Unit C

Module 5 ~ Lesson 5

Krebs Cycle

Glycolysis is anaerobic, but under oxygenated conditions, cellular respiration moves from the cytoplasm of the cell to the matrix of the mitochondria. The products of glycolysis (pyruvate and NADH) move into the matrix where they become a part of the Krebs cycle.

Aerobic respiration can be summarized by this familiar statement:

Both the Krebs cycle and the electron transport chains inside the mitochondria produce ATP, CO₂_(g), and H₂O_(l). (see diagram below)

Pyruvate (a 3-carbon molecule) loses a carbon atom by combining with Coenzyme A to form acetyl Coenzyme A (acetyl CoA) and carbon dioxide.

Acetyl CoA (a 2-carbon molecule) combines with a 4-carbon molecule when it enters the Krebs cycle, forming a 6-carbon molecule. As the molecule is oxidized, CO₂, NADH, FADH₂ and ATP are formed. The ATP is used by cells for energy. The NADH and FADH₂ provide reducing power for the electron transport chain. The CO₂ gas is released as a waste product of the Krebs cycle.

Activity

Use the photosynthesis and cellular respiration activity again. Click on the picture below to activate the activity.

What similarities are there in photosynthesis and cellular respiration?
What differences are there in photosynthesis and cellular respiration?

Read

Read " Inside the Mitochondria" and "The Krebs Cycle" on pages 187 to 188 of your textbook.

Self-Check

What compound derived from glucose actually enters the Krebs cycle?

AcetylCoA is the compound that actually enters the Krebs cycle.

The majority of the ATP from glucose is formed in the electron transport chain. What is the purpose of the Krebs cycle in relation to ATP production?

The Krebs cycle provides the reducing power (NADH and FADH₂) for the electron transport chain.

The carbon atoms derived from glucose are fully oxidized in the Krebs cycle. What is reduced in the Krebs cycle?

NAD⁺ and FAD are reduced to NADH and FADH₂

How does the word catabolism apply to the Krebs cycle?

Catabolism is the breakdown of large molecules; in this case, glucose is broken down into pyruvate (glycolysis), which is then broken down into a 2-carbon molecule before it can be used in the Krebs Cycle. The 6-carbon molecule is then oxidized several times to produce a 4-carbon molecule at the end of the cycle.