Metabolism Explained: How Cells Convert Food Into Energy and Building Blocks

Catabolism: Breaking Down for Energy

Catabolic pathways disassemble large, complex molecules into smaller, simpler ones, releasing energy in the process. The three major fuel molecules, carbohydrates, fats, and proteins, are all broken down through catabolic pathways that converge on a common intermediate: acetyl-CoA. This convergence allows cells to extract energy from diverse food sources using a shared set of downstream reactions.

Carbohydrates are first broken down into monosaccharides like glucose. Glucose enters glycolysis, a ten-step pathway in the cytoplasm that produces pyruvate, a small amount of ATP, and the electron carrier NADH. Pyruvate is then converted to acetyl-CoA and fed into the citric acid cycle in the mitochondrial matrix.

Fats are hydrolyzed into glycerol and fatty acids. The fatty acids undergo beta-oxidation, a repetitive cycle that removes two-carbon units as acetyl-CoA while generating NADH and FADH2. Because fatty acid chains are long and highly reduced, fat yields roughly 9 kilocalories per gram, more than double the 4 kilocalories per gram from carbohydrates or protein.

Proteins are broken down into individual amino acids by proteases. The amino group is removed (a process called deamination) and converted to urea for excretion. The remaining carbon skeleton enters the catabolic pathways at various points, depending on the specific amino acid. Some carbon skeletons become pyruvate, others become acetyl-CoA, and still others enter the citric acid cycle directly as intermediates.

Anabolism: Building Up From Simple Parts

Anabolic pathways use energy, primarily from ATP, to construct complex molecules from simpler precursors. These pathways are essential for growth, tissue repair, and the production of signaling molecules, enzymes, and structural components.

Gluconeogenesis, the synthesis of glucose from non-carbohydrate precursors such as lactate, amino acids, and glycerol, is a key anabolic pathway in the liver. It ensures a steady supply of blood glucose during fasting or intense exercise. Although gluconeogenesis partially reverses glycolysis, it uses different enzymes at the three irreversible steps, allowing the two pathways to be regulated independently.

Fatty acid synthesis occurs primarily in the liver and adipose tissue. It builds long-chain fatty acids from two-carbon acetyl-CoA units using the enzyme fatty acid synthase. The process requires NADPH as a reducing agent, supplied mainly by the pentose phosphate pathway. The newly synthesized fatty acids are esterified with glycerol to form triglycerides for energy storage.

Amino acid synthesis produces the nonessential amino acids from metabolic intermediates. Essential amino acids, which cannot be synthesized by human cells, must be obtained from the diet. Amino acids serve as building blocks for protein synthesis at the ribosome, where mRNA directs the assembly of polypeptide chains.

ATP: The Energy Currency

Adenosine triphosphate (ATP) is the primary energy carrier in all living cells. It consists of adenine, ribose, and three phosphate groups. The bonds between the phosphate groups, particularly the terminal one, are often called "high-energy" bonds because their hydrolysis releases a significant amount of free energy (approximately 7.3 kcal/mol under standard conditions, and often more under cellular conditions).

Cells generate ATP through two main mechanisms. Substrate-level phosphorylation transfers a phosphate group directly from a substrate molecule to ADP, producing ATP. This occurs in glycolysis and the citric acid cycle. Oxidative phosphorylation uses the energy from electron transfer through the mitochondrial electron transport chain to pump protons across the inner mitochondrial membrane, creating a proton gradient that drives ATP synthase. Oxidative phosphorylation accounts for the vast majority of ATP produced during aerobic respiration.

A typical human cell turns over its entire ATP supply roughly every one to two minutes. The adult human body produces and consumes approximately 40 to 70 kilograms of ATP per day, although the total amount of ATP present at any moment is only about 250 grams. This rapid turnover reflects the constant demand for energy to power cellular processes such as ion transport, biosynthesis, and muscle contraction.

Electron Carriers: NADH and FADH2

Energy from catabolic reactions is not transferred directly to ATP. Instead, it is first captured by electron carriers, primarily NAD+ and FAD, which accept electrons and hydrogen atoms from substrates during oxidation reactions. The reduced forms, NADH and FADH2, then carry these high-energy electrons to the electron transport chain, where the energy is used to drive ATP synthesis.

NAD+ (nicotinamide adenine dinucleotide) accepts two electrons and one proton, becoming NADH. It participates in hundreds of metabolic reactions, including multiple steps in glycolysis and the citric acid cycle. FAD (flavin adenine dinucleotide) accepts two electrons and two protons, becoming FADH2. It is the electron acceptor in the succinate dehydrogenase reaction of the citric acid cycle and in the first step of fatty acid beta-oxidation.

NADPH, a close relative of NADH with an extra phosphate group, serves a different purpose. Rather than donating electrons to the electron transport chain for ATP production, NADPH provides reducing power for anabolic reactions, including fatty acid synthesis, cholesterol synthesis, and the detoxification of reactive oxygen species by glutathione reductase.

Metabolic Regulation

Cells regulate metabolism at multiple levels to match energy production with energy demand and to coordinate catabolic and anabolic activities.

Allosteric regulation provides rapid, moment-to-moment control. Key enzymes at branch points and committed steps in metabolic pathways are activated or inhibited by metabolites that reflect the cell's energy status. High ATP levels inhibit catabolic enzymes (signaling that energy is abundant) and activate anabolic enzymes. High AMP levels signal energy deficit and activate catabolic pathways.

Hormonal regulation coordinates metabolism across tissues and organs. Insulin, released by pancreatic beta cells when blood glucose rises, stimulates glucose uptake, glycolysis, glycogen synthesis, and fat synthesis. Glucagon, released by alpha cells when blood glucose falls, stimulates glycogen breakdown and gluconeogenesis in the liver. Epinephrine, released during stress or exercise, rapidly mobilizes glucose from glycogen and fatty acids from adipose tissue to fuel the fight-or-flight response.

Gene expression provides longer-term control by adjusting the amounts of metabolic enzymes. When a cell needs more of a particular enzyme, it increases transcription of the gene encoding that enzyme. This level of regulation operates over hours to days and is important for adaptations to changing nutritional states, such as the switch from fed-state metabolism to fasting metabolism.

Compartmentalization in eukaryotic cells separates competing pathways. Fatty acid oxidation occurs in the mitochondrial matrix, while fatty acid synthesis occurs in the cytoplasm. This spatial separation allows both pathways to operate simultaneously under different regulatory controls, using different enzymes and different electron carriers (NADH for oxidation, NADPH for synthesis).

Metabolic Disorders

When metabolic regulation fails, disease results. Diabetes mellitus is perhaps the most prevalent metabolic disorder. In type 1 diabetes, the immune system destroys insulin-producing beta cells, eliminating the signal for glucose uptake. In type 2 diabetes, cells become resistant to insulin, leading to chronically elevated blood glucose and a cascade of complications affecting the cardiovascular system, kidneys, nerves, and eyes.

Inborn errors of metabolism are genetic conditions in which a specific metabolic enzyme is deficient or absent. Phenylketonuria (PKU) results from a deficiency in phenylalanine hydroxylase, causing the amino acid phenylalanine to accumulate to toxic levels. Gaucher disease results from a deficiency in glucocerebrosidase, leading to the accumulation of glucocerebroside in macrophages. Over 500 inborn errors of metabolism have been identified, and many can be managed through dietary modifications or enzyme replacement therapy.