Lecture 2 ExPhys

BIOENERGETICS & METABOLISM

TERMS TO KNOW

  • Substrates:
      - The three basic fuels or energy sources that get broken down to release stored energy.

  • Bioenergetics:
      - Chemical pathways within the cells that convert these substrates to energy for use by that cell or other cells in the body.

  • Metabolism:
      - All of the chemical reactions in the body collectively.

  • Kilocalories:
      - The unit in which energy is expressed in humans.

ENERGY SUBSTRATES

BREAKING OF BONDS

  • Energy is released when the chemical bonds that hold elements together are broken.

  • Substrates are composed of carbon, oxygen, hydrogen, and nitrogen (only in proteins).

  • When these bonds break, ATP is produced.

WHAT ARE THE 3 SUBSTRATES?
Key Questions:

  • Name three food sources.

  • Which two substrates are primarily used for energy?

  • Which substrate is used for muscle metabolism?

  • What are the building blocks of EACH substrate?

The Three Primary Substrates:

  1. Fats (Triglycerides):
       - Free fatty acids + Glycerol.
       - Lipogenesis: Conversion of excess energy substrates into fat for storage.

  2. Carbohydrates:
       - Glucose, stored as glycogen.
       - Glycogenolysis: Breakdown of glycogen to glucose.

  3. Proteins:
       - Building blocks are Amino acids.
       - Proteins can be converted into glucose through gluconeogenesis, which has minimal contribution to energy needs.

CARBOHYDRATES

Usages During Exercise

  • The quantity of carbohydrates used during exercise correlates with both carbohydrate availability and the muscle’s developed carbohydrate metabolism system.

  • All carbohydrates ultimately convert into glucose, a simple six-carbon sugar that is easily transported through the blood to body tissues.

STORAGE IN BODY

  • Ingested carbohydrates store in the muscles and liver as glycogen (a complex polysaccharide).

  • Glycogenesis: Conversion of glucose to glycogen for storage in muscle cells.

  • Glycogen can also be stored in the liver and converted back to glucose as needed for energy.

  • Glycogen stores are limited and can be depleted or reach maximum capacity.

  • Carbohydrates are the only energy source utilized by brain tissue; hence, significant carbohydrate depletion can cause cognitive impairment.

TYPES OF CARBOHYDRATES

  • Monosaccharides:
      - Examples: Glucose, Fructose.

  • Disaccharides:
      - Examples: Maltose, Sucrose, Galactose, Lactose.

  • Polysaccharides:
      - Examples: Starch, Glycogen, Cellulose.

FATS

Energy Source

  • Fats provide the majority of energy during prolonged, less intensive exercise.

  • Fat stores represent a significantly larger energy reserve compared to carbohydrates.

  • However, fat is less accessible for cellular metabolism because it first requires conversion from triglycerides to free fatty acids (FFAs) and glycerol.

  • Only FFAs can form ATP.

  • The release rate of energy from fat is slower, insufficient to meet the energy demands of intense activity.

NON-ENERGY FUNCTIONS

  • Phospholipids:
      - Key structural components of cell membranes and protective sheaths around nerves.

  • Steroids:
      - Found in cell membranes, functioning as hormone building blocks.

PROTEIN

Energy Usage

  • Proteins serve as a minor energy source and must be converted to glucose via gluconeogenesis for energy.

  • In severe depletion or starvation, proteins can generate FFAs through lipogenesis.

  • Proteins can provide up to 10% of the energy needed during prolonged exercise.

  • Proteins must be broken down into amino acids to be used.

BIOENERGETICS

CONTROLLING ENERGY RATES

  • Free energy must be released from chemical compounds at a controlled rate, primarily influenced by:
      1. The availability of the primary substrate.
      2. Enzyme activity.

Availability of Substrates

  • Diet impacts substrate availability (e.g., ketogenic diet).

  • An abundance of one fuel type (like carbohydrates) may result in the body's reliance on that source over alternatives.

ENZYME ACTIVITY

  • Enzymes:
      - Proteins that accelerate chemical compound breakdown.
      - They do not cause reactions or determine energy output; they merely lower the activation energy required to initiate the reaction.

  • Most enzyme names end with the suffix ‘-ase’ (e.g. ATPase for ATP breakdown).

SPECIFICITY OF ENZYMES

  • Enzymes are specific; each step in a chemical reaction requires different enzymes.

  • Increasing enzyme presence or activity accelerates product formation (influence of temperature and pH).

  • Certain reactions have “manager” enzymes, essential for governing the overall reaction rate, often termed the rate-limiting enzyme.

  • Its activity is influenced by the accumulation of substrates further down the pathway that inhibit activity through negative feedback.

ATP: ADENOSINE TRIPHOSPHATE

  • The immediate energy source for nearly all bodily functions (including muscle contraction).

  • Upon hydrolysis with water and enzyme action (ATPase), the last phosphate group splits, rapidly releasing free energy.

  • This reaction reduces ATP to ADP + Pi (adenosine diphosphate + inorganic phosphate).

  • Phosphorylation: The process of re-synthesizing ATP from ADP + Pi.

THE ENERGY SYSTEMS

Speed and Duration of Energy Systems

  • Power:
      - Duration: 6-10 seconds (ATP-PCr system).

  • Anaerobic glycolysis:
      - Duration: 30-60 seconds (Glycolytic system).

  • Aerobic lipolysis:
      - Duration: 3-4 hours (Oxidative system).

CALORIE SUMMARY

  • 1 gram of carbohydrates: 4 calories

  • 1 gram of protein: 4 calories

  • 1 gram of fat: 9 calories

  • 1 gram of alcohol: 7 calories

BASIC ENERGY SYSTEMS

  1. ATP-PCr system: Simplest energy system utilizing phosphocreatine.

  2. Glycolytic system (glycolysis): ATP production through glycolysis.

  3. Oxidative system (oxidative phosphorylation): Utilizes aerobic metabolism.

  • Anaerobic metabolism: Absence of oxygen; glycolytic (cytoplasm).

  • Aerobic metabolism: Presence of oxygen; oxidative (mitochondria).

ATP-PCr SYSTEM

Features

  • Phosphocreatine (PCr)/creatine phosphate found in cells.

  • This system donates Pi from PCr to ADP to form ATP.

  • The enzyme involved is Creatine Kinase (CK).

  • Activity is enhanced with increased ADP or Pi concentrations, while inhibited with increased ATP concentrations (indicating sufficient ATP supply).

Summary During Exercise

  • During exercise, existing ATP in muscle cells is broken down to ADP + Pi.

  • Elevated ADP and Pi concentrations activate creatine kinase to regenerate ATP from PCr.

  • The ATP-PCr system provides energy for 3-15 seconds before shifting reliance to glycolytic or oxidative systems as exercise continues.

GLYCOLYTIC SYSTEM

Overview

  • The second energy production system, using the breakdown of glucose through glycolytic enzymes (glycolysis).

  • Before ATP can be generated, glucose or glycogen must be converted to glucose-6-phosphate.

  • Glycolysis comprises 10-12 enzymatic reactions, varying depending on whether glucose or glycogen is the starting substrate.

Glycolytic Process
Starting with Glycogen

  • Glycogen is converted to pyruvate, generating ATP in the process.

Starting with Glucose

  • Glucose is converted to pyruvate yielding ATP.

  • Notably, starting with glucose produces 1 less ATP compared to glycogen due to an additional step required to convert glucose to glucose-6-phosphate.

Location & Duration

  • Glycolysis occurs in the cytoplasm and supports 1-3 minutes of physical activity.

  • Pyruvate can be converted to lactate (anaerobic) or enter the Krebs cycle (aerobic) for further ATP production.

  • Rate-limiting enzyme: Phosphofructokinase (PFK), with enhanced activity from increased ADP + Pi and inhibited by increased ATP concentrations.

Pros and Limitations

  • Pros:
      - Operates anaerobically (no oxygen needed) and supports high-intensity activity initially.

  • Limitations:
      - Limited ATP yield and lactate accumulation, potentially leading to muscle fatigue.

Discussion on Lactic Acid

  • Recent research highlighting that lactic acid is not merely a waste product but an energy source, illustrating the adaptive capabilities of trained muscles to utilize lactate for energy, thus enhancing endurance performance.

OXIDATIVE SYSTEM

Characteristics

  • The most complex energy system, which uses oxygen to break down substrates for ATP generation (known as cellular respiration).

  • The oxidative system is activated by endurance exercise or activities lasting over a few minutes.

  • It operates in the mitochondria, with the number of mitochondria in a cell increasing with ATP demand, particularly in response to endurance training.

Process (Using Carbohydrates)

  1. Glycolysis:
       - Converts glucose into 2 pyruvate, generating ATP.

  2. Krebs Cycle:
       - The pyruvate converted into Acetyl CoA enters Krebs Cycle producing ATP and hydrogen ions.
       - Each glucose molecule yields 2 ATPs during Krebs Cycle.

  3. Electron Transport Chain:
       - Takes hydrogen ions from glycolysis and Krebs cycle to produce 32-33 ATP per glucose molecule.

Process (Using Fats)
Lipolysis

  • Breakdown of triglycerides into glycerol and 3 FFAs prior to generating ATP.

Beta-Oxidation

  • FFAs are converted to Acetyl CoA in mitochondria.

  • Unlike glycolysis, this process does not produce ATP.

Importantly:

  • The number of ATP from fat oxidation is dependent on the fatty acid chain length (e.g., a 16-carbon FFA can yield 106 ATP molecules).

Proteins in Metabolism

  • Proteins serve less frequently as energy sources and need to be converted into amino acids then glucose through gluconeogenesis.

  • The use of protein for energy results in ATP loss (costly) and is usually considered during starvation periods.

INTERACTION OF ENERGY SYSTEMS

  • The three energy systems interact continually, with no activity fully supported by a single energy system.

  • A dominant energy system may change during the transition phase between activities.

  • Generally, fats are the primary substrate at rest and during low to moderate intensities, while carbohydrates dominate at high intensities, influenced by training status and muscle fiber types.

OXIDATIVE CAPACITY OF A MUSCLE

Determining Factors

  1. Oxidative enzyme concentration:
       - More oxidative enzymes equal prolonged aerobic activity capacity. Endurance athletes have significantly more oxidative enzymes than untrained individuals.

  2. Fiber type concentration:
       - Type I fibers possess greater aerobic capabilities; more Type I fibers enhance oxidative capacity.

  3. Oxygen availability:
       - As exercise increases, so does ATP demand; sufficient oxygen is essential to meet increased oxidative ATP production requirements.