Higher Biology Respiration Essay Definition

The electron transport chain is the last stage of the respiration pathway and is the stage that produces the most ATP molecules. The electron transport chain is a collection of proteins found on the inner membrane of mitochondria. NADH and FADH2 release the electrons into the transport chain.

The electrons transfer their energy to the proteins in the membrane providing the energy for hydrogen ions to be pumped across the membrane. The flow of the ions across the membrane synthesises ATP by a protein called ATP synthase.

Three ATP are produced from each NADH, and two ATP are produced from each FADH2, which transfers high energy electrons to the electron transport chain. This results in a total gain of 34 ATP molecules in the electron transport chain.

Oxygen is the final electron acceptor. The oxygen combines with the hydrogen to form water. If oxygen is not present then hydrogen cannot pass through the electron transport chain resulting in a reduction of ATP molecules produced.

In total, 38 ATP molecules are produced from one molecule of glucose.

If glucose is not available for the respiration pathway, other respiratory substrates can be used via alternative metabolic pathways.

Starch, glycogen, proteins (amino acids) and fats can all be broken down into intermediates in glycolysis or the citric acid cycle. This provides alternative metabolic pathways to make ATP.

Stages of aerobic respiration

The structure of a mitochondrion

The mitochondria are the site of aerobic respiration.

This has three stages:

  • glycolysis
  • Krebs cycle
  • cytochrome system


This stage takes place in the cytoplasm [cytoplasm: The living substance inside a cell (not including the nucleus).] ; it does not require oxygen:

  • Glycolysis is the breakdown of 6-carbon glucose into two 3-carbon pyruvic acid units.
  • The hydrogens removed join with the hydrogen carrier NAD to form NADH2.
  • Although some energy is needed to start glycolysis there is an overall net gain of 2 ATP.
  • The pyruvic acid (3C) enters the matrix of the mitochondrion where it is oxidized (i.e. 2H removed) and a carbon dioxide is lost. Thus forming a two carbon molecule called acetyl-CoA (2C).
  • The hydrogens which have been removed join with NAD to form NADH2.

Krebs cycle: citric acid cycle; tricarboxylic acid cycle

This stage takes place in the matrix of the mitochondrion and is the aerobic phase and requires oxygen:

  • It begins when the 2-carbon acetyl CoA joins with a 4-carbon compound to form a 6- carbon compound called Citric acid.
  • Citric acid (6C) is gradually converted back to the 4-carbon compound ready to start the cycle once more.
  • The carbons removed are released as CO2.
  • The hydrogens, which are removed, join with NAD to form NADH2.

Cytochrome system: hydrogen carrier system; electron transfer system

Most of the energy produced during respiration is made by the cytochrome system. In this stage of aerobic respiration, the NADH2molecules [molecules: a collection of two or more atoms held together by chemical bonds. The fundamental unit of compounds]  produced during glycolysis and the Krebs cycle transfer the hydrogens to the cytochrome system.

This is a system of hydrogen carriers located on the cristae of the mitochondrion and this stage also requires oxygen, which acts as the final hydrogen acceptor:

  • The oxygen and the hydrogen combine to form metabolic or respiratory water.
  • If oxygen is not present to act as the final hydrogen acceptor, the hydrogen cannot pass through the system and complete oxidation cannot take place

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