B3 Carbohydrates

B.3.1 Describe the structural features of monosaccharides
Monosaccharides 
** contain a carbonyl group (C=O) and at least two -OH groups, and have the empirical formula CH2O
- are the simplest carbohydrates (quickest energy source)
- 2 main types of carbs: simple sugars (monosaccharides) and complex (polysaccharides)
- most common: triose (C3), pentose (C5), hextose (C6)
- large no. -OH groups, hence readily soluble in water


B.3.2 Draw the straight-chain and ring structural formulas of glucose and fructose
Must be aware of the structural difference between α and β isomers.

 

B.3.3 Describe the condensation of monosaccharides to form disaccharides and polysaccharides
Monosaccharides - simple sugars
Disaccharides - two simple sugars linked together
e.g. lactose, maltose and sucrose
Polysaccharides - polymers of sugars
e.g. starch (α-glucose), glycogen (α-glucose) and cellulose (β-glucose)

Disaccharides
- condensation produces disaccharide AND water, -OH group from each sugar molecule reacts together and creates a glycosidic link (essentially an ether bond between the two)
- all soluble
- hydrolysed into 2 monosaccharides, through acid hydrolysis/ enzyme-catalysed reaction
- combining different monos. = different disaccharides

(all have the formula C12H22O11)

Polysaccharides
- held together by glycosidic bonds
- all insoluble, hence ideal for energy storage
- 3 most common glucose based: starch, glycogen, cellulose

Starch
- rich sources: rich, potatoes, flour
- mixture of amylose and amylopectin

 

Glycogen
- α-glucose polymer
- 'animal starch' because it is the main storage carb in animals
- found in liver and muscles
- similar to amylopectin, but more 1-6 branches

Cellulose
- β-glucose polymer
- structural material for plant cell walls
- linear polymer with 1-4 linkage (β-glycosidic links), hence positions sugars at different angles to α-glycosidic links in amylose and amylopectin.
- cellulose chain forms uncoiled linear structure with alternate glucose monomers 'upside down' (to polymerise, one of the glucose molecules in turned upside down to form the glycosidic linkage. The hydroxyl groups form h-bonds with the hydroxyl of another cellulose molecules)
- forms cables called microfibrils, parallel chains that give it its rigid structure (one of the main supports in plant cells)

B.3.4 List the major functions of carbohydrates in the human body
Providing energy (glucose)
- carbs broken into carbon dioxide and water during respiration
- exothermic, hence releases lots of energy

Storing energy (glycogen)
- polysaccharides (starch) are good stores of carbs
- usually within the liver and released when needed

Building blocks
- precursors for other important biological molecules


B.3.5 Compare the structural properties of starch and cellulose, and explain why humans can digest starch but not cellulose.
Both are polymers of glucose units. Starch has 2 forms: amylose (straight chain polymer, α-1,4 linkage), and amylopectin (branched structure with both α-1,4 and α-1,6 linkages)
Cellulose has a β-1,4 linkage, which can be hydrolysed by cellulase (although absent in most animals/mammals)

Polysaccharides
- insoluble, therefore cannot be transported in blood
- needs to be broken into its monos. units
- through hydrolysis, glycosidic links broken and soluble monosaccharides produced - controlled by enzymes that are very specific in their actions.

Enzymes
- produced in the body to break down polys. into glucose that's then absorbed into the body.
- enzyme for β-glycosidic link (in cellulose, hemicellulose etc.) not produced in the body, cellulase needed, maybe secreted in small amounts by bacteria (microflora) living in the gut, but generally not digested and passes through.


B.3.6 State what is meant by the term dietary fibre.
Dietary fibre - mainly plant material that is not hydrolysed by enzymes secreted by the human digestive tract but may be digested by microflora in the gut. E.g. cellulose, hemicellulose, lignin and pectin.


B.3.7 Describe the importance of a diet high in dietary fibre.
Good sources high in dietary fibre includes foods derived from plants with little/no processing
May be helpful in the prevention of conditions of:
- irritable bowel syndrome
- constipation
- obesity
- Crohn's disease
- hemorrhoids
- diabetes mellitus

Cellulose fibrils abrades the wall of the digestive tract and stimulates lining to provide mucus - smoothing the passage of undigesteds through the gut.