Fundamentals

To understand the 3 energy pathways, we must first understand what metabolism and adenosine triphosphate (ATP) are.

Metabolism - the total of all the catabolic (breakdown of molecules) and anabolic (synthesis of molecules) in a biological system. Energy derived from catabolic reactions is used to drive anabolic reactions through an intermediate molecule adenosine triphosphate (ATP). ATP allows the transfer of energy from exergonic (catabolic) to endergonic (anabolic) reactions. Without an adequate supply of ATP, muscular activity and growth would not be possible.

ATP - Adenosine triphosphate is composed of adenosine and three phosphate groups. The breakdown of one molecule of ATP to yield energy is known as hydrolysis, as it requires one molecule of water. The hydrolysis of ATP is catalyzed by the presence of an enzyme called adenosine triphosphatase (ATPase).

We NEED ATP for regular muscle activity and growth, but how do we replenish it? Three basic energy systems exist in our muscle cells to replenish ATP. They are the phosphagen system (ATP-CP), glycolysis, and the oxidative system.

To further break these systems down, there exist two types of metabolism (anaerobic and aerobic). Anaerobic processes do not require the presence of oxygen. The phosphagen system and first phase of glycolysis (fast glycolysis) are anaerobic mechanisms occurring in the sarcoplasm of a muscle cell. The second phase of glycolysis (slow glycolysis, the oxidative system), the Krebs cycle, and electron transport, are aerobic mechanisms occurring in the mitochondria of muscle cells and require oxygen.

We'll now go over exactly what systems in our body do with the aforementioned ATP via the 3 energy pathways.

The first is the Phosphate system. This is the fastest method to produce ATP, and is from the donation of a phosphate group to ADP (adenosine diphosphate - 2 phosphates)

The phosphagen system adds a phosphate group which is known as phosphocreatine (PC) to ADP to form ATP. This looks like:

ADP + PC = ATP + C, where C is a creatine molecule

The system provides energy at the onset of a physical bout and during short term output lasting less than 5 seconds. Examples include short sprints, a football tackle, and max effort lifts.

GLYCOLYSIS: the breakdown of glycogen to rapidly produce ATP without oxygen, although a slower form of ATP production than the phosphagen system.

Glucose or glycogen is broken down by 10 reactions which form pyretic acid and 2-3 ATP molecules. In addition, there are 2 hydrogen molecules produced. These hydrogen molecules then get transported to mitochondria

As an athlete performs, ATP is produced in addition to these 2 “extra” hydrogen molecules. However with so much hydrogen being produced the transporters get backed up, so the hydrogen is returned to the pyretic acid which produces, you guessed it, LACTIC ACID.

Glycolysis occurring at a fast rate leads to lactic acid being formed which is a primary cause of muscular fatigue. Without muscular fatigue lactic acid would be generated indefinitely, leading to an acidic environment and muscle breakdown.

The Oxidative system, or oxidative phosphorylation: Also known as Aerobic ATP production occurs in the mitochondria using two pathways: the Krebs cycle and the electron transport chain. These two pathways oxidize (remove hydrogen) carbohydrates, fats, and proteins. By removing the hydrogen these pathways expose the potential energy contained in hydrogen in food molecules.

This energy is then used to form ATP. Oxygen in the body accepts hydrogen molecules used at the end of the previous pathways to produce the ATP. Without oxygen, none of the metabolic pathways would process and lead to excessive lactic acid production.

Phosphagen

The phosphagen system provides ATP primarily for short-term, high-intensity activities (such as lifting and sprinting) and is active at the start of all exercise regardless of intensity. This energy system relies on the hydrolysis of ATP and breakdown of another high-energy phosphate molecule called creatinephosphate (CP).

ATP in the body stores approximately 80 to 100g of any given time, which does not represent a significant energy reserve for exercise. The skeletal muscle concentration of CP is four to six times higher than ATP concentration. Therefore, the phosphagen system, through CP and the creatine kinase reaction, serves as an energy reserve for rapidly replenishing ATP. In addition, Type II (fast-twitch) muscle fibers contain higher concentrations of CP than Type I (slow-twitch) fibers.

Another important single-enzyme reaction then can rapidly replenish ATP is the adenylate kinase (also called myokinase) reaction:

The reactions of the phosphagen system are largely controlled by the law of mass action. The law of mass action states that the concentrations of reactants or product (or both) in solution will drive the direction of the reaction. For example, as ATP is hydrolyzed to yield energy necessary for exercise, there is a transient increase in ADP concentration in the sarcolemma. This will increase the rate of creatine kinase and adenylate kinase reactions to replenish the ATP supply.

Glycolysis

Glycolysis is the breakdown of carbohydrates-either glycogen stored in the muscle and in the liver or glucose delivered in the blood-to resynthesize ATP. As a result, the ATP resynthesis rate during glycolysis is not as rapid as with the phosphagen system; however, the capacity is much higher due to a larger supply of glycogen and glucose compared to CP.

Pyruvate is the end result of glycolysis, may proceed in one of two directions: 1. Pyruvate can be converted to lactate, or 2. Pyruvate can be sent into the mitochondria. When pyruvate is converted into lactate, ATP resynthesis is slower and it depends on the intensity and duration of your training. This process is called anaerobic glycolysis (fast glycolysis). When pyruvate is transferred into mitochondria to enter the Krebs Cycle, the speed of ATP resynthesis is slower but it can last for a longer time if the intensity of exercise is medium.

This process is often referred to as aerobic glycolysis (slow glycolysis). While glycolysis itself does not depend on oxygen, using the terms anaerobic and aerobic glycolysis is probably not much useful to describe these processes. The need of energy depends primarily on the intensity of training.

The formation of lactate from pyruvate is catalyzed by the enzyme lactate dehydrogenase. Lactate production increases with training intensity and appears to depend on muscle fiber type. The higher rate of lactate production by Type II muscle fibers may reflect a higher concentration of glycolytic enzymes than in Type I muscle fiber. Complete fatigue may occur at blood concentrations between 20 and 25 mmol/L. Peak blood lactate concentration occurs approximately 5-10 minutes after ending a training session.

Oxidative (Aerobic)

The oxidative system, the primary source of ATP at rest and during low-intensity activities, uses primarily carbohydrates and fats as fuel. Following the onset of activity, as the intensity of exercise increases, there is a shift in fuel preference from fats to carbohydrates. During aerobic exercise of high-intensity, energy is mostly gained from carbohydrates, provided there is enough of them.

Glucose and Glycogen Oxidation

The oxidative metabolism of blood glucose and muscle glycogen begins with glycolysis. If oxygen is present in sufficient quantities, the end product of glycolysis, pyruvate, is not converted to lactate but is transported to the mitochondria, where it is taken up, enters the Krebs cycle (slow glycolysis) and subsequently it gets to electron transport chain (ETC) in order to create ATP from ADP. The production of ATP during this process is referred to as oxidative phosphorylation.

Fat Oxidation

Oxidative energy system can use fats as a source of energy. Triglycerides stored in fat cells can be broken down by an enzyme sensitive lipase. This releases free fatty acids from the fat cells into the blood where they circulate and enter muscle fibres, enter the Krebs Cycle directly and subsequently the electron transport chaing (ETC) in order to create ATP from ADP. Creating ATP in this way is also called oxidative phosphorylation.