The metabolic pathway involved in respiration can be split into three main parts:

1. Glycolysis

   Glycolysis is the breakdown of glucose to pyruvate in the cytoplasm.

   ATP is required for the phosphorylation of glucose and intermediates during the energy investment phase of glycolysis.

   → The energy investment phase of glycolysis leads to the generation of more ATP during the energy pay-off stage and results in a net gain of ATP.

   Dehydrogenase enzymes remove hydrogen ions and electrons and pass them to the coenzyme NAD, forming NADH.

   The hydrogen ions and electrons from NADH are passed to the electron transport chain on the inner mitochondrial membrane.

   

1. Citric acid cycle

   The citric acid cycle occurs in the matrix of the mitochondria.

   In aerobic conditions, pyruvate is broken down to an acetyl group that combines with coenzyme A forming acetyl coenzyme A.

   In the citric acid cycle the acetyl group from acetyl coenzyme A combines with oxaloacetate to form citrate.

   During a series of enzyme-controlled steps, citrate is gradually converted back into oxaloacetate which results in the generation of ATP and release of carbon dioxide.

   Dehydrogenase enzymes remove hydrogen ions and electrons and pass them to the coenzyme NAD, forming NADH.

   The hydrogen ions and electrons from NADH are passed to the electron transport chain on the inner mitochondrial membrane.

   

   Krebs Cycle → 2 - 65444

2. Electron transport chain

   The electron transport chain is a series of carrier proteins attached to the inner mitochondrial membrane.

   Dehydrogenase enzymes remove hydrogen ions and electrons and pass them to the coenzyme NAD, forming NADH. This occurs in both glycolysis and the citric acid cycle.

   The hydrogen ions and electrons from NADH are passed to the electron transport chain on the inner mitochondrial membrane.

   For further ATP synthesis, electrons are passed along the electron transport chain releasing energy.

   This energy allows hydrogen ions to be pumped across the inner mitochondrial membrane.

   The flow of these ions back through the membrane protein ATP synthase results in the production of ATP.

   Finally, hydrogen ions and electrons combine with oxygen to form water.

   

Dehydrogenase enzymes remove hydrogen ions and electrons and pass them to the coenzyme NAD, forming NADH. This occurs in both glycolysis and the citric acid cycle.

The hydrogen ions and electrons from NADH are passed to the electron transport chain on the inner mitochondrial membrane.

In the absence of oxygen, fermentation takes place in the cytoplasm.

In animal cells, pyruvate is converted to lactate in a reversible reaction.

In plants and yeast, ethanol and $CO_2$ are produced in an irreversible reaction.

Fermentation results in much less ATP being produced than in aerobic respiration.

ATP is used to transfer energy to cellular processes which require energy.