Understanding the Citric Acid Cycle - AP Biology

Acetyl-CoA is formed when two carbon atoms join to coenzyme A. Ribose is a five carbon sugar in RNA. ATP is an energy compound, and FAD is an electron carrier in the Krebs cycle which gets reduced then passes its electron to an enzyme complex in the electron transport chain.

The citric acid cycle, also known as the Kreb's cycle, occurs within the mitochondria of eukaryotic cells. In prokaryotic cells, it occurs in the cytosol.

Two molecules are produced per one "turn" around the cycle. Note that each glucose results in two pyruvate, which convert to two acetyl-CoA and power the citric acid cycle for two turns.

NAD+ and FADH are used as reactants in the citric acid cycle to make NADH and FADH2, which are used in the electron transport chain to convert additional ADP into ATP. All of the other selections are products in the citric acid cyclce. Protons (H+) are a byproduct when NAD+ is converted to NADH. Carbon dioxide (CO2) is produced during carbohydrate conversions in the cycle. One GTP molecule is produced by the cycle, and contains almost equivalent energy to ATP.

The citric acid cycle takes place in the mitochondrial matrix.

Glycolysis takes place in the cytosol, and the electron transport chain involves both the intermembrane space and the inner mitochondrial membrane.

Pyruvate from glycolysis is transported into the mitochondrial matrix for the citric acid cycle. Energy from the citric acid cycle allows protons to be pumped to the intermembrane space. The electron transport chain involves proteins along the inner mitochondrial membrane, eventually resulting in the activation of ATP synthase due to the influx of protons along their gradient.

In cellular respiration, glucose first undergoes glycolysis and is broken down into two pyruvate molecules. As the pyruvate passes through the citric acid cycle, three molecules of are produced. The radioactive oxygen molecules would be found in the .

is formed when electrons removed from glucose are used to reduce . is produced by the phosphorylation of . The oxygen in enters the mitochondrion as gaseous molecular oxygen from the atmosphere, not from glucose. Finally, is reduced to water in cellular respiration and serves as a reactant, rather than a product, in cell metabolism.

The Krebs cycle takes place in the mitochondrial matrix. The products of glycolysis, which takes place in the cytosol, are brought to the mitochondria for the Krebs cycle and electron transport chain. The electron carriers generated during the Krebs cycle (NADH and FADH2) are then used in the electron transport chain, which takes place on the inner membrane of the mitochondria.

Acetyl-CoA is the molecule that enters as the primary reactant in the Krebs cycle.

During glycolysis glucose is the primary reactant. Glucose contains six carbons. The process of glycolysis converts one molecule of glucose into two molecules of pyruvate with three carbons each. Pyruvate then undergoes a decarboxylation reaction before entering the Krebs cycle. Each pyruvate loses one carbon to create carbon dioxide during this reaction, with the end product of acetyl-CoA. Acetyl-CoA is, thus, a two-carbon chain.

The ratio of carbon in acetyl-CoA to carbon in glucose is two-to-six, or 1:3.

A turn of the Krebs cycle produces one ATP, three NADH, one FADH2, and two CO2.

Acetyl-CoA is not produced during Krebs cycle. It is produced from the decarboxylation of a pyruvate molecule, which occurs before the Krebs cycle can begin. Each turn of Krebs cycle is initiated by one acetyl-CoA molecule. Remember that there are two acetyl-CoA produced from the two pyruvate molecules (end product of glycolysis). For every glucose molecule, the Krebs cycle produces two cycles: two ATP, six NADH, two FADH2, and four CO2.

Carbon dioxide is the gas produced in the Krebs cycle, which animals exhale. Oxygen is used as an electron acceptor, while nitrogen is not a waste gas. Carbon monoxide is not a waste product in the Krebs cycle.

The role of the Krebs cycle is to produce the intermediates NADH and FADH2, which will serve as electron donors in the electron transport chain (ETC). At the same time, the ETC creates NAD+ and FADH+ as byproducts. The products can then be turned around to continue fueling the Krebs cycle. Since the ETC will not function in an anaerobic environment, neither will the Krebs cycle. The reactants will not be replenished, and the cycle will be unable to continue.

Oxygen is not directly involved as a reactant or product of the Krebs cycle. Oxygen is only directly used as an electron receptor in the electron transport chain.

An anabolic reaction is one in which larger molecules are made from combining smaller molecules. Even without knowing the exact mechanics of the reactions given in the answer choices, we know that we are looking for a reaction in which multiple molecules combine to form a single molecule.

Out of the options, there is only one time where a larger molecule is made by the combination of two smaller ones: when acetyl CoA (2 carbons) and oxaloacetate (4 carbons) come together in order to create citrate (6 carbons).

The generation of pyruvate from glucose results in two smaller molecules from one larger molecule; this is a catalysis reaction. The conversion of glucose-6-phosphate to fructose-6-phosphate is an isomerization reaction. The transition from citrate to ketoglutarate is processed through an intermediate, but is ultimately a catalysis reaction.

The Krebs cycle takes place within the mitochondrial matrix of mitochondria. Glycolysis occurs in the cell's cytosol. The stroma is part of plant chloroplasts, thus it is not the site of the Krebs cycle.

Fermentation is a catabolic process which does not require oxygen. In contrast, Krebs cycle (citric acid cycle) and oxidative phosphorylation (chemiosmosis and electron transport) do use oxygen. Aerobic respiration is much more efficient than anaerobic respiration in producing ATP.

The citric acid (Krebs) cycle takes place in the mitochondrial matrix. The Krebs cycle involves using acetyl-CoA as a substrate to produce high energy electron carriers and to later participate in electron transport, ultimately yielding . Glycolysis is the process by which glucose is split into two molecules of pyruvate, and occurs in the cytoplasm. The electron transport chain performs its function in the inner mitochondrial membrane. The nucleus and ribosomes are not part of the citric acid cycle.

Under anaerobic conditions, the 2 molecules of pyruvate produced from glycolysis cannot undergo further oxidation; therefore, they are reduced to lactate (lactic acid) and . This allows glycolysis to continue in order for cells to produce ATP under anaerobic conditions. Once is cycled back through glycolysis, glucose metabolism can continue to produce 2 net molecules of ATP for cellular energy.

The Krebs cycle involves aerobic respiration because oxygen is used. Anaerobic respiration involves fermentation because oxygen is lacking and the compound does not enter the Krebs cycle. Glycolysis is the first step in respiration, which precedes the Krebs cycle and is fully anaerobic. Note that oxygen is not directly required for the Krebs cycle (it is not a reactant), however, without oxygen, the electron transport chain will not proceed and thus the Krebs cycle will also be halted.

Upon completion of the citric acid cycle, 1 molecule of , 2 molecules of , 3 molecules of , and 1 molecule of are produced. is not produced during the citric acid cycle. is the product of the pentose phosphate pathway. is a powerful reducing agent used in several metabolic pathways. For example, it is used in red blood cells to reduce glutathione. Note that the products listed above represent those from one turn of the citric acid cycle; each molecule of glucose produces two molecules of acetyl-CoA, thus the cycle turns twice per glucose molecule.

Glycolysis, is a series of reactions in the first part of cellular respiration. It involves the breaking down of larger molecules such as glucose into smaller ones such as pyruvate. Glucose is a six-carbon molecule that gets broken down into two three-carbon pyruvate molecules during glycolysis. Afterwards, pyruvate is converted to acetyl-CoA, which enters the Krebs cycle in the mitochondria.