The Link Reaction

If oxygen is available energy can be released from pyruvate via

the Krebs Cycle and oxidative phosphorylation

Pyruvate from glycolysis can move into the matrix of the mitochondrion

Pyruvate must cross the outer and inner membranesThis is done by active transport which requires energy

Pyruvate must be converted to Acetyl Coenzyme A (Acetyl CoA)

This used Coenzyme A

Pyruvate + NAD + CoA Acetyl CoA + NADreduced + CO 2

NAD is reduced and CO2 is made

Remember this happens twice for each glucose molecule

3Carbon pyruvate

Pyruvate + NAD + CoA Acetyl CoA + NADreduced + CO2

Coenzyme A NAD

CO2 Acetyl CoAIn effect this is a 2Carbon acetyl moleculeThese 2 carbons will enter the Krebs Cycle

H

NADreduced

Remember this process happens twice per glucose

The Krebs Cycle

Also calledCitric Acid Cycle and the Tricarboxylic Acid Cycle

Described by Hans Krebs in 1937

The cycle is a series of enzyme controlled reactions:

In Step 1 - Citrate (6C) is formed from acetyl CoA + oxaloacetate(4C)

In Steps 2-8 Citrate is decarboxylated to yield CO2 gas and dehydrogenated to release Hydrogen ions

these will reduce NAD and FAD

Oxaloacetate is regeneratedOne ATP is formed by phosphorylation

The Krebs Cycle will go around twice for each molecule of glucose

This is a cycle but for convenience we will start with 4-carbon oxaloacetate This combines with the 2C acetyl group to form 6-carbon Citrate

This is what oxaloacetate looks like There are 4 carbons

This is the structure of citric acid (citrate)

Note it has 6 carbons

There are 3 carboxylic groups so it used to be called tricarboxyilic acid TCA

This is the first step in the Krebs cycle so that is why it is also called the citric acid or tricarboxylic acid cycle

Citric acid is what makes citrus fruit acidicIt is also used as a food additive

In a series of steps citric acid is dehydrogenated (hydrogen is removed and transferred to NAD)And decarboxylated(A carbon and 2 oxygens are removed)

This leaves a 5C cpd

This happens again to reduce another NAD and release another CO2

Leaving a 4C cpd

In a further series of reactions more hydrogens are removed and used to reduce NAD

ATP is made by substrate level phosphorylation

(6C)

Products of the Krebs cycle per glucose molecule

4CO2

2ATP2FAD red6NAD red

NAD reduced is a hydrogen carrier moleculeIt will carry hydrogen to the electron transfer chain where ATP will be madeThe more hydrogens available the more energy in the form of ATP can be made

FAD is also simply a hydrogen carrier moleculeEach molecule of glucose will only make 2 FAD reduced

ATP NAD red

FAD CO2

Glycolysis

2 2 - -

Link - 2(1) - 2(1)

Krebs cycle

2(1) 2(3) 2(1) 2(2)

Total 4 10 2 6

Where it goes next

used To ETC To ETC Diffuses out

Summary so far…(per glucose molecule)