Chapter Questions
Name the following pathways:(a) Pathway for synthesis of glycogen(b) Pathway for release of glucose from glycogen(c) Pathway for synthesis of glucose from lactate
Name the synthetic pathways that have glucose 6 -phosphate as their first reactant.
There are two sets of reactions in glycolysis in which phosphate intermediates are synthesized in the first reaction and their energy harvested as ATP in the second reaction. Identify the two sets of reactions.
Identify each step in glycolysis that is an isomerization.
Verify the isomerization that occurs in step 2 of glycolysis by drawing the open-chain forms of glucose 6 -phosphate and fructose 6 -phosphate.
In Figure $22.3,$ compare the starting compound (glucose) and the final product (pyruvate).(a) Which is oxidized to a greater extent?(b) Are there any steps in the glycolytic pathway in which an oxidation or reduction occurs? Identify the oxidizing or reducing agents that are involved in these steps.
Use curved arrows (like those in Figure 22.3 ) to write an equation for the conversion of fructose to fructose 6 -phosphate by ATP. At what step does fructose 6 -phosphate enter glycolysis?
Compare glucose and galactose (see Table 20.1), and explain how their structures differ.
In alcoholic fermentation, each mole of pyruvate is converted to one mole of carbon dioxide and one mole of ethanol. In the process, about $209 \mathrm{~kJ} / \mathrm{mol}$ of energy is produced. Under the most favorable conditions, more than one-half of this energy is stored as ATP.(a) What happens to the remaining energy produced in alcoholic fermentation?(b) Give two reasons why it would be nearly impossible to reverse the reaction that converts pyruvate to ethanol and carbon dioxide.
Name three ways humans have exploited the ability of microorganisms to ferment carbohydrates.
Pyruvate has three different fates. What are the three different molecules pyruvate is converted into? What conditions exist for the formation of each product?
Glycolysis of one molecule of glucose produces 8 ATP molecules. How many ATP molecules are produced from glycolysis of 10 glucose molecules?
Complete catabolism of one glucose molecule yields 38 ATP molecules. How many moles of ATP are produced by the complete catabolism of one mole of glucose?
Refer to Figure 22.6 and summarize the changes in liver glycogen and blood glucose during the starvation period represented in the figure.
In a diabetic some glucose is converted to sorbitol, an alcohol that accumulates in the eye and can cause cataracts. Draw the open-chain structure of sorbitol, which is identical to that of D-glucose except that the aldehyde group has been reduced to an alcohol group. Can sorbitol form a five- or six-membered cyclic hemiacetal? Explain why or why not. (Hint: The open-chain structure of glucose is found in Section 20.1.)
Ketoacidosis is relieved by rapid breathing, which converts hydrogen carbonate ions and hydrogen ions in the blood to gaseous carbon dioxide and water, as shown in this equation.$$\mathrm{H}^{+}+\mathrm{HCO}_{3}^{-} \longrightarrow \mathrm{H}_{2} \mathrm{CO}_{3} \longrightarrow \mathrm{H}_{2} \mathrm{O}+\mathrm{CO}_{2}(\text { Exhaled })$$(a) Assuming that these reactions can go in either direction, how does a state of acidosis help to increase the generation of carbon dioxide?(b) What principle describes the effect of added reactants and products on an equilibrium?
What is the difference between glycogenesis and glycogenolysis?
Why is glycogenesis necessary? Why is glycogenolysis necessary?
What two types of reactions convert glycerol to dihydroxyacetone phosphate?
What is the purpose of the Cori cycle?
Why is gluconeogenesis necessary?
What class of enzymes catalyzes the majority of the reactions involved in carbohydrate digestion?
Glucose 6 -phosphate is in a pivotal position in metabolism. Depending on conditions, glucose 6 -phosphate follows one of several pathways. Under what conditions do the following occur?(a) Glycolysis(b) Hydrolysis to free glucose(c) Pentose phosphate pathway(d) Glycogenesis
What "chemical investments" are made to get glycolysis started, and why are they made? What happens in the middle of the pathway to generate two 3 -carbon compounds? What are the outcomes of the reactions of these 3 -carbon compounds?
Outline the conditions that direct pyruvate toward the following:(a) Entry into the citric acid cycle(b) Conversion to ethanol and $\mathrm{CO}_{2}$(c) Conversion to lactate(d) Glucose synthesis (gluconeogenesis) In what tissues or organisms is each pathway present?
Classify each enzyme of glycolysis into one of the six classes of enzymes. What class of enzymes has the most representatives in glycolysis? Why is this consistent with the goals of glycolysis? Why are ligases not represented in glycolysis?
When blood glucose levels rise following a meal, the following events occur. Arrange these events in the appropriate sequence.(a) Glucagon is secreted.(b) Glycolysis replenishes ATP supplies.(c) Glucose is absorbed by cells.(d) The liver releases glucose into the bloodstream.(e) Glycogen synthesis (glycogenesis) occurs with excess glucose.(f) Blood levels pass through normal to below normal (hypoglycemic).(g) Insulin levels rise.
Name the molecules used for gluconeogenesis. What are the sources of these molecules? Under what conditions would gluconeogenesis occur?
Fatty acids from stored triacylglycerols (fat) are not available for gluconeogenesis. Speculate why we do not have the enzymes to directly convert fatty acids into glucose. Plants (especially seeds) do have enzymes to convert fatty acids into carbohydrates. Why are they so lucky?
The pathway that converts glucose to acetyl-CoA is often referred to as an "aerobic oxidation pathway." (a) Is molecular oxygen involved in any of the steps of glycolysis? (b) Thinking back to Chapter 20 , where does molecular oxygen enter the picture?
Where does digestion occur in the body, and what kinds of chemical reactions does it involve?
Complete the following word equation:$$\text { Lactose }+\mathrm{H}_{2} \mathrm{O} \longrightarrow ?+?$$Where in the digestive system does this process occur?
What are the major monosaccharide products produced by digestion of carbohydrates?
What are the products of digestion of proteins, triacylglycerols, maltose, sucrose, lactose, and starch?
What do the words aerobic and anaerobic mean?
What three products are formed from pyruvate under aerobic, anaerobic, and fermentation conditions?
Name the substrate and the product of (a) glycolysis and(b) gluconeogenesis.
Name the substrate and the product of (a) glycogenesis and (b) glycogenolysis.
What is the major purpose of the pentose phosphate pathway? What cofactor (coenzyme) is used?
Depending on the body's needs, into what type of compounds is glucose converted in the pentose phosphate pathway?
Where in a liver cell do the following pathways occur?(a) Glycolysis(b) Gluconeogenesis(c) Glycogenesis(d) Glycogenolysis
Which cells, liver, muscle, or brain, use the following pathways?(a) Glycolysis(b) Gluconeogenesis(c) Glycogenesis(d) Glycogenolysis
Although the catabolism of glucose produces energy, the first step uses energy. Explain why.
Glycolysis can occur under both aerobic and anaerobic conditions. Why is glycolysis called an anaerobic pathway?
Which glycolysis reactions are catalyzed by the following enzymes?(a) Pyruvate kinase(b) Glyceraldehyde 3 -phosphate dehydrogenase(c) Hexokinase(d) Phosphoglycerate mutase(e) Aldolase
Review the 10 steps in glycolysis (Figure 22.3 ) and then answer the following questions:(a) Which steps involve phosphorylation?(b) Which step is an oxidation?(c) Which step is a dehydration?
How many moles of ATP are produced by phosphorylation in the following?(a) Glycolysis of $1 \mathrm{~mol}$ of glucose(b) Aerobic conversion of $1 \mathrm{~mol}$ of pyruvate to $1 \mathrm{~mol}$ of acetyl-CoA(c) Catabolism of $1 \mathrm{~mol}$ of acetyl-CoA in the citric acid cycle
For each reaction in Problem $22.47,$ tell if the ATP formed is produced by oxidative phosphorylation or substrate-level phosphorylation. What is the difference in the two types of ATP formation?
Why is pyruvate converted to lactate under anaerobic conditions?
Lactate can be converted into pyruvate by the enzyme lactate dehydrogenase and the coenzyme $\mathrm{NAD}^{+} .$ Write the reaction in the standard biochemical format, using a curved arrow to show the involvement of $\mathrm{NAD}^{+}$.
How many moles of $\mathrm{CO}_{2}$ are produced by the complete catabolism of $1 \mathrm{~mol}$ of sucrose?
How many moles of acetyl-CoA are produced by the complete catabolism of $1 \mathrm{~mol}$ of sucrose?
Differentiate between the effect of insulin and glucagon on blood sugar concentration.
Differentiate between blood sugar levels and resulting symptoms in hyperglycemia and hypoglycemia.
What molecules are used initially during starvation or fasting to produce glucose?
(Fill in the blank) As starvation continues, acetyl-CoA is converted to $\quad$ to prevent buildup of acetyl-CoA in the cells.
Where is most of the glycogen in the body stored?
What major site of glycogen storage is not able to release glucose to the bloodstream?
How is UTP used in the formation of glycogen from glucose?
Why does glycogenolysis use fewer steps than the reverse process, glycogenesis? Which process uses less energy?
Name the anabolic pathway for making glucose.
Name the two molecules that serve as starting materials for glucose synthesis.
(Fill in the blanks.) Pyruvate is initially converted to _____ in the anabolism of glucose. That molecule in turn is converted to _____.
Explain why pyruvate cannot be converted to glucose in an exact reverse of the glycolysis pathway.
Explain how the energy-releasing steps of glycolysis are reversed in gluconeogenesis.
How many steps in gluconeogenesis are not the exact reversal of the steps in glycolysis? What kind of conversion of substrate to product does each involve? What is the common theme in each of these reactions?
What is the Cori cycle?
Explain why the Cori cycle is necessary and when your cells would use this cycle.
Why can pyruvate cross the mitochondrial membrane but no other molecule after step 1 in glycolysis can?
Look at the glycolysis pathway (Figure 22.3 ). With what type of reactions are kinase enzymes usually associated?
Explain why one more ATP is produced when glucose is obtained from glycogen rather than used directly from the blood.
Why is it important that glycolysis be tightly controlled by the cell?
How many moles of ATP are generated from the catabolism of fructose (by glycolysis) in (a) liver cells and (b) muscle cells?
Which of the following conversions would you expect to consume energy and which would you expect to yield energy based on the final oxidation state of the coenzymes involved in each reaction?(a) pyruvate lactate(b) pyruvate acetyl-CoA $+\mathrm{CO}_{2}$
Why is it important for muscle cells to export lactate into the bloodstream during heavy exercise?
Under which physiological condition is the following pathway the predominant one?(a) glycolysis(b) gluconeogenesis
Why is it important for the cell that the NADH produced when pyruvate is converted to lactate be converted back to NAD $^{+} ?$
What are the characteristics of Type I diabetes?
What are the characteristics of Type II diabetes
Explain the relationship between metabolic syndrome and diabetes.
Many diabetics suffer blindness due to cataracts. Why is this condition associated with this disease?
A primary function of liver cells is to synthesize new glucose via gluconeogenesis, while working muscle cells use glucose via glycolysis. Why is this a good physiological strategy for your body?
In liver cells, galactose is converted to glucose 6 -phosphate in a four-step process; it then enters glycolysis. People with the genetic disease galactosemia lack one or more of the enzymes necessary to convert galactose to glucose 6-phosphate; galactose instead is converted to undesirable molecules that damage various organs. This disease can be controlled by a careful diet. What food or foods are a major source of galactose in the diet?
It is important to avoid air when making wine, so a novice winemaker added yeast to fresh grape juice and placed it in a sealed bottle to avoid air. Several days later, the lid exploded off the bottle. Explain the biochemistry responsible for the exploding lid.
Is the same net production of ATP observed in the complete oxidation of fructose as is observed in the complete oxidation of glucose? Why or why not?