Show three metabolic pathways by which glucose gets oxidized in a living cell.
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Answer:
Cellular respiration is a collection of three unique metabolic pathways: glycolysis, the citric acid cycle, and the electron transport chain.
Glycolysis is the first pathway in cellular respiration. This pathway is anaerobic and takes place in the cytoplasm of the cell. This pathway breaks down 1 glucose molecule and produces 2 pyruvate molecules. There are two halves of glycolysis, with five steps in each half. The first half is known as the “energy requiring” steps. This half splits glucose, and uses up 2 ATP. If the concentration of pyruvate kinase is high enough, the second half of glycolysis can proceed. In the second half, the “energy releasing: steps, 4 molecules of ATP and 2 NADH are released. Glycolysis has a net gain of 2 ATP molecules and 2 NADH.
In eukaryotes, pyruvate oxidation takes place in the mitochondria. Pyruvate oxidation can only happen if oxygen is available. In this process, the pyruvate created by glycolysis is oxidized. In this oxidation process, a carboxyl group is removed from pyruvate, creating acetyl groups, which compound with coenzyme A (CoA) to form acetyl CoA. This process also releases CO2.
The citric acid cycle (also known as the Krebs cycle) is the second pathway in cellular respiration, and it also takes place in the mitochondria. The rate of the cycle is controlled by ATP concentration. When there is more ATP available, the rate slows down; when there is less ATP the rate increases. This pathway is a closed loop: the final step produces the compound needed for the first step.The citric acid cycle is considered an aerobic pathway because the NADH and FADH2 it produces act as temporary electron storage compounds, transferring their electrons to the next pathway (electron transport chain), which uses atmospheric oxygen. Each turn of the citric acid cycle provides a net gain of CO2, 1 GTP or ATP, and 3 NADH and 1 FADH2
Most ATP from glucose is generated in the electron transport chain. It is the only part of cellular respiration that directly consumes oxygen; however, in some prokaryotes, this is an anaerobic pathway. In eukaryotes, this pathway takes place in the inner mitochondrial membrane. In prokaryotes it occurs in the plasma membrane.
The electron transport chain is made up of 4 proteins along the membrane and a proton pump. A cofactor shuttles electrons between proteins I–III. If NAD is depleted, skip I: FADH2 starts on II. In chemiosmosis, a proton pump takes hydrogens from inside mitochondria to the outside; this spins the “motor” and the phosphate groups attach to that. The movement changes from ADP to ATP, creating 90% of ATP obtained from aerobic glucose catabolism.