Carbohydrates Metabolism

We’ve spoke about the importance of a high metabolism over a fast metabolism before, and how fast you can burn fat and the pace you can build muscle at. But what about the metabolism of carbohydrates? A quick fact: Whenever you digest something, carbs, protein, fats, a cardboard box, it is ALL digested at a different pace and completely different. For example: let’s say you have an energy drink that will be in your system within seconds of digesting it, along with a lot of insulin! Add a bit of protein in here, and this will blunt the effect slightly.

A general rule of thumb a lot of bodybuilders like to do is in the morning or after a workout (even during) take a fast digesting protein shake around 5/10 minutes after drink an energy drink, and if timed right, the carbohydrates will cause an insulin spike, which will force the protein/amino acids into the body much faster.

On the other hand, let’s say we take a fast acting carbohydrate with high fat, such as what’s found in pork or eggs. What will this do? This tends to blunt the insulin release slightly, but not enough. It basically forces the FAT into your body quicker to be stored as, well, fat!

Now, let’s look at a slower carbohydrate such as oats. If you were to start the day with a protein shake and some oats, supplementing with some fish oil will slow the insulin release down even further which is great for fat loss.

Confusing? Well I’ll try explaining it further.

So we all know that glucose is the principal carbohydrate available as an energy source to humans. Other carbohydrates consumed by humans include fructose, galactose, sucrose, lactose, maltose, and starch, all of which are converted to glucose or a related compound for use in energy. In the process, carbon dioxide and water are given off as a waste products, and oxygen gas is required as an electron acceptor. The process involves four subdivisions: glycolysis, the Krebs cycle, the electron transport system, and chemiosmosis. To describe them, in glycolysis, glucose molecules are converted to pyruvic acid; in the Krebs cycle, pyuvic acid molecules are broken down further, and the energy in the molecules is used to form a high-energy compounds such as NADH; in the electron transport system, electrons are transported among coenzymes and cytochromes and the energy is released; in chemiosmosis, the energy is used to pump protons across a membrane and provide the energy for ATP synthesis.


In glycolysis a glucose molecule is metabolized through a multistep pathway to two molecules of pyruvic acid. The process occurs in the cytoplasm of cells. At least six enzymes operate in the metabolic pathway.

In the first and third steps of the pathway, ATP molecules are employed to energize the chemical reactions. Further along in the process, the 6-carbon glucose molecule is converted into an intermediary compound, which splits into 3-carbon compounds. The latter undergo additional conversions and eventually form pyruvic acid at the conclusion of the process.

Also during the process of glycolysis, another chemical reaction yields high-energy electrons and a hydrogen atom (H). These are transferred to the coenzyme molecule NAD, thereby changing it to NADH. The reduced coenzyme (NADH) will later be used in the electron transport system. During glycolysis two NADH molecules are produced. Glycolysis does not require oxygen, so the process is considered anaerobic.

When anaeronic conditions occur in muscle cells, an enzyme converts the pyruvic acid of glycolysis to lactic acid. This chemical reaction frees up the NAD for the reuse in glycolysis, while providing the cells with two ATP molecules. Eventually, the lactic acid causes intense fatigue and the muscle cell stops contracting.

The Krebs Cycle

After glycolysis, cellular respiration involves another multistep process called the Krebs cycle, also called the citric acid cycle and tricarboxylic acid (TCA) cycle. The Krebs cycle utilizes the two molecules of pyruvic acid fromed in glycolysis. The cycle yields high-energy molecules of NADH and FADH, as well as some ATP. It occurs in the mitochondrion of a cell. This cellular organelle possesses inner and outer membranes, organized to compartments. This inner membrane is folded over itself many times to form cristae. Along the cristae are the important enzymes necessary for the proton pump and ATP production.

Entering the Krebs cycle, the pyruvic acid molecules are processed. An enzyme acts on the pyruvic acid molecule and releases one carbon atom as a carbon dioxide molecule. The remaining two carbon atoms (the acetyl group) combine with a coenzyme known as coenzyme A to form acetyl-CoA. In the process, electrons and a hydrogen ion are transferred to NAD to form high-energy NADH. Acetyl-CoA is now ready for entry to the Krebs cycle. It unites with a 4-carbon acid called oxaloacetic acid. The combination results in a 6-carbon acid called the citric acid. Citric acid now undergoes a series of enzyme-catalyzed conversions. The converions involve up to 10 chemical reactions and are brough about by enzymes. In many of the steps, high-energy electrons are released to NAD molecules. The NAD molecules also acquire hydrogen ions and become NADH molecules. In the one of the steps, FAD serves as the electron acceptor. In the process it acquires two hydrogen ions to become FADH2. Also in one of the reactions, enough energy is released to synthesize a molecule of ATP. Since there are two pyruvic acid molecules entering the Krebs cycle, two ATP molecules form.

In the reactions of the Krebs cycle, the two carbon atoms of acetyl-CoA are released. Each atom is used to form a carbon dioxide molecule. Since there are two acetyl-CoA molecules entering the Krebs cycle and each has two carbon atoms, four CO2 molecules result. Added to the two CO2 molecules formed in the conversion of pyruvic acid to acetyl-CoA, total is six CO2 molecules given off as a waste gas in the Krebs cycle. The six molecules of CO2 represent the six carbons of glucose that originally entered the process of glycolysis. The CO2 molecules are transported to the lungs for disposal.

At the end of the Krebs cycle, the last chemical compound formed is oxaloacetic acid. This compound is identical to the oxaloacetic acid that began the cycle. It is now ready to accept another acetyl-CoA molecule to begin another turn of the cycle. Note that for two molecules of pyruvic acid metabolized in the Krebs cycle, two ATP molecules have formed, plus there is a large number of NADH molecules and some FADH2 molecules. The NADH and the FADH2 will now be used in the electron transport system.

The Electron Transport System

The electron trannsport system occurs along the cristae of the mitochondria, which is where the participating cytochromes abd coenzymes are located. In the electron transport system, NADH and FADH2 molecules are used from the Krebs cycle and glycolysis. The molecules give up their electrons to a series of iron-containing pigments (cytochromes) and other coenzymes. The cytochromes and coenzymes transport the elctrons among one another, and the energy in the electrons is gradually lost. But it is not lost to the environment. Rather, the energy form electron passages is used to pump protons across the mitochondrial membrane into the outer compartment of the mitochondria. Each NADH molecule contains enough energy to transfer six protons into the outer compartment. Each FADH2 molecule has enough energy to transfer four protons.

The electrons passed among the cytochromes and coenzymes ultimately are taken up by an oxygen atom. Having acquired two electrons, the oxygen atom becomes negatively charged. To balance the charges, the atom takes on two protons from the solution to form a molecule of water (H2O). Water is thus an important waste product of the metabolism. As the final electron receptor, oxygen is responsible for removing electrons from the system. If oxygen were not available at the end of the transport system, electrons could not be released from the coenzymes and cytochromes, and they would be unable to function any further. Then the energy in electrons could not be released; the proton pump could not be established; and ATP cou;d not be produced. In humans, breathing is the essential process that brings oxygen into the body for delivery to the cells for use in cellular respiration.


The actual production of ATP in cellular respiration takes place through chemiosmosis. As noted previously, chemiosmosis involves proton pumping across the membranes of mitochondria to establish a proton gradient. Once the gradient is established, protons pass down the gradient through particles designated F1. In these particles, enzymes use the energy of the protons to generate ATP, using ADP and phosphate ions as the starting points.

The energy production in cellular respiration during chemiosmosis is substantial. it is widely agreed among scientists that a total 34 molecules of ATP can be produced during cellular respiration as a result of the reactions of the Krebs cycle, the electron transport system, and chemiosmosis. Two ATP molecules are formed during the Krebs cycle, and two molecules of ATP are produced in glycolysis for a total of 38 molecules of ATP. These ATP molecules may be utilized in the cell for cellular needs. However, they cannot be stored for long periods of time, and cellular respiration must continue constantly in order to regenerate the ATP molecules as they are used.

Physiology of Glucose Metabolism

The glucose molecules used in cellular respiration are absorbed from the small intensine into the bloodstream. They and other monosaccharides, such as fructose and galactose, are transported to the liver by the hepatic portal vein. In the liver, frucotse and galactose are converted to glucose, and the glucose molecules can then be used in cellular respiration.

At the tissue cells, the hormone insulin facilitates the transfer of glucose molecules across cell membranes by increasing the affinity of membrane carrier molecules for glucose molecules. In the absence of insulin, type I diabetes, or insulin-dependent diabetes, occurs in the patient.

Glucose molecules are also stored in the liver as glycogen when the level of blood glucose is high. The process of glycogen formation is called glycogenesis. When the level of blood glucose is low, glycogen breaks down and releases glucose molecules into the bloodstream. This process is glycogenolysis. The hormone glucagon and epinephrine accelerate glycogenolysis.

Glucose molecules can also be constructed in the liver from non-carbohydrate sources. For example, amino acids can be used to form glucose molecules by an intricate process involving reversal of some of the steps of glycolysis and the Krebs cycle. This process of glucose formation is called gluconeogenesis. Certain fatty acids as well as glycerol molecules and lactic acid can be changed into glucose molecules through gluconeogenesis.

There you have it.

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