ML4: Carbohydrates (CHO) and CHO catabolism

Question 1

What is the general formula for carbohydrates?

Answer 1

(CH₂O)_n

Question 2

What is the plasma glucose concentration in mammals maintained at?

Answer 2

Approx. 5 mM

Question 3

Which groups do carbohydrates contain?

Answer 3

Aldehyde (–CHO) or keto (–C=O)

Multiple –OH

Question 4

What are stereoisomers with reference to trioses?

Answer 4

Molecules that differ in spatial organisation of the asymmetric carbon atom furthest from the aldehyde or ketone group

Question 5

What are the names of monosaccharides with 3, 4, 5, 6, and 7 carbons?

Answer 5

3 = triose

4 = tetrose

5 = pentose

6 = hexose

7 = pentose

Question 6

In what form do hexoses usually exist? Give an example.

Answer 6

Cyclic forms, e.g. glucose, fructose

Question 7

What are epimers?

Answer 7

Each of two isomers with different configurations of atoms about one of several asymmetric carbon atoms present

Question 8

What conformation do sugar molecules assume?

Answer 8

A ‘chair’ or ‘boat’ conformation

Question 9

What is the most important monosaccharide metabolically in mammals?

Answer 9

Glucose

(Other important ones include fructose (from fruit) and galactose (from dairy))

Question 10

How are monosaccharides linked to form complex carbohydrates?

Answer 10

Through α-1,4-glycosidic bonds

Question 11

What are the monosaccharides that make up sucrose, maltose, and lactose?

Answer 11

Sucrose = glucose + fructose

Maltose = glucose + glucose

Lactose = glucose + galactose

Question 12

How are glucose molecules joined in glycogen? Which bond forms branches?

Answer 12

Through α-1,4- and α-1,6-glycosidic bonds

α-1,6-glycosidic bonds cause branches

Question 13

What are the bonds between the monosaccharides in cellulose?

Answer 13

β-1,4-glycosidic bonds only (unbranched)

Question 14

What are the three polymers of glucose?

Answer 14

• Glycogen
  
  • Main storage polysaccharide of animal cells (liver and skeletal muscle)
  • α-1,4- and α-1,6-glycosidic bonds
  • Highly branched
• Starch
  
  • ​Plants
  • α-1,4- and α-1,6-glycosidic bonds
  • Less branched
• Cellulose
  
  • ​Plants
  • β-1,4-glycosidic bonds
  • Unbranched

Question 15

Name seven dietary carbohydrates.

Answer 15

1. Starch
2. Glycogen
3. Lactose
4. Maltose
5. Sucrose
6. Glucose
7. Fructose

Question 16

Give details of Stage I catabolism of carbohydrates.

Answer 16

• α-1,4-bonds of glycogen and starch digested in the mouth by salivary amylase to produce dextrins
• Other enzymes in small intestine (attached to brush border membrane of epithelial cells)
  
  • Pancreatic amylase (α-1,4 bonds)
  • Isomaltase (α-1,6 bonds)
  • Sucrase (sucrose)
  • Lactase (lactose)
• Monosaccharides released (glucose, fructose, and galactose), absorbed and further metabolised (stage II)

Question 17

How are monosaccharides absorbed during Stage I catabolism?

Answer 17

• Actively transported into intestinal epithelial cells and then, via blood supply, to target tissues
• Uptake into cells via facilitated diffusion using transport proteins (GLUT1 – GLUT5)
  
  • Different distribution and affinities
  • Can be hormonally controlled (insulin)

Question 18

What are the glucose requirements of tissues?

Answer 18

• All tissues can metabolise glucose
• Blood glucose concentration regulated (approx. 5 mM)
  
  • Because some tissues (RBC, WBC, kidney medulla, lens of the eye) have an absolute requirement and uptake by these tissues is dependent on blood glucose concentration
• CNS (brain) prefers glucose