Chapter 12: Bioenergetics and Regulation of Metabolism

______________ is used to describe energy states in biological systems.

Bioenergetics

Most biochemical studies are performed on the cellular or sub cellular level rather than in an entire organism. These systems can be considered _______________ because there is no exchange of matter with the environment. Such a system we can make useful simplifications about the internal energy, U.

closed

In the cytoplasm, there are several molecules that act as ______________________. These are soluble and include NADH, NADPH, FADH2, ubiquinone, cytochromes, and glutathionine. Some of these electron carriers are used by the mitochondrial ETC, which leads to oxidative phosphorylation of ADP to ATP. As electrons are passed down the ETC, they give up their free energy to form the proton-motive force across the inner mitochondrial membrane. In addition to soluble electron carriers, there are membrane-bound electron carriers embedded within the innermitochondrial membrane. One such carrier is flavin mononucleotide (FMN), which is bonded to complex I of the ETC and can also act as soluble electron carrier. In general, proteins with prostetich groups containing iron-sulfur clusters are particularly well suited for the transport of electrons.

high-energy electron carriers

Wether a chemical reaction proceeds is determined by the degree to which enthalpy and entropy change during a chemical reaction. ____________________ measures the overall change in heat of a system during a reaction. At constant pressure and volume, enthalpy (deltaH) and thermodynamic heat exchange (Q) are equal. Changes in entropy (deltaS) measure the degree of disorder or energy dispersion in a system. Entropy carries units of J/K.

Enthalpy

_______________ from the adrenal cortex are responsible for part of the stress response. In order to make a getaway in the fight-or-flight response, glucose must be rapidly mobilized from the liver in order to fuel actively contracting muscle cells while fatty acids are released from adipocytes.

Glucocorticoids

____________________ allows accurate measurement of the respiratory quotient, which differs depending on the fuels being used by the organism.

Respirometry

While different energy sources provide greater or less caloric values, the end goal is to have energy in a readily available form. For the cell, this is ________________.

adenosine triphosphate (ATP)

While the adrenal cortex produces steroid hormones (glucocorticoids, mineralocorticoids, and sex hormones), the adrenal medulla produces ________________

catecholamines

The human body can make use of different energy sources with roughly the same efficiency, but all nutrient molecules are not created equally. For example, ________ are much more energy-rich than carbs, proteins, or ketones. Complete combusiton of fat results in 9kcal/g of energy, compared with only 4 kcal/g derived from carbohydrates, proteins , or ketones. Because fats are so much more energy-dense than other biomolecules, they are preferred for long term energy storage.

fats

ATP can not get back the "leftover" _____________ after a reaction, so it's best to use a carrier with a smaller free energy. ATP provides about 30 kJ/mol of energy under physiological conditions. If a reaction only requires 10kJ/mol to overcome a positive deltaG value, then 20 kJ/mol have been wasted. The waste would be even higher with a higher energy compound like creatine phosphate.

free energy

Changes in ____________ (deltaG) provide information about chemical rxns and can predict wether a chemical reaction is favorable and will occur. In biological systems, ATP plays a crucial role in transferring energy from energy-releasing catabolic processes to energy-requiring anabolic processes.

free energy

Important: Some tissues that require insulin actively store glucose when it is present in high concentrations, while other tissues that do not require insulin must still be able to absorb _______________ even when the glucose concentration is low.

glucose

Regulation of Body Mass: Body mass is determiend by several factors, including water, carbs, proteins, and lipids, while nucleic acids to not contribute significantly. The overall mass of carbs tends to be stable over time, although it can be modified slightly by periods of prolonged starvation or by significant muscle building activities. Water is very quickyly adjusted to the endocrine system and kidneys, therefore it does not factor into our discussion of obesity and weight regualtion. Water is the primary source of frequent minor weight fluctutations because it is subject to rapid adjustment. Therefore, lipids, stored in adipocytes, are the primary factor in gradual change of body __________ over time

mass

Biological systems are often considered ________________ because they can exchange both energy and matter with the environment.

open systems

glucagon, cortisol, epinephrine, norepinephrine, and growth hormone oppose the action of insulin. These hormones are sometimes termed counterregulatory hormones because of their effect on skeletal muscle, adipose tissue, and the liver, which are opposite to the actions of insulin. In the liver, glycogen degradation and the release of glucose into the blood are stimulated. Hepatic gluconeogenesis is also stimulated by glucagon, but the response is slower that that of glycogenolysis. Whereas glycogenolysis begins almost immediately at the begging of the postabsorbative state, gluconeogeneisis takes about 12 hours to hit maximum velocity. The release of amino acids from skeletal muscle and fatty acids from adipose tissue are both stimulated by the decrease in insulin and by an increase in levels of epinephrine. Once carried into the liver, amino acids and fatty acids can provide the necessary carbon skeletons and energy required for gluconeognesis.

postabsorptive (fasting) state

___________________ can measure basal metabolic rate (BMR) based on heat exchange with the environment. Human calorimetry makes use of large insulated chambers with specializes heat sinks to determine energy expenditure. Because of the isolationist nature of testing and the expense of creating a calorimetry chamber, other measures of BMR are preferred. Because of the previous experimentation, BMR can be estimated based on age, weight, height and gender.

Calorimetry

________ is the major energy currency in the body. It is a mid-level energy carrier and is formed from substrate-level phosphorylation as well as oxidative phosphorylation.

ATP

___________ is the transfer of a high-energy phosphate group from ATP to another molecule. Generally, this activates or inactivates the target molecule. With these phosphoryl group transfers, the overall free energy of the reaction will be determined by taking the sum of the free energies of the individual reactions.

ATP cleavage

____________________is most likely to be encountered in the context of coupled reactions. Many coupled reactions use ATP as an energy source. For example, the movement of sodium and potassium against their electrochemical gradients requires energy, which is harnessed from hydrolysis of ATP.

ATP hydrolysis

________________ are secreted by the adrenal medulla and include epinephrine and norepinephrine, also known as adrenaline and noradrenaline. Catecholamines increase the activity of liver and muscle glycogen phosphorylase, thus promoting glycogenolysis. This increases glucose output by the liver. Glycogenolysis also increases in skeletal muscle, but because muscle lacks glucose-6-phosphatase, glucose cannot be released by skeletal muscle in the bloodstream; instead. it is metabolized by the muscle tissue itself. Catecholamines act on adipose tissue to increase lipolysis by increasing the activity of hormone-sensative lipase. Glycerol, from triacylglycerol breakdown is a minor substrate for gluconeogeneisis. Epinephrine also acts directly on target organs like the heart to increase the basal metabolic rate through the sympathetic nervous system. This increase in metabolic function is often associated with an adrenaline rush.

Catecholamines

______________________ contain a modified vitamin B2, or riboflavin. They are nucleic acid derivatives, generally either flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN). Flavoproteins are most notable for their presence in the mitochondria and chloroplast as electron carriers. Flavoprotiens are also involved in the modification of other B vitamins to active forms. Finally, flavoproteins function as a coenzyme for enzymes in the oxidation of fatty acids, the decarboxylation of pyruvate, and the reduction of glutathionine.

Flavoproteins

_______________ is a peptide hormone secreted by a-cells of the pancreatic islets of Langerhans. The primary target for glucagon action is the hepatocyte. Glucagon acts through second messengers to cause the following effects: -Increased liver glycogenlysis. Glucagon activates glycogen phosphorylase and inactivates glycogen synthase. -Increased liver gluconeogeneis. Glucagon promotes the conversion of pyruvate to phosphoenolpyruvate by pyruvate carboxylase and phosphoenolpyruvate carboxykinase (PEPCK). Glucagon increases the conversion of fructose 1,6-bisphosphate to fructose 6-phosphate by fructose-1,6-bisphosphate. -Increased liver ketogenesis and decreased lipogenesis. -Increased lipolysis in the liver. Glucagon activates hormone-sensitive lipase in the liver. Because the action is on the liver and not the adipocyte, glucagon is not considered a major fat-mobilizing hormone.

Glucagon

_________________ is a physiological tendency toward a relatively stable state that is maintained and adjusted, often with the expenditure of energy. Most compounds in the body are actually maintained at a homeostatic level that is different than equilibrium, which allows us to store potential energy.

Homeostasis

____________ is a peptide hormone secreted by the B-cells of the pancreatic islets of Langerhans. It is a key player in the uptake and storage of glucose. Glucose is absorbed by peripheral tissues via facilitated transport mechanisms that utilize glucose transporters located in the cell membrane. The tissues that require insulin for effective uptake of glucose are adipose tissue and resting skeletal muscle. Tissues in which glucose uptake is not affected by insulin include: -Nervous tissue -Kidney tubules -Intestinal mucosa -RBC (erythrocytes) -B-cells of pancreas

Insulin

______________, associated with a well-fed, absorbitive metabolic state, and glucagon, associated with a postabsorbitive metabolic state, usually oppose each other with respect to pathways of energy metabolism. Enzymes that are phosphorylated by glucagon are generally dephosphorylated by insulin; enzymes that are phosphorylated by insulin are generally dephosphorylated by glucagon.

Insulin

______________ is the sum of all of the different interactions between and within atoms in a system; vibration, rotation, linear motion, and stored chemical energies all contribute.

Internal energy

Levels of glucagon and epinephrine are markedly elevated during starvation. Increased levels of glucagon relative to insulin result in rapid degradation of glycogen stores in the liver. As liver glycogen stores are depleted, gluconeogenic activity continues and plays an important role in maintaining blood glucose levels during prolonged fasting; after about 24 hours, gluconeogenisis is the predominant source of glucose for the body. Lipolysis is rapid, resulting in excess acetyl-CoA that is used in the synthesis of ketone bodies. Once levels of fatty acids and ketones are high enough in the blood, muscle tissue will utilize fatty acids as its major fuel source and the brain will adapt to using ketone bodies for energy. After several weeks of fasting, the brain derives approximately 2/3 of its energy from ketones and one third from glucose. The shift from glucose to ketones as the major fuel reduces the quantity of amino acids that must be degraded to support gluconeogenesis which spares proteins that are vital for other functions. Cells that have few, if any, mitochondria, like RBC, continue to be dependent on glucose for their energy.

Prolonged Fasting (Starvation)

_________________ activity is largely permissive. In other words, thyroid hormone levels are kept more or less constant, rather than undulating with changes in metabolic state. Thyroid hormones increase the basal metabolic rate, as evidence by increased O2 consumption and heat production when they are secreted. The increase in metabolic rate produced by a dose of thyroxine (T4) occurs after a latency of several hours but may last for several days, while triiodothyronine (T3) produces a more rapid increase in metabollic rate and has a shorter duration of activity. The subscript numbers refer to the number of iodine atoms in the hormone. Thyroid hormones have their primary effects in lipid and carbohydrate metabolism. They accelerate cholesterol clearance from the plasma and increase the rate of glucose absorption from the small intestine. Epinephrine requires thyroid hormones to have a significant metabolic effect.

Thyroid hormone

__________ in thermodynamics refers to change in pressure and volume. These are constant in most living systems, so the only quantity of interest in determining internal energy is heat

Work

After a meal, elevated insulin levels stimulate glucose uptake by __________________. Insulin also triggers fatty acid release from VLDL and chylomicrons (which carry triacylglycerols absorbed from the gut). Lipoprotein lipase, an enzyme found in the capillary bed of adipose tissue, is also induced by insulin. The fatty acids that are released from lipoproteins are taken up by adipose tissue and re-esterfied to triacylglycerols for storage. The glycerol phosphate required for triacylglycerol synthesis comes from glucose that is metabolized in adipocytes as an alternative product of glycolysis. Insulin can also effectively suppress the release of fatty acids from adipose tissue. During the fasting state, decreased levels of insulin and increased epinephrine activate hormone-sensitive lipase in fat cells, allowing fatty acids to be released in circulation.

adipose tissue

Low plasma glucose (hypoglycemia) is the most important physiological promoter of glucagon secretion, and elevated plasma glucose (hyperglycemia) is the most important inhibitor. Amino acids, especially ____________ amino acids (arginine, lysine, histidine), also promote the secretion of glucagon. Thus glucagon is secreted in response to the ingestion of a meal rich in proteins.

basic

(Brain)Although the ________ represents only 2 percent of total body weight, it obtains 15 percent of cardiac output, uses 20 percent of total O2, and consumes 25 percent of total glucose, the brain's primary fuel. Blood glucose levels are tightly regulated to maintain a sufficient glucose supply for the brain (and sufficient concentration while studying). Normal function depends on a continous glucose supply from the blood stream. In hypoglemic conditions (<70mg/dL), hypothalamic centers in the brain sense a fall in blood glucose level, and the release of glucagon and epinephrine is triggered. Fatty acids cannot cross the blodd-brain barrier and are therefore not used at all as an energy source. Between meals, the brain relies on blood glucose supplied by either hepatic glycogenolysis or gluconeogenesis. Only during prolonged fasting does the brain gain the capacity to use ketone bodies for energy, and even then, the ketone bodies only supply approximately 2/3 of the fuel; the remainder is glucose.

brain

Glucocorticoids, especially ______________, are secreted with many forms of stress, including exercise, cold, and emotional stress. Cortisol is a steroid hormone that promotes the mobilization of energy stores through the degradation and increased delivery of amino acids and increased lipolysis. Cortisol also elevates blood glucose levels, increasing glucose availability for nervous tissue through 2 mechanisms. First, cortisol inhibits glucose uptake in most tissue (muscle, lymphoid, and fat) and increases hepatic output of glucose via gluconeogenesis, particularly from AAs. Second, cortisol has a permissive function that enhances the activity of glucagon, epinephrine, and other catecholamines. Long-term exposure to glucocorticoids may be required clinically, but causes persistent hyperglycemia, which stimulates insulin. This actually promotes fat storage in adipose tissue, rather than lipolysis.

cortisol

(Active Muscle) The primary fuel used to support muscle contractions depend on the magnitude and duration of excersize as well as the major fibers involved. A very short-lived source of energy (2-7 seconds) comes from ____________________, which transfers a phosphate group to ADP to form ATP. Skeletal muscle has stores of both glycogen and some triacylglycerols. Blood glucose and free fatty acids may also be used. Short burst of high-intensity excersize are also supported by anaerobic glycolysis drawing on stored muscle glycogen. During moderately high-intensity, continous excersize, continous excersize, oxidation of glucose and fatty acids are both important, but after 1 to 3 hours of continious excersize at this level, muscle glycogen stores become depleted and the intesnity of excersize declines to a rate that can be supported by oxidation of fatty acids.

creatine phosphate

Insulin impacts the metabolism of different nutrient classes in different ways. For carbs, insulin creates the uptake of glucose and increases carbohydrate metabolism in muscle and fat. Increased glucose in muscle can be used as additional fuel to burn during excersize, or can stored as glycogen. Insulin also increases glycogen synthesis in the liver by increasing the activity of glucokinase and glycogen synthase, while ________________ the activity of enzymes that promote glycogen breakdown (glycogen phosphorylase and glucose6phosphatase). While the primary effects of insulin are on the carbohydrate metabolism, it also changes the way that the body processes other macromolecules. For instance, insulin increases amino acid uptake by muscle cells, thereby increasing levels of protein synthesis and decreasing breakdown of essential proteins. Insulin also exhibits a significant impact on the metabolism of fats, especially in the liver and adipocytes. The effects of insulin on the metabolism of fats are described below: Insulin increases: -Glucose and triacylglycerol uptake fat cells -Lipoprotein lipase activity, which clears VLDL and chylomicrons from the blood -Triacylglycerol synthesis (lipogenesis) in adipose tissue and the liver from acetyl-CoA Insulin decreases: -Triacylglycerol breakdown (lipolysis) in adipose tissue -Formation of ketone bodies by the liver The most important controller of insulin secretion is plasma glucose. Above a threshold of 100mg/dL, or about 5.6mM glucose, insulin secretion is directly proportional to plasma glucose. For glucose to promote insulin secretion, it must not only enter the B-cell, but also be metabolized, increasing intracellular ATP concentration. Increased ATP leads to calcium released in the cell, which promotes exocytosis of preformed insulin from intracellular vesicles. Insulin secretion is also affected by signaling initiated by other hormones, such as glucagon and somatostatin.

decreasing

When combined together mathematically, along with temperature (T), these quantities can be related through Gibbs free energy equation _______________which predicts the direction in which a chemical reaction proceeds spontaneously. Spontaneous reactions proceed in the forward direction, exhibit a net loss of free energy and have a positive delta G. Free energy approaches zero as the reaction proceeds to equilibrium and there is no net change in concentration of reactants and products.

delatG=deltaH-TdeltaS

The change in free energy (deltaG) predicts changes occurring at any concentration of products and reactants and at any temperature. In contrast, standard free energy (deltaGprime) is the energy change that occurs at standard concetrations of 1 M, pressure of 1 atm, and temperature of 25'C. These can be related by the equation ___________________ where R is the universal gas constant, T is the temperature, and Q is the reaction of the quotient. Biochemical analysis works well under all standard conditions except 1: pH. A 1 M concetration of protons would correspond to a pH of 0, which is far too acidic for most biochemical reactions. Therefore, in the modified standard state, [H+]=10^-7 M and the pH is 7. With this additional condition, deltaGprime is given the special symbol deltaGprime', indicating that it is standardized to the neutral buffers used in biochemistry. Note that is the concentration of other reactants and products differ from 1 M, these must still be adjusted for the equation above. The shift in deltaG as a result of changing concentration is not universally toward or away from spontaneity. There is a general trend that reactions with more products than reactants have a more negative deltaG, while reactions with more reactants that products have a more positive delta G.

deltaG=deltaGprime + RTlnQ

Because the system is closed, the change in internal energy can only come in the form of work or heat. This can be represented mathematically in the first law of thermodynamics:

deltaU=Q-W

Tissues have evolved so that their metabolic needs are met in a way corresponding to their form and function. The major sites of metabolic activity in the body are in the liver, skeletal and cardiac muscles, brain, and adipocytes. Connective tissue and _________________cells do not make major contributions to the consumption of energy. Remember though, that epithelial cells are the primary secretory cells, so they are involved in the regulation of metabolism. We have already discussed how the body operates under different nutritional conditions.

epithelial

Diet (energy intake) and excersize( energy expenditure), genetics, socioeconomic status, and geography all play key roles in weight control. Hormonal control by thyroid hormones, cortisol, epinephrine glucagon, and insulin is critical to the integration of metabolism. In addition, there are hormones that control hunger and satiety, including ghrelin, orexin, and leptin. Have you ever wondered why you don't feel hungry and then you walk into your favorite restraunt and are? This is ghrelin and orexin. Ghrelin is secreted by the stomach in response to signals of an impending meal. Sight, sound, taste, and smell especially act as signals for its release. _________________ increases appetite and also stimulates secretion of orexin. Orexin further increases apetite, and is also involved in alertness and the sleep-wake cycle. Hypoglycemia is also a trigger for orexin release. Leptin is a hormone secreted by fat cells that decrease apetite by supressing orexin produciton. Genetic varitions in the leptin molecule and its receptors have been implicated in obesity.

ghrelin

As individuals ____________ in mass, basal metabolic rate (the amount of energy required for one sedentary day) also increases. Thus, a caloric excess will cause an increase in body mass until equilibrium is reached between the new basal metabolic rate and the existing intake. In weight loss the reverse trend is seen. This effect does not have a threshold that differs between individuals. Small adjustments in intake, even over a prolonged period of time, are partially or fully compensated by changes in energy expenditure. Similarly, a small increase or decrease in activity level will be compensated by changes in hunger. Deliberate alterations of body mass require alterations above this threshold level, which is larger in negative energy balance than in positive energy balance- in other words, larger changes must be made to lose weight than to gain it.

increase

(postprandial state continued...)just after eating, blood glucose levels rise and stimulate the release of __________. The three major target tissues for insulin are the liver, muscle and adipose tissue. Insulin promotes glycogen synthesis in liver and muscle. After the glycogen stores are filled, the liver converts excess glucose to fatty acids and triacylglycerols. Insulin promotes triacylglycerol synthesis in adipose tissues. After a meal, protein synthesis in muscle, as well as glucose entry into both tissues. After a meal, most of the energy needs of the liver are met by the oxidation of excess amino acids. Two types of cells-nervous tissue and red blood cells- are notably insensitive to insulin. Nerovous tissue derives energy from oxidizing glucose to CO2 and water in both the well-fed and normal fasting states. Only in prolonged fasting does this situation change. Red blood cells can only use glucose anaerobically for all their energy needs, regardless of the individuals metabolic state.

insulin

(Cardiac muscle)Unlike other tissues of the body, cardiac myocytes prefer fatty acids as their major fuel, even in the well-fed state. When ____________ are present during prolonged fasting, they can also be used. Thus, not surprisingly, cardiac myocytes most closely parallel skeletal muscle during extended periods of excersize. In patients with caridac hypertrophy (thickining of heart muscle), this situation reverses to some extent. In a failing heart, glucose oxidation increases and B-oxidation falls.

ketones

2 major roles of the ____________ in fuel and metabolism are to maintain a constant level of blood glucose under a wide range of conditions and to synthesize ketones when excess fatty acids are being oxidized. After a meal, glucose concentration in the portal blood is elevated. The liver extracts excess glucose and uses it to replenish its glycogen stores. Any glucose remaining in the liver is then converted to acetyl-CoA and used for fatty acid synthesis. The increase in insulin after a meal stimulates both glycogen synthesis and fatty acid synthesis in the liver. The fatty acids are converted to triacylglycerols and released into the blood as very-low-density lipoproteins (VLDL). In the well-fed state, the liver derives most of its energy from the oxidation of excess amino acids. Between meals and during prolonged fasts, the liver releases glucose in the blood. The increase in glucagon during fasting promotes both glycogen degradation and gluconeogensisis. Lactate from anaerobic merabolism, glycerol from triacylglycerols, and amino acids provide carbon skeletons for glucose synthesis.

liver

There are several methods for analyzing _______________ control of an organism. In humans, levels of glucose, thyroid hormones, and thyroid stimulating hormone, insulin, glucagon, oxygen, and carbon dioxide can all be measured in the blood. Because these hormones and substrates have a predictable effect on metabolism, they can be used as indicators of metabolic function. They can also be used as indicators for disorders, as in the case of blood glucose or thyroid-stimulating hormone.

metabolic

Most of the ATP in a cell is produced by _________________, but some ATP is produced during glycolysis and indirectly from GTP in the citric acid cycle. ATP consist of an adenosine molecule attached to three phosphate groups, and is generated from ADP and Pi with energy input from an exergonic reaction or electrochemical gradient. ATP is consumed either through hydrolysis or the transfer of a phosphate group to another molecule. If one phosphate group is removed, adenosine diphosphate (ADP) is produced; if two phosphate groups are removed, adenosine monophosphate (AMP) is the result. In a single day, an averaged sized person uses about 90% of her weight in ATP but only has about 50 grams of ATP available at any given time. Continous recycling of ATP, ADP, and Pi more than 1000 times per day accounts for this discrepancy. What makes ATP such a good energy carrier is its high-energy phosphate bonds. The negative charge on the phosphate groups experience repulsive forces with one another, and the ADP and Pi molecules that form after hydrolysis are stabilized by resonance. While ATP doesn't rapidly break down on its own in the cell, it is much more stable after hydrolysis. This accounts for a very negative value of deltaG. Under standard conditions deltaGprime is about -55kJ/mol. At pH 7 and with excess magnesium, the standard free energy change is still-30.5 kJ/mol. ADP, which also displays charge repulsion and resonance stabilization after hydrolysis, has similar deltaG values, but AMP has a much smaller deltaGprime near -9.2 kJ/mol.

mitochondrial ATP synthase

The pathways that are operational in fuel metabolism depend on the _____________ status of the organism. Shift between storage and mobilization of a particular fuel, as well as shifts among the type of fuel being used, are very pronounced when going form a well-fed state to an overnight fast, and finally to a prolonged state of stavation.

nutritional

Many key enzymes in ATP synthesis and other biochemical pathways have ____________________ activity

oxidoreductase

If each cell were acting independently of one another, metabolism would be a random process that could not be coordinated with outside events like meals or exertion. In order to make the most efficient use of the resources available, metabolism must be regulated across the entire organism. This regulation is accomplished best through hormonal means. Water-soluble ____________________, like insulin, are able to rapidly adjust the metabolic processes of cells via second messenger cascades, while certain fat-soluble amino acid-derivative hormones, like thyroid hormones, and steroid hormones, like cortisol, enact longer-range effects by exerting regulatory actions at the transcriptional level. Hormone levels are regulated by feedback loops with other endocrine structures, such as the hypothalamic-pituitary axis, or by the biomolecule upon which they act; for example, insulin causes a decrease in blood glucose, which removes the trigger for continued insulin release. Next, we'll examine the specific actions of several hormones involved in the regulation of metabolism and in maintaining homeostasis, including insulin and glucagon, epinephrine, glucocorticoids, and thyroid hormones.

peptide hormones

The ___________, also called the absorptive or well-fed state, occurs shortly after eating. This state is marked by greater anabolism (Synthesis of biomolecules) and fuel storage than catabolism ( breakdown of biomolecules for energy). Nutrients flood in from the gut and make their way vie the hepatic portal vein to the liver, where they can be stored or distributed to other tissues of the body. The postprandial state generally lasts three to five hours after eating a meal.

postprandial state

Energy is exchanged in the form of mechanical work when something is moved over a distance, or as heat energy. Matter is exchanged through food consumption and elimination, as well as __________________.

respiration

(Resting muscle)The major fuel of skeletal muscle are glucose and fatty acids. Because of its enormous bulk, _________________ is the bodies major consumer of food. After a meal, insulin promotes glucose uptake in skeletal muscle, which replenishes glycogen stores and amino acids used for protein synthesis. Both excess glucose and amino acids can also be oxidized for energy. In the fasting state, resulting muscle uses fatty acid derived from free fatty acids circulating in the bloodstream. Ketone bodies may also be used if the fasting is prolonged.

skeletal muscle