Basic Energy System

8. Basic Energy Systems

BASIC ENERGY SYSTEMS

A. Energy All plants & animals depend on energy to sustain life. Humans derive this energy from food. Many forms: chemical, electrical, electromagnetic, thermal, mechanical &

nuclear. All energy forms are interchangeable; e.g. chemical energy used to create

electrical energy stored in battery. Never lost or newly created – it undergoes steady degradation from one form

to another, ultimately becoming heat. 60%-70% of the total energy in humans is degraded to heat.

1. Energy for Cellular Activity All energy originates from the sun as light energy. Chemical reactions in plants convert light into stored chemical energy. Humans obtain energy by eating plants, or animals that feed on plants. Energy is stored in food in the form of carbohydrate, fats & proteins. Human cells can break down these 3 basic food components to release the

stored energy.

Energy Sources Foods are composed of carbon, hydrogen, oxygen, & nitrogen (protein). Molecular bonds in foods are weak & provide little energy when broken. Food is NOT used directly for cellular activity. Energy in food molecules’ bonds chemically released within cells, then stored

in the form of a high-energy compound called adenosine triphosphate (ATP).

At rest, energy that body needs is derived almost equally from the breakdown of CHO & fats.

Proteins provide little energy for cellular function/activity. During mild to severe exercise, more CHO is used. In maximal, short-duration exercise, CHO is used exclusively to produce ATP.

Carbohydrate (CHO) CHO – to be useful must be converted into glucose (monosaccharide) that is

transported to all body tissue via blood. During rest, ingested CHO taken up by muscle & liver, then converted into

glycogen (a more complex glucose molecule). Glycogen is stored in cytoplasm until cells use it to form ATP. Liver & muscle glycogen reserves are limited & can be depleted unless CHO

is increase. CHO stores in liver & skeletal muscle are limited to < 2,000 kcal of energy.

Fats Fat provides 2 times more energy than CHO but less accessible for cellular

metabolism because it must first be reduced from its complex form (triglyceride) to its basic components: glycerol & free fatty acids (FFA).

Only FFA are used to form ATP. Fat is a good source of energy, can be stored exceeding 70,000 kcal of

energy.

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Protein Protein can be used as energy source if convert into glucose. Protein converted into glucose through gluconeogenesis. In severe energy depletion (starvation), protein can be converted to FFA for

cellular energy through lipogenesis. Protein can supply up to 5-10% of the energy needed to sustain prolonged

exercise. Protein can be used as energy source in basic form of amino acids.

Energy Yield 1 g of CHO (C6H12O6) yields 4 kcal of energy. 1 g of fat (C16H18O2) yields 9 kcal of energy. 1 g of protein (NH2 + CO2H) yields 4.1 kcal of energy.(Though 1 g of fat can generate 2.25 times as much as a similar amount of CHO, it also takes substantially more oxygen to metabolize fat than CHO)

B. Bioenergetics

The chemical processes involved with the production of cellular ATP by converting foodstuffs (i.e., carbohydrates, fats, proteins) into a biologically usable form of energy.

ATP Production An ATP molecule consists of adenosine (adenine joined to ribose) combined

with 3 inorganic phosphate (Pi) groups. When acted on by enzyme ATPase (adenosine triphosphatase), the last

phosphate group splits away from the ATP molecule, rapidly releasing a large amount of energy (7.6 kcal per mole of ATP). This reduces the ATP to ADP & Pi.

ATPaseATP ADP + P i

The process of storing energy by forming ATP from other chemical sources is called phosphorylation.

Through various chemical reactions, a phosphate (Pi) groups is added to a relatively low-energy compound, ADP, converting it to ATP.

ADP + Pi ATP

When these reactions occur without oxygen, the process is called anaerobic metabolism.

With the aid of O2, the overall process is called aerobic metabolism & the aerobic conversion of ADP to ATP is oxidative phosphorylation.

Cells generate ATP by 3 methods:1. ATP-PC system2. Glycolytic system3. Oxidative system

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1. ATP-PC system: (Anaerobic ATP Production)

Formation of ATP by PC breakdown. The simplest of the energy system. Phosphocreatine (PC) is a high-energy phosphate molecule that store

in the muscle cells. Energy is released when PC is breakdown / separate to Pi and creatine

by enzyme creatine kinase (CK). This energy is not used directly to accomplish cellular work.

Energy released from the breakdown of PC is used to combine Pi with ADP to form ATP.

This system is anaerobic that functions to maintain the ATP levels. 1 mole of PC will yield 1 mole of ATP. Provides energy for short-term and high-intensity exercise that lasting

about 3-15 seconds.

creatine kinasePC Pi + C + energy

ADP + Pi + energy ATP

Figure: ATP-PC system

2. Glycolytic system

Occurs in the sarcoplasm of the muscle cells. Use only carbohydrate as the main source of fuel. Involves glycolysis the breakdown (lysis) of glucose or liver glycogen to

pyruvic acid via glycolytic enzymes. Glycogen is synthesized from glucose by a process called glycogenesis &

stored in the liver or in muscle until needed. Before either glucose 0r glycogen can be used to generate energy, they must

be converted to a compound called glucose-6-phosphate. Conversion of a molecule of glucose requires 1 mole of ATP. 1 mole of glucose produces 2 ATPs or 1 mole of glycogen produces 3

ATPs. Provides energy for high-intensity exercise (80-90% max) up to 2 minutes. If O2 is not available to accept the hydrogen ions in the mitochondria,

pyruvic acid can accept the hydrogen ions to form the lactic acid. This accumulation of lactic acid is a major limitation of anaerobic glycolysis. This acidification of muscle fibers inhibits further glycogen breakdown

because it impairs glycolytic enzymes functions. In addition, the acid decreases the fibers’ calcium-binding capacity & thus

may impede muscle contraction.

Glucose or Glycogen(Need 1 ATP)

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Glucose-6-phosphate

ATP

Pyruric acid

Lactic acid

Figure: Glycolytic system

(Lactic acid is an acid with the chemical formula C3H6O8. Lactate is any salt of lactic acid. When lactic acid releases H+, the remaining compound joins Na+ or K+ to form a salt. Anaerobic glycolysis produces lactic acid, but it quickly dissociates & the salt (lactate) is form.)

3. Oxidative system The body’s most complex energy system, which generates energy by

breakdown of fuels with the aid of O2 (cellular respiration). Because O2 is used, this is an aerobic process. Has a very high-energy yield and yields more energy than the ATP-PC or

glycolytic system. Oxidative production of ATP occurs within the mitochondria. Main energy production during endurance activities.

Oxidative production of ATP involves:i. Oxidation of CHO ii. Oxidation of Fat

i Oxidation of Carbohydrate Involves 3 processes:

a. Aerobic glycolysisb. The Krebs cyclec. The electron transport chain

Aerobic glycolysis In CHO metabolism, glucose or glycogen is broken down to pyruvic acid via

glycolytic enzymes. Hydrogen is released as glucose is metabolized to pyruvic acid. In the presence of O2, the pyruvic acid is converted into acetyl coenzyme A

(acetyl CoA). 1 mole of glucose produces 2 moles of ATP or 1 mole of glycogen produces 3

moles of ATP.

The Krebs cycle 4


Once the acetyl CoA is formed, it enters the Krebs cycle (citric acid cycle), a complex series of chemical reactions that permits the complete oxidation of acetyl CoA.

At the end of the Krebs cycle, 2 moles of ATP have been formed. The substrate (CHO) has been broken down into carbon (C) & hydrogen (H). Remaining C then combine with O2 to form CO2. H+ released combines with 2 coenzymes: NAD (nicotinamide adenine

dinucleotide) & FAD (flavin adenine dinucleotide) to enter electron transport chain (Supplies electrons to be passed through the electron transport chain).

The Electron Transport Chain (Respiratory chain or cytochrome chain) The coenzymes carry the H atom (NADH & FADH) to the electron transport

chain, split into protons & electrons. At the end of the chain, H+ combines with O2 to form H20 (O2–accepting

electrons), thus preventing acidification. The electrons that were split from the H pass through a series of reactions

(ETC) & ultimately provide energy for the phosphorylation of ADP, thus forming ATP.

This process relies on O2, referred to as oxidative phosphorylation.

Energy yield from Carbohydrate- 1 mole of glycogen generates up to 39 moles of ATP.- If 1 mole of glucose, the net gain is 38 ATP (1 mole of ATP is used for

conversion to glucose-6-phosphate before glycolysis).

ii Oxidation of Fat Muscle & liver glycogen stores provide only 1,200 – 2,000 kcal of energy. Fat stored inside the muscle fibers (fat cells) can supply about 70,000 –

75,000 kcal. Triglycerides (major energy sources) stored in fat cells in the skeletal muscle

fibers. Triglycerides break down to its basic units to be used for energy: 1 mol of

glycerol to 3 moles of free fatty acids/FFA (= process lipolysis with lipases enzymes).

FFA can enter blood & be transported throughout the body, entering muscle fibers by diffusion.

ß Oxidation - Upon entering the muscle fibers, FFA are enzymatically activated with

energy from ATP, preparing FFA for catabolism (breakdown) within the mitochondria.

- This enzymatically catabolism of fat (FFA) by the mitochondria = beta oxidation (ß oxidation).

- The carbon chain of FFA is cleaved into separate 2-carbon units of acetic acid. eg. FFA with 16-carbon chain, ß oxidation yields 8 moles of acetic acid. Each acetic acid converted to acetyl CoA.

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The Krebs Cycle & the Electron Transport Chain - Fat metabolism follows the same path as CHO metabolism.- Acetyl CoA formed by ß oxidation enters the Krebs cycle, - Krebs cycle generates H+ that is transported to the electron transport

chain, along with H+ generated during ß oxidation, to undergo oxidative phosphorylation – produce ATP, H2O & CO2.

- The complete combustion of FFA molecule requires more O2 because FFA contains more carbon (C) than a glucose molecule.

More carbon in FFA, more acetyl CoA is formed from the metabolism of fat, so more enters the Krebs cycle & more electrons are sent to the e. t. chain. (Fat metabolism generate more energy than glucose metabolism)

Eg. Palmitic acid, 16-carbon FFA. The combine reaction of oxidation, Krebs cycle, & e. t. chain produce 129 molecules of ATP from 1 mole of palmitic acid. (1 mol of glucose/glycogen = 38/39 moles of ATP)

40% of the energy released by metabolism is captured to form ATP, 60% is given off as heat.

4. Protein Metabolism Proteins (amino acids) are also used as body fuels. Some amino acids can be converted into glucose (gluconeogenesis) Some can be converted into various intermediates of oxidative metabolism

(such as pyruvate or acetyl CoA) to enter the oxidative process. Protein’s energy yield is not easy because it contains nitrogen (N). When amino acids are catabolized, some of the released N is used to form

new amino acids, but remaining N cannot be oxidized by body. N is converted into urea & then excreted in the urine. This conversion use

ATP, so some energy is spent in this process. In laboratory, 1 gram of protein = 5.65 kcal of energy. When metabolized in the body, energy used to convert N to urea, energy

yield is only about 5.20 kcal per gram (8% less than the lab. Value). Healthy body utilizes little protein during rest & exercise (< 5-10% of total

energy expended). Estimates of energy expenditure generally ignore protein metabolism.

5. The Oxidative Capacity of Muscle Oxidative metabolism has the highest energy yields. Oxidative capacity (QO2) – A measure of the muscle’s maximal capacity to

use oxygen. Oxidative capacity depends on:

a. Enzyme Activity b. Fiber-type Composition c. Oxygen Needs

Enzyme Activity Many enzymes are required for oxidation. The enzyme activity of the muscle fibers provides an indication of the

oxidative potential.

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The enzymes most frequently measured are SDH (succinate dehydrogenase), CS (citrate synthase) & mitochondria enzymes in the Krebs cycle.

Endurance athletes’ muscles have oxidative enzyme activities 2-4 times greater than those untrained men & women.

Fiber-type Composition Muscle’s fiber-type composition determines its oxidative capacity. Slow-twitch (ST) fibers have a greater capacity for aerobic activity than the

Fast-twitch (FT) fibers because ST fibers have more mitochondria & higher concentrations of oxidative enzymes.

More ST fibers, the greater oxidative capacity in the muscle. FT fibers are better suited for glycolytic energy production. Elite distance runners have reported to process more ST fibers, more

mitochondria & higher muscle oxidative enzyme activity than untrained individuals.

Endurance training enhances the oxidative capacity of fibers, especially FT fibers.

Training that places demands on oxidative phosphorylation stimulates the muscle fibers to develop more mitochondria that are also larger & contain more oxidative enzymes.

By increasing the fiber’s enzymes for ß oxidation, this training also enables the muscle to rely more heavily on fat for ATP production.

With endurance training, even people with large % of FT fibers can increase their muscles’ aerobic capacities.

Endurance-trained FT fiber will not develop the same high-endurance capacity as a similarly trained ST fiber.

Oxygen Needs Oxidative metabolism depends on an adequate supply of O2. When at rest, body’s need for ATP is small, requiring minimal O2 delivery. As exercise intensity increases, to meet the energy demands, the rate of

oxidative ATP production also increases. In an effort to satisfy the muscle need for O2, the rate & depth of the

respiration increase, improving gas exchange in the lungs, & heart beats faster, pumping more oxygenated blood to the muscle.

C. Causes of Fatigue

1. Depletion of PC or glycogen. The depletion of PC or glycogen will impairs ATP production, thus fatigue

is caused by inadequate energy supply.

2. Accumulation of metabolic by-products. Accumulation of hydrogen (H+) decreases muscle pH, causes muscle

acidification (acidosis), which impairs the cellular processes that produce

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energy (inhibits the action of glycolytic enzyme, slowing the rate of glycolysis & ATP production) & muscle contraction.

3. Failure of neural transmission in the muscle fiber. Fatigue may occur at the motor end plate, preventing nerves impulse transmission to the muscle fiber membrane, thus cause the neuromuscular block and leads to neuromuscular fatigue.

4. CNS may cause fatigue. Perceived fatigue usually leads to psychologically exhausted/fatigue and the exhausted feeling can often be psychologically trauma and may inhibit the

athlete’s willingness to tolerate further pain or to continue exercise.

SUMMARY

1. About 60% to 70% of the energy in human body is degraded to heat. The remainder is used for mechanical work & cellular activities.

2. Humans derive energy from food sources – CHO, fats, & proteins.

3. The energy humans derive from food is stored in a high-energy compound – ATP.

4. CHO provides about 4 kcal of energy per gram, compared to about 9 kcal of energy per gram for fat; but CHO is more accessible. Protein can also provide energy.

5. ATP is generated through 3 energy systems: The ATP-PC system The glycolytic system The oxidative system

6. In the ATP-PC system, Pi is separated from phosphocreatine through the action of creatine kinase. The Pi can then combine with ADP to form ATP. This system is anaerobic, and its main function is to maintain ATP levels. The energy yield is 1 mole of ATP per 1 mole of PC.

7. The glycolytic system involves the process of glycolysis, through which glucose or glycogen is broken down to pyruvic acid via glycolytic enzymes. When conducted without oxygen, the pyruvic acid is converted to lactic acid. 1 mole of glucose yields 2 moles of ATP, but 1 mole of glycogen yields 3 moles of ATP.

8. The ATP-PC and glycolytic systems are major contributors of energy during the early minutes of high-intensity exercise.

9. The oxidative system involves breakdown of fuels with aid of oxygen. This system yields more energy than the ATP-PC or glycolytic system.

10. Oxidation of carbohydrate involves glycolysis, the Krebs cycle, and the electron transport chain. The end result is H2O, CO2, and 38 or39 ATP molecules per carbohydrate molecule.

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11. Fat oxidation begins with ß oxidation of free fatty acids, then follows the same path as carbohydrate oxidation: the Krebs cycle and the electron transport chain. The energy yield for fat oxidation and it varies with the free fatty acid being oxidized.

12. Protein oxidation is more complex because protein (amino acids) contains nitrogen, which cannot be oxidized. Protein contributes relatively little to energy production, so its metabolism is often overlooked.

13. Your muscles’ oxidative capacity depends on their oxidative enzyme levels, their fiber-type composition, and oxygen availability.

14. Fatigue may result from depletion of PC or glycogen. Either of these situations impairs ATP production.

15. Lactic acid has often been blamed for fatigue, but it is actually the H+ generated by lactic acid that leads to fatigue. The accumulation of H+ decreases muscle pH, which impair the cellular processes that produce energy & muscle contraction.

16. Failure of neural transmission may be a cause of some fatigue. Many mechanisms can lead to such failure, & all need further research.

17. The CNS may also cause fatigue, perhaps as a protective mechanism. Perceived fatigue usually leads to physiological fatigue, and athletes who feel psychologically exhausted can often inhibit their willingness to continue exercise or to tolerate further pain.

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Basic Energy System