Bioenergetics & Exercising
Bioenergetics & Exercising Muscle: Chapter 2 Part 1
Introduction to Bioenergetics
Chemical Energy and ATP
Chemical energy in the body is primarily derived from Carbohydrates, Fats, and other less utilized sources.
ATP (Adenosine Triphosphate) is the body's universal energy currency, directly fueling almost all bodily functions.
Metabolism refers to the chemical reactions that convert food and drink into energy for the body's survival and function.
Chemical waste products of metabolism include Carbon dioxide, Water, and Heat.
Key Terms
Bioenergetics: The study of complex chemical pathways that convert substrates (like food) into usable energy within biological organisms.
Metabolism: The sum of all chemical processes that convert food and drink into energy to sustain life and function.
Substrates: Basic fuel sources (Carbohydrates, Fats, Proteins) that are broken down in the body and used to produce ATP.
Measuring Energy
Energy is released when chemical bonds are broken.
Energy is measured by the amount of heat produced.
In biological reactions, energy is equated to heat.
Calorie (1 \text{ cal}): The amount of heat energy required to raise the temperature of 1 \text{ gram} of water by 1^\text{o} \text{C}.
In humans, energy is typically measured in kilocalories (kcal).
1 \text{ kcal} = 1000 \text{ cal}.
Energy Substrates
Substrates are primarily composed of carbon, hydrogen, oxygen, and in the case of protein, nitrogen.
The molecular bonds within substrates are considered weak, providing little direct energy when broken.
Therefore, food is not direct energy; the energy contained in our food's molecular bonds is chemically released within our cells and subsequently stored as ATP.
food does not equal cellular function
Substrates for Fuel
Carbohydrates (CHO)
Energy yield: 4\text{ kcal/g} .
Storage: Approximately 500\text{g} of carbohydrates are stored in the liver and skeletal muscle as glycogen.
Primary ATP source for the brain.
Dietary consumption is crucial for replenishing carbohydrate stores.
Glycogen stored in the liver, and ultimately all dietary CHO, are converted to glucose, a monosaccharide (one-unit sugar) transported via the blood to all body tissues.
Fat (Free Fatty Acids; FFA)
Energy yield: {}9\text{ kcal/g} .
Preferentially used for long-duration, low-intensity activity.
Contains high amounts of energy but is generated at a slow rate.
Fats must first be broken down from Triglycerides (TG) into Glycerol and Free Fatty Acids (FFAs).
FFAs are then utilized to produce ATP.
Protein (PRO)
Energy yield: {}4\text{ kcal/g} .
Not a preferred energy source for the body.
Only the most basic units of protein, Amino Acids (AA), can be used for energy.
Gluconeogenesis: The process where protein is converted to glucose for energy.
Lipogenesis: The process where protein is converted to fatty acids for energy storage.
Summary of Substrates
Carbohydrates (CHO):
{}4\text{ kcal/g} .
500\text{g} stored in liver and skeletal muscle as glycogen.
Primary unit for metabolism: Glucose.
Fat (Triglyceride; TG):
9\text{ kcal/g} .
>70,000\text{ kcal} stored in the body.
Primary unit for metabolism: Free Fatty Acids (FFA).
Protein (PRO):
4\text{ kcal/g} .
Not a preferred energy source.
Primary unit for metabolism: Amino Acid (AA).
Metabolic Pathways of Fuel Storage and Utilization
Food intake (Carbohydrates, Fats, Proteins) leads to their respective pools:
Carbohydrates—>\text{ Glucose pool }—→ Glycogenesis (storage as Glycogen in stores)—> Glycogenolysis (breakdown into Glucose pool).
Fats (Triglycerides)—→ Lipolysis (breakdown into Free fatty acids + Glycerol) —→\text{ FFA pool }—→ Lipogenesis (storage as Fat stores).
Proteins—→ Protein breakdown—→\text{ Amino acid pool }—> Protein synthesis (storage as Body protein).
Excess components from the Glucose pool or FFA pool can undergo Lipogenesis (conversion to fat stores).
Amino acids can undergo Gluconeogenesis (conversion to glucose pool), although this is a minimal contribution.
Controlling the Rate of Energy Production
Free Energy: How and Why
Free Energy: The portion of energy in a system that is available to perform work (e.g., muscle contraction, ion transport, biosynthesis).
Cells capture free energy mainly by breaking down high-energy compounds like ATP to power physiological processes.
For physiological processes to occur efficiently, free energy must be released at a controlled rate.
The rate of energy production is primarily determined by two factors:
Substrate availability
Enzyme activity
Mechanisms of Rate Control
Availability of Primary Substrate
Mass Action Effect: This principle states that the influence of substrate availability directly impacts the rate of metabolism. More substrate generally leads to a faster reaction rate.
An increase in substrate availability (e.g., glucose, fatty acids, oxygen) leads to an increase in pathway activity.
Different metabolic pathways (e.g., glycolysis, Krebs cycle, beta-oxidation) are activated depending on the availability and demand for energy.
A higher abundance of one substrate causes cells to increase their reliance on that particular substrate.
For example, during increased exercise intensity, there is a greater need for energy, leading to an increased reliance on specific pathways (like glycolysis) based on how readily available the corresponding substrate (e.g., glucose) is.
Enzyme Activity
Enzymes: These are biological catalysts (proteins) that significantly speed up biochemical reactions, particularly the catabolism (breakdown) of substrates.
Enzyme names typically end with the suffix “-ase” (e.g., ATPase, Creatine Kinase).
Enzymes increase reaction rates by lowering the activation energy required to start the reaction.
Higher enzyme activity results in increased product formation.
Enzyme Regulation in Metabolic Pathways
Metabolic pathways involve multiple enzyme-catalyzed steps.
A rate-limiting enzyme, typically found early in a pathway, controls the overall speed or rate of the entire reaction sequence.
The concentration of products within a pathway acts as a signal to either speed up or slow down the rate-limiting enzyme's activity, effectively preventing an overaccumulation of products or unnecessary energy expenditure.
Rate limiting enzyme in PCr bionenergetic pathway is:
Creatine kinase (CK)
Major Energy Systems
1. ATP-PCr System (Phosphagen System)
Primary Role: Provides immediate, rapid ATP for short-duration, very high-intensity activities (e.g., $0-15 seconds).
Mechanism: The stored phosphocreatine (PCr) in muscle cells donates a phosphate group to ADP, regenerating ATP.
PCr + ADP \xrightarrow{\text{Creatine Kinase}} Cr + ATP
Location: Cytoplasm (sarcoplasm).
Oxygen Requirement: Anaerobic (does not require oxygen).
ATP Yield: Very limited (1 ATP per PCr molecule), but produced very quickly.
Rate-limiting enzyme: Creatine Kinase (CK).
2. Glycolytic System (Glycolysis)
Primary Role: Provides rapid ATP for short-to-medium duration, high-intensity activities (e.g., $15 seconds to 2-3 minutes).
Mechanism: Breaks down glucose (from blood or muscle glycogen) into pyruvate.
Net ATP gain: 2 ATP from glucose; 3 ATP from glycogen.
If oxygen is insufficient, pyruvate is converted to lactic acid/lactate, leading to muscle fatigue.
Location: Cytoplasm (sarcoplasm).
Oxygen Requirement: Anaerobic.
ATP Yield: Limited (2-3 ATP per glucose/glycogen molecule), but faster than the oxidative system.
Rate-limiting enzyme: Phosphofructokinase (PFK).
3. Oxidative System (Aerobic Metabolism)
Primary Role: Provides a large amount of ATP for long-duration, low-to-moderate intensity activities (e.g., >2-3 minutes to hours).
Mechanism: Involves three main processes:
Glycolysis (initial breakdown of glucose to pyruvate, which then enters mitochondria).
Krebs Cycle (Citric Acid Cycle): Pyruvate is converted to acetyl-CoA, which enters the Krebs cycle, producing ATP, CO2, and electron carriers (NADH, FADH2).
Electron Transport Chain (ETC): NADH and FADH2 deliver electrons to the ETC, creating a proton gradient that drives ATP synthesis (oxidative phosphorylation).
Substrates: Can metabolize carbohydrates, fats, and, to a lesser extent, proteins.
Location: Mitochondria.
Oxygen Requirement: Aerobic (requires oxygen).
ATP Yield: Very high (approx. 32-33 ATP per glucose molecule; much more from fats), but produced slowly.
Rate-limiting enzymes: Isocitrate Dehydrogenase (for Krebs Cycle), Cytochrome Oxidase (for Electron Transport Chain).