3.5.2 Respiration

There are four stages to aerobic respiration and two stages to anaerobic respiration. Respiration produces energy in the form of ATP

Aerobic respiration

Glycolysis
Glycolysis occurs in the cytoplasm of cells and is an anaerobic process (it does not require oxygen). It occurs in four stages:

1. Phosphorylation of glucose to glucose phosphate: Glucose is made more reactive by the addition of two phosphate molecules (phosphorylation). The phosphate comes from the hydrolysis of two ATP molecules. This provides the energy to activate glucose and lower the activation energy for the following enzyme controlled reactions.
2. Splitting of the phosphorylated glucose: Each glucose molecule is split into two triose phosphate molecules (3 carbon molecules)
3. Oxidation of triose phosphate: Hydrogen is removed from each of the triose phosphate molecules and transferred to NAD (NAD reduces to NADH/reduced NAD).
4. The production of ATP: Enzyme controlled reactions convert triose phosphate into pyruvate, two molecules of ATP are formed from ADP+Pi in the process. In total, 4 ATP molecules are produced as there are two triose phosphate molecules each producing two ATP molecules.

In total, glycolysis produces net two molecules of ATP (4 in total but 2 are used to phosphorylate the glucose), two molecules of NADH, and 2 molecules of pyruvate.


Link reaction
The pyruvate molecules are actively transported into the mitochondrial matrix. Here, they are oxydised in the link reaction so that potential energy can be released in the krebs cycle. A series of reactions known as the link reaction follows:

1. The pyruvate is oxidised to acetate: The 3C pyruvate molecule loses a carbon and two hydrogens. The hydrogens are accepted by NAD forming NADH/reduced NAD.
2. The 2C molecule combines with coenzyme A producing acetylcoenzyme A.

The overall equation:

pyruvate + NAD + coA --> acetyl coA + reduced NAD + CO2

Krebs cycle
This involves a series of oxidation and reduction reactions:

1. The 2C acetylcoA combines with a 4C molecule producing a 6C molecule
2. The 6C molecule loses CO2 and hydrogen in a series of oxidation-reduction to give a 4C molecule and a single ATP molecule (substrate level phosphorylation)
• NAD is reduced to NADH
• 2xCO2 is removed
• FAD os redced to FADH
• ATP produced
3. The 4C molecule can now combine with a new acetlycoA

For each pyruvate molecule the link reaction and krebs cycle produces:

• Reduced coenzymes (NADH, FADH) which have the potential to provide energy to produce ATP molecules by oxidative phosphorylation (the electron transport chain).
• One ATP molecule
• 3 CO2 molecules

The krebs cycle is important for many reasons including the following:

• Breaks down macromolecules into smaller ones (e.g pyruvate into co2)
• Produces hydrogen ions that are carried by NAD and FAD to the etc for oxidative phosphorylation
• Regenerates the 4C molecule so acetylcoA does not accumulate
• It is a source of intermediate compounds that cells can use to manufacture important substances such as chlorophyll/amino acids/fatty acids.

Oxidative phosphorylation (ATP synthesis)
Within the inner folded mitochondrial membrane sits enzymes and proteins involved in oxidative phosphorylation - it follows that cells that need to respire lots have a lot of mitochondria present.This is the last stage in aerobic respiration and is the mechanism by which some of the energy of electrons within hydrogen atoms is conserved in the formation of ATP. That was a bit of a mouthful. We think it works by chemiosmotic theory - here is the process:

1. Hydrogen atoms produced during glycolysis and the krebs cycle are currently being transported by NADH and FADH.
2. The electrons of the hydrogen atoms are donated to the first molecule in the etc
3. The electrons pass along the etc in a series of oxidation-reduction reactions
4. The energy released by this flow is used to actively transport protons across the inner mitochondrial membrane into the inter-membranal space
5. The protons accumulate in the inter-membranal space and a diffusion gradient is established
6. They diffuse back into the mitochindrial matrix through ATP synthase channels embeded in the inner mitochondrial membrane
7. Electrons at the end of the etc combine with these protons and oxygen forming water - oxygen is the final electron acceptor.

Anaerobic respiration

Glycolysis
As above.


Fermentation
In the absence of O2, krebs and the etc cannot take place as soon all the FAD anf NAD will be reduced and none will be available to tae up the hydrogen atoms produced during the krebs cycle so enzymes will stop working. For glycolysis to continue pyruvate and hydrogen must constantly be removed.

The NAD is replenished because pyruvate accept the hydrogen from NADH. The NAD can then be used in glycolysis again. In plants (and microorganisms such as yeast) the pyruvate is converted into ethanol and lactic acid, in animals it is converted into lactate.

Ethanol production:
The pyruvate molecule loses a carbon dioxide and accepts a hydrogen from NADH which produces ethanol.

pyruvate + NADH --> ethanol + CO2 + NAD

Lactate production:

Each pyruvate molecule takes up two hydrogen atoms from NADH.

pyruvate + NADH --> lactate + NAD

The only ATP produced from anaerobic respiration is formed by glycolysis (net 2 ATP molecules).

Alternative respiratory substrates

Sugars (glucose) are not the only substances that can be oxidised to release energy.

Respiration of lipids
Lipids are hydrolysed to glycerol and fatty acids. Glycerol is phosphorylated and converted to triose phosphate which enters the glycolysis pathway and so on. The fatty acid is broken down into 2C fragments which are converted to acetylcoenzymeA which enters the krebs cycle. The oxidation of lipids produces 2C fragments and lots of hydrogen atoms which can be used to produce ATP during oxidative phosphorylation. This is why lipids release more than double the energy of the same mass of carbohydrate.

Respiration of protein
Protein is hydrolysed to amino acids. They have their amino group removed (deamination) and enter the respiratory pathway at different points depending on the number of C atoms they contain. 3C convert to pyruvate, 4C and 5C convert to intermediates in the Krebs cycle.