Question

State if each of the following processes produce or consume ATP: a. citric acid cycle b. glucose forms two pyruvates c. pyruvate forms acetyl CoA d. glucose forms glucose-6-phosphate e. oxidation of \(\alpha\) -ketoglutarate f. transport of NADH across the mitochondrial membrane \(\mathrm{g}\). activation of a fatty acid

Short answer

a. Indirectly produces; b. Produces; c. Neither; d. Consumes; e. Produces indirectly; f. Consumes; g. Consumes.

Step-by-step solution

- Citric Acid Cycle

The citric acid cycle itself does not directly produce or consume ATP, but it does produce GTP, which can be converted to ATP. Hence, it is considered to indirectly produce ATP.

- Glycolysis: Glucose to Pyruvate

The conversion of glucose to two pyruvates in glycolysis involves both the consumption and production of ATP. Net, it produces 2 ATP.

- Pyruvate to Acetyl CoA

The conversion of pyruvate to acetyl CoA does not consume or produce ATP directly.

- Phosphorylation of Glucose

The conversion of glucose to glucose-6-phosphate consumes 1 ATP molecule.

- Oxidation of \(\beta\)-Ketoglutarate

The oxidation of \(\beta\)-ketoglutarate to succinyl-CoA is a part of the citric acid cycle; it does not produce or consume ATP directly but produces NADH and FADH2, which are used later to generate ATP.

- Transport of NADH

The transport of NADH across the mitochondrial membrane from the cytoplasm requires energy and consumes ATP.

- Activation of Fatty Acids

The activation of fatty acids for oxidation consumes 2 ATP molecules.

Key concepts

Citric Acid Cycle

The citric acid cycle, also known as the Krebs cycle or TCA cycle, plays a critical role in cellular respiration. This cycle is a series of chemical reactions to release stored energy through the oxidation of acetyl-CoA. While it does not directly produce or consume ATP, it generates GTP. This GTP can be readily converted to ATP. Additionally, the cycle produces high-energy molecules like NADH and FADH2 that transfer electrons to the electron transport chain, ultimately leading to ATP production.

Glycolysis

Glycolysis is the process by which one glucose molecule is converted into two molecules of pyruvate. This pathway occurs in the cytoplasm and is essential for both aerobic and anaerobic respiration. Glycolysis consumes 2 ATP molecules in the initial steps and produces 4 ATP molecules in later steps, resulting in a net gain of 2 ATP. It also produces NADH, which can be used in the electron transport chain to generate further ATP.

Pyruvate Metabolism

Pyruvate metabolism involves the conversion of pyruvate into acetyl-CoA before entering the citric acid cycle. This conversion occurs in the mitochondria and is catalyzed by the pyruvate dehydrogenase complex. The process does not directly produce or consume ATP; however, it generates NADH. This NADH is essential for the electron transport chain, indirectly contributing to ATP production.

Cellular Respiration

Cellular respiration is a multi-step process that cells use to convert biochemical energy from nutrients into ATP. The stages include glycolysis, the citric acid cycle, and oxidative phosphorylation. During oxidative phosphorylation, electrons carried by NADH and FADH2 are passed through the electron transport chain, generating a proton gradient that drives the synthesis of ATP. The complete oxidation of one glucose molecule can produce up to 36-38 ATP molecules.

Mitochondrial Transport

Mitochondrial transport refers to the movement of molecules such as NADH from the cytoplasm into the mitochondria. This process is crucial for ATP production, as NADH needs to enter the mitochondria to donate electrons to the electron transport chain. The transport of NADH into the mitochondrial matrix consumes ATP, making it an energy-demanding process. Shuttle systems such as the malate-aspartate shuttle are often employed to facilitate this transfer efficiently.

Fatty Acid Activation

Fatty acid activation is the initial step in fatty acid oxidation, also known as beta-oxidation. This process takes place in the cytoplasm and involves the conversion of fatty acids into fatty acyl-CoA. The activation consumes 2 ATP molecules, allowing the fatty acids to be transported into the mitochondria for subsequent oxidation. The breakdown of fatty acyl-CoA in the mitochondria generates acetyl-CoA, NADH, and FADH2, which are essential for ATP production through the citric acid cycle and electron transport chain.