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Ch.5 Introduction to Energy Transfer/Ch.6 Energy Transfer in the Body…
Ch.5 Introduction to Energy Transfer/Ch.6 Energy Transfer in the Body
Bioenergetics (the flow and exchange of energy within a living system)
First Law of Thermodynamics (energy cannot be created nor destroyed but transforms from one form to another without being depleted)
This essentially describes the Conservation of Energy Principle (pertains to both living and nonliving systems)
The body does not produce, consume or use up energy, instead it transforms it from one state into another.
Photosynthesis is an example of energy conversion in living cells. (A endergonic process powered by sunlight.)
In the sun, nuclear fusion releases part of the potential energy stored in the nucleus of the hydrogen atom. This energy, in the form of gamma radiation, then converts to radiant energy.
The pigment chlorophyll, contained in large chloroplast organelles within a leaf's cells, absorbs radiant (solar) energy to synthesize glucose from carbon dioxide and water, while oxygen flows to the environment. Plants also convert carbohydrates to lipids and proteins for storage as a future reserve for energy and to sustain growth.
Respiration is the opposite of photosynthesis. Represents an exergonic reaction.
Plant's stored energy in the form of ATP transfers for mechanical work, chemical work, and transport work. This produces a negative free energy change.
A portion of the energy released during cellular respiration is conserved in other chemical compounds for use in energy-requiring processes; the remaining energy flows to the environment as heat.
In the body, chemical energy within the bonds of macronutrients does not immediately dissipate as heat during energy metabolism; instead a large portion remains as chemical energy, which the musculoskeletal system changes into mechanical energy and ultimately to heat energy.
Potential Energy include Bound Energy within an internal structure like a battery, a stick of dynamite or a macronutrient before releasing its stored energy in metabolism.
The release of potential energy transforms into kinetic energy of motion
In some cased, bound energy in one substance directly transfers to other substances to increase the substance's potential energy
Energy transfer of this type provide the energy for the body's chemical work of biosynthesis
In biosynthesis, specific building-block atoms of carbon, hydrogen, oxygen and nitrogen become activated to join other atoms and molecules to synthesize biologic compounds and tissues.
Exergonic describes any physical or chemical process that releases (frees) energy to its surroundings. Also known as "downhill" processes due to the decline in free energy stored after reaction releasing energy.
Endergonic reactions store or absorb energy. These are "uphill" processes that increase free energy available for biologic work.
Second Law of Thermodynamics (the total entropy of an isolated system can never decrease over time, and is constant if and only if all processes are reversible.)
The irreversibility of natural processes causes an increase in entropy. The transfer of potential energy in any spontaneous process always proceeds in a direction that decreases the capacity to perform work. Food represent excellent stores of potential energy. This energy continually decreases as the compounds decompose through normal oxidative processes.
There are six forms of energy: chemical, mechanical, heat, light, electrical and nuclear.
Mechanical work in humans refers to muscle action
Chemical work in humans synthesizes cellular molecules such as glycogen, triacylglycerol and protein
The sustained pace of a marathon runner at close to 90% aerobic capacity or the sprinter's rapid speed in all-out running directly reflects the body's capacity to transfer chemical energy into mechanical work.
Transport work in humans refers the work that concentrates substances such as sodium (NA+) and potassium (K+) ions in the intracellular and extracellular fluids.
Enzymes and Coenzymes
Enzymes are highly specific and large protein catalysts, they accelerate the forward and reverse rates of chemical reactions without themselves being consumed or changed during the reaction. Enzymes reduce required activation energy. Enzymes action takes place without altering free energy change.
Enzymes possess a unique property of not being readily altered by the reactions they affect. Enzyme turnover in the body remains slow and specific enzymes are continually reused.
Coenzymes active dormant enzymes.
Coenzymes are nonprotein organic substances that facilitate enzyme action by binding the substrate with a specific enzyme. The coenzymes then regenerate to assist in further similar reactions.
The metallic ions iron and zinc play coenzyme roles, as do the B vitamins or their derivatives.
Competitive inhibitors inhibit enzyme activity. They closely resemble the structure of the normal substrate for an enzyme. They bind with the enzyme's active site but the enzyme cannot change them.
Noncompetitive inhibitors do not resemble the enzymes substrate and do not bind to its active site. Instead, they bind to the enzyme at a site other than the active site. This changes the enzyme's structure and ability to catalyze the reaction because of the inhibitors presence.
Some drugs used in the treatment of cancer, depression and immunodeficiency syndromes act as noncompetitive inhibitors.
Hydrolysis and Condensation
Hydrolysis reactions digest or degrade complex molecules into simpler subunits.
Condensation reactions build larger molecules by bonding their subunits.
Also called dehydration synthesis as a water molecule forms in this process
Oxidation and Reduction
Oxidation reactions transfer oxygen atoms, hydrogen atoms or electrons. A loss of electrons always occurs in oxidation reactions, with a corresponding net gain in valence.
A oxidizing agent is the substance reducing or gaining electrons. Also called the electron acceptor.
Reduction involves any process in which the atoms in an element gain electrons, with a corresponding net decrease in valence.
The term reducing agent describes the substance that donates or loses electrons as it oxidizes.
Electron transfer requires both oxidizing and reducing agents. The process of oxidation and reduction reactions are coupled.
The term redox reaction commonly describes a coupled oxidation-reduction reaction. Whenever oxidation occurs, the reverse reduction also takes place; when one substance loses electrons, the other substance gains them.
One example of a redox reaction would be during strenuous exercise when oxygen supply (or use) becomes inadequate, some pyruvate formed in energy metabolism gains two hydrogens (two electrons) and becomes reduced to a new compound, lactate. In recovery, when oxygen supply (or use) becomes adequate, lactate loses two hydrogens (two electrons) and oxidizes back to pyruvate. This shows how a redox reaction continues energy metabolism, despite limited oxygen availability (or use) in relation to exercise energy demands.
Phosphate Bond Energy
The human body cannot use heat energy like a mechanical engine.
Human energy dynamics involve transferring energy via chemical bonds. Potential energy within carbs, fat and protein bonds releases with the splitting of chemical bonds.
Adenosine Triphosphate (ATP) serves as the ideal energy-transfer agent. The potential energy within this molecule powers all of the cell's energy-requiring processes
Energy from macronutrient oxidation is harvested and funneled through ATP.
The cells two major energy-transforming activities are:
1) Extract potential energy from food and conserve it within the bonds of ATP
2) Extract and transfer the chemical energy in ATP to power biologic work
Energy from ATP hydrolysis powers all forms of biologic work; ATP constitutes the cell's "energy currency".
ATP splits almost instantly without the need for molecular oxygen. The ability to hydrolyze ATP without oxygen (anaerobically) generates rapid energy transfer.
Rate of energy transfer depends on movement intensity. Transitioning from sitting in a chair to slow walking increases energy transfer fourfold. Changing from a slow walk to an all-out sprint changes rate by 120-fold.
Six fuel sources that supply substrate for ATP formation:
1) Triaclyglycerol and glycogen molecules stored within muscle cells
2) Blood glucose (derived from liver glycogen)
3) Free fatty acids (derived from triacylglycerols in liver and adipocytes)
4) Intramuscular- and liver-derived carbon skeletons of amino acids
5) Anaerobic reactions in the cytosol in the initial phase of glucose or glycogen breakdown (small amount of ATP)
6) Phosphorylation of ADP by PCr under enzymatic control by creatine kinase and adenylate kinase
Carbohydrate provides the only macronutrient substrate whose stored energy generates ATP without oxygen (anaerobically).
Central nervous system requires an uninterrupted supply of carbohydrate to function properly The brain normally uses blood glucose almost exclusively as its fuel. After about 8 days of low carbohydrates it will metabolize fat (as ketones) for an alternative fuel source.
Two forms of carb breakdown occur in a series of fermentation reactions collectively termed glycolysis ("the dissolution of sugar")
In one form, lactate, formed from pyruvate, becomes the end product. Glycolysis that results in lactate formation (referred to as anaerobic [without oxygen] glycolysis) represents rapid but limited ATP production.
Lactic acid and lactate are different substances. Lactic acid is an acid formed during anaerobic glycolysis that in the body quickly dissociates to release a hydrogen ion (H+). The remaining compound binds with a positively charged sodium or potassium ion ro form the acid salt called lactate.
During rest and moderate activity some lactate continually forms in two ways.
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Once lactate forms in muscle it takes two different routes:
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When blood and muscle lactate levels increase and ATP formation fails to keep pace with its rate of use. The end results, fatigue, soon sets in and exercise performance diminishes. Increased intracellular acidity under anaerobic conditions mediates fatigue by inactivating various enzymes in energy transfer, thus impairing the muscle's contractile properties.
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During intense physical activity when hydrogen oxidation fails to keep pace with its production, pyruvate temporarily binds hydrogen to form lactate. This allows progression of anaerobic glycolysis for an additional duration.
In the other form, pyruvate remains the end product. With pyruvate as the end substrate, carbohydrate catabolism further breaks down in the citric acid cycle with subsequent electron transport production of ATP. Carb breakdown of this form (sometimes termed aerobic [with oxygen] glycolysis) is a relatively slow process resulting in substantial ATP formation.
Glucose degradation occurs in two stages. Stage one, glucose breaks down rapidly into two molecules of pyruvate. Energy transfer for phosphorylation occurs without oxygen (anaerobic). Stage two, pyruvate degrades further to carbon dioxide and water. Energy transfers from these reactions require electron transport and accompanying oxidative phosphorylation (aerobic)
Fast-twitch (type II) fibers contain large quantities of PFK; this makes them ideally suited for generating anaerobic energy via glycolysis.
In normal conditions following a meal, glucose does not accumulate in the blood. Rather, surplus glucose either enters the pathways of energy metabolism, stores as glycogen, or converts too fat.
In high cellular activity, available glucose oxidizes via the glycolytic pathway, citric acid cycle, and respiratory chain to form ATP.
In low cellular activity and/or depleted glycogen reserves inactivate key glycolytic enzymes. This causes surplus glucose to form glycogen.
Glycolysis is regulated by three factors;
1) Concentrations of the four key glycolytic enzymes: hexokinase, phosphorylase, phosphofructokinase, and pyruvate kinase.
Skeletal muscle possesses the largest quantity of glycolytic enzymes. Erythrocytes and adipocytes also contain glycolytic enzymes.
2) Levels of the substrate fructose 1,6-disphosphate
3) Oxygen, which in abundance inhibits glycolysis
Glucose locates in the surrounding extracellular fluid for transport across the cell's plasma membrane. A family of five proteins, collectively termed facilitative glucose transporters, mediates this process of facilitative diffusion.
Muscle fibers and adipocytes contain an insulin-dependent transporter known as GLUT 4. In response to both insulin and physical activity (independent of insulin), this transporter migrates from vesicles within the cell to the plasma membrane. its action facilitates glucose transport into the sarcoplasm, where it catabolizes to from ATP.
Another glucose transporter GLUT 1, accounts for basal levels of glucose transport into muscle.
Glycogenolysis is the cleavage of glucose from the glycogen molecule.
Epinephrine's action has been termed the glycogenolysis cascade because this hormone affects progressively greater phosphorylase activation to ensure rapid glycogen mobilization. This is at its highest level with intense activity where sympathetic activity increases and carbs are the optimal fuel.
Just as the hypoglycemic state coincides with neural or central fatigue, muscle glycogen depletion probably causes "peripheral" or local muscle fatigue during exercise.
Gluconeogenesis does not replenish or even maintain glycogen stores without adequate carbohydrate consumption. Appreciably reducing carbohydrate availability seriously limits energy transfer capacity.
Energy release from fat
The fat fuel reserves come form two main places
1) Between 60K and 100K kcal (enough energy to power 25-40 marathon runs) from triacylglycerol in fat cells (adipocytes) distributed throughout the body.
Three specific energy sources for fat catabolism include;
1) Triacylglycerols stored directly within the muscle fiber in close proximity to the mitochondria (more in slow-twitch than in fast-twitch muscle fibers)
2) Circulating triacylglycerols in lipoprotein complexes that become hydrolyzed on the surface of a tissue's capillary endothelium
3) Circulating free fatty acids mobilized from triacylglycerols in adipose tissue
The availability of fatty acid molecules regulates fat breakdown or synthesis. After a meal, when energy metabolism remains relatively low, digestive processes increase FFA and triacylglyerol delivery to cells; this in turn stimulates triacylglycerol synthesis.
2) About 3000 kcal come from intramuscular tricyglycerol.
In contrast, carbohydrate reserves amount to less than 2000 kcal.
Prior to energy release from fat, hydrolysis (lipolysis) in the cell's cytosol splits the triacylglycerol molecule into a glycerol molecule and three water-insoluble fatty acid molecules.
Lipid mobilization and catabolism involves seven discrete processes:
1) Breakdown of triacylglycerol to free fatty acids
Adipose tissue release of FFAs and their subsequent use for energy in light and moderate physical activity increase directly with blood flow through adipose tissue (threefold increase not uncommon) and active muscle.
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2) Transport of free fatty acids in the blood
3) Uptake of free fatty acids from blood to muscle
One inside the muscle fiber, FFAs accomplish two tasks:
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4) Preparation of fatty acids for catabolism (energy activation)
5) Entry of activated fatty acid into muscle mitochondria
6) Breakdown of fatty acid to acetyl-CoA via Beta-oxidation and the production of NADH and FADH2
Acetyl-CoA functions as the starting point for synthesizing cholesterol and many hormones.
7) Coupled oxidation in citric acid cycle and electron transport chain
Glycerol provides carbon skeletons for glucose synthesis.
Glycerol breakdown goes through the glycolysis and citric acid cycle pathways. While the fatty acids go through Beta-oxidation and citric acid cycle pathways.
The liver can use glycerol for glucose synthesis
Fatty acid breakdown relates directly to oxygen consumption. Oxygen must join with hydrogen for Beat-oxidation to proceed. Under anaerobic conditions, hydrogen remains with NAD+ and FAD, thus halting fat catabolism.
"Fats burn in a carbohydrate flame". The degradation of fatty acids in the citric acid cycle continues only if sufficient oxaloacetate and other intermediates from carbohydrate breakdown combine with the acetyl-CoA formed during Beta-oxidation. These intermediates are continually lost or removed from the cycle and need to be replenished. Pyruvate formed during glucose catabolism plays an important role in maintaining a proper level of oxaloacetate.
Due to the rate limit for fatty acid's use by active muscle, the power generated solely by fat breakdown represents only about one-half that achieved with carbohydrate as the chief aerobic energy source.
Lipogenesis is the formation of fat (mostly in the cytoplasm of liver cells) when ingested glucose or protein not used to sustain energy metabolism converts into stored triacylglycerol.
Lipogenesis begins with carbons from glucose and the carbon skeletons from amino acid molecules that metabolize to acetyl-CoA. Liver cells bond the acetate parts of the acetyl-CoA molecules in a series of steps to from the 16-carbon saturated fatty acid palmitic acid. This molecule then lengthens to an 18-20 carbon chain fatty acid in either the cytosol or the mitochondria. Three fatty acid molecules join (esterify) with one glycerol molecule (produced during glycolysis) to yield one triacylglycerol molecule.
Energy release from protein
Deamination (the removal of nitrogen from the amino acid molecule) mainly happens in the liver though skeletal muscle also contains enzymes that remove nitrogen and pass it to other compounds which transamination.
The muscle will use the carbon skeleton byproducts of donor amino acids to form ATP.
The levels of enzymes for transamination increase with training to further facilitate protein's use as an energy substrate.
Some amino acids are glucogenic, when deaminated, they yield pyruvate, oxaloacetate, or malate (all intermediates for glucose synthesis via gluconeogenesis.)
Muscle protein (amino acids) degrades to gluconeogenic constituents to sustain plasma glucose levels. Excessive muscle protein catabolism eventually produces a muscle wasting effect. Reliance on protein catabolism, coincident with depleted glycogen, continues because fatty acids from triacylglycerol hydrolysis in muscle and adipose tissue fail to provide gluconeogenic substrates.
Some amino acids are ketogenic; when deaminated, they yeild the intermediates acetyl-CoA or acetoacetate. These compounds cannot be used to synthesize glucose; rather, they synthesize to triacylglycerol or catabolize for energy in the citric acid cycle.
When protein provides energy, the body must eliminate the nitrogen-containing amine group and other solutes produced from protein breakdown. These waste products leave the body dissolved in "obligatory" fluid (urine). For this reason, excessive protein catabolism increases the body's water needs.
Phosphocreatine (PCr) is another high-energy compound like ATP.
Some energy for ATP resynthesis also comes directly from the anaerobic splitting of a phosphate from PCr (PCr interacts with ADP to form ATP).
PCr has a larger free energy of hydrolysis than ATP. Cells store 4-6 times more PCr than ATP.
Both the splitting of PCr (into P and Cr) and ATP (into ADP and P) are reversible reactions.
Oxidation and reduction reactions constitute the biochemical mechanism that underlies energy metabolism.
Oxidative Phosphorylation
Phosphorylation refers to energy transfer via phosphate bonds as ADP with creatine continually recycle into ATP and PCr.
More than 90% of ATP synthesis takes place in the respiratory chain by oxidative reactions coupled with phosphorylation
No single chemical regulator dominates mitochondrial ATP production.
An increase in ADP signals a need to supply energy to restore depressed ATP levels. ADP concentrations function as a cellular feedback mechanism to maintain the appropriate level of energy currency for biologic work. Conversely, high cellular ATP levels indicate a relatively low energy requirement.
Oxidative phosphorylation synthesizes ATP by transferring electrons from NADH and FADH2 to oxygen.
Three prerequisites exist for the continual resynthesis of ATP during coupled oxidative phosphorylation
1) Tissue availability of the reducing agent NADH (or FADH2)
2) Presence of the oxidizing agent oxygen in the tissues
3) Sufficient concentration of enzymes and mitochondria to ensure that energy transfer reactions proceed at their appropriate rate
During strenuous physical activity, inadequacy in oxygen delivery (condition 2) or its rate of use (condition 3) creates an imbalance between hydrogen release and its terminal oxidation. In both cases, electron flow down the respiratory chain "backs up". Lactate formation allows electron transport-oxidative phosphorylation to continue to provide energy as needed.
Citric acid cycle, electron transport and oxidative phosphorylation represent the three components of aerobic metabolism.