Principal sources and dietary importance of carbohydrates, fats and protein

Question 1

Fats (Principal sources)

Answer 1

Animal fats are found in meat, milk, cheese, butter and egg-yolk
Plant fats occur in fruits(palm oil) and seeds(sunflower seed oil)

Question 2

Fats (Dietary importance)

Answer 2

• Forms part of the cell surface membrane
• An efficient source of energy that can be oxidised in respiration to carbon dioxide and water, liberating twice as much energy in the form of ATP as can be obtained from the same weight of carbohydrate
• Means of long-term storage of energy in fat depots(adipose tissue)
• Act as an insulating material, beneath the skin, to prevent excessive heat loss
• A solvent for fat-soluble vitamins and sex hormones
• Surround and protect vital organs such as the heart and kidney
• Sebum, the oily secretion produced by sebaceous glands in the skin forms a thin layer over the surface of the skin, thus restricting water loss

Question 3

Proteins (Principal sources)

Answer 3

Animal proteins: lean meat, fish, eggs, milk and cheese

Plant proteins: Peas, beans, nuts and bean curd

Question 4

Proteins (Dietary importance)

Answer 4

When digested, amino acids are absorbed into the blood stream, enter the cell and used to build up different proteins

• Enzyme of cells
• Protein that form part of the protoplasm of new cells(during growth) and worn-out cells(during cell repair)
• Membrane proteins embedded in phospholipid bilayer
• Actin and myosin in muscles
• Keratin in hair and nails
• Haemoglobin in blood
• Peptide hormones(insulin)
• Antibodies that defend the body against infections by bacteria and viruses

Question 5

Carbohydrates (Principal sources)

Answer 5

Monosaccharides (Glucose/Fructose and Galactose: Fruits and component of milk, sugar or lactose
Disaccharides (Sucrose, Lactose and Maltose): Sugarcane stems, sweet fruits, transportable form of sugar in plants/ Milk of all mammals/ Malted cereals and germinating seeds
Polysaccharides (Starch): Rice, cereals, potatoes and tapioca

Question 6

Carbohydrates (Dietary importance)

Answer 6

Monosaccharides
- Chief source of energy in the form of ATP molecules which are released during cellular respiration
- Formation of nucleic acid, of which ribose sugar is a constituent
Disaccharides
- A source of energy when it is hydrolysed to its constituent monomers
Sucrose + water = glucose + fructose
Maltose + water = glucose + glucose
Lactose + water = glucose + galactose
(Hydrolytic reaction: Breakdown of disaccharides to two monosaccharides with the the addition of water)
Polysaccharides
- Starch is a storage form of carbohydrates in plants. It is also a source of energy when starch is hydrolysed to release glucose units
- Glycogen is a compact storage form of carbohydrates in animals (livers and muscles) and fungi
Starch and glycogen are ideal storage materials
- Insoluble in water so they do not change the osmotic pressure in the cell
- Large molecules which are unable to diffuse through the cell surface membrane
- Can be broken down to glucose molecules when required
- Compact shapes so they occupy less space
- Cellulose chains are arranged parallel to one another
(Major component of the tough walls that enclose a plant cell)