Bioenergetics

energy

The ability or capacity to perform physical work requires

Bioenergetics

or the flow of energy in a biological system, primarily concerns the conversion of food—or large carbohydrate, protein, and fat molecules that contain chemical energy—into biologically usable forms of energy.

chemical bonds

The breakdown of , , in these molecules releases the energy necessary to perform physical activity.

catabolic

The process of breaking down large molecules into smaller molecules, such as the breakdown of carbohydrates into glucose, is generally accompanied by the release of energy and is termed

anabolic

The synthesis of larger molecules from smaller molecules can be accomplished using the energy released from catabolic reactions. This building-up process is termed

metabolism

The human body is in a constant state of anabolism and catabolism, which is defined as

adenosine triphosphate (ATP)

Energy obtained from catabolic reactions is used to drive anabolic reactions through an intermediate molecule,

would not be possible

Without an adequate supply of ATP, muscular activity and muscle growth

adenine, ribose, three phosphate groups

Adenosine triphosphate is composed of

two terminal phosphate groups

Adenosine triphosphate is classified as a high-energy molecule because it stores large amounts of energy in the chemical bonds of the

constant supply of ATP

Because muscle cells store ATP only in limited amounts and activity requires a, , to provide the energy needed for muscle actions, ATP-producing processes must also occur in the cell.

phosphagen system

is the primary source of ATP for short-term, high-intensity activities (e.g., jumping and sprinting) but is active at the start of all types of exercise regardless of intensity

creatine phosphate

phosphagen system relies on the chemical reactions of ATP and , , both phosphagens, which involve the enzymes myosin adenosine triphosphatase (ATPase) and creatine kinase.

Myosin ATPase

increases the rate of breakdown of ATP to form ADP and inorganic phosphate (Pi) and releases energy, all of which is a catabolic reaction.

Creatine kinase

increases the rate of synthesis of ATP from creatine phosphate and ADP by supplying a phosphate group that combines with ADP to form ATP, which is an anabolic reaction.

in small amounts

These reactions provide energy at a high rate; however, because ATP and creatine phosphate are stored in the muscle , , the phosphagen system cannot supply enough energy for continuous, long-duration activities.

phosphagens

Generally, type II (fast-twitch) muscle fibers contain greater concentrations of , , than type I (slow-twitch) fibers

ADP

An increase in the muscle cell concentration of , , promotes creatine kinase activity;

inhibits

An increase in the muscle cell concentration of ATP , , creatine kinase activity

broken down to ADP

At the beginning of exercise, ATP is, , releasing energy for muscular actions.

high intensity

Creatine kinase activity remains elevated if exercise continues at a

decrease

If exercise is discontinued, or continues at an intensity low enough to allow glycolysis or the oxidative system to supply an adequate amount of ATP for the muscle cells’ energy demands, the muscle cell concentration of ATP will likely increase. This increase in ATP then results in a , , in creatine kinase activity.

Glycolysis

is the breakdown of carbohydrates

glycogen

either , , stored in the muscle or glucose delivered in the blood, to produce ATP

high-intensity muscular activity

The ATP provided by glycolysis supplements the phosphagen system initially and then becomes the primary source of ATP for, , that lasts up to about 2 minutes

cytoplasm

The enzymes for glycolysis are located in the , , of the cells

ATP, carbon dioxide, and water

Glycolysis is the first step in a multiphase pathway that ultimately converts glucose to

slow (aerobic) / fast (anaerobic) glycolysis.

the process of glycolysis may occur in one of two ways, termed

oxygen-dependent process

This is not a technically accurate way of describing the bioenergetics of glycolysis because glycolysis itself is not an

two (glucose) or three (glycogen) ATP molecules, two pyruvate molecules, and two electron transporters (NADH)

under conditions of low energy demand (i.e., rest to moderate-intensity exercise), glycolysis will yield three end products:

Krebs cycle (pyruvate) and electron transport chain (NADH)

The aerobic ATP can be used immediately for energy, while the pyruvate and NADH enter the mitochondria to continue the process of extracting energy via the

sufficient quantity

During times of intense muscular action, the aerobic system (i.e., oxidative phosphorylation) is not capable of generating a , , of ATP to maintain the given activity level or intensity.

This effectively creates an energetic bottleneck, resulting in pyruvate molecules being converted into lactate via the enzymatic action of lactate dehydrogenase (LDH).

pyruvate molecule

The defining factor that differentiates slow from fast glycolysis is whether the resulting , , will be converted

one molecule of glucose

Glycolysis produces a net of two molecules of ATP from

phosphorylating (adding a phosphate group to) glucose

glycogen (the stored form of glucose) is used, there is a net production of three ATPs because the reaction of , , which requires one ATP, is bypassed

AMP

Glycolysis is stimulated during intense muscular activity by ADP, Pi, ammonia, and a slight decrease in pH and is strongly stimulated by

ATP, creatine phosphate, citrate, and free fatty acids

Glycolysis is inhibited by the markedly lowered pH that may be observed during periods of inadequate oxygen supply and by increased levels of , , at rest.

hexokinase

The phosphorylation of glucose by , , primarily controls glycolysis

phosphorylase

the rate of glycogen breakdown to glucose, which is controlled by , , in the regulation of glycolysis

slowed

In other words, if glycogen is not being broken down into glucose quickly enough and the supply of free glucose has already been depleted, glycolysis will be

rate-limiting step

important consideration in the regulation of any series of reactions is the , , (i.e., the slowest reaction in the series).

fructose-6-phosphate to fructose-1,6-biphosphate

The rate-limiting step in glycolysis is the conversion of , , a reaction controlled by the enzyme phosphofructokinase (PFK).

PFK

Thus the activity of , , is the primary factor in the regulation of the rate of glycolysis.

energy production

Activation of the phosphagen energy system stimulates glycolysis (by stimulating PFK) to contribute to the , , of high-intensity exercise

Ammonia

produced during high-intensity exercise as a result of increased AMP or amino acid deamination (removing the amino group of the amino acid molecule) can also stimulate PFK.

lactate

Fast glycolysis occurs during periods of reduced oxygen availability in the muscle cells, typically during higher-intensity activity, and results in the formation of the end product

conjugate base

Lactate is the , , of lactic acid, meaning it has one less hydrogen ion (H+).

decreased tissue pH from metabolic by-products

Thus, muscular fatigue experienced during exercise, previously thought to be associated with high concentrations of lactic acid in the muscle tissue, is actually a result of , , such as H+ ions, that are acidic in nature as compared with lactate (a base)

inhibiting calcium binding to troponin or by interfering with actin–myosin crossbridge formation

As pH decreases (becomes more acidic), it is believed to inhibit glycolytic reactions and directly interfere with muscle action, possibly by

enzyme activity

Also, the decrease in pH levels inhibits the , , of the cell’s energy systems.

energy substrate

Lactate, however, is used as an , , especially in type I and cardiac muscle fibers

gluconeogenesis

Lactate is also used in , , the formation of glucose from non-sugar substances, during extended exercise and recovery

ability to recover

Although lactate does not contribute to muscular fatigue, monitoring its clearance from blood can indicate a person’s

oxidized

Lactate can be cleared by oxidation within the muscle fiber in which it was produced, or it can be transported in the blood to other muscle fibers to be

Cori cycle

Lactate can also be transported in the blood to the liver, where it is converted to glucose via gluconeogenesis. This process is referred to as the

0.5 to 2.2 mmol/L

Normally there is a low concentration of lactate in blood and muscle. The reported normal range of lactate concentration in blood is , , at rest

muscle fiber type

Lactate production increases with increasing exercise intensity and appears to depend on

glycolytic enzymes

The higher rate of lactate production by type II muscle fibers may reflect a concentration or activity of , , that is higher than that of type I muscle fibers

hour

Blood lactate concentrations normally return to preexercise values within an , , after activity

lactate clearance rates

Light activity during the postexercise period has been shown to increase

5 minutes

Peak blood lactate concentrations occur approximately , ,after the cessation of exercise, a delay frequently attributed to the time required to transport lactate from the tissue to the blood

inflection

It is widely accepted that there are specific , , points in the lactate accumulation curve as exercise intensity increases

lactate threshold (LT)

The exercise intensity or relative intensity at which blood lactate begins an abrupt increase above the baseline concentration has been termed the

anaerobic mechanisms

The LT represents an increasing reliance on

50% to 60%

The LT typically begins at , , of maximal oxygen uptake in untrained subjects

70% to 80%

The LT typically begins at , , in trained subjects

second increase

A , , in the rate of lactate accumulation has been noted at higher relative intensities of exercise.

blood lactate accumulation (OBLA)

This second point of inflection, termed the onset of

4 mmol/L

onset of blood lactate accumulation (OBLA), generally occurs when the concentration of blood lactate is near

intermediate and large motor units

The breaks in the lactate accumulation curve may correspond to the points at which, , are recruited during increasing exercise intensities.

type II fibers

The muscle cells associated with large motor units are typically

LT or OBLA

It has been suggested that training at intensities near or above the , ,changes the LT and OBLA so that lactate accumulation occurs later at a higher exercise intensity

increased mitochondrial content

This shift in the LT/OBLA curve probably occurs as a result of several factors but particularly as a result of the , , which allows for greater production of ATP through aerobic mechanisms.

blood

This shift in the LT/OBLA curve allows the individual to perform at higher percentages of maximal oxygen uptake without as much lactate accumulation in the

The oxidative system

the primary source of ATP at rest and during aerobic activities, uses primarily carbohydrates and fats as substrates

Protein

when relying on the oxidative system , , is normally not metabolized significantly except during long-term starvation and long steady-state bouts (>90 minutes) of exercise

fats

At rest, approximately 70% of the ATP produced is derived from , , and 30% from carbohydrates.

carbohydrates

After the onset of activity, as the intensity of the exercise increases, there is a shift in substrate preference from fats to

100%

During high-intensity aerobic exercise, almost , , of the energy is derived from carbohydrates if an adequate supply is available.

fats and protein

However, during prolonged, submaximal, steady-state work there is a gradual shift from carbohydrates back to , , as energy substrates

acetyl-CoA (CoA stands for coenzyme A)

When pyruvate enters the mitochondria, it is converted to , , and can then enter the Krebs cycle for further ATP production.

two molecules of NADH

Also transported there are , , produced during the glycolytic reactions.

guanine triphosphate (GTP)

The Krebs cycle, another series of reactions, produces two ATPs indirectly from , , for each molecule of glucose

six molecules of NADH and two molecules of reduced flavin adenine dinucleotide (FADH2).

Also produced in the Krebs cycle from one molecule of glucose are an additional

fat or protein

The number of ATPs and amount of NADH and FADH2 are different if , , enters the Krebs cycle, although all of these substrates must be converted to acetyl-CoA before entering the Krebs cycle.

electron transport chain (ETC)

These molecules transport hydrogen atoms to the , , to be used to produce ATP from ADP

rephosphorylate

The ETC uses the NADH and FADH2 molecules to , , ADP to ATP

cytochromes

The hydrogen atoms are passed down the chain, via a series of electron carriers known as , , to form a concentration gradient of protons to provide energy for ATP production, with oxygen serving as the final electron acceptor (resulting in the formation of water).

ability

Because NADH and FADH2 enter the ETC at different sites, they differ in their , , to produce ATP.

three molecules

One molecule of NADH can produce , , of ATP

two molecules

one molecule of FADH2 can produce only , , of ATP

oxidative phosphorylation

The production of ATP during NADH and FADH2 ability to produce ATP is referred to as

38

The oxidative system, beginning with glycolysis, results in the production of approximately , , ATPs from the degradation of one glucose molecule

hormone-sensitive lipase

Triglycerides stored in fat cells can be broken down by an enzyme known as

circulate and enter muscle fibers

hormone-sensitive lipase releases free fatty acids from the fat cells into the blood, where they can

source of free fatty acids

Additionally, limited quantities of triglycerides are stored within the muscle, along with a form of hormone-sensitive lipase, to serve as a , , acids within the muscle

beta oxidation

Free fatty acids enter the mitochondria, where they undergo, , a series of reactions in which the free fatty acids are broken down, resulting in the formation of acetyl-CoA and hydrogen atoms

to the ETC

The acetyl-CoA enters the Krebs cycle directly, and the hydrogen atoms are carried by NADH and FADH2

constituent amino acids

protein can be broken down into its , , by various metabolic processes.