Respiration

Introduction to respiration

Respiration is the process by which cells obtain energy in the form of adenosine tri-phosphate or ATP.

ATP transfers chemical energy from the energy rich substances in the cell to the cell's energy requiring reactions.

When ATP breaks down, the energy created is used by the cell for processes such as active transport, DNA replication and muscle contraction.

The main respiratory substrate used by cells is 6-carbon glucose.

Respiration is a series of reactions in which 6-carbon glucose is oxidised to form carbon dioxide. The energy released due to the oxidation of glucose is used to synthesize ATP from adenosine diphosphate or ADP and inorganic phosphate or Pi.

Fats and proteins can also be used as respiratory substrates.

Respiration is a stepwise series of reactions involving oxidation and reduction. The mnemonic OIL RIG may help you remember.

In oxidation, energy is released:

· Oxidation

· Is

· Loss; the removal of hydrogen.

In reduction, energy is used up:

· Reduction

· Is

· Gain; the addition of hydrogen.

Stages of aerobic respiration

The structure of a mitochondrion

The structure of a mitochondrion

The mitochondria are the site of aerobic respiration.

This has three stages:

· glycolysis

· Krebs cycle

· cytochrome system

Glycolysis

Conversion of 1 molecule of glucose to 2 molecule of pyruvic acid with net gain of 2 x ATP

This stage takes place in the cytoplasm; 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 mitochondrion where it is oxidised (i.e. 2H are removed) to form a two carbon molecule called acetyl CoA (2C).

· The hydrogens which are removed join with NAD to form NADH₂ and the carbon which is removed is released as CO₂.

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 NADH₂ molecules 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

Anaerobic respiration

When anaerobic respiration occurs there is no oxygen to act as the final hydrogen acceptor and so the hydrogen cannot pass through the cytochrome system. As a result, both the Krebs cycle and the cytochrome system stages cannot take place:

· The only ATP produced is formed during glycolysis, that is, 2 ATP.

· The pyruvic acid produced following glycolysis undergoes a different anaerobic pathway depending on the organism.

In plants: pyruvic acid 2C ethanol + CO₂

In animals: pyruvic acid (3C) lactic acid

Anaerobic respiration produces only 2ATP compared to the 38 molecules of ATP produced during aerobic respiration.

Glycolysis animation

Krebs Cycle animation

Electron Transport Chain animation

Deriving Energy From Food - Web Tutorial

Electron Transport and ATP Synthesis Animation

Cellular Energy References

Basic Metabolic Processes of Life- Good Animations on Respiration