IB Sports, exercise and health science A.2.3.2 Phosphagen, Glycolytic, and Oxidative Energy Systems During Exercise Notes

The Three Energy Systems

1. Phosphagen System

1. The Phosphagen System, also known as the ATP-PC System, is the body’s quickest source of ATP production.
2. It utilizes stored ATP and creatine phosphate (PC) to provide immediate energy for very high-intensity activities lasting for a few seconds.

Fuel Source

1. ATP stored in muscles (immediate, limited supply).
2. Creatine phosphate (PC) is used to quickly regenerate ATP from ADP. PC stores are replenished during recovery periods.
3. $$ \text{PCr + ADP} \xrightarrow{\text{creatine kinase}} \text{ATP + Creatine} $$

Recovery Time

1. Full recovery of the phosphagen system takes about 2-5 minutes, allowing creatine phosphate to regenerate.
2. During recovery, a period of low-intensity exercise (e.g., walking or light jogging) helps clear waste products and assist in faster recovery.

Benefits

1. Provides immediate ATP production.
2. Supports very high-intensity efforts, such as sprints, weightlifting, or jumping.
3. No oxygen required (anaerobic).

Limitations

1. Limited capacity: once stored ATP and PC are used up, the system is depleted in under 20 seconds at maximal intensity.
2. Requires long recovery time for the complete restoration of stores, which can take several minutes.

2. Glycolytic System

1. The glycolytic system provides energy for activities lasting from 20 seconds to 2 minutes.
2. It involves the breakdown of glucose or glycogen into pyruvate to produce ATP.

Fuel Source

1. Glucose (from blood or muscle glycogen) is broken down into pyruvate to produce ATP.
2. Since it occurs in the absence of oxygen (anaerobic), the byproduct of this process is lactate.

Recovery Time

1. Partial recovery can occur within 30 minutes to 1 hour, but full recovery may take several hours.
2. Lactate clearance is essential for full recovery. Active recovery (e.g., light jogging) speeds up the removal of lactate from the bloodstream.

Benefits

1. Provides quick ATP production for medium-duration activities (e.g., 400m sprints, intense interval training).
2. Supports high-intensity efforts lasting 20 seconds to 2 minutes.
3. No oxygen required (anaerobic).

Limitations

1. Lactate accumulation: lactate accumulates when production outpaces clearance. The associated hydrogen ion (H⁺) accumulation, not lactate itself, contributes to fatigue and the burning sensation.
2. Inefficient ATP production: Compared to the oxidative system, the glycolytic system produces less ATP per molecule of glucose.
3. Limited by the need to clear lactate after intense activity.

3. Oxidative System

1. The oxidative system is the most efficient way the body produces ATP and operates during low-to-moderate intensity activities.
2. It involves the use of oxygen to fully oxidize glucose or fatty acids into carbon dioxide, water, and large amounts of ATP.
3. Glucose → Pyruvate → Acetyl-CoA → Citric Acid Cycle → Electron Transport Chain (ETC) → ATP

Glucose Oxidation

1. When exercise intensity is moderate and oxygen is plentiful, pyruvate from glycolysis enters the mitochondria, is converted to acetyl-CoA, then enters the Krebs cycle, which feeds the electron transport chain.

Fat Oxidation

1. Triglycerides are first broken into glycerol and fatty acids in adipose tissue (lipolysis, covered in A.2.3.1). Fatty acids are transported into the mitochondria and broken down in cycles of beta-oxidation, producing acetyl-CoA molecules that enter the Krebs cycle alongside those from carbohydrate.
2. Fat oxidation is slower: the body cannot use fat as a fuel during high-intensity, anaerobic activities.
3. Intensity limit: above about 90% of maximal exercise intensity, no free fatty acids are used at all.

Fuel Source

1. Carbohydrates (glucose/glycogen) and fats are the primary fuels.
2. Proteins can also be used in the case of extended exercise or starvation, but they are less efficient.
3. Oxygen is required, making it an aerobic system.

Recovery Time

1. Full recovery can take several hours or even days, depending on exercise duration and intensity.
2. Recovery continues after exercise as the body restores PCr, oxygen stores, and metabolic balance.

Benefits

1. Sustains energy production for extended periods (hours of continuous activity).
2. Efficient ATP production: produces far more ATP per glucose molecule than glycolysis (which yields only 2 ATP).
3. Utilizes fat as a fuel source, making it ideal for prolonged, moderate-intensity activities.

Limitations

1. Slower ATP production: The oxidative system activates more slowly than the phosphagen and glycolytic systems.
2. Requires oxygen: Thus, it cannot be used for high-intensity, short-duration activities like sprints or weightlifting.
3. Efficiency depends on aerobic conditioning: Less trained individuals may rely more on anaerobic systems.

How the energy systems compare

All three energy systems work together on a continuum

1. The Energy Continuum explains that all three energy systems, phosphagen, glycolytic, and oxidative, contribute to ATP production at all times, but the dominance of each system depends on exercise intensity and duration.
2. First 2 seconds: pre-existing ATP in the muscle is used directly.
3. Up to ~20 seconds: phosphagen system dominates. PCr donates phosphate to regenerate ATP. Used for short, explosive activities (e.g., 100m sprint).
4. ~30 seconds to 2 minutes: glycolytic system dominates. Anaerobic glycolysis breaks down glucose to lactate. Used for sustained high-intensity efforts (e.g., 400m sprint).
5. Beyond 2 minutes: oxidative system dominates. Carbohydrate and fat are fully oxidised in the mitochondria. Used for longer, lower-intensity activities (e.g., a marathon).

Energy system dominance changes across activities

Oxidative metabolism dominates at rest and shifts during exercise

Energy Systems at Rest

1. At rest, the oxidative system is the dominant energy system since energy demands are low.
2. The body does not need rapid ATP production for movement, and oxygen is readily available.
3. Therefore, the oxidative system uses fatty acids as the primary fuel source to produce ATP.
4. This system is highly efficient and capable of sustaining prolonged ATP production at low intensity.
5. At rest, the body does not need quick bursts of energy, so ATP production is slow and steady through the oxidative pathway.

During Exercise

The energy system contributions change dynamically based on the intensity and duration of exercise.

1. Sudden bursts of high-intensity movement (e.g., sprinting, lifting heavy weights) predominantly activate the phosphagen system, which provides immediate ATP by breaking down stored creatine phosphate and ATP.
2. Sustained high-intensity exercise (lasting 30 seconds to 2 minutes) relies primarily on the glycolytic system. This system breaks down glucose to produce ATP, and produces lactate as a byproduct.
3. Extended low-to-moderate intensity exercise (lasting several minutes to hours) uses the oxidative system. This system provides a steady supply of ATP using aerobic respiration (glucose and fatty acids) to sustain energy production over long periods.