WEEK 4: CARBOHYDRATES: DIGESTIBLE AND INDIGESTIBLE

Question 1

Carbohydrates Classification

Answer 1

Monosaccharides
– simplest carbohydrates 
– C6H12O6 (glucose) 
Disaccharides 
– 2 monosaccharides bound 
– C12 H22O11 (maltose) 
Polysaccharides 
– many monosaccharides bound 
– complex carbohydrates: glycogen, starch, fibre 
– (C6 H12 O6)n

Question 2

Monosaccharide

Answer 2

6 CARBON SINGLE UNITS:
– Glucose 
– Fructose 
– Galactose 
– Sugar alcohols: xylitol, mannitol and sorbitol (= low calorie sweeteners) 
5 CARBON SINGLE UNITS: 
- Ribose 
- Deoxyribose

Question 3

Glucose

Answer 3

• A hexose, C6H12O6
• Most abundant monosaccharide
• Consumed as sucrose
  (glucose + fructose)
• Constitutes “blood sugar level”
• GLUT4 is insulin dependent
• Undergoes glycolysis or glycogenesis

Question 4

Fructose

Answer 4

• Varies from glucose in that the carbonyl group is on C-2 rather then C-1
• Is the sweetest monosaccharide
• Found in fruit, honey, vegetables
• Cell uptake is not insulin dependent
• Thought to be a good sugar because blood sugar level only measures glucose… but it does get transformed to glucose inside the cell

Question 5

Galactose

Answer 5

• Differs from glucose only in the orientation of the OH group around C-4
• With glucose, forms lactose (milk sugar)
• Also present in some vegetables
• Is the least sweet of the
  monosaccharides
• Converted to glucose in the liver

Question 6

Role of Carbohydrates

Answer 6

• The basis of structural tissue in plants e..g cellulose
• Source of energy and blood glucose homeostasis
• Substrate in the biosynthesis of other compounds: e.g. fat, ribose in ribonucleic acid
• Cell to cell recognition and communication: saccharides are used in the formation of glycoproteins that sit on the surface of cells

Question 7

Glycoprotein cell receptors

Answer 7

• surface carbohydrates on cells serve as points of attachment for other cells

Question 8

Galactosemia

Answer 8

• reduction in the metabolism of galactose to glucose due to deficient galactokinase or other metabolism genes
• galactose build-up leads to bacterial infections, mental retardation and cataracts
• management: switch to soy formula at infancy, throughout life avoid dairy product, organ meats and some fruit, nuts and vegetables containing galactose

Question 9

Sugar Alcohols

Answer 9

• derivatices of mono, di or polysaccharides
• class of polyols, 1 OH group attached to each carbon
• not fully absorbed in SI, femented by bacteria in LI - therefore used as low energy sweeteners
• with large quantities may induce bloating, diarrhoea and flatulence as water is drawn into the colon and bacterial fermentation
• occurs naturally in some fruit and vegetable but usually issues from hydrogenation of sugars (sorbitol, mannitol, xylitol) or via fermentation of sugars (erythritol)

Question 10

Pentose Monosaccharides

Answer 10

• ribose and deoxyribose
• contains 5 carbons
• essential part of cells genetic material - RNA and DNA
• body makes own sypply via pentose phosphate pathway

Question 11

Pentose Monosaccharides

Answer 11

• ribose and deoxyribose
• contains 5 carbons
• essential part of cells genetic material - RNA and DNA
• body makes own sypply via pentose phosphate pathway
• found in nuts, meat, dairy, veges and is part of riboflavin

Question 12

Disaccharides

Answer 12

• two monosaccarides linked together by a condensation reaction
• alpha or beta glycosidic bonds
• maltose, lactose, sucrose

Question 13

Maltose

Answer 13

• glucose to glucose with alpha 1,4 glycosidic bond
• energy tor sprout growth
• used in food production in cereals, candies and brewing

Question 14

Lactose

Answer 14

• galactose to glucose with beta 1,4 glycosidic bond

- milk sugar

Question 15

Sucrose

Answer 15

• glucose to fructose with alpha 1,2 glycosidic bond

- table sugar, sugarcane, in fruit

Question 16

Lactose Intolerance

Answer 16

The inability to digest lactose
- primary: shortage of enzyme lactase in adulthood because of lactose non-persistence, which is default program in humans post weaning
- secondary: in lactase persistence phenotypes (in certain ethnicities and population with dairy farming ancestry), LI is associated with damage of lactase producing cells
symptoms (occur 30mins-2hours after digesting) - nausea, cramps, distension, gas, diarrhoea

Question 17

Oligosaccharides

Answer 17

• contains 3-10 saccharides
• e.g. raffinose and starchyose (onions, legumes, cabbage, broccoli)
• mainly indigestible, may cause discomfort

Question 18

Polysaccharides

Answer 18

• contains >10 saccharides
• includes starch, glycogen, cellulose
• digestibility depends on alpha or beta glycosidic bonds

Question 19

Starch

Answer 19

• digestible
• storage form of carbohydrate in plant
• amylose: continuous chain of glucose molecules linked by alpha-1,4 glycosidic bonds
• amylopectin: branched chain of glucose molecules linked by alpha-1,4 and alpha-1,6 glycosidic bonds. More quickly digested as more sites available to enzymes activity