Respiration

The chemistry of respiration

Respiration is the process by which chemical energy is released from a foodstuff by oxidation. This is when ATP is regenerated, it involves a complex series of metabolic reactions.

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Glycolysis 
  • This is the first stage of respiration and it occurs in the cytoplasm of a living cell. It involves with breaking a 6-carbon glucose being brokken down into two 3-carbon pyruvic acid. 
  • This process requires two molecules of ATP to activate the process, although this process does create four ATP, giving a net gain of two ATP.
  • During glycolysis hydrogen is released and becomes temporarily bound to the hydrogen receptor NAD (NADH2 in its reduced state). 
  • Glycolysis does not require oxygen, yet if oxygen is present then the ATP is regenerated at a later stage. If oxygen is not present then anaerobic respiration occurs.
Mitochondria

When oxygen is present, aerobic respiration occurs in the mitochondria. Mitochondria are found in every living cell, they contain a folded inner lining with many extensions called cristae this increases surface area. The central fluid filled matrix is teeming with enzymes, the cristae are the site of ATP production.










Krebs' Cycle

Pyruvic acid is produced during glycolysis  it then diffuses into the matrix of the mitochondrion, it is then converted into acetyl CoA, once again hydrogen is released and attaches to NAD (the co-enzyme acting as a hydrogen acceptor. Each 2-carbon acetyl CoA combines with a 4-carbon intermediate to create 6-carbon citric acid. This then goes through a number of stages shown below to until it reaches back to the 4-carbon intermediate, during this cycle carbon dioxide and hydrogen is released.
 The enzyme that is present during this stage is dehydrogenases.


Hydrogen Transfer System

Hydrogen is released and bound to the acceptor NAD at six seperate points. NADH2 transfers the hydrogen to the cytochrome system (shown below) every mitochondrion has many of these attached to the cristae.

This transfer of hydrogen from NADH2 along the cytochrome system releases enough energy to create 3 ATP, this process is called oxidative phosphorylation, 38 ATP are regenerated from on glucose molecule, 2 during glycolysis and 36 from oxidative phosphorylation.



The role of oxygen is extremely important - it acts as the final hydrogen acceptor. Oxygen and Hydrogen combine in the cytochrome system to produce water. Without oxygen being present respiration would fail to go beyond glycolysis. 

Aerobic respiration is a metabolic pathway with a series of enzyme-controlled reactions which oxidises a 6-carbon glucose molecule to form carbon dioxide which is accompanied by the production of ATP from ADP+Pi.

Anaerobic respiration

As you will know from reading the above information, if oxygen is not present anaerobic respiration occurs and the breakdown of the glucose molecule does not go past glycolysis. This process is extremely inefficient and only regenerates 2 ATP, therefore an oxygen debt builds up and lactic acid (2 x 3-carbon) is produced. Once the oxygen debt is repaid then lactic acid forms pyruvic acid and respiration can continue.
Subpages (1): Oxidation and Reduction
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