43
Chapter 3 – Carbohydrates
Multiple-Choice
Key Page(s)
c 63 1. The majority of energy in the typical American diet comes from:
a. fat.
b. protein.
c. carbohydrate.
d. vitamins.
a 64 2. A ketopentose is a carbohydrate containing
a. 5 carbons and a ketone group.
b. 6 carbons and an aldehyde group.
c. 3 carbons and an aldehyde group.
d. 7 carbons and a ketone group.
b 64 3. When compounds with 1 or more chiral carbon atoms and the same formula are arranged as mirror images they are said to be _____.
a. symmetrical
b. enantiomers
c. rotated
d. linear
b 65 4. What is the nutritional significance of stereoisomers?
a. Some isomers cannot cyclize.
b. Certain metabolic enzymes require a particular structure.
c. Certain isomers cannot polarize light.
d. Some isomers have no anomeric carbon.
a 66 5. Name the model that depicts cyclized monosaccharides as lying in a horizontal plane with the hydroxyl groups pointing down or up from the plane.
a. Haworth
b. Fischer projection
c. cyclized Fischer projection
d. stereoisomer
a 63, 67 6. What are the major dietary energy sources that are composed of two simple sugars?
a. disaccharides
b. polysaccharides
c. monosaccharides
d. trioses
d 68 7. What is the most common digestible homopolysaccharide existing as both amylose and amylopectin?
a. glycogen
b. cellulose
c. hemicellulose
d. starch
b 68 8. Which of the following homopolysaccharides made of glucose contributes the most energy to the ordinary diet?
a. amylose
b. amylopectin
c. glycogen
d. cellulose
d 68 9. A homopolysaccharide that is important in human diets is _____ and the end product formed from the complete digestion of this homopolysaccharide is _____.
a. cellulose; glucose
b. lactose; galactose
c. glycogen; glucose
d. starch; glucose
c 69 10. In what form do carbohydrates enter the bloodstream?
a. starch
b. disaccharides
c. monosaccharides
d. ketoses
d 70 11. What is the key enzyme in digestion of polysaccharides?
a. starch amylopectin
b. β-amylase
c. α-dextranase
d. α-amylase
a 70 12. Which of the following carbon bonds is digested by α-amylase?
a. α (1-4)
b. α (1-6)
c. β (1-4)
d. β (1-6)
d 63, 64, 70 13. Which of the following is the predominant monosaccharide produced by the digestion of all dietary carbohydrates?
a. ribose
b. fructose
c. galactose
d. glucose
d 70 14. The enzyme needed to hydrolyze the α (1,6) bond of amylopectin is _____, which is secreted from the _____.
a. amylase, enterocyte
b. sucrase, pancreas
c. lactase, enterocyte
d. isomaltase, enterocyte
c 70 15. The disaccharidases are synthesized by the _____.
a. pancreas
b. liver
c. enterocyte
d. chief cell
c 70 16. Sucrose digestion is initiated in the _____.
a. pylorus
b. fundus
c. duodenum
d. mouth
d 64, 70 17. The final molecules that are absorbed from the digestion of the carbohydrate in spaghetti (noodles) and a cream sauce are _____.
1. sucrose
2. glucose
3. maltose
4. galactose
5. lactose
6. fructose
a. 1, 4
b. 1, 4, 6
c. 1, 5, 3
d. 2, 4
d 72, 73 18. Choose the best description of the main method of glucose absorption.
a. passive diffusion down a concentration gradient
b. active transport with fructose
c. facilitated transport
d. active transport with sodium
b 73 19. What is a likely reason that large quantities of fructose may cause gut discomfort?
a. Diffusion of fructose only occurs from low to high concentrations.
b. Facilitated diffusion is slower than active transport.
c. Active transport is saturable.
d. Pinocytosis reverses fructose and releases it to the gut.
c 75 20. The glucose transporter that is sensitive to insulin is
a. GLUT1.
b. GLUT2.
c. GLUT4.
d. SGLT1.
b 77 21. Which hormonal changes occur in response to a fall in blood glucose concentration?
a. increased insulin, decreased glucagon
b. decreased insulin, increased glucagon
c. decreased insulin, decreased glucocorticoids
d. increased insulin, increased glucagon
d 77 22. Glycemic load considers the _____ in the food.
a. protein and carbohydrate content
b. fat and carbohydrate content
c. fat to carbohydrate ratio
d. quantity and quality of carbohydrate
d 81 23. People with type 1 diabetes have
a. slow translocation of GLUT4 receptors from the Golgi body.
b. lack of mRNA synthesis for GLUT4 receptors in adipocytes or myocytes.
c. increased glycogenesis in myocytes after a meal.
d. below-normal glucokinase activity because of low insulin levels.
c 77, 80, 81 24. Insulin is an anabolic hormone. Which process does insulin stimulate?
a. lipolysis
b. gluconeogenesis
c. glycogenesis
d. ketogenesis
b 82 25. Due to a lack of the enzyme glucose-6-phosphatase, which tissue capable of glycogenesis cannot contribute to blood glucose levels between meals?
a. liver
b. muscle
c. brain
d. kidney
c 79, 83 26. Glycolysis is a process involving:
a. synthesis of fatty acids.
b. reactions that convert glucose to glycogen.
c. reactions that convert glucose to pyruvate.
d. reactions that convert glycogen to protein.
b 83 27. The major end products of cellular oxidation of carbohydrate include:
a. carbon, nitrogen, and hydrogen.
b. water, carbon dioxide, and energy.
c. glucose and amino acids.
d. dietary fiber and ammonia.
b 83 28. In what part of the cell does glycolysis occur?
a. mitochondrion
b. cytosol
c. nucleus
d. endoplasmic reticulum
d 83 29. In which cellular site is most of the energy released when carbohydrates are oxidized to carbon dioxide and water?
a. cytoplast
b. reticuloendothelium
c. Golgi body
d. mitochondrion
b 83 30. Two hormones that stimulate glycogenolysis in the muscle and liver, respectively, are _____.
a. cortisol and epinephrine
b. epinephrine and glucagon
c. insulin and epinephrine
d. glucagon and insulin
c 86 31. In what organelle are the enzymes that catalyze the citric acid cycle located?
a. cytoplasmic matrix
b. endoplasmic reticulum
c. mitochondrion
d. lysosome
a 85, 86 32. In the _____ galactose is eventually converted to _____.
a. hepatocyte, glucose
b. Kupffer cell, fructose
c. enterocyte, glucose
d. chylomicron, glucose
b 86, 89 33. Under anaerobic conditions within a cell, how many net ATPs are formed from one glucose molecule by substrate-level phosphorylation?
a. one
b. two
c. four
d. six
d 89 34. In the complete oxidation of 1 mol of glucose, how many ATPs are formed?
a. 4-6
b. 8-12
c. 18-24
d. 32-38
b 89, 103 35. When oxygen is present in a tissue, less glucose is metabolized to pyruvate. Why?
a. Less lactate can accumulate in the presence of oxygen.
b. ATP accumulates and inhibits phosphofructokinase.
c. Metabolism slows in the presence of oxygen.
d. More glucose is converted to glycogen.
b 89-90 36. Alcohol in beverages is degraded mainly in the liver cytosol with the production of one NADH for each ethanol molecule. Predict what effect consumption of alcohol would have on the activity of the liver malate-aspartate shuttle.
a. no change
b. increased
c. decreased
d. reversed
d 90 37. The formation of ATP by compounds with a more negative phosphate group transfer potential than -7,300 cal is called _____.
a. transfer phosphorylation
b. oxidative phosphorylation
c. coupled reactions
d. substrate-level phosphorylation
b 91 38. During the end reaction of the electron transport chain, molecular oxygen becomes _____.
a. oxidized to carbon dioxide
b. reduced to water
c. NADH + H+
d. FADH2
a 91, 93 39. Electron flow through Complexes I, III, and IV is accompanied by the translocation of protons
a. from the matrix into the intermembrane space.
b. from the iron-sulfur centers into the cytosol.
c. from cytochrome c to cytochrome c1.
d. from cytochrome b to the iron-sulfur center.
c 93-94 40. Electrons carried by one mole NADH + H+ enter the electron transport chain at Complex I and generate _____ mole(s) ATP as they are sequentially oxidized.
a. 0.5
b. 1.5
c. 2.5
d. 3.5
d 95-96 41. The enzyme ATP synthase is believed to catalyze the formation of ATP, as described by the _____ theory.
a. leakage
b. hydrogen pump
c. protein channel
d. chemiosmotic
a 97 42. The purpose of the hexose monophosphate shunt is to produce
a. pentose phosphates and NADPH.
b. DNA and RNA.
c. fatty acids.
d. reducing substrates.
d 97 43. Which tissue has the least activity of the pentose phosphate pathway?
a. liver
b. adrenal cortex
c. mammary gland
d. skeletal muscle
b 98 44. Gluconeogenesis is essentially the reversal of which pathway?
a. glycogenesis
b. glycolysis
c. TCA cycle
d. lipogenesis
a 98 45. All of the following are substrates for gluconeogenesis EXCEPT
a. fatty acids.
b. lactate.
c. glycerol.
d. glucogenic amino acids.
c 99 46. Muscle lactate is converted to glucose in the liver and returned to the muscle by means of the _____.
a. malate-aspartate shuttle
b. electron transport chain
c. Cori cycle
d. pentose phosphate pathway
c 81-83, 102 47. If an individual with no blood sugar abnormalities when eating regularly presented with severe hypoglycemia after 30 hours of fasting, which enzyme would you suspect might be malfunctioning?
a. phosphofructokinase
b. pyruvate kinase
c. fructose-1,6-bisphosphatase
d. glucose-6-phosphatase
c 80, 103 48. All of the following enzymes are negatively regulated (allosterically inhibited) by increasing levels of ATP EXCEPT _____.
a. phosphofructokinase
b. pyruvate dehydrogenase complex
c. hexokinase
d. citrate synthase
a 84, 103 49. Glucagon stimulates hepatic gluconeogenesis and suppresses glycolysis by reducing the concentration of _____, a positive modulator of phosphofructokinase.
a. fructose-2,6-bisphosphate
b. fructose-1,6-bisphosphate
c. glucose-6-phosphate
d. glucose-1-phosphate
True/False
Indicate whether each of the following statements is true or false. If the statement is false, explain why it is false.
Key Page(s)
F 75, 76 1. The abundance of GLUT4 is increased by induction in response to a high-CHO meal.
Explanation: Translocation, not induction.
F 78, 97 2. The purpose of the pentose phosphate pathway is to generate ribose, for nucleic acid synthesis, and NAD, for oxidizing power.
Explanation: It generates NADPH for reducing power.
F 102 3. Phosphorylation of a protein always results in its inactivation, whereas dephosphorylation activates it.
Explanation: Phosphorylation can sometimes inactivate a protein/enzyme (e.g., glycogen synthase) and sometimes activate a protein/enzyme (e.g., glycogen phosphorylase), with dephosphorylation respectively activating and inactivating these enzymes.
T 72 4. Glucose is transported from the lumen into the enterocyte by active transport using SGLT1 protein, which also requires Na as a co-transporter.
T 99 5. The process of gluconeogenesis occurs partially in the mitochondria, and partially in the cytosol.
F 79, 82, 99 6. The muscle is an important tissue in gluconeogenesis, because it can use amino acids from protein breakdown and convert them to glucose, which it then secretes into the circulation for other tissues.
Explanation: The muscle cannot convert amino acids to glucose and then secrete it because of a lack of glucose-6-phosphatase.
F 83 7. The signal for glycogenolysis to occur in liver and muscle is glucagon, which is secreted by the pancreas in response to low blood glucose levels.
Explanation: The signal is glucagon for the liver, epinephrine for muscle.
T 89-90 8. The reason we require the malate-aspartate shuttle is because NADH cannot enter the mitochondria to donate its electrons to the electron transport chain.
T 99 9. The Cori cycle would be active under anaerobic conditions, such as excessive muscle exertion.
T 99-101 10. Although many enzymes in a given pathway are bidirectional, often the key enzymes are unidirectional (i.e., only work in one direction).
F 83, 99 11. Because they do not have mitochondria, red blood cells generate a lot of lactate from glycolysis, which they in turn must convert into glucose to meet their energy needs.
Explanation: The lactate must travel from the RBC via the blood to the liver; in the liver, lactate is converted into glucose (via gluconeogenesis), which is then secreted back into the bloodstream.
F 83 12. All cells have mitochondria, which act as the main site for ATP production.
Explanation: RBCs do not have mitochondria; hence, they rely on glycolysis for energy needs and produce a lot of lactate.
T 72 13. Active transport requires energy for the Na-K pump to transport Na out of the cell, thereby driving the transport of something else (e.g., glucose) when the Na re-enters the cell down its concentration gradient.
Matching
Enzymes: Match the enzymes with the pathway in which they function in carbohydrate metabolism.
Key / Page(s) / Enzyme / Pathwayb / 84 / 1. Phosphofructokinase / a. glycogenesis
c / 89, 99 / 2. Pyruvate carboxylase / b. glycolysis
d / 82 / 3. Glycogen phosphorylase / c. gluconeogenesis
e / 97 / 4. Glucose-6-phosphate dehydrogenase / d. glycogenolysis
a / 80-81 / 5. Branching enzyme / e. pentose phosphate pathway
Fill-in-the-Blank
Page(s)
68-69 1. A homopolysaccharide that is important as a storage compound in human body is _____, which can be enzymatically dismantled to produce _____.
Key: glycogen, glucose
98 2. When dietary intake of carbohydrate is reduced or blood glucose concentration declines, the pathway that is stimulated in the liver by glucagon and corticosteroids is called _____. Four non-carbohydrate precursors that serve as substrates for this pathway are: _____, _____, _____ and amino acids.
Key: gluconeogenesis, pyruvate, lactate, glycerol
80 3. Glucokinase located in the _____ is induced by insulin, and hexokinase located in the _____ is inhibited by glucose-6-phosphate.
Key: liver (and pancreas), muscle
101 4. ADP can positively modulate the activity of the rate-limiting allosteric enzyme in glycolysis, _____.
Key: phosphofructokinase
101 5. Pyruvate dehydrogenase is positively modulated by _____ and _____.
Key: AMP, NAD+
Short Answer (with suggested answer key)
Page(s)
75-76 1. Often you will hear the statement, “Insulin is important for the uptake of blood glucose by all tissues.” What is wrong with this statement? Be specific.
Key: This is really a GLUT4 question—all tissues can take up glucose because they have transporters (even when there isn’t any insulin). Insulin stimulates increased uptake in muscle and adipose because it binds to the insulin receptor on these cells and causes the GLUT4 to translocate from the cytosol to the membrane. Note that insulin still binds to the insulin receptor on all the other cells—they all have changes in glycolytic enzymes, etc.—they just don’t have the right type of GLUT to translocate in response to insulin. Also note that insulin does not enter cells—it (like glucagon) binds to its receptor and various signals are transmitted into the cell (i.e., signal transduction), which results in various responses (induction of genes; stimulation of enzymes; translocation of GLUT4); some of these responses occur in all cells, some in only specific cells.