Biological energy systems

Bioenergetics is the study of how energy flows within biological systems. In exercise and training, it explains how the body converts food into energy to fuel muscle contractions and sustain physical activity. An understanding of energy systems, ATP production, and their relationship to exercise intensity and duration is critical for designing effective training programs. This chapter covers the three major energy systems—phosphagen, glycolytic, and oxidative—as well as the metabolic processes involved in energy production, substrate utilization, and recovery. It’s important to note that these energy systems never work in isolation; instead, they operate along a continuum, with their relative contributions varying based on the intensity and duration of activity.

Bioenergetics

Bioenergetics refers to the process of converting macronutrients—carbohydrates, proteins, and fats—into usable energy for cellular functions. It involves:

• Catabolism: Breakdown of larger molecules into smaller ones to release energy (e.g., glycogen to glucose).
• Anabolism: Synthesis of larger molecules from smaller ones, requiring energy (e.g., protein synthesis).
• Exergonic reactions: Energy-releasing reactions, such as ATP hydrolysis.
• Endergonic reactions: Energy-consuming reactions, such as muscle contraction.

Adenosine triphosphate (ATP)

ATP is the primary energy currency of the body. It consists of an adenosine molecule bound to three phosphate groups. Energy is released when ATP is hydrolyzed into adenosine diphosphate (ADP) and inorganic phosphate (Pᵒ). This energy fuels muscle contractions and other cellular processes.

• $\text{ATP}\to \text{ADP}+\text{P}+\text{Energy}$

The body uses three primary energy systems to replenish ATP during exercise. Each system’s contribution depends on the intensity and duration of the activity. Because ATP stores in muscle last only a few seconds, it must be continually resynthesized to sustain activity.

1. Phosphagen system

The phosphagen system provides ATP for short-term, high-intensity activities, such as sprinting or heavy lifting. It relies on:

• Creatine phosphate (CP): A molecule stored in muscles that donates a phosphate group to ADP to regenerate ATP.
• Enzyme: Creatine kinase catalyzes the reaction.

Key characteristics:

• Operates anaerobically (without oxygen).
• High rate of ATP production but low capacity.
• Supports activities lasting up to 10 seconds.

Equation:

$\text{ADP}+\text{CP}\to \text{ATP}+\text{Creatine}$

ATP stores:

• Muscle ATP stores are limited and depleted rapidly.
• Full resynthesis of ATP occurs within 3 to 5 minutes during recovery.

2. Glycolytic system

The glycolytic system involves the breakdown of carbohydrates to produce ATP. Glycolysis can proceed via two pathways:

Fast glycolysis

• Pyruvate is converted to lactate in the sarcoplasm.
• Predominates during high-intensity activities lasting 15 seconds to 2 minutes.

Slow glycolysis

• Pyruvate is transported to the mitochondria for further oxidation via the Krebs cycle.
• Supports moderate-intensity activities lasting several minutes.

Equation:

$\text{Glucose}+2\text{P}+2\text{ADP}\to 2\text{Lactate}+2\text{ATP}+2{\text{H}}_2\text{O}$

Key enzymes: Phosphofructokinase (PFK) regulates the rate of glycolysis.

Byproducts:

• Hydrogen ions (${\text{H}}^{+}$) contribute to metabolic acidosis.
• Lactate is a usable energy source in the Cori cycle, not a cause of fatigue.

3. Oxidative (aerobic) system

The oxidative system is the primary energy source for prolonged, low-intensity activities. It utilizes:

• Carbohydrates: Oxidized through glycolysis, the Krebs cycle, and the electron transport chain (ETC).
• Fats: Broken down via beta-oxidation to produce acetyl-CoA, which enters the Krebs cycle.
• Proteins: Contribute minimally, except during prolonged exercise.

Key features:

• High capacity for ATP production but slower rate.
• Requires oxygen and is most efficient for sustained activities.

Krebs cycle and electron transport chain (ETC)

The Krebs cycle, also known as the citric acid cycle, is a series of enzyme-catalyzed chemical reactions that occur in the mitochondria. It plays a key role in cellular respiration by oxidizing acetyl-CoA to produce energy in the form of ATP. The cycle generates high-energy electron carriers, NADH and FADH${}_2$, which are subsequently used in the electron transport chain to produce ATP through oxidative phosphorylation.

Key steps of the Krebs cycle:

1. Acetyl-CoA combines with oxaloacetate to form citrate.
2. Citrate undergoes a series of transformations, releasing two molecules of CO${}_2$.
3. NADH and FADH${}_2$ are generated through redox reactions.
4. ATP (or GTP) is produced through substrate-level phosphorylation.
5. The cycle regenerates oxaloacetate, allowing the process to continue.

ATP yield:

• 1 glucose molecule produces ~38 ATP in the oxidative system.
• Each turn of the Krebs cycle yields 1 ATP, 3 NADH, and 1 FADH${}_2$.
• 1 glucose molecule produces ~38 ATP in the oxidative system.

Energy systems and exercise intensity

Energy systems work together in a continuum based on the intensity and duration of exercise:

While one system may dominate depending on the activity, all three energy systems contribute simultaneously to varying degrees.