Energy Systems in the Body

Module 1: Bioenergetics

• Bioenergetics is the process of converting food (fats, carbohydrates, protein) into a usable form of energy.

What is Metabolism?

• Metabolism is the total sum of all chemical reactions that occur in the body.
• These reactions involve enzymes.

Two Main Types of Metabolism

• Anabolic metabolism: Synthesis or "building" of molecules.
• Catabolic metabolism: Breakdown of molecules.

ATP

• Adenosine Triphosphate (ATP) is the energy currency of the cell.
• Resting intramuscular concentration of ATP is 5 mmol/kg.
• ATP is not an energy store (~90 g total).
• Muscle contraction can increase cellular demand for ATP by 500-1,000 times.

Macronutrients

• Protein: 4 kcal/g
• Carbohydrates: 4 kcal/g
• Fats: 9 kcal/g

Enzymes

• Enzymes are protein molecules that speed up reactions without being consumed in the process.
• They do not cause reactions.
• They lower the activation energy.
• Enzymes can go either way (forward or backward)
• Enzymes activity is dependent on factors such as pH and temperature.

Enzymes Factors: pH

• Different enzymes have different optimal pH ranges.
• Examples: Pepsin (pH ≈ 2), Urease (pH ≈ 7), and Trypsin (pH ≈ 8).
• Optimal pH is where the enzyme's activity is maximum.

Enzymes Factors: Temperature

• Enzymes have an optimal temperature to function.
• At high temperatures, the enzyme becomes denatured (loses its shape) which also causes loss in function.
• At low temperatures, the enzyme becomes inactive.
• Temperature can impact the folding of the protein, potentially impacting its ability to interact with substrates.

The Big Three Energy Systems

• Phosphagen: 3-15 seconds. First and shortest.
• Anaerobic Glycolysis: 15-120 seconds. Second and with two types.
• Oxidative Phosphorylation: > 120 seconds. Third and longest duration.

Other Energy Systems

• The primary energy systems used depend on the intensity and duration of the event.
• The table below summarizes the primary energy systems used for different durations of exercise. Duration | Intensity | Primary Energy System
• -- | --- | --- 0-6 s | Extremely high | Phosphagen 6-30 s | Very high | Phosphagen & Fast Glycolysis 30 s -2 min | High | Fast Glycolysis 2–3 min | Moderate | Fast Glycolysis & Oxidative system

3 min | Low | Oxidative system

Phosphagen Energy System

• The phosphagen system utilizes creatine phosphate (PCr) to rapidly regenerate ATP from ADP.
• This system is used during short-duration, high-intensity activities.

Anaerobic Glycolysis

• This system breaks down glucose/glycogen into pyruvate without oxygen.
• It is used for medium-duration high-intensity activities.

Oxidative Phosphorylation

• This system utilizes oxygen to efficiently produce large amounts of ATP by breaking down carbohydrates, fats, or proteins. It is most efficient for energy over longer time periods.

Energy Systems and ATP Production (Summary)

• Energy systems differ in duration and rate of ATP production.
• They are utilized during aerobic and anaerobic conditions.

Glycolysis

• Glycolysis is the breakdown of glucose or glycogen to pyruvate in cells.
• Two main phases: energy investment and energy payoff
• Net yield from Glucose: 2 ATP (2 are invested, 4 are created) and 2 NADH.
• Net yield from Glycogen: 3 ATP (1 are invested, 4 are created) and 2 NADH.

Anaerobic Glycolysis (Lactate Formation)

• Under conditions of insufficient oxygen, pyruvate can form lactate.
• Lactate is a byproduct of anaerobic respiration.
• It serves as a temporary hydrogen acceptor for NADH+H+
• Lactate is then exported to the blood, and from there to the liver
• The Cori cycle helps regenerate glucose from lactate.

Lactate Threshold

• The lactate threshold (LT) is the point where blood lactate accumulation increases markedly.
• This is an important indicator of the amount of anaerobic metabolism occurring.
• At rest, lactate levels are less than 2 mmol/L.
• During maximal exercise, lactate levels can exceed 10 mmol/L.
• Lactate threshold can be expressed as a percentage of VO2 max.

VO2max

• VO2max is the maximum amount of oxygen someone can intake, transport, and utilize.

Respiratory Quotient (RQ)

• RQ is calculated by dividing the volume of carbon dioxide produced by the volume of oxygen consumed.
• The RQ value can indicate the type of fuel being used for energy production.

The Krebs Cycle (Citric Acid Cycle)

• The Krebs cycle is a series of reactions that occurs in the mitochondrial matrix.
• The acetyl-CoA from pyruvate enters the cycle to produce ATP, NADH, and FADH2.
• Citrate is the starting substrate for the cycle.
• Key intermediates: citrate, isocitrate, α-ketoglutarate, succinyl-CoA, succinate, fumarate, malate, and oxaloacetate.
• Net yield: 2 ATP, 2 FADH2, 6 NADH.

The Electron Transport Chain (ETC)

• The ETC occurs at the inner membrane of the mitochondria.
• Electrons from NADH and FADH2 are passed through a series of complexes, pumping protons (H+) into the intermembrane space creating a concentration gradient.
• This gradient drives ATP synthesis by ATP synthase.
• Net yield from ETC: 30-34 ATP.

ATP Yield Breakdown

• Glycolysis net yield: 2 ATP.
• Conversion of pyruvate to Acetyl-CoA yields 5 ATP.
• TCA cycle (from two Acetyl CoAs) yields 20 ATP.
• Electron Transport Chain yields 30-34 ATP.
• Glycogen yields 2 ATP less than glucose metabolism.
• Total yield from one glucose molecule: 30-32 ATP.
• Total yield from one glycogen molecule: 31 ATP.

Metabolism Wrap Up

• Students should be able to draw glycolysis and label it correctly.
• They should also know the fates of pyruvate and how those reactions work, the intermediates and order of the Krebs cycle, the five steps of the ETC, and the net yields of each of the four stages.

Test your knowledge on the various energy systems utilized by the body during physical activities. This quiz covers key concepts surrounding ATP, anaerobic glycolysis, and muscle contraction. Find out how well you understand the energetic demands of exercise!