogenesis). Thus anabolic pathways are important during periods of excess energy availability. During
starvation, anabolic pathways are used to synthesize glucose from noncarbohydrate sources
(gluconeogenesis). Substances used for gluconeogenesis include glucogenic amino acids, lactate, and
glycerol. However, high rates of gluconeogenesis deplete the amount of oxaloacetate. When this occurs,
acetyl-CoA cannot participate in the citric acid cycle. Rather, acetyl-CoA is diverted to ketone production.
Some amino acids are converted to ketones as well. Tissues such as muscle, brain, and kidney have
enzymes that allow them to use ketones for ATP production.
Energy metabolism pathways are responsive to intermittent states of feeding and fasting, called the
absorptive state, postabsorptive state, acute starvation, and prolonged starvation. Most of the major
anabolic pathways operate during the absorptive state, including those promoting protein, triglyceride,
and glycogen synthesis. During this time, glucose is the major source of energy for all tissues. The
postabsorptive state, which is the period 4 to 24 hours after the last intake of food, relies heavily on
energy supplied by the breakdown of stored energy reserves, especially glycogen. As the body enters
acute starvation, defined as the first five days after the postabsorptive state, the body begins to rely more
on the mobilization of adipose reserves for ATP production, while producing sufficient glucose via
gluconeogenesis. The body enters the nutritional state of prolonged starvation when a person is deprived
of food for more than one week. During this time, the body must preserve lean tissue from its
gluconeogenic fate by increasing the use of ketones as an energy source. This is accomplished via
ketogenic pathways. However, prolonged starvation can eventually result in the extensive use of muscle
for glucose and, ultimately ATP production. If refeeding is not resumed, the consequences of starvation
can be severe.