The Krebs Cycle

9-2

At the end of glycolysis, about 90% of the chemical energy available in glucose is still unused

To extract the rest, cells need oxygen Powerful electron acceptor Required for final steps of cellular

respiration b/c they require oxygen - aerobic

Cellular respiration should not be confused with respiration

Cellular respiration is the releasing of energy stored in food

Respiration gives our cells oxygen The oxygen needed to power cellular

respiration

The Krebs Cycle

In the presence of oxygen, pyruvic acid from glycolysis is passed to the Krebs cycle

Second stage of cellular respiration

Named after Hans Krebs

During the Krebs cycle - pyruvic acid is broken down into carbon dioxide

Also known as citric acid cycle Citric acid is the first compound formed

Step A

Begins with pyruvic acid produced by glycolysis entering the mitochondria

1 carbon atom from pyruvic acid becomes part of a CO2 molecule Eventually released into the air

Remaining 2 carbon atoms from pyruvic acid are joined to a compound called coenzyme A Forms acetyl-CoA Acetyl is made of 2 C atoms, 1 O atom,

and 3 H atoms

Acetyl-CoA adds the 2 carbon acetyl group to a 4-carbon molecule Produces 6-carbon citric acid

Step B

Citric acid is broken down into a 4-carbon molecule

More CO2 is released

Electrons are transferred to electron carriers NAD+ and FAD

At each turn of the cycle, a molecule similar to ADP is converted to a molecule similar to ATP

At 4 places in the cycle, a pair of high-energy electrons is accepted by electron carries Changes NAD+ to NADH and FAD to

FADH2

What happens to the products? CO2 released to atmosphere

ATP produced can be used for cellular activities

High-energy electrons can be used to generate large amounts of ATP in electron transport chain