Option B.3 – Carbohydrates
B.3.1 – Describe the structural features of monosaccharides
Monosaccharides have a general formula of CH2O. They have multiple alcohol groups (-OH) as well as a carbonyl group (-C=O). Since they are polar molecules, they are soluble in water. Although they occur as straight-chain molecules, in aqueous solution, monosaccharides like glucose will react to form ringed molecules.
B.3.2 – Draw the straight-chain and ring structural formulas of glucose and fructose
Straight chain D-glucose
α-Glucose ring
B.3.3 – Describe the condensation of monosaccharides to form disaccharides and polysaccharides
Monosaccharides will react in a condensation reaction to form disaccharides and polysaccharides. The monomers link together to form long carbohydrate chains, releasing molecules of water in the process.
B.3.4 – List the major functions of carbohydrates in the human body
Store Energy – Energy is stored in carbohydrates to be used by the body when it is needed, such as in the form of glycogen.
Metabolism – Glucose is a respiratory substrate and is used for the release of energy in ATP
Soluble – Small monosaccharides are soluble in water and can be easily absorbed by cells.
Structure – Cellulose in plants is used to maintain cell structure.
B.3.5 – Compare the structural properties of starch and cellulose, and explain why humans digest starch but not cellulose
Polysaccharides are broken down in a hydrolysis reaction into their monosaccharides. This reaction is made possible by the presence of specific enzymes. Humans have enzymes that aid the digestion of starch and glycogen into glucose.
Amylase, on the other hand, is found in human saliva, as well as being secreted from the pancreas. This means that starch can be digested by humans, whilst cellulose cannot. Starch is broken down into glucose monosaccharides. The bacteria in the intestine may secrete some cellulose, but on the whole, cellulose passes through the human body undigested.
B.3.6 – State what is meant by the term dietary fibre
Dietary fibre includes molecules like cellulose that are not able to be digested by the body. Humans do not have the necessary enzymes to break them down into usable molecules.
B.3.7 – Describe the importance of a diet high in dietary fibre
Dietary fibre helps to maintain the health of the large intestine by stimulating the production of mucous, allowing for the easier passage of food. These are found in abundance in unprocessed plant matter. This helps to prevent conditions such as:
- constipation
- haemorrhoids
- irritable bowel syndrome
- colorectal cancer