Metabolic pathways that contribute to the production of ATP molecules in cells are collectively referred to as cellular respiration.

Cellular respiration equation

When a molecule of glucose undergoes aerobic cellular respiration, 36 molecules of ATP are produced.

Glucose is an energy-rich molecule. The breakdown of glucose results in the formation of low-energy molecules and energy.

ATP synthesis requires energy; it involves a series of endothermic reactions.

The exothermic breakdown of glucose is coupled (linked) to the endothermic reactions involved in the synthesis of ATP.

This coupling of reactions results in about 40 percent of the chemical energy in the glucose molecule being transformed into energy in ATP molecules.

The rest of the energy is waste thermal energy.

Cellular respiration steps

4 distinct steps of cellular respiration include:

  1. Glycolysis pathway (Embden–Meyerhof pathway)
  2. The transition reaction (oxidative decarboxylation)
  3. Krebs cycle (citric acid cycle)
  4. Oxidative phosphorylation in mitochondria

Cellular respiration steps

Glycolysis pathway (Embden–Meyerhof pathway)

“Glycolysis” - breaking sugar.

This process is anaerobic (without oxygen) and occurs in the cytosol of cells.

During this stage, the six-carbon glucose is broken down into 2 molecules of three-carbon pyruvate.

The pyruvate can be used without oxygen in the process of fermentation, but no further ATP is produced during this process.

If oxygen is present, the pyruvate molecules enter the mitochondria and the process of aerobic cellular respiration can occur.

The transition reaction

In the transition reaction each pyruvate is decarboxylated by the oxidative activity of NAD+.

This reaction changes a three-carbon pyruvate to a two-carbon acetyl group. This smaller molecule combines with coenzyme A to form acetyl-CoA.

The Krebs cycle

The Krebs cycle is a cyclical metabolic pathway located in the matrix of a mitochondrion. Only one ATP molecule results from one cycle of this metabolic pathway.

Oxidative phosphorylation

Oxidative phosphorylation in mitochondria requires oxygen to produce ATP by chemiosmosis - the movement of concentrated H+ ions through a special protein complex.

Oxidative phosphorylation relies on the electron transport chain. This is a series of molecules that are embedded on the inner membrane of the mitochondrion.