Question

Describe the steps whereby a molecule of sucrose is oxidized to CO₂ in glycolysis, the pentose phosphate pathway, and the citric acid cycle. During which steps is CO₂ released, and during which steps is energy conserved?

Answer 1

Sucrose, which is mainly glucose and Fructose.

In Glycolysis, end product is the two molecules of pyruvate which is generated through a series of 10 steps. It occurs in the cytosol and divided into two phases. Phase 1 is the synthesis of glyceraldehyde 3 phosphate using 2 ATP molecules and phase 2 is the synthesis of pyruvate from glyceraldehyde 3 phosphate and also 4 molecules of ATP is released. So there will be a net gain of 2 ATP molecules per one glucose molecule.

Breakdown of Pyruvate

In order for pyruvate, the product of glycolysis, to enter the next pathway, it must undergo several changes to become acetyl Coenzyme A (acetyl CoA). Acetyl CoA is a molecule that is further converted to oxaloacetate, which enters the citric acid cycle (Krebs cycle). The conversion of pyruvate to acetyl CoA is a three-step process.

Step 1. A carboxyl group is removed from pyruvate, releasing a molecule of carbon dioxide into the surrounding medium. The result of this step is a two-carbon hydroxyethyl group bound to the enzyme pyruvate dehydrogenase; the lost carbon dioxide is the first of the six carbons from the original glucose molecule to be removed. This step proceeds twice for every molecule of glucose metabolized , there are two pyruvate molecules produced at the end of glycolysis; thus, two of the six carbons will have been removed at the end of both of these steps.

Step 2. The hydroxyethyl group is oxidized to an acetyl group, and the electrons are picked up by NAD+, forming NADH (the reduced form of NAD+). The high- energy electrons from NADH will be used later by the cell to generate ATP for energy.

Step 3. The enzyme-bound acetyl group is transferred to CoA, producing a molecule of acetyl CoA. This molecule of acetyl CoA is then further converted to be used in the next pathway of metabolism, the citric acid cycle.

Acetyl CoA to CO2

The acetyl carbons of acetyl CoA are released as carbon dioxide in the citric acid cycle.

Acetyl CoA links glycolysis and pyruvate oxidation with the citric acid cycle. In the presence of oxygen, acetyl CoA delivers its acetyl group to a four-carbon molecule, oxaloacetate, to form citrate, a six-carbon molecule with three carboxyl groups. During this first step of the citric acid cycle, the CoA enzyme, which contains a sulfhydryl group (-SH), is recycled and becomes available to attach another acetyl group. The citrate will then harvest the remainder of the extractable energy from what began as a glucose molecule and continue through the citric acid cycle.

In the citric acid cycle, the two carbons that were originally the acetyl group of acetyl CoA are released as carbon dioxide, one of the major products of cellular respiration, through a series of enzymatic reactions. For each acetyl CoA that enters the citric acid cycle, two carbon dioxide molecules are released in reactions that are coupled with the production of NADH molecules from the reduction of NAD+ molecules.

Two carbon atoms come into the citric acid cycle from each acetyl group, representing four out of the six carbons of one glucose molecule. Two carbon dioxide molecules are released on each turn of the cycle. The two acetyl carbon atoms will eventually be released on later turns of the cycle; thus, all six carbon atoms from the original glucose molecule are eventually incorporated into carbon dioxide.

In pentose phosphate pathway, Glucose 6 phosphate is converted to 6-phosphogluconolactone which is again converted to 6-phosphogluconate in the second step. one molecule of carbon dioxide is released during the synthesis of Ribulose 5 phosphate which is the third step of the pathway.

In glycolysis energy is conserved in the following steps

1. 1, 3 bis phospho glucerate to 3-phospho glycerate
2. PEP to Pyruvate

In Citric acid cycle energy is conserved in the following steps

1. Oxidative decarboxylation of isocitrate to alpha - ketoglutarate by isocitrate dehydrogenase with release of NADH+H+
2. Oxidative decarboxylation of alpha - ketoglutarate to succinyl coA by alpha - ketoglutarate dehydrogenase complex with release of NADH+H+
3. Conversion of succinyl coA to succinate by succinyl coA synthetase with release of GTP
4. Conversion of succinate to fumarate by succinate dehydrogenase with release of FADH2
5. Conversion of malate to oxaloacetate by malate dehydrogenase with release of NADH+H+