Carbohydrate Classification, Structure, Digestion, Absorption, and Metabolism

Question 1

What are the monosaccharides?

Answer 1

Glucose, galactose, fructose

Question 2

Name 3 common disaccharides

Answer 2

Sucrose (1 glucose and 1 fructose)
Maltose (2 glucose moieties)
Lactose (1 glucose and 1 galactose)

Question 3

Are complex carbs water soluble or insoluble?

Answer 3

Insoluble with high molecular weights.

Question 4

What are oligosaccharides?

Answer 4

Polysaccharides containing fewer than 10 glucose units

Question 5

What are the straight chains in starch called?

Answer 5

Amylose

Question 6

What are the branched chains in starch called?

Answer 6

Amylopectin

Question 7

How do glucose and galactose enter enterocytes?

Answer 7

Through sodium-glucose transporter 1 (SGLT1), an adenosine triphosphate (ATP)-dependent active transporter

Question 8

Which protein transports fructose?

Answer 8

GLUT5 (glucose transporter 5)

Question 9

What are the 2 classes of glucose transport proteins?

Answer 9

The sodium dependent glucose transporters and the facilitative transporters

Question 10

Describe the function of facilitative transporters

Answer 10

Function as passive diffusions channels (channels dependent on the gradient of glucose concentration between extra- and intracellular compartments) and do not require sodium or ATP. Present in many different tissues

Question 11

Describe the function of sodium dependent glucose transporters

Answer 11

Involved in the active, ATP-dependent absorption of glucose from the intestinal lumen into cells. Also function in the reabsorption of filtered glucose in the proximal tubule of the kidney

Question 12

Where are the highest concentrations of GLUT1?

Answer 12

Endothelial (major part of the blood-brain barrier) and glial cells of the brain

Question 13

Which monosaccharides are transported by GLUT1?

Answer 13

Glucose and galactose

Question 14

Where is GLUT2 mainly located?

Answer 14

Liver, small intestine, and kidneys

Question 15

Describe the action of GLUT2 in the liver (1), kidneys (2), small intestine (3)

Answer 15

1. In hepatocytes, GLUT2 has a low affinity but high capacity for glucose, allowing quick equilibration of intracellular and extracellular glucose concentrations across the membrane during fasting and fed states.
2. GLUT2 transports the reabsorbed glucose back into the bloodstream from proximal tubular epithelia.
3. GLUT2 is important in the transepithelial transport of the absorbed glucose

Question 16

Where is the highest concentration of GLUT3?

Answer 16

The neuronal membranes of the brain

Question 17

Describe the action of GLUT3 in neuronal cells

Answer 17

Controls the rate of glucose entry into neuronal cells

Question 18

Where is GLUT4 mainly located?

Answer 18

Adipose (brown and white) and muscle (skeletal and cardiac) tissues. Mainly within the intracellular vesicles of these tissues but can be translocated to the cell surface in response to a rise in plasma insulin concentrations or muscle contraction

Question 19

Where is GLUT5 mainly located?

Answer 19

The apical membrane of the jejunum

Question 20

What is the formula that expresses the net chemical reaction of glucose catabolism?

Answer 20

C6H12O2 + 6O2 –> 6CO2 + 6H2O + Energy

Question 21

Where in the cell does glycolysis take place?

Answer 21

The cytoplasm

Question 22

Where in the cell does the TCA cycle take place?

Answer 22

Inside mitochondria

Question 23

Glucose absorption into which organs is dependent on insulin? Independent?

Answer 23

Skeletal muscle and adipose tissue glucose absorption is insulin dependent.
Liver, kidney, brain, other tissue glucose absorption is insulin independent.

Question 24

Name the substrates and major sites of expression for GLUT1

Answer 24

Substrates are glucose, galactose, mannose, and glucosamine
Major sites of expression are erythrocytes, CNS, blood-brain barrier, placenta, fetal tissues in general

Question 25

Name the substrates and major sites of expression for GLUT2

Answer 25

Substrates are glucose, galactose, fructose, mannose, glucosamine
Major sites of expression are liver, beta-cells of pancreas, kidney, small intestine

Question 26

Name the substrates and major sites of expression for GLUT3

Answer 26

Substrates are glucose, galactose, mannose, xylose, dehydroascorbic acid
Major sites of expression are brain (neurons), spermatozoa, placenta, preimplantation embryos

Question 27

Name the substrates and major sites of expression for insulin dependent GLUT4

Answer 27

Substrates are glucose, glucosamine, dehydroascorbic acid
Major sites of expression are muscle, heart, brown and white adipocytes

Question 28

Name the substrates and major sites of expression for GLUT5

Answer 28

Substrates are fructose, but not glucose
Major sites of expression are intestine, kidney, brain, skeletal muscle, adipose tissue

Question 29

The net chemical reaction of glucose catabolism results in:

Answer 29

An equal number of moles of CO2 and oxygen, creating a respiratory quotient of 1

Question 30

Glycosis converts glucose into:

Answer 30

Pyruvate

Question 31

Where does the glycolytic pathway in the cell take place?

Answer 31

Cytoplasm

Question 32

Approximately __ kcals are released per mole of ATP

Answer 32

7

Question 33

What are the 3 major enzymes in glycolysis?

Answer 33

Glucokinase, phosphofructokinase, and pyruvate kinase

Question 34

Where in the cell does the TCA cycle occur?

Answer 34

Mitochondria, only under aerobic conditions

Question 35

What is the metabolic reaction of cell respiration?

Answer 35

The TCA cycle

Question 36

Using the pyruvate-lactate pathway, how many ATPs can be generated per molecule of pyruvate?

Answer 36

2

Question 37

The oxidation of 1 glucose molecule produces __ molecules of ATP total from which 2 types of reactions?

Answer 37

36 molecules of ATP. 4 molecules via substrate phosphorylation (2 from glycolysis and 2 from pyruvate decarboxylation) and 32 molecules via oxidative phosphorylation

Question 38

Which pathway allows some organisms to survive under anaerobic conditions?

Answer 38

Pyruvate-lactate pathway

Question 39

What are the 4 distinctive, irreversible reactions of gluconeogensis?

Answer 39

1. Pyruvate is carboxylated in mitochondria to oxaloacetate
2. The formed oxaloacetate is phosphorylated in the cytosol to phosphoenolpyruvate
3. Special phosphatases hydrolyze fructose 1,6-biphosphate
4. Special phosphatases hydrolyze glucose 6-phosphate

Question 40

When can amino acids and fat provide carbon for gluconeogenesis?

Answer 40

During fasting or carbohydrate deprivation

Question 41

What is the purpose of gluconeogenesis?

Answer 41

It is the endogenous formation of glucose and is important to maintain plasma glucose concentration in the normal range during fasting and allows constant provision of energy for metabolic needs

Question 42

The body stores carbohydrate as:

Answer 42

Glycogen

Question 43

In a 70 kg healthy person, the liver contains approximately __ gm of glycogen

Answer 43

100 gm (has the potential to provide 390 kcal)

Question 44

Skeletal muscle contains about ___ gm of glycogen which suggests that an adult has enough glycogen to last for how long while performing normal activities?

Answer 44

Contains 300-400 gm glycogen, yielding less than 1560 kcal, suggesting that an adult stores enough glycogen for about a day of normal activities

Question 45

The formation of glycogen is called ___, and the breakdown of glycogen is called ___

Answer 45

Glycogenesis
Glycogenolysis

Question 46

Glycogen synthesis and glycogenolysis are regulated by the rate-limiting enzymes:

Answer 46

Glycogen synthase and glycogen phosphorylase, respectively

Question 47

What are the primary substrates to gluconeogensis?

Answer 47

Lactate, glutamine, alanine, and glycerol

Question 48

After 14 days of fasting, adipose tissue can provide more than what percent of daily energy requirements?

Answer 48

90%

Question 49

After how many weeks of starvation is the brain able to adapt to using ketones as a source of energy?

Answer 49

1-2 weeks

Question 50

Describe how stress hyperglycemia occurs?

Answer 50

During periods of trauma and illness, there is increased production of stress hormones such as epinephrine and cortisol, accompanied by an elevation in growth hormone and glucagon. These counterregulatory hormones oppose insulin action, causing increased glucose production by the liver (may exceed 500 gm glucose per day) and decreased utilization of glucose in peripheral tissues. Acute and chronic disease and injury can also increase production of cytokines which further contribute to hyperglycemia through stimulation of counterregulatory hormone release and suppression of insulin action

Question 51

How can gastric emptying be delayed in regard to fiber intake?

Answer 51

Increased viscosity of gastric contents, which reduces pyloric flow

Question 52

A minimum of __ gm of carbohydrates per day is required to avoid ketone production

Answer 52

50 gm

Question 53

During moderate exercise, approximately __% of energy is provided by glucose

Answer 53

60%

Question 54

What should be done if a pregnant patient with gestational diabetes loses weight?

Answer 54

If patient’s response to diagnosis of gestational diabetes is to self-starvation to avoid hyperglycemia, test urine for ketones and increase carb intake