fitnessASSIST

"Krebs cycle and ATP

The oxidative production of ATP involves three processes:
• Glycolysis
• Krebs cycle
• Electron transport chain

The glycolytic system produces ATP from the breakdown of glucose. One glucose molecule is metabolized to form two pyruvate molecules. The pyruvate molecules are transferred to one of the many mitochondria within the muscle cells. In the presence of oxygen the pyruvate acid is converted into a compound called acetyl coenzyme A (acetyl CoA).

In the process of the conversion of pyruvate to acetyl CoA a molecule of carbon dioxide is produced for the first of three for each turn in the Krebs cycle. Utilisation of oxygen and production of carbon dioxide takes places at this cellular level.

The Krebs cycle consists of a series of 10 chemical reactions that commence with acetyl CoA (2 carbon compound) combining with oxaloacetate (4 carbons) to produce a 6 carbon compound named citric acid (other name for the Krebs cycle). Isocitrate dehydrogenase (IDH) is the rate- limiting enzyme involved in this succession of reactions. During the route of the chain of reactions carbon dioxide molecules are formed resulting in a loss of two carbon molecules. This results in a 4 carbon oxaloacetate which is then available to combine with another acetyl CoA to repeat the cycle of chemical reactions again.

The Electron transport chain
During gylcolysis whereby glucose is metabolized to pyruvic acid, hydrogen is released. There is also an additional release of hydrogen during the Krebs cycle. An accumulation of hydrogen ions will consequently cause a decrease in cellular PH, making the cells acidic.  For this purpose the Krebs cycle is coupled with a series of chemical reactions known as the electron transport chain.  The released hydrogen combines with two coenzymes: NAD (nicotinamide adenine dinucleotide) and FAD (Flavin adenine dinucleotide). These coenzymes then transport the hydrogen to the electron transport chain, where they are divided into protons and electrons. At the last part of the chain the hydrogen joins with oxygen to form water, reducing and avoiding acidification. The divided electrons from the hydrogen undertake a series of reactions (electron transport chain) consequently producing energy for phosphorylation of ADP, forming ATP. This process is referred to as oxidative phosphorylation. This process can generate up to 39 molecules of ATP from one molecule of glucose.

Fibre-Type and composition

Muscles that depend predominantly on oxidative phosphorylation for ATP require a plentiful supply of oxygen. Slow twitch fibres have a great capacity for aerobic metabolism due to their high concentration of mitochondria and oxidative enzymes. They also contain a high content of myoglobin which gives oxidative muscles their red appearance. Fast twitch fibres rely more on glycolytic reactions. They lack a high number of myoglobin and therefore appear white. Endurance training can enhance oxidative capacity in the fast twitch fibres by stimulating the fibres to produce more mitochondria and oxidative enzymes. An increase in oxidative enzymes will also increase the muscles ability to utilize fat to produce ATP."



"Muscle fibre types
All muscle fibres differ in their characteristics and predominant fuel source.  The two types of muscle fibres are slow twitch and fast twitch. The slow twitch fibres take 110ms to reach peak tension whereas fast-twitch fibres take approximately 50ms to reach peak tension. Fast twitch fibres consist if fast-twitch types a (FTa) and fast-twitch type b (FTb) and less identified fast-twitch fibre c (FTc).  The average individual has 50% slow-twitch fibres and 25% fast-twitch a (FTa) fibres. The additional 25% is made up of mainly fast-twitch type b and fast-twitch type c making up a small percentage of 1-3%.
Characteristics of each muscle fibre
The slow-twitch fibre types have a slow form of myosin ATPase (the enzyme that splits ATP to produce energy), in comparison to fast-twitch fibres.  The motor units which stimulate slow-twitch muscle fibres have a small cell body and stimulate 10 to 180 muscle fibres. However, fast-twitch motor units have a larger cell body and more axons, innervating approximately 300-800 muscle fibres.  This enables fast-twitch muscle fibres to generate more force and reach peak tension at a higher rate than slow-twitch muscle fibres.  The fast-twitch fibres also have a greater developed sarcoplasmic reticulum; enabling more calcium to be released for faster muscle contractions. 
Slow –twitch muscle has a high oxidative capacity as they are most often recruited during low intensity endurance events such as the marathon. Their contractile speed, glycolytic capacity and motor unit strength is low. However their fatigue resistance is high making them predominantly suited to long, endurance events.  Slow-twitch muscle fibres have a large number of myoglobin and mitochondria.  They are also surrounded by a high density of blood capillaries. They have a high capacity to generate ATP via oxidative metabolic processes.
Fast-twitch type a have a moderately high oxidative capacity and a very high glycolytic capacity to produce ATP.  Their contractile speed is fast and they are relatively fatigue resistant.  This makes them incredibly beneficial for both endurance and speed athletes, however most humans have a small percentage of these fibres. 
Fast-twitch type b has a very low oxidative capacity to produce ATP. However they are most effective muscle group to produce ATP via glycolytic metabolic processes.  Their contractile velocity is fast due to their high motor unit strength.  However FTb have a low resistance to fatigue.
Endurance athletes tend to have a higher level of slow-twitch muscle fibres, whereas sprinters are predominantly made up of fast-twitch.  Middle distance events such as 800m require a combination of both types.