Krebs Cycle

Krebs Cycle Definition

The Krebs Cycle describes how living cells generate the energy used by the body. It is associated with the process of aerobic respiration which, in animal cells, refers to the cellular conversion of one substance to another substance. Most commonly, this transformation refers to changing adenosine diphosphate (ADP) to adenosine triphosphate (ATP).

Krebs Cycle Products

In simplest terms, the Krebs Cycle results in adenosine triphosphate (ATP).

In more detailed terms, the Krebs Cycle produces carbon dioxide (CO2), nicotinamide-adenine-dinucleotide (NADH), and flavin-adenine-dinucleotide (FADH2), as well as a small amount of ATP.

Krebs Cycle Steps

  1. Before beginning the Krebs Cycle, pyruvic acid, a substance commonly found in fruits, undergoes a process called glycolysis, in which it is either converted into a carbohydrate or stored in fat cells. In technical terms, this carbohydrate is called acetyl-coenzyme A (acetyl-CoA). It is acetyl-CoA that undergoes the Krebs Cycle.
  2. Acetyl-CoA combines with a molecule called oxaloacetic acid. Together, acetyl-CoA and oxaloacetic acid form what is commonly known as citric acid.
  3. Acetyl-CoA loses its carbon atoms when combining with oxaloacetic acid. The lost carbon atoms from acetyl-CoA combine with oxygen and create CO2 molecules. These molecules are the first products of the Krebs Cycle. They are identical to those the catalysts, or initiators of glycolysis, and prepare the body to undergo glycolysis again.
  4. Citric acid undergoes a series of reactions that change its molecular shape. During these reactions, citric acid releases hydrogen (H) molecules. These H molecules readily attach to two other molecules, nicotinamide-adenine-dinucleotide (NAD) and flavin-adenine-dinucleotide (FAD), creating NADH and the FADH2. These are the second products of the Krebs Cycle. They assist in electron transport.
  5. The reactions that create NADH and FADH2 usually release ATP. While technically a “waste” product, ATP is nonetheless a product of the Krebs Cycle.
  6. The acetyl-CoA eventually breaks completely down, leaving only NADH, FADH2, ATP, and oxaloacetic acid. Oxaloacetic acid is final product of the Krebs Cycle. Like CO2, it prepared the body to undergo the Krebs Cycle a second time.

It is important to note that these steps function as a guide to the Krebs Cycle. In reality, the Krebs Cycle passes far more quickly, making these reactions – and their products – happen and appear almost instantaneously.

Related Biology Terms

  • Pyruvic acid – A substance that helps metabolize carbohydrates and sugars. Chemical formula: CH3COCO2H.
  • Glycolysis – The chemical process that turns sugar into ATP in the human body.
  • Electron transport – The movement of electrons through the body, usually to assist in chemical processes like metabolism and digestion.

Quiz

1. Glycolysis is a process that happens before the Krebs Cycle. It functions to:
A. Prepare pyruvic acid to undergo the Krebs Cycle by encouraging and supporting it emotionally.
B. Prepare pyruvic acid to undergo the Krebs Cycle by reducing it to an enzyme that combines with another enzyme to make citric acid.
C. Prepare pyruvic acid to undergo the Krebs Cycle by freeing some of its hydrogen bonding sites.
D. Prepare pyruvic acid to undergo the Krebs Cycle by bonding it with carbon.

Answer to Question #1

2. ATP and CO2 are products of the Krebs Cycle. ATP _______ and CO2 _________.
A. Prepares the body to undergo glycolysis a second time; does nothing significant, it is a waste product.
B. Prepares the body to undergo the Krebs Cycle a second time; does everything, it has universal benefits.
C. Does nothing significant, it is a waste product; prepares the body to undergo glycolysis a second time.
D. Provides energy to females only; prepares the body to undergo glycolysis a second time.

Answer to Question #2

3. Two molecules produced by the Krebs Cycle, NADH and FADH2, form what function in the body?
A. They transport electrons to perform other chemical functions.
B. They transport protons to perform other chemical functions.
C. They transport neutrons to perform other chemical functions.
D. They transport electrons, protons, and neutrons to perform other chemical functions.

Answer to Question #3
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