Bioenergetics and Human Energy Systems Study Guide

Introduction to Bioenergetics and Biological Work

• Definition of Energy: Energy is most simply defined as the ability to do work. This definition is fundamental to understanding physical and biological processes (Yoke, 2010).
• Types of Biological Work: The human body requires energy for various forms of physical or biological work, including:
  • Muscular Contraction: Specifically skeletal muscle contraction that facilitates movement, walking, and exercise.
  • Tissue Growth and Repair: This includes the growth of new tissue in children and the healing of tissue in adults.
  • Electrical Impulses: The conduction of electrical signals through the body to control heart rate, regulate the release of hormones, and manage the constriction of blood vessels.
• Energy Conversion: For physical movement to occur, the body must convert chemical energy into physical energy. This process is essential for exercise and general human function.
• Bioenergetics: This field of study focuses on the body’s ability to acquire, convert, store, and utilize energy. In a biological context, it primarily involves the conversion of macronutrients—carbohydrates, fats, and proteins—into usable forms of energy through the breaking down of nutrient molecules.
• The Ultimate Source of Energy: The sun is the primary source of all energy. However, humans do not obtain energy directly from sunlight for exercise. Instead, energy undergoes multiple transformations:
  • Photosynthesis: Plants absorb light energy and transform it into chemical energy, producing complex food molecules (carbohydrates, fats, and proteins).
  • Ingestion: Humans and animals ingest these plants or other animals that have eaten plants, thereby consuming the stored chemical energy.
  • Metabolic Transformation: Once ingested, energy is either used for biological work (e.g., standing up, lifting weights) or stored for later use in adipose tissue (fat), skeletal muscle, and the liver.
• Energy Efficiency and Heat: Humans utilize or store less than half of the original energy available from food. During heavy energy release, such as exercise, a significant amount of energy is lost as heat, which is sufficient to increase body temperature (Yoke, 2010).

Cellular Metabolism: Catabolism and Anabolism

• Metabolism: This is the result of the body's continuous state of catabolism and anabolism.
• Catabolism: The metabolic process of breaking down large molecules into smaller molecules.
  • Example: The breakdown of carbohydrates into glucose, a process that results in the release of energy.
• Anabolism: The metabolic process in which larger molecules are synthesized from smaller molecules.
  • Requirement: Anabolism uses the energy released during catabolism.
  • Example: The formation of protein from amino acids.

Adenosine Triphosphate (ATP): The Body's Energy Currency

• Definition: Adenosine Triphosphate (ATP) is the chemical compound in which food energy is stored within every cell of the body. It is the only direct source of energy that the body can use for muscular contraction.
• Necessity: Without an adequate supply of ATP, muscular activity and muscle growth are impossible. All stored energy from fats or carbohydrates must be converted into ATP before use.
• Chemical Structure: ATP is composed of three distinct parts:
  • One molecule of Adenosine.
  • One molecule of Ribose.
  • Three Phosphate groups.
• High-Energy Bonds: The phosphate groups are attached to the ATP molecule via "high-energy" bonds. When ATP is broken down, a high-energy bond is severed, and a phosphate group is released, providing energy for cellular work.
• ATP Breakdown Reaction:   $ATP \rightarrow ADP + P + \text{Energy Released}$
• ATP Resynthesis: Once ATP is used, it must be replenished. This requires Adenosine Diphosphate (ADP), a phosphate group ($P$), and new energy. ADP is the compound produced when ATP is broken down, retaining only two phosphate groups.
• ATP Resynthesis Reaction:   $ADP + P + \text{Energy Required} \rightarrow ATP$
• Metabolic Pathways: The body uses three distinct pathways to produce the energy required to resynthesize ATP:
  1. The Phosphagen System.
  2. The Lactic Acid System (Fast Glycolysis).
  3. The Aerobic System (Oxidative System).
• System Dominance: Which energy system dominates depends entirely on the intensity and duration of the exercise being performed.

The Phosphagen System (ATP-CP System)

• Overview: Also known as the ATP-CP system, this is the simplest and fastest energy system. It relies on chemical fuel stored directly in the muscles (muscle phosphagens).
• Function: The system supplies energy rapidly and is immediately available at the onset of all types of exercise, regardless of intensity. It is the primary source of ATP for short-term, high-intensity activities like jumping or sprinting.
• Chemical Mechanism: The system relies on a coupled reaction involving Creatine Phosphate (CP). CP consists of a creatine molecule and a phosphate group joined by a high-energy bond.
  • The Reaction:     $\text{Creatine Phosphate} + ADP \rightleftharpoons \text{Creatine} + ATP$
• Limiting Factors: The capacity of the phosphagen system is limited because ATP and CP are stored in small amounts in the muscle.
  • Fatigue occurs once phosphagen levels are depleted.
  • In an all-out effort, the system can only sustain activity for approximately $30$ seconds.
• Muscle Fiber Specifics: Type II (fast-twitch) muscle fibers generally contain higher concentrations of phosphagens than Type I (slow-twitch) fibers.
• Thresholds and Reliability:
  • $1-5$ seconds: Extremely heavy reliance on phosphagens (e.g., power lifting, shot put, $100\text{m}$ sprints).
  • $15-20$ seconds: Primary energy source, providing over $50\%$ of energy requirements.
  • $30-45$ seconds: Reliance on the system decreases as intensity drops.

The Lactic Acid System (Fast Glycolysis)

• Overview: Also known as fast glycolysis, this system involves the rapid breakdown of carbohydrates (glucose or glycogen) to produce ATP without the immediate need for oxygen.
• ATP Production: It produces more ATP than the phosphagen system because the supply of glucose is greater than the supply of muscle phosphagens.
• Function: It is the primary energy source for sustained higher-intensity exercise lasting from a few seconds up to about $2$ minutes (e.g., an intense tennis volley).
• Glycolysis Process: Carbohydrates (blood glucose or muscle glycogen) are broken down into a compound called pyruvate.
• Fate of Pyruvate:
  • Slow Glycolysis: If oxygen is sufficient and energy demand is low, pyruvate enters the mitochondria and is combusted aerobically.
  • Fast Glycolysis: If oxygen is insufficient or energy demand is high, pyruvate is converted into lactate (lactic acid).
• The Role of Lactic Acid / Hydrogen Ions: High concentrations of lactic acid lead to muscle fatigue. Specifically, the accumulation of hydrogen ions increases acidity (decreased pH), which inhibits glycolytic reactions, interferes with muscle contraction, and slows enzyme activity.
• Limiting Factor: The primary limiting factor is the accumulation of lactic acid, not the depletion of fuel.
• Lactate Threshold: The point where blood lactate begins an abrupt increase above baseline.
  • Untrained subjects: Begins at $50\%-60\%$ of maximal oxygen uptake ($VO_2\text{ max}$).
  • Trained subjects: Begins at $70\%-80\%$ of maximal oxygen uptake ($VO_2\text{ max}$).
• Relevant Activities: Prolonged sprints ($400-800\text{m}$ running, $1,000-2,000\text{m}$ cycling) and sustained high-intensity rallies in sports like soccer or basketball.

The Aerobic System (Oxidative System)

• Overview: This system relies on oxygen and is the primary ATP source at rest and during low-to-moderate intensity activities. It has an unlimited capacity to produce energy.
• Fuels Used: It can synthesize ATP from carbohydrates, fats (lipids), and proteins.
• Energy Shifts:
  • At Rest: Approximately $70\%$ of ATP is derived from fats and $30\%$ from carbohydrates.
  • High-Intensity Aerobic: Shifts toward almost $100\%$ carbohydrate usage if available.
  • Prolonged Sub-maximal Activity: Shifts gradually from carbohydrates back to fats and proteins.
• Components of Aerobic ATP Production:
  1. Oxidative Process: The initial step where glucose, fat, or protein is oxidized.
  2. Krebs Cycle: Electrons and protons are removed from the cells for subsequent reactions.
  3. Electron Transport System: Facilitates the flow of electrons to provide energy for ATP production.

Specific Oxidation of Substrates

• Glucose Oxidation: In the presence of sufficient oxygen, pyruvate is converted to coenzyme A and enters the Krebs cycle. This is referred to as "slow" or "aerobic" glycolysis, dominant in events lasting $30\text{ seconds}$ to $3\text{ minutes}$.
• Fat Oxidation: Fats are stored as triglycerides in adipose tissue and skeletal muscle.
  • Potential Energy: Fat can supply $70,000-75,000\,kcal$ of energy, whereas glycogen only supplies $1,200-2,000\,kcal$.
  • Beta Oxidation: Triglycerides are broken down by the enzyme lipase into free fatty acids, which enter the blood and then the mitochondria to be converted into acetyl CoA.
• Protein Oxidation: Protein is broken down into amino acids and converted to glucose or Krebs cycle intermediates. It is only utilized significantly during long-term starvation or exercise bouts exceeding $90\text{ minutes}$. Using muscle protein for fuel is generally disadvantageous.

Fat versus Carbohydrate Utilization

• Resting state: An average person burns about $1\,kcal$ per minute at rest, with a $50:50$ ratio of carbohydrate and fat usage.
• Intensity Impact: As intensity increases, lactic acid production inhibits fat usage, and the body shifts toward carbohydrates because the cardiorespiratory system can no longer supply oxygen efficiently enough for fat metabolism.
• Weight Management Note: Higher intensities burn a higher percentage of carbohydrates, but they burn more total calories. Instructors should focus on total caloric expenditure for weight loss rather than low-intensity training aimed at "burning more fat" as a percentage, which results in fewer total calories burned.

Comprehensive Energy System Comparison

Characterization of Systems

Primary Energy System by Duration

Questions & Discussion

• Exercise Review: Identify the Dominant Energy System:

  • Sleeping: Aerobic System.
  • The first few seconds of running: Phosphagen System.
  • In the middle of a 10k run: Aerobic System.
  • 2 repetitions of a push-up: Phosphagen System.
  • Standing up from a chair: Phosphagen System.

• Case Study: Mark wants to do jumping jacks for $3\text{ minutes}$. After $20\text{ seconds}$, he is unable to continue.

  • Analysis: At $20\text{ seconds}$, Mark is likely hitting a fatigue point in the phosphagen system or transitioning into fast glycolysis at too high an intensity.
  • Solution: To complete the full $3\text{ minutes}$, Mark needs to lower the intensity of the jumping jacks so that his body can rely more on the Aerobic system and the Lactic Acid system without accumulating fatiguing levels of hydrogen ions too quickly.