The Krebs Cycle (Citric Acid Cycle)
If oxygen is present, the pyruvate produced in glycolysis is oxidized further in the Krebs cycle, a continuation of aerobic respiration. The cycle involves a series of changes in organic acids with 4‐, 5‐, and 6‐ carbon atoms (citric acid has 6 carbons). The cycle takes place in the mitochondria, separated from the reactions of glycolysis, which occur within the cytoplasm. The cycle starts (and ends) with acetyl coenzyme A ( acetyl CoA) as the substrate. As the cycle turns, NADH, ATP, FADH 2 and CO 2 are released and the substrate is regenerated to be used again. Although carbon dioxide—an end product of respiration—is generated and the glucose completely oxidized during the cycling, this is not the final stage in aerobic respiration; most of the energy still remains in the high‐energy electron carriers NADH and FADH 2.
Role of the mitochondria
A mitochondrion has two membranes, an outer and an inner, the latter deeply creased and folded thereby increasing its surface area on which the specialized reactions occur. The folds of the mitochondrion are called cristae. The inner cavity—the matrix—is filled with a watery solution containing all but one of the needed materials of the Krebs cycle. The electron carriers associated with the uptake of oxygen (used in the next step of respiration, see below) are embedded in the cristae.
Steps in the cycle
Preparation of pyruvate. The pyruvate produced in glycolysis is transported to the matrix of the mitochondria where it is converted to acetyl‐CoA, a three‐step conversion.
- The two 3‐carbon pyruvate molecules are oxidized to two 2‐carbon acetyl groups (CH 3CO).
- Two molecules of CO 2 are released.
- Two molecules of NADH are formed.
- The acetyl groups are then attached to molecules of coenzyme A, forming acetyl‐CoA, and the cycle begins.
Cyclical reactions. Acetyl‐CoA, the substrate starting point, releases its 2‐carbon portion to the 4‐carbon organic acid, oxaloacetic acid, making the 6‐carbon citric acid. The seven steps are repeated and the cycle is back to oxaloacetic acid—in the process having lost two carbon molecules to two molecules of CO 2 and releasing electrons to the high‐energy compounds NADH, ATP, and FADH. Two turns of the cycle are necessary to release the six carbons of the original glucose molecule.
The energy input into the Krebs cycle comes from the oxidation (i.e. the removal of electrons) of NADH and FADH 2 and their regeneration back to NAD + and FAD. Without a constant supply of the latter compounds, the Krebs cycle stops.