Chapter 21:Carbohydrate Metabolism

explain reaction 3 in glycolysis

A second energy investment is catalyzed by the enzyme phosphofructokinase. A phosphoanhydride bond in ATP is broken, and a phosphoester linkage between the phosphoryl group and the C-1 hydroxyl group of fructose-6-phosphate is formed. The product is fructose-1,6-bisphosphate.

Breaking the terminal phosphoanhydride bond of ATP yields _____________ __________________, a phosphoryl group, and energy. The energy released by this reaction is then used to drive biological processes; like the phosphorylation of glucose or fructose.

Adenosine diphosphate (ADP)

is a sex-linked genetic disorder (on the X chromosome). As a result, more males suffer from it. There are many clinical features but rarely are all found in the same patient. These range from mental challenge and seizures to a slowly progressive myopathy and hemolytic anemia.

Phosphoglycerate kinase deficiency

what hormones control glycogenolysis?

Two hormones control glycogenolysis, the degradation of glycogen. These are glucagon, a peptide hormone synthesized from the pancreas, and epinephrine, produced in the adrenal glands. Glucagon is released from the pancreas in response to low blood glucose, and epinephrine is released from the adrenal glands in response to a threat or stress. Both situations require an increase in blood glucose, and both hormones function by altering the activity of two enzymes, glycogen phosphorylase and glycogen synthase

Fat arrives in the duodenum, the first portion of the SI, in the form of large fat globules. Bile salts produced by the liver break these up into an emulsion of tiny fat droplets. Because the small droplets have a greater surface area, the lipids are now more accessible to the action of pancreatic lipase. This enzyme hydrolyzes the fats into fatty acids and glycerol, which are taken up by the intestinal cells by a transport process that does NOT require energy. This process is called

passive transport

The second major segment of glycolysis involves the remaining reactions of the pathways (6-10), those that

result in a net energy yield.

explain reaction 8 in glycolysis

3-phosphoglycerate is isomerized to produce 2-phosphoglycerate in a reaction catalyzed by the enzyme phosphoglycerate mutase. The phosphoryl group attached to the third carbon of 3-phosphoglycerate is transferred to the second carbon.

how does lactate fermentation affect the food industry

A variety of bacteria are able to carry out lactate fermentation under anaerobic conditions. This is of great importance in the dairy industry, because these organisms are used to produce yogurt and some cheeses. The tangy flavor of yogurt is contributed by the lactate produced by these bacteria. Unfortunately, similar organisms also cause milk to spoil.

is the universal energy currency of all cells

ATP

what type of molecule is ATP? what does it contain?

ATP is a nucleotide, which means it is a molecule composed of a nitrogenous base; a five-carbon sugar; and one, two or three phosphoryl groups

what happens in stage 3 of the catabolic processes?

Acetyl CoA carries acetyl groups, two-carbon remnants of the nutrients, to the citric acid cycle. Acetyl CoA enters the cycle, and electrons and hydrogen atoms are harvested during the complete oxidation of the acetyl group to CO2. coenzyme A is released (recycled) to carry additional acetyl groups to the pathway. The electrons and hydrogen atoms that are harvested are used in the process of oxidative phosphorylation to produce ATP.

biosynthesis of metabolic intermediates and macromolecules

Anabolism

explain reaction 5 in glycolysis

Because G3P is the only substrate that can be used by the next enzyme in the pathway, the DHAP is rearranged to become a second molecule of G3P. the enzyme that mediates this isomerization is triose phosphate isomerase.

why must pyruvate and NADH must be used?

Both pyruvate and NADH must be used in some way so that glycolysis can continue to function and produce ATP. The buildup of pyruvate would cause glycolysis to stop, thereby stopping production of ATP. If NADH were to build up, there would be no NAD+, available for step 6, in which glyceraldehyde-3-phosphate is oxidized and NAD+ is reduced (accepts the hydride anion). This would stop glycolysis and thus NADH must be reoxidized so that glycolysis continues to produce ATP for the cell.

what is the net gain of ATP during glycolysis

Chemical energy as ATP. four ATP molecules are formed by the process of substrate-level phosphorylation. In this process a high energy phosphoryl group from one of the substrates is transferred to ADp to form ATP. the two substrates involved in these transfer reactions are 1,3biphosphoglycerae and phosphoenolpyruvate (steps 7 and 10). Although four ATP molecules are produced during glycolysis, the net gain is only two ATP molecules because two ATP molecules are used early in glycolysis.

when is NADH used for energy?

Chemical energy in the form of reduces NAD+, NADH. Nicotinamide adenine dinucleotide (NAD+) is a coenzyme derived from the vitamin niacin, the reduced form of NAD+, NADH, carries hydride anions, hydrogen atoms with two electrons (H:-) removed during the oxidation of glyceraldehyde-3-phosphate. Under aerobic conditions, the electrons and hydrogen atom are transported from the cytoplasm into the mitochondria. Here they enter an electron transport system for the generation of ATP by oxidative phosphorylation. Under anaerobic conditions, NADH is used as a source of electrons in fermentation reactions

The conversion of lactate into glucose is important in mammals. As the muscles work the produce lactate, which is converted back to glucose in the liver. The glucose is transported into the blood and from there back to the muscle. In the muscle, it can be catabolized to produce ATP, or it can be used to replenish the muscle stores of glycogen. This cyclic process between the liver and skeletal muscles is called the _________ _______________. Through this cycle, gluconeogenesis produces enough glucose to restore the depleted muscle glycogen reservoir within 48H.

Cori Cycle

when/how can fructose enter into glycolysis?

Depending on the tissue, fructose enters glycolysis in different ways. In the muscle, where hexokinase is abundant, the enzyme phosphorylates fructose to fructose-6-phosphate, which directly enters glycolysis. There is much less hexokinase in the liver, but fructokinase is present. Fructokinase phosphorylates fructose to produce fructose-1-phosphate. This product is cleaved into dihydroxyacetone phosphate (DHAP) and glyceraldehyde by the enzyme fructose-1-phosphate aldolase. The glyceraldehyde is phosphorylated by triose kinase to produce glyceraldehyde-3-phosphate (G3P). The DHAP and G3P enter glycolysis directly.

what helps regulate glycolysis?

Energy-harvesting pathways such as glycolysis are responsive to the energy needs of the cell. Reactions of the pathway speed up when there is a demand for ATP and slow down when there is abundant ATP to meet the energy requirements of the cell. One of the major mechanisms for the control of the rate of glycolysis is the use of allosteric enzymes. In addition to the active site, which binds the substrate, allosteric enzymes have an effector binding site, which binds a chemical signal that alters the rate at which the enzymes catalyze the reaction. Effector binding may increase (positive allosterism) or decrease (negative allosterism) the rate of the reaction, The chemical signals, or effectors, that indicate the energy needs of the cell include molecules such as ATP. when the ATP concentration is high, the cell must have sufficient energy. Similarly, ADP and AMP, which are precursors of ATP, are indicators that the cell is in need of ATP. in fact, all of these molecules are allosteric effectors that alter the rate of irreversible reactions catalyzed by enzymes in the glycolytic pathway.

is involved in glycogen degradation and is activated during degradation

Glycogen phosphorylase

explain reaction 4 in glycolysis

Fructose-1,6-bisphosphate is split into three-carbon intermediates in a reaction catalyzed by the enzyme aldolase. The products are glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP).

why is it important that glycolysis and gluconeogenesis are regulated?

If glycolysis and gluconeogenesis were not regulated in some fashion, the two pathways would occur simultaneously. Three convenient sites for this regulation are the three bypass reactions. Step 3 of glycolysis is catalyzed by the enzyme phosphofructokinase. This enzyme is stimulated by high concentrations of AMP, ADP, and inorganic phosphate, which signals that the cell needs energy. When the enzyme is active, glycolysis proceeds. On the other hand, when ATP is plentiful, phosphofructokinase is inhibited, and fructose-1,6biphosphate is stimulated. The net result is that in time of energy excess, gluconeogenesis will occur.

explain reaction 7 in glycolysis

In this reaction, energy is harvested in the form of ATP. The enzyme phosphoglycerate kinase catalyzes the transfer of the phosphoryl group of 1,3-biphosphoglycerate to ADP. This is the first substrate-level phosphorylation of glycolysis and it produces ATP and 3-phosphoglycerate. It is a coupled reaction in which the high-energy bond is broken and the energy released is used to drive the synthesis of ATP.

explain reaction 6 in glycolysis

In this reaction, the aldehyde glyceraldehyde-3-phosphate is oxidized to a carboxylic acid in a reaction catalyzed by glyceraldehyde-3-phosphate dehydrogenase. This is the first step in glycolysis that harvests energy, and it involves the reduction of the coenzyme nicotinamide adenine dinucleotide (NAD+). This reaction occurs in two steps. First, NAD+ is reduced to NADH as the aldehyde group of glyceraldehyde-3-phosphate is oxidized to a carboxyl group. Second, an inorganic phosphate group is transferred to the carboxyl group to give 1,3-biphosphoglycerate. Notice that the new bond is denoted with a squiggle, indicating it is high-energy. This, and all remaining reactions of glycolysis occur twice for each glucose because each glucose has been converted into two molecules of glyceraldehyde-3-phosphate

explain reaction 9 in glycolysis

In this step, the enzyme enolase catalyzes the dehydration (removal of water molecule0 of 2-phosphoglycerate. The energy-rich product is phosphoenolpyruvate, the highest phosphorylated compound in our metabolism.

has been mapped on chromosome 7. The disorder is found in US African American, Italian, and Japanese families. The clinical features include exercise intolerance, muscle pain, and myoglobinuria after intense exercise.

Phosphoglycerate mutase deficiency

what are the two alcohol fermentation reactions that pyruvate endures?

Pyruvate decarboxylase removes CO2 from the pyruvate, producing ethanal (acetaldehyde) Alcohol dehydrogenase catalyzes the reduction of ethanal to ethanol, and the oxidation of NADH to NAD+ The regeneration of NAd+ allows glycolysis to continue The two products of alcohol fermentation, then, are ethanol and CO2, we take advantage of this fermentation in the production of wines and other alcoholic beverages and in bread making

which reactions can happen twice?

Reactions 6-10 occur twice per glucose molecule because the starting six-carbon sugar is split into two three-carbon molecules. Thus glycolysis produces two NADh molecules and a total of four ATP molecules. The net ATP gain from this pathway is only two ATP molecules because of the energy investment of two ATP molecules in the early steps of the pathway.

We can organize catabolic processes into 3 stages, which are:

Stage I: Hydrolysis of Dietary Macromolecules into Small Subunits Stage II: Conversion of Monomers into a Form that can be completely oxidized Stage III: The complete oxidation of Nutrients and the Production of ATP

what are the steps for glycogen degradation?

Step 1: The enzyme glycogen phosphorylase catalyzes phosphorolysis of a glucose at one end of a glycogen polymer. The reaction involves the displacement of a glucose unit of glycogen by a phosphate group. As a result of phosphorolysis, glucose-1-phosphate is produced without using ATP as the phosphoryl group donor Step 2: Glycogen contains many branches bound to the alpha (1→4) backbone by alpha (1→6) glycosidic bonds. These branches must be removed to allow the complete degradation of glycogen. The extensive action of glycogen phosphorylase produces a smaller polysaccharide with a single glucose bound by an alpha (1→6) glycosidic bond to the main chain. The enzyme alpha (1→6) glycosidase, also called the debranching enzyme, hydrolyzes the alpha (1→6) glycosidic bond at a branch point and frees one molecule of glucose. This molecule of glucose can be phosphorylated and utilized in glycolysis, or it may be released into the bloodstream for use elsewhere. Hydrolysis of the branch bond liberates another stretch of alpha (1→4) linked glucose for the action of glycogen phosphorylase. Step 3: Glucose-1-phosphate is converted to glucose-6-phosphate by phosphoglucomutase. Glucose originally stored in glycogen enters glycolysis through the action of phosphoglucomutase. Alternatively, in the liver and kidneys it may be dephosphorylated for transport into the bloodstream.

is caused by a deficiency of phosphofructokinase. Although this is not a sex-linked diosrder, its more common in males. The disorder is most frequently found in Jews and Italian families. Onset of symptoms typically occur between the ages of 20 and 40. Patients experiencing the late-onset typically experience exercise intolerance when they are younger. Vigorous exercise results in myoglobinuria and severe muscle pain. Meals high in carbohydrates worsen the exercise intolerance. Early onset disease is often associated with respiratory failure, cardiomyopathy, seizures, and cortical blindness.

Tarui's disease

which enzymes are allosterically connected to glycolysis?

The enzyme hexokinase, which catalyzes the phosphorylation of glucose, is allosterically inhibited by the product of the reaction it catalyzes, glucose-6-phosphate. A build-up of this product indicates that the reactions are not proceeding at a rapid rate, presumably because the cell has enough energy. Phosphofructokinase, the enzyme that catalyzes the third reaction in glycolysis, is a key regulatory enzyme in the pathway. ATP is an allosteric inhibitor of phosphofructokinase, whereas AMP and ADp are allosteric activators. Another allosteric inhibitor of phosphofructokinase is citrate. Citrate is the first intermediate in the citric acid cycle, a pathway that results in the complete oxidation of pyruvate. A high concentration of citrate signals that sufficient substrate is entering the citric acid cycle. The inhibition of phosphofructokinase by citrate is an example of feedback inhibition: the product, citrate, allosterically inhibits the activity of an enzyme early in the pathway. The last enzyme of glycolysis, pyruvate kinase, is also subject to allosteric regulation. In this case, fructose 1,6-bisphosphate, the product of the reaction catalyzed by phosphofructokinase is the allosteric activator. Thus, activation of phosphofructokinase results in the activation of pyruvate kinase. This is an example of feedforward activation because the product of an earlier reaction causes the action of an enzyme later in the pathway

explain reaction 10 in glycolysis

The final substrate-level phosphorylation is in the pathway catalyzed by pyruvate kinase. Phosphoenolpyruvate serves as a donor of the phosphoryl group that is transferred to ADP to produce ATP. This is another coupled reaction in which breaking the phosphoester bond in phosphoenolpyruvate provides energy for the formation of the phosphoanhydride bond of ATP. the final product of glycolysis is pyruvate

what is the first reaction in glycogen synthesis?

The first reaction of glycogen synthesis in the liver traps glucose within the cell by phosphorylating it. In this reaction, catalyzed by the enzyme glucokinase, ATP serves as a phosphoryl donor and glucose-6-phosphate is formed:

explain reaction 2 in glycolysis

The glucose-6-phosphate formed in the first reaction is rearranged to produce the structural isomer fructose-6-phosphate. The enzyme phosphoglucose isomerase catalyzes this isomerization. The result is that the C-1 carbon of the six carbon sugars is exposed; it is no longer part of the ring structure. Examination of the open-chain structures reveals that this isomerization converts an aldose into a ketose.

what happens in stage 2 of the catabolic processes

The monosaccharides, amino acids, fatty acids, and glycerol must now be assimilated into the pathways of energy metabolism. The two major pathways are glycolysis and the citric acid cycle. Sugars usually enter the glycolysis pathway in the form of glucose or fructose. They are eventually converted to acetyl CoA, which is a form that can be completely oxidized in the citric acid cycle. Amino groups are removed from amino acids, and the remaining carbon skeletons enter the catabolic processes at many steps. Fatty acids are converted to acetyl CoA and enter the citric acid cycle in that form. Glycerol, produced by the hydrolysis of fats, enters energy metabolism via glycolysis.

what is the purpose of the first stage of catabolic processes?

The purpose of the first stage of catabolism is to degrade large food molecules into their components of subunits, simple sugars, amino acids, fatty acids, and glycerol, which are then taken into the cells for the body for use as an energy source.

what is the second reaction of glycogenesis

The second reaction of glycogenesis is the reverse of one of the reactions of glycogenolysis. The glucose-6-phosphate formed in the first step is isomerized to glucose-1-phosphate. The enzyme that catalyzes this step is the phosphoglucomutase:

explain reaction 1 in glycolysis

The substrate, glucose, is phosphorylated by the enzyme hexokinase in a couple phosphorylation reaction. The source of the phosphoryl group is ATP. At first, this reaction seems contrary to the overall purpose of catabolism, production of ATP. The expenditure of ATP in these early reactions must be thought of as an investment. The cell actually goes into energy "debt' int these early reaction but this is necessary to get the pathway started

how many pyruvate molecules are created in glycolysis?

Two pyruvate molecules. At the end of glycolysis, the six-carbon glucose molecules has been converted into two three-carbon pyruvate molecules. The fate of the pyruvate also depends on whether the reactions are occurring in the presence or absence of oxygen. Under aerobic conditions, it is used to produce acetyl CoA destine for the citric acid cycle and complete oxidation. Under anaerobic conditions, it is used as an electron acceptor in fermentation reactions.

Polysaccharides are hydrolyzed to monosaccharides. Salivary amylase begins the hydrolysis of starch in the mouth. In the small intestine, pancreatic amylase further hydrolyzes the starch into maltose. Maltase catalyzes the hydrolysis of maltose, producing two glucose molecules. Similarly, sucrose is hydrolyzed to glucose and fructose by the enzyme sucrase, and lactose (milk sugar) is degraded into glucose and galactose by the enzyme lactase in the small intestine. The monosaccharides are taken up by the epithelial cells of the intestine in an energy-requiring process called

active transport

has been appreciated since the dawn of civilization.the fermentation process itself was discovered by Pateur. Under anaerobic conditions, yeast ferment the sugar produced by fruit and grains

alcohol femernation The pyruvate produced by glycolysis undergoes two reactions of the alcohol fermentation

The lactate produced in the working muscle passes into the blood. Eventually, if strenuous exercise is continued, the concentration of lactate becomes so high that this fermentation can no longer continue. Glycolysis and this ATP production stops. The muscle, deprived of energy, can no longer function. This point of exhaustion is called the

anaerobic threshold

Of the 3 classes of food molecules, _____________- are the most readily used.

carbohydrates

Our diet includes 3 major sources of energy:

carbohydrates, fats and proteins.

is the set of metabolic pathways that break down complex macromolecules into simpler ones and in the process harvest part of their potential energy for use by the cell.

catabolism

If the cell is functioning under aerobic conditions, NADH will be reoxidized, and pyruvate will be completely oxidized by aerobic respiration. Under anaerobic conditions, different types of _______________________ reaction accomplish this. they are catabolic reactions that occur with no net oxidation. Pyruvate or an organic compound produced from pyruvate is reduced as NADH is oxidized.

fermentation

is produced in response to low blood glucose levels, hypoglycemia, and has an effect opposite to that of insulin. It stimulates glycogen phosphorylase, which catalyzes the first stage of glycogen degradation. This accelerates glycogenolysis and release of glucose into the bloodstream. The effect is further enhanced because ________________ inhibits glycogen synthase.

glucagon

another symptomof genetic defects in enzymes is is ___________________ anemia (anemia that results from the lysis of RBCs). RBCs are completely dependent on glycolysis of ATP. a defect in one of the enzymes of glycolysis results in insufficient ATP and resultant cell death.

hemolytic

Glucose is produced by the process of ________________, the production of glucose from noncarbohydrate starting materials. it is an anabolic pathway, occurs primarily in the liver. Lactate, all the amino acids except leucine and lysine, and glycerol from fatas can all be used to make glucose. However, the amino acids and glycerol are generally only used in starvation conditions

gluconeogenesis Gluconeogenesis is NOT the reverse of glycolysis because steps 1, 3, and 10 of glycolysis are irreversible and therefore the reverse reaction must be carried out by other enzymes. In step 1 of glycolysis, hexokinase catalyzes the phosphorylation of glucose. In gluconeogenesis, the dephosphorylation of glucose-6-phosphate is carried out by the enzyme glucose-6-phosphatase, which is found in the liver but not in muscle. Similarly, reaction 3, the phosphorylation of fructose-6-phosphate catalyzed by phosphofructokinase, is irreversible. That step is bypassed in gluconeogenesis by using the enzyme fructose-1,6-bisphosphatase. Finally, the phosphorylation of ADP catalyzed by pyruvate kinase in step 10 of glycolysis cannot be reversed. The conversion of pyruvate to phosphoenolpyruvate actually involves two enzymes and some unusual reactions. First, the enzyme pyruvate carboxylase adds CO2 to pyruvate. The product is the four-carbon compound oxaloacetate. Then phosphoenolpyruvate carboxykinase removes the CO2 and adds a phosphoryl group. The donor of the phosphoryl group in this unusual reaction is guanosine triphosphate (GTP). This is a nucleotide like ATP, except the nitrogenous base is guanine.

is a long, branched chain polymer of glucose. Stored in the liver and skeletal muscles, it is the principal storage form of glucose.

glycogen is a highly branched glucose polymer in which the "main chain" is linked by alpha (1→ 4) glycosidic bonds. The polymer alpha (1→6) glycosidic bonds, which provide many branch points along the chain. Glycogen granules with a diameter of 10-40nm are found in the cytoplasm of the liver and muscle cells. These granules exist in complexes with the enzymes that are responsible for glycogen synthesis and degradation

involves glycogen synthesis and is inactivated during degradation

glycogen synthase

21.7 Glycogen Synthesis and Degradation Glucose is the sole source of energy of mammalian RBCs and the major source of energy for the brain. Neither RBCs nor the brain can store glucose; thus, a constant supply must be available as blood glucose. This is provided by dietary glucose and by the production of glucose either by gluconeogenesis or by __________________________, the degradation of glycogen

glycogenolysis

The pathway for the first stage of carbohydrate breakdown is

glycolysis

also known as the Embden-Meyerhof pathway, is a pathway for carbohydrate catabolism that begins with the substrate D-glucose. The pathway evolved at a time when the earth's atmosphere was anaerobic, no free oxygen was available. As a result, _______________ requires no oxygens; it is an anaerobic process. It must have evolved in very simple, single-celled organisms that lacked complex organelles, much like bacteria. As a result, __________ is a process carried out by enzymes that are free in the cytoplasm.

glycolysis

When the blood glucose level is too high, ________________________, insulin stimulates the uptake of glucose via a transport mechanism. It further stimulates the trapping of glucose by the elevated activity of glucokinase. Finally, it activates glycogen synthase, the last enzyme in the synthesis of glycogen chains. To further accelerate the storage, insulin inhibits the first enzyme in glycogen degradation, glycogen phosphorylase. The net effect is that the glucose is removed from the bloodstream and converted into glycogen in the liver. When the glycogen stores are filled, excess glucose is converted to fat and stored in adipose tissue.

hyperglycemia

The hormone_____________, produced by the pancreas in response to high blood glucose levels, stimulates the synthesis of glycogen, glycogenesis. Insulin is perhaps one of the most influential hormones in the body because it directly alters the metabolism and uptake of glucose in all but a few cells.

insulin When blood glucose rises, as after a meal, the beta cells of the pancreas secrete insulin. It immediately accelerates the uptake of glucose by all the cells of the body except the brain and certain blood cells. In these cells, the uptake of glucose is insulin-independent. The increased uptake of glucose is especially marked in the liver, heart, skeletal muscle, and adipose tissue. In the liver, insulin promotes glycogen synthesis and storage by inhibiting glycogen phosphorylase, thus inhibiting glycogen degradation. It also stimulates glycogen synthase and glucokinase, two enzymes that are involved in glycogen synthesis. -Although glycogenesis and glycogenolysis share some reactions in common, the two pathways are not simply the reverse of one another. Glycogenesis involves some very unusual reactions.

Glycolysis can be divided into two major segments. The first set of steps, reactions 1-5, is the

investment of ATP energy.

is familiar to anyone who has performed strenuous exercise. If you exercise so hard that your lungs and circulatory system can't deliver enough oxygen to the working muscles, your aerobic energy-harvesting pathways are not able to supply enough ATP to your muscles. But the muscles still demand energy. Under these anaerobic conditions, lactate fermentation begins, in this reaction, the enzyme lactate dehydrogenase reduces pyruvate to lactate. NADh is the reducing agent for this process. As pyruvate is reduced, NADH is oxidized, and NAD+ is again available permitting glycolysis to continue.

lactate fermentation

Another symptom of rhabdo is the release of myoglobin into the blood and eventually into the urine, ____________________ which results in the urine that is the color of cola soft drinks and may damage the kidneys.

myoglobinuria

are sent from cell to cell in your nervous systems

neurotransmitters

is an alternative pathway for glucose oxidation. It provides the cell with energy in the form of NADPH, which is the reducing agent required for many biosynthetic pathways.

pentose phosphate pathway The pentose phosphate pathway provides several molecules that are important in biosynthesis. The first is reducing power in the form of NADPH. It also provides sugar phosphates that are required for biosynthesis. For instance, ribose-5-phosphate is used for the synthesis of nucleotides such as ATP. The four-carbon sugar phosphate, erythrose-4-phosphate, is produced in the thor stage of the pentose phosphate pathway. It is a precursor of the amino acids phenylalanine, tyrosine, and tryptophan. The pentose phosphate pathway is most active in tissue involved in cholesterol and fatty acid biosynthesis. These two processes require abundant NADPH. Thus, the liver, which is the site of cholesterol synthesis and a major site for fatty acid biosynthesis, and adipose (fat) tissue where active fatty acid synthesis also occurs, have very high levels of pentose phosphate pathway enzymes.

Symptoms can arise if a person has a genetic defect in one of the enzymes. Symptoms include muscle myopathy which involves damage to the muscle as a result of the inability to extract energy from food molecules. Myopathy can cause exercise intolerance, which is the onset of fatigue when exercising or it can even cause muscle breakdown called in which the muscle cells, starved of energy, begin to die

rhabdomyolysis

Digestion of fats does not begin until the food reaches the__________ ____________

small intestine.

In ATP, a phosphoester bond joins the first phosphoryl group to the five-carbon sugar ribose. The next two phosphoryl groups are joined to one another by phosphoanhydride bonds (high-energy bonds). When it is broken, a large amount of energy is released. When the phosphoanhydride bond of ATP is broken, the energy released can be used for cellular work. These high energy bonds are indicated by

squiggles (~).

The digestion of proteins begins in the __________, where the low pH denatures the proteins so that they are more easily hydrolyzed by the enzyme pepsin. They are further degraded in the small intestine by trypsin, chymotrypsin, elastase, and other proteases. The products of protein digestion-amino acids and short oligopeptides, are taken up by the cells lining in the intestine. This uptake also involves an active transport mechanism.

stomach