5 Metabolism and Energy Use

Metabolism and energy transfer

Metabolism is the sum of chemical reactions that sustain the body. Catabolism breaks molecules down and captures some of their energy; anabolism uses energy and small molecules to build and maintain tissues.

Carbohydrates, fats, and proteins can all contribute to energy production, but they enter shared pathways at different points. Their energy is transferred mainly to ATP (adenosine triphosphate), the cell’s immediately usable energy currency, and to electron carriers such as $\mathrm{NADH}$ and $\mathrm{FADH_2}$.

The shared route from glucose to ATP

After digestion and absorption, nutrients are processed inside cells. Carbohydrates are converted to simple sugars, fats to fatty acids and glycerol, and proteins to amino acids. Many of these products become acetyl-CoA or intermediates that feed into shared energy pathways.

From glucose to pyruvate

Glycolysis takes place in the cytosol and splits glucose into two molecules of pyruvate. It produces a small amount of ATP and $\mathrm{NADH}$. If oxygen-dependent metabolism cannot keep pace with demand, cells can convert pyruvate to lactate, allowing glycolysis to continue producing ATP.

From pyruvate to acetyl-CoA

When conditions permit, pyruvate enters mitochondria and is converted to acetyl-CoA. This pyruvate oxidation produces $\mathrm{NADH}$ and carbon dioxide ($\mathrm{CO_2}$).

The citric acid cycle and ATP synthesis

In the mitochondrial matrix, acetyl-CoA is oxidized in the citric acid cycle. The cycle releases $\mathrm{CO_2}$ and generates $\mathrm{NADH}$, $\mathrm{FADH_2}$, and a small amount of ATP-equivalent energy.

On the inner mitochondrial membrane, $\mathrm{NADH}$ and $\mathrm{FADH_2}$ deliver electrons to the electron-transport chain. The released energy helps create a proton gradient that drives ATP synthesis. Oxygen accepts electrons at the end of the chain, forming water.

Complete oxidation of one glucose molecule yields roughly $30\text{–}32$ ATP, although the exact total varies with cellular conditions. Glycolysis alone yields a net of two ATP per glucose. Energy conversion is not perfectly efficient, and some energy is released as heat.

Carbohydrate use and glucose storage

Glucose can be used promptly in glycolysis and, when oxygen-dependent metabolism proceeds, in the citric acid cycle and oxidative phosphorylation. When glucose supply exceeds immediate needs, the body can store it as glycogen, especially in the liver and skeletal muscle.

Glycogenesis builds glycogen, while glycogenolysis breaks it down. The liver can release glucose into the blood; muscle glycogen is mainly used by the muscle itself.

When glycogen availability is limited, the liver—and, in some circumstances, the kidneys—can make glucose through gluconeogenesis. This process uses precursors such as lactate, glycerol, and certain amino acids.

Fat and protein as fuels

Most stored fat is in the form of triglycerides. Lipolysis splits triglycerides into fatty acids and glycerol. Fatty acids enter mitochondria and are broken down through beta-oxidation, which removes two-carbon units to form acetyl-CoA while generating $\mathrm{NADH}$ and $\mathrm{FADH_2}$. Acetyl-CoA can then enter the citric acid cycle, and the electron carriers support ATP production. Because fatty acids are highly energy-rich, fat is an important fuel and a concentrated form of stored energy.

During prolonged fasting or very low carbohydrate availability, the liver can convert some acetyl-CoA into ketone bodies. Other tissues can use ketone bodies as fuel. During prolonged fasting, the brain also increases its use of ketones.

Proteins are continually built and broken down to maintain and renew body tissues. Amino acids are primarily used to make proteins and other nitrogen-containing compounds. If amino acids are in excess or needed for energy, their nitrogen-containing amino group is removed or transferred.

The liver converts much of the resulting nitrogen into urea, which travels in the blood to the kidneys for excretion. The remaining carbon skeleton can enter energy pathways, be used to make glucose, or contribute to fat synthesis. Unlike carbohydrates and fats, protein has no dedicated storage depot; using it extensively for energy also diverts amino acids from their other roles.

Pathway integration and hormonal regulation

Metabolic pathways are interconnected rather than isolated. For example, a fatty acid can yield acetyl-CoA, while some amino-acid carbon skeletons can enter the citric acid cycle directly. Intermediates from energy pathways can also be used to build other molecules. The body regulates these competing uses according to tissue needs and nutrient supply.

Hormones help coordinate this regulation. After a meal, insulin generally supports glucose uptake in responsive tissues, glycogen formation, and the use of nutrients for synthesis and storage. Between meals, glucagon helps maintain blood glucose, especially by promoting liver glycogen breakdown and gluconeogenesis; fasting also favors the use of stored fat.

These are broad patterns: regulation differs by tissue and changes with exercise, fasting duration, and other conditions. Overall, carbohydrates, fats, and proteins enter metabolism through different routes but converge on shared pathways. Glycolysis begins glucose breakdown, beta-oxidation processes fatty acids, and amino-acid carbon skeletons can join central energy pathways after nitrogen is removed and disposed of. The citric acid cycle and oxidative phosphorylation capture much of the energy in these fuels as ATP, while hormonal regulation helps balance immediate energy use, storage, synthesis, and repair.