Site hosted by Angelfire.com: Build your free website today!

Carbohydrate Metabolism

The functions of metabolism are:

The functions of metabolism are:

To release and use energy from foods
To synthesize one substance from another
To prepare waste products for excretion
Vitamins and minerals are "keys" to releasing energy

There are numerous metabolic pathways, or reactions from beginning to end, which occur in our cells to keep every life process continuing. Anabolic pathways are those which build compounds or synthesize substances for the body, while catabolic pathways result in breaking down compounds. Intermediates are compounds which are formed as pathways proceed. An example of an intermediate in energy production is water-it is recycled through the body as a source of metabolic water. The byproducts of energy production in humans are carbon dioxide gas, water, and heat.

Glucose is the key food molecule for most organisms, and it is the central substance in carbohydrate metabolism. During digestion, carbohydrates are hydrolyzed to the monosaccharaides glucose, fructose, and galaxies, which are absorbed into the bloodstream through the lining of the small intestine.

ATP (adenosine triphosphate) is the main energy source for cells, and is the goal of energy production from the nutrients we consume. The mitochondria is the major site for ATP production in the cell, and is often referred to as the "powerhouse" of the cell because of this. It is an aerobic cell organelle that is responsible for most energy production in eukaryotic cells. The inner mitochondrial membrane is the site where oxidative phosphorylation occurs. The enzymes of the citric acid cycle are located in the matrix space of the mitochondrion.

Anaerobic respiration occurs when no oxygen is present. Aerobic respiration is the oxygen-requiring degradation of food molecules and production of ATP, and is the one we shall be concerned with in carbohydrate metabolism.

There are three basic steps involved with carbohydrate metabolism:

1.Glycolysis

2.Citric Acid Cycle (Kreb's Cycle)

3.Electron Transport Chain/Oxidative Phosphorylation

Glycolysis

Glycolysis is the pathway for the catabolism of glucose that leads to pyruvate. A net of two molecules of ATP per molecule of glucose are produced by substrate level phosphorylation. (Phosphate transfers from organic compounds to ADP, forming ATP). Two molecules of ATP are consumed in the conversion of glucose to fructose-1,6-biphosphate. The first substrate-level phosphorylation of glycolysis is a phosphoryl group transfer from 1,3-biphosphoglycerate to ADP. The second is a phosphoryl group transfer from phosphoenolpyruvate to ADP. NAD+ is also reduced to NADH as glyceraldehyde-3-phosphate is oxidized. The reactions of glycolysis and an interactive glycolytic pathway are available.

Citric Acid Cycle

Pyruvate from glycolysis enters into the Citric Acid Cycle . This cycle occurs in the mitochondria of the cell in aerobic conditions. The pyruvate loses a carbon dioxide group, forming acetyl-CoA, the compound which forms a link for many other pathways and helps build other compounds. The citric acid cycle pathway consists of eight reactions that process incoming molecules of Acetyl CoA. The carbon atoms leave the cycle in the form of molecules of carbon dioxide. The hydrogen atoms and electrons leave the cycle in the form of reduced coenzymes NADH and FADH2. The cycle is regulated by three allosteric enzymes in response to cellular levels of ATP. One Acetyl CoA molecule entering the citric acid cycle produces three molecules of NADH, one of FADH2, and one of GTP. Click here to see the citric acid cycle.

Oxidative Phosphorylation/Electron Transport Chain

The electron transport chain is the primary site for ATP synthesis, and occurs in the mitochondria. This pathway involves a series of reactions that pass electrons from NADH and FADH to molecular oxygen. Each carrier in the series has an increasing affinity for electrons. Four of the carriers, known as cytochromes, contain iron, which accepts and then transfers the electrons. As NADH and FADH2 release their hydrogen atoms and electrons, NAD+ and FAD are regenerated for return to the citric acid cycle.

There are three sites within the electron transport chain where the decrease in free energy is sufficient to convert ADP to ATP. Oxidative phosphorylation is the process by which NADH and FADH2 are oxidized, with concomitant production of ATP. Two molecules of ATP are produced when FADH2 is oxidized, and 3 molecules of ATP are produced when NADH is oxidized. The synthesis of ATP occurs because of a flow of protons across the inner mitochondrial membrane. The complete oxidation of one glucose molecule by the citric acid cycle and oxidative phosphorylation yields 36 molecules of ATP, vs. two molecules of ATP by glycolysis. For an excellent tutorial on oxidative phosphorylation, click here.