Biochemistry and Metabolism - MCAT Biological and Biochemical Foundations of Living Systems

Cells need a constant supply of NAD+ to accept electrons during glycolysis in order to produce pyruvate from glucose.

Glycolysis occurs in the cytosol and does not require oxygen. The pyruvate dehydrogenase complex (PDC) and Kreb's cycle require oxygen indirectly, while the electron transport chain and oxydative phosphorylation require oxygen directly. After glycolysis produces pyruvate, either aerobic respiration or anaerobic respiration can proceed depending on the availability of oxygen.

Anaerobic respiration creates the byproduct lactic acid. Accumulation of lactic acid in the muscles due to lack of oxygen results in the pain we experience during exercise. Remember that aerobic respiration creates 36 ATP molecules per glucose, while anaerobic repiration forms only 2 ATP molecules per glucose. Since both processes begin with glycolysis, pyruvate is still generated.

Note that while lactic acid is responsible for the "burn" in muscles during exercise, other agents are responsible for muscle soreness after exercise.

In the absence of available oxygen, the body conducts metabolism anaerobically in a process known as fermentation. During strenuous exercise, like running a marathon, the body needs to generate ATP at a rate faster than oxygen is becoming available.

To combat this issue, the body converts pyruvate and NADH, generated in glycolysis, into lactic acid and NAD+, respectively. This regenerated NAD+ can participate in further glycolysis to generate more ATP, even in the absence of oxygen. Oxygen only becomes a necessary reactant in the electron transport chain; thus, glycolysis can continue to generate limited amounts of ATP in an anaerobic environment as long as NAD+ is present.

Pyruvate can be decarboxylated to make acetyl-CoA. This is the process that initiates the citric acid cycle. Pyruvate can also undergo fermentation, and be reduced to either lactic acid or acetaldehyde. Acetaldehyde can then be reduced to ethanol, however, pyruvate cannot directly be converted to ethanol.

Fermentation occurs in the absence of oxygen, and reduces pyruvate to the end product of either ethanol or lactic acid. Since pyruvate is being reduced, NADH is oxidized to NAD+, which is needed for the initial glycolysis reaction to produce pyruvate. During anaerobic respiration, glycolysis is still able to function, but only if NAD+ is available; thus, fermentation allows the regeneration of NAD+ in order for glycolysis to proceed in the absence of oxygen.

In an anaerobic environment, two net ATP are produced from glycolysis. Since glycolysis requires an investment of two ATP and produces four ATP, it has a total net yield of two ATP. In an aerobic environment, however, the cell performs glycolysis, pyruvate decarboxylation, the citric acid cycle, and oxidative phosphorylation. These processes together yield a net of 36 ATP.

Two net molecules of ATP are produced via anaerobic cellular respiration.

During anaerobic conditions only glycolysis occurs. Glycolysis alone produces 4 ATP per glucose, but requires an input of 2 ATP per glucose. Thus, 2 ATP per glucose are yielded through glycolysis.

In order for oxaloacetate to be formed, malate must lose electrons, which is the definition of an oxidation reaction. Alternately, NAD+ is reduced (gains electrons) to form NADH and H+.

NADH is produced during glycolysis, which occurs in the cytoplasm. NADH is also produced during the Krebs cycle, which occurs in the mitochondrial matrix. The protons generated in the production of NADH are later used in the intermembrane space to power ATP synthase during oxidative phosphorylation.

Of the answer choices, only carbon dioxide is a product of the Krebs cycle. If the cycle is overstimulated, too much of the products will be formed and the body will have too much carbon dioxide.

Glucose is the reactant that fuels glycolysis to produce pyruvate, which is then converted to acetyl CoA for the Krebs cycle. As such, each of these would be depleted as reactants fueling an overstimulation of the Krebs cycle.

During the Krebs cycle, or citric acid cycle, acetyl CoA is oxidized to CO2 and NAD+ and FADH are reduced to NADH and FADH2, respectively. This process is carried out in the mitochondrial matrix of eukaryotic cells.

The electron transport chain is carried out in the inner membrane of the mitochondria, while glycolysis is carried out in the cytosol.

The citric acid (Krebs) cycle and glycolysis yield high energy intermediates that can then be used to make ATP. Each turn of the citric acid cycle generates NADH and FADH2, and each cycle of glycolysis generates NADH. These intermediates can then donate their electrons and become oxidized in the electron transport chain. Production of these electron donors is essential to the function of the electron transport chain.

Each turn of the citric acid cycle is powered by one molecule of acetyl-CoA, resulting in three and one . The net reaction is:

Since twelve are produced, there must have been an input of four acetyl-CoA molecules and four total turns in the cycle. As a result, four molecules were produced.

Oxygen is needed for the electron transport chain to occur which oxidizes and . If there is no oxygen available then Krebs cycle would not occur since there would be no oxidized electron carriers. Therefore oxygen is only indirectly required for the Krebs cycle to occur, not directly.

The cell body of a neuron is where the mitochondria and all other organelles are located. Recall from the Krebs cycle that carbon dioxide is produced as a byproduct. Anaerobic respiration, which occurs in the cytoplasm does not release carbon dioxide (in humans) and produces lactic acid instead. Note that in certain organisms like yeast, fermentation produces ethanol (two-carbons) and carbon dioxide since pyruvate, the product of glycolysis is a three-carbon molecule.

NADH is produced during glycolysis, which occurs in the cytoplasm. NADH is also produced during the Krebs cycle, which occurs in the mitochondrial matrix. The protons generated in the production of NADH are later used in the intermembrane space to power ATP synthase during oxidative phosphorylation.

In order for oxaloacetate to be formed, malate must lose electrons, which is the definition of an oxidation reaction. Alternately, NAD+ is reduced (gains electrons) to form NADH and H+.

Of the answer choices, only carbon dioxide is a product of the Krebs cycle. If the cycle is overstimulated, too much of the products will be formed and the body will have too much carbon dioxide.

Glucose is the reactant that fuels glycolysis to produce pyruvate, which is then converted to acetyl CoA for the Krebs cycle. As such, each of these would be depleted as reactants fueling an overstimulation of the Krebs cycle.