Bioenergetics and Exercising - Chapter 2
Bioenergetics & Exercising Muscle
01 Intro to Bioenergetics
Definition of Bioenergetics
Bioenergetics: The study of chemical pathways that convert substrates to energy within biological organisms.
Metabolism: The process by which the body converts food and drink (via chemical reactions) into energy for the body to live and function.
Substrates: Basic fuel sources (i.e., carbohydrates, fats, protein) broken down in the body and used as energy to produce ATP.
Chemical Energy: The energy derived from macronutrients (carbohydrates, fats, proteins), which is then converted into ATP, the body's energy currency.
Metabolism: This process generates chemical waste products such as carbon dioxide, water, and heat.
Measuring Energy
Energy is released when chemical bonds are broken.
Energy is measured by the amount of heat produced.
In biological reactions, energy is equivalent to heat.
1cal(calorie) = the amount of heat energy needed to raise 1{ g} . of water 1oC . (centigrade).
In humans, energy is measured in kilocalories (kcal).
1k{cal}=1000{ cal} .
Energy Substrates
Substrates are primarily composed of carbon, hydrogen, oxygen, and (in the case of protein) nitrogen.
Molecular bonds store energy; providing little energy when broken directly.
Thus, food is not a direct source of energy.
Energy in our food's molecular bonds is chemically released within our cells and stored as ATP.
Substrates for Fuel
Carbohydrates (CHO):
Yield approximately 4{ kcal/g} .
Approximately 500 ext{ g} are stored in the liver and skeletal muscle as glycogen.
Primary ATP source for the brain.
Rely on dietary consumption to replenish.
Glycogen stored in the liver, and ultimately all CHO, are converted to glucose.
Glucose is a monosaccharide (one-unit sugar) transported via blood to all body tissues and is the primary circulating blood sugar (approx. 99 ext{%}).
Fat (Free Fatty Acids; FFA):
Yield approximately 9 ext{ kcal/g}.
Used for long, low-intensity activity.
High amounts of ATP are generated at a slow rate.
Must first be broken down: Triglycerides (TG)—→Glycerol + FFAs—→ATP.
Over 70,000{ kcal} are stored in the body as triglycerides.
Protein (PRO):
Yield approximately 4 ext{ kcal/g}.
Not the preferred energy source.
Only the most basic units of protein, amino acids (AA), can be used for energy.
PRO $\rightarrow$ glucose = gluconeogenesis.
PRO $\rightarrow$ fatty acids = lipogenesis.
Food Intake and Energy Storage Diagram
Food Intake (Carbohydrates, Fats, Proteins) leads to:
Carbohydrates—→Glucose pool—→Glycogenesis (storage as Glycogen in liver/muscle) or usage.
Fats (Triglycerides)—→ Lipolysis—→ FFA pool + Glycerol—→ Fat stores or usage. Excess glucose can undergo Lipogenesis to become fat stores.
Proteins—→Protein breakdown—→ Amino acid pool—→ Body protein or usage. Minimal contribution to Gluconeogenesis (PRO—→ glucose) or Lipogenesis (PRO—→fatty acids).
All these metabolic pathways are interconnected to maintain energy balance.
02 Controlling the Rate of Energy Production
Free Energy: How and Why
Free Energy: The portion of energy in a system available for work (e.g., muscle contraction, ion transport, biosynthesis).
Cells harness free energy primarily through the breakdown of high-energy compounds like ATP to fuel physiological processes.
Free energy must be released at a controlled rate to be usable for physiological processes.
Two primary factors determine this rate:
Availability of primary substrate.
Enzyme activity.
Controlling the Rate of Energy Production
Occurs in two main ways:
Availability of primary substrate:
Mass Action Effect: The influence of substrate availability on the rate of metabolism.
Increased substrate availability leads to increased pathway activity.
Different metabolic pathways (e.g., glycolysis, Krebs cycle, beta-oxidation) are activated based on the availability and demand for energy.
An abundance of one substrate leads to the cells' increased reliance on that substrate.
Exercise Intensity: Higher exercise intensity increases the need for energy, leading to increased reliance on certain pathways (e.g., glycolysis) based on how readily available the corresponding substrate (e.g., glucose) is.
Enzyme activity:
Enzymes: Proteins that help speed up breakdown (catabolism) of substrates by lowering the activation energy required to start a reaction.
Enzyme names typically end with the suffix "-ase" (e.g., ATPase, Creatine Kinase).
Increased enzyme activity leads to increased product formation.
Enzyme Regulation in Metabolic Pathways
Metabolic pathways consist of multiple enzyme-catalyzed steps.
Rate-limiting enzyme: One key enzyme, typically early in the pathway, serves as the control point for the overall reaction rate/speed.
The concentration of products in a pathway can signal the enzyme to speed up or slow down, creating a "bottleneck."
Negative feedback regulates the rate-limiting enzyme:
Accumulation of end-products or high levels of ext{ATP} / ext{ADP} / ext{Pi} ratio inhibits further enzyme activity.
This ensures efficient energy use and prevents overproduction.
Example: High [ ext{product 3}] leads to decreased enzyme 1 activity. Low [ ext{product 3}] leads to increased enzyme 1 activity.
03 Energy Systems - Anaerobic
Source of Energy = ATP
Adenosine Triphosphate (ATP): The immediately available source of energy for almost all bodily functions.
ATP consists of adenosine and three phosphate groups, making it a high-energy compound.
ADP (Adenosine Diphosphate) has two phosphate groups.
AMP (Adenosine Monophosphate) has one phosphate group.
Examples of ATP reactions:
2 ext{ADP}
ightleftharpoons ext{ATP} + ext{AMP}ext{ATP}
ightarrow ext{ADP} + ext{Pi} ext{ (Energy Release)}
Synthesis and breakdown of ATP can occur in the absence (anaerobic) or presence (oxidative phosphorylation/aerobic) of ext{O}_2 .
Hydrolysis of ATP releases energy.
Phosphorylation of ADP creates ATP from byproducts.
Basic Energy Systems
Stored ATP is limited!
ATP synthesis pathways:
ATP-PCr System (anaerobic metabolism): Substrate-level phosphorylation, occurs without oxygen. This is the simplest and fastest system.
Anaerobic Glycolysis (anaerobic metabolism): Also without oxygen, involves different substrates (glucose/glycogen). Provides ATP faster than oxidative phosphorylation but slower than PCr.
Oxidative Phosphorylation / Aerobic Metabolism (aerobic metabolism): Requires oxygen and is the most energy-efficient.
Conclusion: ATP is both produced and consumed rapidly, and its constant recycling is critical for maintaining energy supply during physical activity.
Anaerobic: ATP-PCr System
ATP-PCr is the simplest form of energy system.
Replenishes ATP stores during rest.
Recycles ATP during exercise until phosphocreatine (PCr) is used up.
Duration: Fuels approximately 3-15 seconds of maximal exercise.
Reaction: ext{ADP} + ext{PCr}
ightleftharpoons ext{ATP} + ext{Cr}.Enzyme: Creatine Kinase (CK).
Regulation of Creatine Kinase (CK):
Decreased [ ext{ATP}] pulls the reaction to the right (towards ATP production).
Increased [ ext{ATP}] pulls the reaction to the left (towards PCr regeneration).
Fuels short-duration, maximal-exertion exercise (e.g., 100 ext{m} dash, punt return, 40 ext{-yd} dash).
Maintains cellular ATP levels from PCr.
Anaerobic: Glycolysis
Glycolysis: The breakdown of glucose.
Glycogenolysis: The breakdown of liver glycogen.
Digestion of CHO and breakdown of liver glycogen $\rightarrow$ glucose.
Duration: 15 ext{ s} - 2 ext{ min}. of maximal exercise (e.g., 400 ext{m} dash, 800 ext{m} dash, 100 ext{m} backstroke).
All steps occur in the cytoplasm of the cell.
Anaerobic Glycolysis – Steps
Step 1: Energy Costing Step
Fuel source: Glucose OR Glycogen.
Both must be converted to Glucose 6-phosphate (G6P).
Glucose $\rightarrow$ G6P requires 1 ext{ ATP} via Hexokinase enzyme.
Glycogen $\rightarrow$ G6P does not directly consume ATP from this step.
Step 2: Rearrangement from Glucose to Fructose
Glucose 6-phosphate $\rightarrow$ Fructose 6-phosphate via Glucose 6-phosphate isomerase.
Step 3: Energy Costing Step
Fructose 6-phosphate $\rightarrow$ Fructose 1,6-bisphosphate via Phosphofructokinase (PFK).
This step requires 1 ext{ ATP}.
PFK is the anaerobic glycolysis rate-limiting enzyme.
Increased [ ext{ADP}] and [ ext{Pi}] enhance PFK activity, speeding up glycolysis.
Increased [ ext{ATP}] inhibits PFK activity, slowing down glycolysis (negative feedback).
Net ATP loss up to this point:
For glucose: 2 ext{ ATP}.
For glycogen: 1 ext{ ATP}.
Steps 4-10: Energy Creating Steps
Result in the production of high-energy compounds.
Net Gain: +2 ext{ NADH}^+ ext{, } +4 ext{ ATP, and } +2 ext{ Pyruvate} (per glucose/glycogen molecule).
In anaerobic glycolysis, pyruvate is converted to lactate/lactic acid due to the lack of ext{O}_2 .
Anaerobic Glycolysis – Net Gain
From 1 molecule of Glucose:
Uses 2 ext{ ATP} (Steps 1 & 3).
Generates 4 ext{ ATP}.
Net ATP gain: 2 ext{ ATP}.
Generates 2 ext{ NADH}^+ .
Generates 2 ext{ Pyruvate}.
From 1 molecule of Glycogen:
Uses 1 ext{ ATP} (Step 3 only, as conversion to G6P is different).
Generates 4 ext{ ATP}.
Net ATP gain: 3 ext{ ATP}.
Generates 2 ext{ NADH}^+ .
Generates 2 ext{ Pyruvate}.
Anaerobic Glycolysis – Summary
Total of 10-12 reactions.
All steps occur in the cytoplasm of the cell.
Enzymes involved: Hexokinase (for glucose) and PFK.
Generated ATP fuels approximately 2 ext{ min} of maximal exercise.
Anaerobic Glycolysis – Pros/Cons
Pros:
Muscles can contract when ext{O}_2 is limited.
Provides for longer-term, higher-intensity exercise than the ATP-PCr system can sustain.
Lactate can be used as an energy source (e.g., in aerobic metabolism).
Cons:
Low ATP yield, inefficient use of CHO.
Lack of ext{O}_2 converts pyruvate to lactic acid (a waste product).
Lactic acid can impair glycolysis and muscle contraction, contributing to fatigue.
04 Energy Systems - Aerobic
Aerobic: Oxidative Phosphorylation
Occurs primarily in the mitochondria.
Oxidative Phosphorylation: The process by which the body breaks down substrates with the aid of ext{O}_2 to generate energy.
It is slow to "turn on" but has a greater energy (ATP) capacity compared to anaerobic systems.
Primary method of energy production during endurance activities.
Phases of Oxidative Phosphorylation
Oxidative production of ATP from CHO or fats involves three main phases:
Glycolysis (for CHO) or \beta-oxidation (for fats).
Krebs Cycle (also known as the citric acid cycle).
Electron Transport Chain (ETC).
Aerobic: Oxidative Phosphorylation – Phase 1: CHO Oxidation - Glycolysis
Glycolysis occurs with or without ext{O}_2 .
The ATP yield at the end of glycolysis (net 2 ext{ ATP} from glucose, 3 ext{ ATP} from glycogen) is still the same.
With ext{O}_2 present:
Pyruvate, generated in the cytosol, is transported into the mitochondria.
Inside the mitochondria, pyruvate is converted to Acetyl-CoA.
Acetyl-CoA can then enter the Krebs Cycle.
Net Gain (Glycolysis to Acetyl-CoA):
2 ext{ Acetyl-CoA}
2 ext{ NADH}^+
2-3 ext{ ATP} (depending on glucose or glycogen origin)
Aerobic: Oxidative Phosphorylation – Phase 2: CHO Oxidation – Krebs Cycle
1 ext{ Glucose/Glycogen}
ightarrow 2 ext{ Pyruvate}
ightarrow 2 ext{ Acetyl-CoA}.1 ext{ Acetyl-CoA} molecule results in 1 "turn" of the Krebs Cycle.
Since 2 ext{ Acetyl-CoA} are produced per glucose, the Krebs Cycle turns twice.
Net Gain per 1 ext{ Acetyl-CoA} turn:
1 ext{ ATP}
3 ext{ NADH}^+
1 ext{ FADH}_2
Net Gain per Glucose/Glycogen (2 Acetyl-CoA turns):
2 ext{ ATP} (1 ext{ ATP} imes 2 )
6 ext{ NADH}^+ (3 ext{ NADH}^+ imes 2)
2 ext{ FADH}2 (1 ext{ FADH}2 imes 2)
Regulation of Krebs Cycle:
Negative feedback regulates the Krebs Cycle.
Rate-limiting enzyme: Isocitrate Dehydrogenase.
Increased [ ext{ATP}] inhibits isocitrate dehydrogenase activity.
Decreased [ ext{ATP}] enhances isocitrate dehydrogenase activity.
Aerobic: Oxidative Phosphorylation – Phase 3: Electron Transport Chain (ETC)
The ETC uses the ext{NADH}^+ and ext{FADH}_2 produced in glycolysis and the Krebs cycle to generate a large amount of ATP via a series of protein complexes and oxygen as the final electron acceptor.
This process occurs on the inner mitochondrial membrane.
Major Points on Oxidative System
The oxidative system begins in the cytosol (glycolysis) and ends in the mitochondria (Krebs cycle, ETC).
It involves three stages: Glycolysis (in cytosol/mitochondrial entry), Krebs Cycle (in mitochondria), and Electron Transport Chain (in mitochondria).
Fat oxidation yields more ATP than carbohydrates, but at a slower rate.
Tradeoff: Faster ATP production corresponds to shorter duration activity. Slower ATP production corresponds to longer duration activity.
As exercise intensity increases, the body shifts from relying primarily on fat (FAT) to relying more on carbohydrates (CHO).
Endurance training improves the oxidative capacity of Type I fibers, helping increase ATP yield and duration.
Review Questions (Integrated Answers)
Primary Substrates: The three primary substrates are Carbohydrates, Fats, and Proteins.
Readily Available Substrate for ATP Synthesis: While glucose is readily available from stored glycogen and circulating blood sugar, phosphocreatine (PCr) is most immediately available for ATP synthesis via the ATP-PCr system, due to its direct phosphate donation to ADP.
Methods for Controlling Energy Production Rate: The two primary methods are the availability of primary substrate (Mass Action Effect) and enzyme activity.
How Enzymes Work: Enzymes make chemical reactions easier to complete by lowering the activation energy required to start the reaction.
Rate-Limiting Enzyme Feedback: Rate-limiting enzymes mostly work from a negative feedback loop, where product accumulation inhibits enzyme activity.
Glycolysis and ext{O}2 : Glycolysis does not need ext{O}2 to work (it is an anaerobic process).
Duration of Anaerobic Systems: Anaerobic glycolysis can fuel exercise for approximately 15 ext{ s} - 2 ext{ min} . The ATP-PCr system can sustain exercise for about 3-15 ext{ s}.
Main Substrate in Anaerobic Glycolysis: The main substrate utilized in anaerobic glycolysis is glucose (or glycogen).
Regulator of ATP-PCr: The enzyme Creatine Kinase (CK) regulates the ATP-PCr system, responding to [ ext{ADP}] and [ ext{ATP}] levels.
Location of Anaerobic Glycolysis: Anaerobic glycolysis works within the cytoplasm of the cell.
Rate-Limiting Enzyme of Anaerobic Glycolysis: The rate-limiting enzyme of anaerobic glycolysis is Phosphofructokinase (PFK).
Fate of Pyruvate in Absence of ext{O}2 : When ext{O}2 is absent, pyruvate is converted to lactate/lactic acid.