L14- Digestion and Uptake

Question 1

What is the energy value for dietary glucose and carbohydrates in Cal/gram?

Answer 1

4 Cal/gram.

Question 2

What is the energy value for dietary fat in Cal/gram?

Answer 2

9 Cal/gram. It is the highest value of dietary macromolecules.

Question 3

Which bond is cleaved by the enzyme alpha-amylase?

Answer 3

Glu-alpha(1,4)-glu in starch.

Question 4

Which bond is cleaved by the enzyme maltase?

Answer 4

Glu-alpha(1,4)-glu in maltose.

Question 5

Which bond is cleaved by the enzyme isomaltase?

Answer 5

Glu-alpha(1,6)-glu in isomaltose and dextrins.

Question 6

Which bond is cleaved by the enzyme sucrase?

Answer 6

Glu-alpha(1,2)-beta-fru in sucrose.

Question 7

Which bond is cleaved by the enzyme lactase?

Answer 7

Gal-beta(1,4)-glu.

Question 8

How is salivary amylase inactivated?

Answer 8

Salivary amylase is inactivated by the low pH in the lumen of the stomach.

Question 9

What are the end products of starch digestion by alpha-amylase?

Answer 9

Maltose, maltrotriose and small branched oligosaccharides. Note that amylase does not generate glucose.

Question 10

Where are the enzymes that cleave disaccharides into monosaccharides located?

Answer 10

In the mucosal brush border of the small intestine.

Question 11

True or False. Sucrase and isomaltase are distinct enzymes encoded by two different genes.

Answer 11

False. Sucrase and isomaltase are indeed distinct enzymes but they are encoded by a single gene. An alteration of this gene can therefore affect the activity of both enzymes.

Question 12

Insufficiency of which enzyme causes lactose intolerance?

Answer 12

Lactase.

Question 13

What is the biochemical composition of dietary fiber?

Answer 13

Dietary fibers are undigested polysaccharides.

Question 14

Epidemiological studies have suggested a correlation between high dietary fiber intake and resistance to colon cancer and reduced risk of coronary heart disease. What is the physiological mechanism behind this claim?

Answer 14

Dietary fibers, especially ones that soluble, may be able to absorb and remove carcinogenic substances and cholesterol from digested foods in the intestine, and promote the excretion of these substances.

Question 15

List some physiological roles of dietary fiber.

Answer 15

It affects gastric emptying, intestinal absorption of monosaccharides, intestinal mobility and transit time. It also contributes to stool bulk.

Question 16

Cellulose is a prominent dietary fiber. It is found in the cell wall of many green plants. Why are humans not able to digest cellulose?

Answer 16

Cellulose is composed of glucose monomers bound in a beta(1-4) conformation. Humans do not have an enzyme that can cleave the glu-beta(1, 4)-glu bonds in cellulose.

Question 17

What are lignins?

Answer 17

They are non-carbohydrate plant cell wall materials that are also found in dietary fiber.

Question 18

Which monosaccharides are absorbed in the small intestine via simple diffusion?

Answer 18

Xylose and arabinose. Note that these monosaccharides are only produced in small amounts by the action of intestinal bacteria on polysaccharides that are not digested by human enzymes.

Question 19

Which monosaccharides are absorbed in the small intestine via facilitated transport?

Answer 19

Fructose, mannose and glucose. Note that glucose is moved into the cell by both facilitated and active transport.

Question 20

Which monosaccharides are absorbed in the small intestine via active transport?

Answer 20

Glucose and galactose.

Question 21

During active transport of glucose (and galactose) into mucosal cells of the small intestine, glucose (and galactose) are transported against their concentration gradient. What is the source of the energy required for this transport?

Answer 21

Transport of glucose and galactose into the mucosal cells of the small intestine is coupled to the entry of Na+ down its concentration gradient. The Na+ gradient is maintained by a Na+/K+ ATPase located on the basolateral membrane of the mucosal cells.

Question 22

Glucose and galactose enter the mucosal cells of the small intestine by ________ transport and leave the cells to enter the bloodstream by ________ transport.

Answer 22

Active, facilitated.

Question 23

What is the major metabolite used for energy in intestinal mucosal cells?

Answer 23

Glutamine. This reduces the need for mucosal intestinal cells to metabolize glucose for energy.

Question 24

Insulin stimulates glucose uptake in which tissues?

Answer 24

Skeletal and cardiac muscle, fat cells and white blood cells.

Question 25

How does insulin stimulate glucose absorption?

Answer 25

Insulin binds to a tyrosine kinase receptor, leading to the mobilization of glucose transporters to the cell membrane from the endoplasmic reticulum (especially GLUT4).

Question 26

Which glucose transporter is sodium-linked and where is it found?

Answer 26

SLGLUT1 is sodium-linked. It is found in intestinal and kidney epithelium.

Question 27

What is the difference between the GLUT1 and SLGLUT1 transporters?

Answer 27

SLGLUT1 is a sodium-linked glucose transporter that is found in intestinal and kidney epithelium. GLUT1 is also a glucose transporter but it is not sodium-linked. GLUT1 is found in erythrocytes, brain tissue and fetal tissue.

Question 28

What is the main glucose transporter of the liver and beta-cells of the pancreas?

Answer 28

GLUT2. Note that this transporter is not responsive to insulin.

Question 29

Which glucose transporter is responsive to insulin?

Answer 29

GLUT4.

Question 30

After a meal rich in carbohydrates, peak blood glucose in normal individuals is reached within _____ minutes, and the concentration returns to baseline by _____ hours. How are these values different in subjects affected by diabetes?

Answer 30

Peak = 30-45 minutes; Return to baseline = 1.5-2 hours. Subjects affected with diabetes have both a higher baseline concentration and peak concentration of blood glucose. Elevated concentrations of blood glucose also persist longer in subjects with diabetes when compared to normal individuals.

Question 31

Define the term ‘glycemic index.’

Answer 31

A glycemic index (GI) measures how much a carbohydrate raises blood glucose concentration. It is also defined on the basis of the area under the curve of the post-meal rise and fall of blood glucose concentration.

Question 32

What is the glycemic index of glucose? What is the glycemic index of other carbohydrates?

Answer 32

Glucose has a glycemic index of 100. All other carbohydrates have a glycemic index less than 100.

Question 33

What are isoenzymes?

Answer 33

Isoenzymes (also called isozymes) are enzymes that catalyze the same reaction even though they differ in amino acid sequence, kinetic behavior and/or regulatory properties.

Question 34

Once glucose is transported into a cell, it is quickly phosphorylated to glucose-6-phosphate. What is the reason for this?

Answer 34

Adding a phosphate group to glucose produces glucose-6-phosphate, a molecule that is much more hydrophilic than glucose because the phosphate group has a negative charge. This effectively traps glucose in the cells and maintains the gradient required for facilitated transport.

Question 35

How do hexokinase and glucokinase differ in terms of their tissue distribution?

Answer 35

Hexokinase is present in all tissues, whereas glucokinase is present mainly in the liver and beta-islet cells of the pancreas.

Question 36

How do hexokinase and glucokinase differ in terms of their Km for glucose?

Answer 36

Hexokinase has a much lower Km for glucose (0.2 mM) than glucokinase (10-20 mM).

Question 37

How do hexokinase and glucokinase differ in terms of their regulation?

Answer 37

Hexokinase is regulated by product feedback inhibition (i.e. glucose-6-phosphate inhibits hexokinase). Glucokinase is not inhibited by glucose-6-phosphate. Instead, it is regulated by substrate concentration and the amount of enzyme present in the cell.

Question 38

How do hexokinase and glucokinase differ in terms of their substrate specificity?

Answer 38

Hexokinase can phosphorylate many hexoses, whereas glucokinase can only phosphorylate glucose.

Question 39

Explain why hexokinase is not sensitive to changes in blood glucose levels after a meal.

Answer 39

The hexokinase Km for glucose (less than 0.2 mM) is much lower than blood glucose concentrations before and after a meal (can vary from 4 to 8 mM). Therefore, hexokinase will bind to glucose no matter the variation in blood glucose concentration after eating.

Question 40

What is the main function of hexokinase in all cells and how is it different than the function of glucokinase?

Answer 40

Hexokinase catalyzes the phosphorylation of hexoses to meet the metabolic needs of cells. Glucokinase regulates blood glucose levels by increasing glucose uptake in the liver and acting as part of the glucose sensor system in the beta-cells of the pancreas.

Question 41

How does insulin affect the activity of glucokinase in the liver?

Answer 41

In the liver, insulin stimulates gene transcription and translation of glucokinase. Since glucokinase activity is regulated by the substrate concentration and amount of enzyme in the cell, insulin indirectly increases the activity of glucokinase.

Question 42

Maturity-onset-diabetes of the young (MODY) is associated with a mutation in a gene that codes for which enzyme?

Answer 42

Glucokinase.