Energy Systems for Exercise Performance
Phosphagen System
Provides energy for very high-intensity, short-duration activities, and the initial phase of any activity. The most important substrates are ATP and Phosphocreatine.
ATP
ATP is hydrolyzed by the ATPase enzyme located in the myosin heads. This hydrolysis triggers the movement of actin, resulting in muscle contraction.
The energy released during hydrolysis in exercise is approximately 7300 calories (under specific temperature and pH conditions).
This energy is used for muscle work and metabolic synthesis. ATP stores are depleted in about 1 second during maximal physical exertion.
Phosphocreatine
Phosphocreatine (PC) allows for rapid resynthesis of ATP after its use. This energy transformation requires the enzyme Creatine Kinase (CK), which is activated by an increasing concentration of ADP.
Muscle PC reserves would be exhausted in about 2 seconds during very intense exercise if this were the sole energy source.
Anaerobic Glycolysis
Through this system, only carbohydrates can be metabolized in the muscle cell cytosol to produce energy without direct oxygen involvement.
It provides enough power to sustain exercise intensity from a few seconds up to about 1 minute.
Glucose enters cells via facilitated transport (facilitated diffusion) through a membrane transporter called GLUT4.
Lactate Dehydrogenase
The enzyme Lactate Dehydrogenase (LDH), located in the cytosol, is involved in converting pyruvate to lactate during anaerobic glycolysis.
Cori Cycle
Also known as the alanine-glucose cycle.
Alanine, synthesized in muscle from glucose-derived pyruvate, travels via the blood to the liver. In the liver, alanine is converted into glucose and urea.
The newly formed glucose is then returned to the blood for transport back to the muscle as an energy substrate.
During exercise, increased production and output of alanine from muscle helps maintain blood glucose levels for the needs of the nervous system and muscles.
Aerobic System
This system utilizes oxygen to produce large amounts of ATP, primarily through the metabolism of carbohydrates and fats.
Pyruvate Oxidation
Pyruvate formed during glycolysis enters the mitochondria. It is then converted into Acetyl-CoA by the enzyme Pyruvate Dehydrogenase, allowing it to enter the Krebs Cycle.
Krebs Cycle
Carbohydrates, fats, and to a lesser extent, proteins can be used to obtain energy through the Krebs Cycle (also known as the Citric Acid Cycle).
The energy yield from the Krebs Cycle is much higher than that obtained from glycolysis alone.
In the Krebs Cycle, ATP is produced, and CO2 and hydrogen atoms are formed. The electrons from these hydrogen atoms are transferred to the mitochondrial respiratory chain.
The most important function of this cycle is to generate electrons that pass through the respiratory chain, where a large amount of ATP is resynthesized via Oxidative Phosphorylation.
The limiting enzyme in the Krebs Cycle is Isocitrate Dehydrogenase, which is inhibited by ATP and stimulated by ADP.
Both ADP and ATP also stimulate and inhibit, respectively, the electron transport chain.
Lipid Metabolism
For energy use, triglycerides (TG) must be broken down into their basic units: one molecule of glycerol and three molecules of free fatty acids (FFA).
This process is called Lipolysis and is carried out by lipase enzymes. Moderate-intensity exercise stimulates lipolysis.
An increased concentration of FFA in the blood promotes their uptake into muscle fibers.
Once inside the muscle fibers, FFA are enzymatically activated using ATP. They then undergo Beta-Oxidation, producing Acetyl-CoA, which enters the Krebs Cycle to generate energy.