Carbohydrate metabolism is the name for the different biochemical processes that make, break down, and change carbohydrates in living things.

The most important carbohydrate is glucose, a simple sugar (monosaccharide) that is metabolized by nearly all known organisms.

Since all digestible carbs are eventually turned into glucose, it’s important to think about how glucose gives energy to cells and tissues in the form of adenosine triphosphate (ATP).

Glucose is metabolized in three stages in carbohydrate metabolism.

Basic Overview of Carbohydrate Metabolism

They are…

  1. Glycolysis
  2. Krebs Cycle
  3. Oxidative phosphorylation and ETC

Carbohydrate Metabolism Basic Overview

1. Glycolysis

During exercise, hormone levels change, which upsets homeostasis and changes how glucose and other molecules that provide energy are broken down. The breakdown of glucose to provide energy begins with glycolysis. To begin with, glucose enters the cytosol of the cell, or the fluid inside the cell, not including cellular organelles.

Next, glucose is converted into two, three-carbon molecules of pyruvate through a series of ten different reactions.

  • A specific enzyme catalyses each reaction along the way, and a total of two ATP molecules are generated per glucose molecule.
  • Since ADP is changed into ATP when glucose is broken down, this process is called substrate-level phosphorylation.
  • During the sixth reaction, glyceraldehyde 3-phosphate is oxidised to 1,3-bisphosphoglycerate while reducing nicotinamide adenosine dinucleotide (NAD) to NADH, the reduced form of the compound.
  • NADH is then shuttled to the mitochondria of the cell, where it is used in the electron transport chain to generate ATP via oxidative phosphorylation.
  • The most important enzyme in glycolysis is called phosphofructokinase (PFK), which catalyses the third reaction in the sequence. Since this reaction is so favourable under physiologic conditions, it is known as the “committed step” in glycolysis. In other words, glucose will be completely degraded to pyruvate after this reaction has taken place.
  • With this in mind, PFK seems as if it would be an excellent site of control for glucose metabolism. In fact, this is exactly the case.

When there is a lot of ATP, or energy, in the cell, PFK is turned off. This slows down the breakdown of glucose into energy. So, PFK can control how glucose is broken down to meet the energy needs of the cell. This type of regulation is a recurring theme in biochemistry.

2.Krebs Cycle:

Kreb’s Cycle is the central metabolic cycle of carbohydrate metabolism and all metabolic pathways. There are many compounds that are formed and recycled during the Krebs cycle (the citric acid cycle). These include the oxidized forms of nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD) and their reduced counterparts, NADH and FADH2. In the Krebs Cycle, the substrates become oxidized and give up their electrons, while NAD+ and FAD accept electrons and become reduced.

The Krebs Cycle starts when the pyruvate made by glycolysis in the cytoplasm of the cell is moved to the mitochondria. This is where most of the energy in glucose is taken out. In the mitochondria, pyruvate is converted to acetyl CoA by the enzyme pyruvate carboxlase.

In general, acetyl-CoA condenses with a four-carbon compound called oxaloacetate to form a six-carbon acid. This six-carbon compound is degraded to a five- and four-carbon compound, releasing two molecules of carbon dioxide. At the same time, two molecules of NADH are formed.

Lastly, the C-4 carbon skeleton goes through three more reactions that make guanosine triphosphate (GTP), FADH2, and NADH. This is how oxaloacetate is made again. FADH2 and NADH are sent to the electron transport chain (see below), which is part of the inner membrane of the mitochondria.

3. Oxidative phosphorylation/electron transport chain:

GTP is a high-energy compound that is used to regenerate ATP from ADP. So, the main job of the Krebs cycle is to make FADH2 and NADH, which contain high-energy electrons that are then passed on to the electron transport chain.

NADH and FADH2 have a lot of high-energy electrons that are sent to a series of enzyme complexes in the mitochondrial membrane.

Three enzyme complexes work in sequence to harvest the energy in NADH and FADH2 and convert it to ATP: NADH-Q reductase, cytochrome reductase, and cytochrome oxidase. The final electron acceptor in the electron transport chain is oxygen. Each successive complex is at a lower energy level than the previous one so that it can accept electrons and effectively oxidize the higher-energy species.

In fact, each complex uses the energy in these electrons to pump protons across the inner mitochondrial membrane, creating a proton gradient. This electropotential energy is then converted to chemical energy by allowing protons to flow back down the chemical gradient and through specific proton channels that synthesize ATP from ADP.

Approximately two molecules of ATP are produced during the Kreb’s cycle reactions, while approximately 26–30 molecules of ATP are generated by the electron transport chain. In short, ATP is made when glucose is burned off through the reduction of NAD+ and FADH and the phosphorylation of ADP. Hence, the process is known as oxidative phosphorylation.

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