These are revision cards on respiration which includes the processes Glycolysis, Link Reaction, Kreb's Cycle and Oxidative Phosphorylation

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Glycolysis - Occurs in the cytoplasm of cells.

It can occur under both aerobic and anaerobic conditions.

It is the process of breaking down a 6 carbon compound (Glucose) into a 3 carbon compound (Pyruvate) through a series of steps.


Glucose - 6 - phosphate

Fructose - 6 - phosphate

Hexose - 1,6 - bisphosphate

Triose Phosphate


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Glucose is a hexose sugar - it contains 6 carbons.

 1. One ATP molecule is hydrolysed, a phosphate group is released which attaches to the glucose molecule forming glucose-6-phosphate.

2. Glucose-6-phosphate is changed to Fructose-6-phosphate.

3. Another ATP is hydrolysed and the phosphate group released attaches to the Fructose-6-phosphate forming Fructose-1,6-bisphosphate.

4. The energy from the hydrolysed ATP activates the hexose sugar preventing it from being transported out of the cell. The compound is now Hexose-1,6-bisphosphate.

At this stage 2 ATP molecules have been used.

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Conversion to Pyruvate

Each 1X Hexose-1,6-bisphosphate is broken down into 2X Triose Phosphate

The Triose Phosphate is then oxidated in which 2X hydrogen atoms are removed from each Triose Phosphate molecule.

This involves the use of dehydrogenase enzymes and is aided by co-enzyme NAD which is a  hydrogen acceptor:

NAD + Hydrogen atoms ----> Reduced NAD (rNAD)

Also two ATP molecules are formed via substrate level phosphorylation.

Then there is a series of 4X enzyme catalysed reactions which convert the Triose Phosphate into the Pyruvate.

During this conversion 2X ADP is converted to 2X ATP

Overall Products of Glycolysis:

2 ATP, 2 rNAD, 2 molecules of Pyruvate

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Link Reaction

The link reaction occurs in the matrix of the mitochondria.

In the link reaction the pyruvate is dehydrogenated and decarboxlyated to Acetate.

There are two enzymes involved in the decarboxylation and dehydrogenation of pyruvate:

  • Pyruvate Dehydrogenase - removes H atoms from pyruvate.
  • Pyruvate Decarboxylase - removes a carboxyl group which eventually forms CO2.

There are also two co-enzymes involved:

  • NAD
  • CoA (Coenzyme A)

The coenzyme A accepts the acetate forming acetyl CoA. The function of the CoA is to carry the acetate to Kreb's Cycle.

2pyruvate + 2NAD + 2CoA ---------->2CO2 + 2rNAD + 2acetylCoA

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Kreb's Cycle

The Kreb's cycle takes place in the mitochondria.

The acetate is offloaded from CoA (which is then free to transport more acetate from the link reaction). 

The acetate (4 carbon compound) then joins with a 4 carbon compound (Oxaloacetate) to form a 6 carbon compound (Citrate).

Citrate is then decarboxylated - (One molecule of CO2 removed) and dehydrogenated - (A pair of hydrogen atoms removed) to form a 5-carbon compound. 

The 5-carbon compound is decarboxylated and dehydrogenated to form a 4 carbon compound is changed into another 4 carbon compound. During this reaction a molecule of ADP is phosphorylated to produce a molecule of ATP.

The second 4-carbon compound is changed to another 4 carbon compound and a pair of hydrogen atoms are removed and accepted by FAD which is reduced.

The third 4 carbon compound is the further dehydrogenated forming oxaloacetate. The compound is reduce using NAD

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Electron Transport Chain and Chemiosmosis

In the electron transport chain, the electrons are passed along a chain of electron carriers and then donate to molecular oxygen, the final electron acceptor.


1. As electrons flow through the electron transport chain, energy is released and use by coenxymes associated with some of the electron carriers to pump protons across the intermembrane space.

2. This builds up a proton gradient which is also a pH gradient and an electrochemical gradient.

3. Thus potential energy builds up in the intermembrane space.

4. The hydrogen ions cannot diffuse through the lipid part of the inner membrane but can diffuse through ion channels in it. These channels are associated with the enzmye ATP synthase. This flow of electrons is called Chemiosmosis.

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Oxidative Phosphorylation

Oxidative Phosphorylation  is the formation of ATP by the addition of inorganic phospate to ADP in the prescence of oxygen.

ADP + inorganic phosphate --------> ATP

The electrons are passed through the last electron carrier to the molecular oxygen which is the final hydrogen acceptor.

The hydrogen ions join with the oxygen forming water

4H+ions + 4 e- +O2---------->2H2O

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