Metabolism: Glycolysis, Citric Acid Cycle, and Energy Production

Glycolysis breaks down one molecule of glucose ($C_6H_{12}O_6$) into two molecules of pyruvate. The process consists of two main phases:

1. Energy Investment Phase: The cell spends 2 ATP to split the glucose molecule.
2. Energy Payoff Phase: The cell produces 4 ATP and 2 NADH.

The net yield of glycolysis is 2 ATP per glucose molecule, produced via substrate-level phosphorylation (where an enzyme transfers a phosphate group directly from a substrate to ADP).

Pyruvate Oxidation and the Citric Acid Cycle

If oxygen is available, the pyruvate produced in glycolysis enters the mitochondrion to undergo further oxidation. First, pyruvate is converted into acetyl CoA through a process catalyzed by pyruvate dehydrogenase. This step releases the first molecule of $CO_2$ and reduces $NAD^+$ to NADH.

The acetyl CoA then enters the citric acid cycle (also known as the Krebs cycle). In this cycle, the acetyl group joins with oxaloacetate to form citrate, which is then decomposed back into oxaloacetate through eight enzymatic steps. For every turn of the cycle, the cell generates:

• 1 ATP
• 3 NADH
• 1 FADH$_2$
• 2 $CO_2$ (as waste)

Since one glucose molecule produces two pyruvates, the cycle must run twice per glucose molecule.

Metabolic Intersections: Catabolism and Anabolism

Glycolysis and the citric acid cycle are not just "burners" for glucose; they are major intersections for many different metabolic pathways.

Catabolic Pathways (Breaking Down)

Cells can funnel many different types of organic molecules into these central pathways:

• Carbohydrates: Can be converted into various sugars for glycolysis.
• Proteins: Must undergo deamination (removal of amino groups) to become amino acids, which can then enter the respiratory pathway.
• Fats: These are highly energy-dense. Fats are digested into glycerol and fatty acids. Fatty acids undergo beta oxidation to yield acetyl CoA, NADH, and FADH$_2$. An oxidized gram of fat produces more than twice the ATP of a gram of carbohydrate.

Anabolic Pathways (Building Up)

Conversely, the intermediates of these pathways can be diverted toward biosynthesis (anabolic pathways). For example, a cell might take a molecule from the citric acid cycle to build an amino acid for a protein or a lipid for a membrane.

Metabolic Control and Feedback Inhibition

Because cells must balance the need for energy with the need to conserve resources, metabolism is tightly regulated. The most common mechanism is feedback inhibition.

In feedback inhibition, the end product of a pathway (such as ATP) acts as an inhibitor for an enzyme used earlier in the pathway.

• If ATP concentrations are high, the cell has plenty of energy, and respiration slows down to prevent wasteful production.
• If ATP concentrations drop, the inhibition is lifted, and respiration speeds up to replenish the supply.

This ensures that the cell maintains a steady state of energy production tailored to its immediate work requirements.