🧪 Biochemistry intermediate Lesson 3 of 3 4 min read

Metabolism: How Cells Make and Use Energy

An overview of anabolism and catabolism, ATP as the cell's energy currency, and how glycolysis, the Krebs cycle, and the electron transport chain extract energy from glucose.

Reading level

What you'll learn

  • Distinguish anabolism from catabolism and give an example of each.
  • Explain why ATP is described as the cell's energy currency.
  • Outline the three stages of aerobic glucose breakdown and where each occurs.
  • Compare aerobic respiration with anaerobic fermentation in terms of oxygen use and ATP yield.

Overview

Metabolism is the entire network of chemical reactions that keeps a cell alive. It has two directions. Anabolism builds larger molecules from smaller ones and requires an energy input, such as linking amino acids into a protein. Catabolism breaks larger molecules into smaller ones and releases energy, such as digesting food. These two are coupled: catabolism releases energy that anabolism uses. The molecule that connects them is ATP.

ATP: The Energy Currency

ATP (adenosine triphosphate) is how cells carry energy in a form ready for immediate use. It consists of adenosine plus a chain of three phosphate groups. The bond to the last phosphate stores energy; when it is broken, ATP becomes ADP (adenosine diphosphate) plus a free phosphate, releasing energy that powers muscle contraction, nerve signaling, and building new molecules. Cells then use energy from food to reattach the phosphate, recharging ADP back into ATP. This cycle turns over constantly, which is why ATP is called a currency rather than a savings account: it is spent and replaced continuously.

Extracting Energy from Glucose

Cells commonly break down glucose to make ATP. Full aerobic respiration happens in three stages.

1. Glycolysis

Glycolysis takes place in the cytoplasm and does not require oxygen. It splits one six-carbon glucose molecule into two three-carbon pyruvate molecules. It produces a small net gain of 2 ATP and loads two molecules of the electron carrier NADH.

2. The Krebs Cycle

If oxygen is present, pyruvate enters the mitochondrion. There it is converted to a molecule called acetyl-CoA, which feeds the Krebs cycle (also called the citric acid cycle) in the mitochondrial matrix. The cycle releases carbon dioxide as waste and captures high-energy electrons onto the carriers NADH and FADH2, along with a small amount of ATP.

3. The Electron Transport Chain

The loaded carriers deliver their electrons to the electron transport chain, a set of proteins in the inner mitochondrial membrane. As electrons pass down the chain, they pump hydrogen ions across the membrane, and the ions flow back through an enzyme that forges the bulk of the cell’s ATP. The final electron acceptor is oxygen, which combines with hydrogen to form water. This is why we breathe: oxygen keeps the chain running.

StageLocationOxygen needed?Main ATP contribution
GlycolysisCytoplasmNoSmall (net 2 ATP)
Krebs cycleMitochondrial matrixYes (indirectly)Small, plus loaded carriers
Electron transport chainInner mitochondrial membraneYesLarge (most of the ATP)

Altogether, aerobic respiration of one glucose molecule can yield roughly 30 or more ATP, far more than glycolysis alone.

Aerobic vs. Anaerobic

When oxygen is scarce, the electron transport chain backs up and cannot accept more electrons. Cells then rely on fermentation, which does not use oxygen. Fermentation does not make more ATP itself; instead it regenerates the carrier NAD+ so that glycolysis can keep producing its small 2 ATP. In human muscle during intense exercise, fermentation produces lactic acid (lactate), which contributes to muscle fatigue and is later cleared when oxygen returns. Yeast, by contrast, ferments sugar into ethanol and carbon dioxide, the basis of baking and brewing.

FeatureAerobic respirationAnaerobic fermentation
OxygenRequiredNot required
ATP yieldHigh (about 30+)Low (net 2)
End productsCO2 and waterLactic acid (in muscle) or ethanol + CO2 (in yeast)

Clinical Relevance

Metabolism explains why oxygen and glucose matter so urgently in emergencies. The brain relies almost entirely on a steady supply of glucose and oxygen; interrupt either, as in a stroke or cardiac arrest, and cells run out of ATP within minutes and begin to die. This is why restoring breathing and circulation is the first priority in emergency care. Inherited enzyme deficiencies in these pathways cause metabolic diseases, since a single missing enzyme can block ATP production. Lactose intolerance is a familiar example of a metabolic gap: without enough lactase, lactose passes undigested into the large intestine, where bacteria ferment it and cause gas, bloating, and diarrhea. Finally, cyanide is deadly precisely because it blocks the electron transport chain, halting aerobic ATP production even when oxygen is plentiful.

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P/O ratio and why ATP yield is ~30-32

ATP is made two ways: substrate-level phosphorylation transfers a phosphate directly from a high-energy intermediate (as in glycolysis and the succinyl-CoA synthetase step), while oxidative phosphorylation couples electron transport to ATP synthase via the proton-motive force. Modern measurements of the P/O ratio give roughly 2.5 ATP per NADH and 1.5 per FADH2, rather than the older integer values of 3 and 2. Combined with the ATP cost of shuttling cytosolic NADH into mitochondria, this revises the total yield per glucose down to about 30 to 32 ATP.

Regulation, the Cori cycle, and fasting ketogenesis

Phosphofructokinase-1 (PFK-1) catalyzes the committed step of glycolysis and is the main control point: it is inhibited by ATP and citrate (signals of energy surplus) and activated by AMP and fructose-2,6-bisphosphate. During anaerobic effort, muscle lactate travels to the liver, where gluconeogenesis regenerates glucose in the Cori cycle, exporting the metabolic burden at a net ATP cost to the liver. In prolonged fasting, high acetyl-CoA and low oxaloacetate drive hepatic ketogenesis, producing acetoacetate and beta-hydroxybutyrate as water-soluble fuels that spare glucose for the brain, an adaptation that becomes pathological in diabetic ketoacidosis.

Key terms

Metabolism
The sum of all chemical reactions in a cell, including those that build molecules and those that break them down.
Anabolism
Metabolic reactions that build larger molecules from smaller ones and require energy input.
Catabolism
Metabolic reactions that break larger molecules into smaller ones and release energy.
ATP
Adenosine triphosphate, the molecule cells use to store and transfer energy for immediate use.
Glycolysis
The breakdown of glucose into two pyruvate molecules in the cytoplasm, producing a small net amount of ATP.
Krebs cycle
The citric acid cycle in the mitochondrial matrix that oxidizes fuel and loads electron carriers with high-energy electrons.
Electron transport chain
A series of proteins in the inner mitochondrial membrane that uses electrons to drive the bulk of ATP production, with oxygen as the final acceptor.
Aerobic respiration
Energy release that requires oxygen and yields a large amount of ATP from glucose.
Fermentation
Anaerobic breakdown of glucose that regenerates carriers for glycolysis without oxygen, producing little ATP.

Check your understanding

6 questions · answers reveal instantly.

  1. 1.Which term describes reactions that build larger molecules and consume energy?
  2. 2.Why is ATP called the cell's energy currency?
  3. 3.Where does glycolysis take place?
  4. 4.In aerobic respiration, what is the final acceptor of electrons in the electron transport chain?
  5. 5.Compared with aerobic respiration, anaerobic fermentation:
  6. 6.In human muscle, what does fermentation produce when oxygen runs short during intense exercise?

Citations & References

Links open publicly available educational and peer-reviewed sources.

  1. OpenStax. Biology 2e. ↗
  2. OpenStax. Concepts of Biology. ↗
  3. MedlinePlus, U.S. National Library of Medicine. ↗