HSS 486 Bioenergetics

Substrates

Fuel sources from which we make energy (adenosine triphosphate [ATP])

• Macronutrients: carbohydrates, fats, proteins

• Energy from chemical bonds in food → stored in ATP

Bioenergetics

Conversion of substrates into energy (cellular-level process)

Metabolism

All chemical reactions in the body (anabolic [building up] and catabolic [breaking down])

What is the preferred fuel source for long-term, low-intensity exercise?

Fats

What is the preferred fuel source for short-term, high-intensity exercise?

Carbohydrates

Respiratory exchange ratio (RER)

Ratio that measures the proportion of fat, carbohydrate, and protein used during aerobic processes

What RER is representative of a 100% carbohydrate metabolism?

1.00

What RER is representative of a 100% fat metabolism?

0.71

How does intensity effect RER?

RER increases as intensity increases; higher intensity reflects a higher reliance on carbohydrate fuel sources and therefore a higher RER (closer to 1.00)

Carbohydrates as a substrate

The primary ATP substrate (for muscles, the brain); ~4.1 kcal/g

• All consumed carbohydrate converted to glucose

• Extra glucose is stored as glycogen in the liver & muscles

• Glycogen stores are limited (~2,500 kcal); dietary carbohydrate is needed to replenish stores

Fat as a substrate

Preferred energy source for prolonged, less intense exercise (yields high net ATP but slow ATP production); ~9.4 kcal/g

• Must be broken down into free fatty acids (FFAs) & glycerol

• Extra FFAs stored as adipose

• Efficient substrate with efficient storage; relatively unlimited (70,000+ kcal) stores

Protein as a substrate

Energy substrate only major contributor during starvation; ~4.1 kcal/g

• Broken down to amino acids (AAs), some of which can enter directly into Krebs cycle, OR can be converted into glucose (gluconeogenesis)

• Extra AAs stored as body protein

Mass action effect

Substrate availability effects metabolic rate, with more available substrate = higher pathway activity and excess of given substrate = cells relying on that substrate more than others

• The body will use what is available to make ATP when there is a lack of the preferred fuel source

Breakdown of ATP

ADP + water + ATPase → ADP + P_i + energy

• ATP breakdown RELEASES energy

Synthesis of ATP

ADP + P_i + energy → ATP

• ATP is the only usable form of energy; all substrates must be converted to ATP first

• Limited ATP storage = constant synthesis of new ATP

What are the three ATP synthesis pathways?

• ATP-PCr system

• Glycolytic system

• Oxidative system

ATP-PCr system

Anaerobic system that recycles ATP during exercise until used up (~3-15 sec. maximal exercise)

• PCr + creatine kinase → Cr + P_i + energy

• ATP yield: 1 ATP/1 mol PCr

• Fastest but least efficient energy system

Examples of activities where ATP-PCr system would dominate

Sprinting, jumping, & powerlifting

Glycolytic system

Anaerobic system used from 15 sec. to 2 min.; all steps occur in the cytoplasm

• Breakdown of glucose → 2 pyruvate (or 2 lactate if no O₂ available)

• ATP yield: net 2-3 ATP/1 glucose

• Creates electron carriers important for oxidative system

Pros of the glycolytic system

• Allows muscles to contract when O₂ is limited

• Permits shorter-term, higher-intensity exercise than oxidative metabolism can sustain

Cons of the glycolytic system

• Low ATP yield (inefficient use of substrate)

• Lack of O₂ → lactic acid, which impairs glycolysis & muscle contraction

Oxidative system

Aerobic system consisting of the Krebs cycle + electron transport chain (ETC) and occuring in the mitochondria; utilized after 2+ min. (steady supply for hours)

• Kreb’s cycle makes electron carriers & ETC makes most ATP

  • 2 pyruvate → 2 acetyl CoA → e- carriers + ATP

• ATP yield dependent on substrate…

  • 32-33 ATP/1 glucose

  • 100+ ATP/1 FFA

• Most complex of the 3 bioenergetic systems

Oxidation of carbohydrate

1. Glycolysis (glucose → pyruvate)

2. Krebs cycle (pyruvate → acetyl CoA)

3. ETC

Oxidation of fat

1. Lipolysis (triglycerides → FFAs)

2. Beta oxidation (FFAs → acetyl CoA)

3. Oxidative system (Krebs + ETC)

**Yields 3-4x more ATP than glucose, but slower than glucose oxidation

Oxidation of protein

Rarely used as a substrate, but…

1. Can be converted to glucose → glycolysis

2. Some AAs → Krebs cycle

Lactate

Lactate can be an important fuel during exercise, with muscles utilizing it in 3 ways:

1. Lactate produced in cytoplasm can be taken up by mitochondria of the same muscle fiber and oxidized

2. Lactate can be transported to another cell and oxidized there (lactate shuttle)

3. Lactate can recirculate back to the liver and be reconverted to pyruvate then glucose through gluconeogenesis (Cori cycle)

Primary energy system(s) during 0-6 s of very intense exercise

ATP-PCr system

Primary energy system(s) during 6-30 s of intense exercise

ATP-PCr system & fast (anaerobic) glycolysis

Primary energy system(s) during 30 s to 2 min. of heavy exercise

Fast (anaerobic) glycolysis

Primary energy system(s) during 2-3 min. of very moderate exercise

Fast (anaerobic) glycolysis & oxidative system

Primary energy system(s) during 3+ min. of light exercise

Oxidative system

Bioenergetic demands & sport/event specific training

Bioenergetic demands dictate training by aligning exercises with a sport's primary energy systems (ATP-PCr, glycolytic, oxidative) for efficiency, leading to specific adaptations like power for sprints (ATP-PCr), lactate tolerance for repeated bursts (glycolytic), or sustained endurance (oxidative).