All living organisms require energy to stay alive. Green plants are able make use of light energy from the sun to convert raw materials from their surroundings into stores of chemical energy such as carbohydrates (in the form of glucose or starch), in a process known as photosynthesis. Animals must consume green plants, or organisms that feed on green plants, in the form of food in order to get stored energy.
Nutrients are chemical substances found in the food we consume which provides energy and the raw materials needed by the body for growth and repair of worn out body parts. They can be classified based on the presence (organic) or absence (inorganic) of carbon. There are five main types of nutrients that we consume – carbohydrates, fats, proteins, water and vitamins and minerals. In this topic, we are only looking at carbohydrates, fats and proteins.
Carbohydrates – inorganic compounds made up of carbon, hydrogen and oxygen with the general formula of CnH2mOm, one gram of carbohydrate provide an average of 16 kJ of energy
There are three main groups of carbohydrates – monosaccharides, disaccharides and polysaccharides.
Obtained from Perfect Guide: ‘O’ Level Biology 2nd Edition, Marshall Cavendish
Functions of carbohydrates:
- substrate for formation of supporting sturctures
- substrate for respiration, provides energy for cellular activities
- substrate for the synthesis of other organic compounds such as amino acids and fats
- required for the synthesis of nucleic acids
- main component of nectar in flowers
- used to synthesise natural lubricants
Food tests involving carbohydrates:
- Benedict’s test – Blue Copper (II) sulfate solution (Benedict’s solution) when boiled with reducing sugar (any sugar that is capable of acting as a reducing agent; glucose, maltose, fructose, galactose or lactose), produce brick-red precipitate of copper (I) oxide
- Iodine test – Triiodide anion (I3–) present in the iodine solution form a complex with amylose of starch, resulting in the formation of a blue-black complex
Fats – organic compounds made up of carbon, hydrogen and oxygen, contains less oxygen in proportion to hydrogen, have no general formula (proportion of elements not fixed)
There are two main kinds of fats – saturated or unsaturated fats. Unsaturated fats can be further characterised into monounsaturated or polyunsaturated fats.
Functions of fats:
- Insulating layer that reduces heat loss
- Essential component of cell membranes
- Reduce water loss from skin surface by forming a hydrophobic layer which prevents evaporation of water
- Solvent for fat-soluble vitamins and vital substances, such as hormones
Food test involving fats:
- Emulsion test – Lipids are dissolved in alcohol before it gets precipitated out, in the form of a cloudy emulsion, in the presence of water
Proteins – complex organic compounds made up amino acid subunits with carbon, hydrogen, oxygen and nitrogen, sulfur may sometimes be present
An amino acid has an amino group (-NH3), and acidic (or carboxyl) group (-COOH) and a variable side chain (-R). Hence the general formula for an amino acid is:
Amino acids can be linked together by peptide bonds, which is formed through a condensation reaction between a amino group and a carboxyl group, to give a polypeptide or peptone.
Functions of proteins:
- Synthesis of antibodies required for adaptive immunity
- Growth and repair of body cells and tissues
- Key components of some enzymes and hormones
- Synthesis of haemoglobin, a protein in red blood cell that binds and carries oxygen to all parts of the body
Food test involving proteins:
- Biuret test – Biuret solution (blue solution of sodium hydroxide and copper (II) sulfate) turns deep purple in the presence of protein and pink in the presence of short chain polypeptides
In order for us to obtain the chemical energy stored in the nutrients of the food we consume, our body must break the nutrients down into simpler substances.
Hydrolysis (or hydrolytic reaction) is a chemical reaction in which water molecules is used to break up a complex molecule into simpler molecules.
Reversibly, simpler molecules can also be chemically combined to form a more complex substance, through a process known as condensation (or dehydration reaction).
Hydrolysis of carbohydrates:
- Glycosidic bonds between monosaccharides broken down
- For complex carbohydrates such as starch, a two step reaction is adopted. Amylase first digests starch into smaller maltose molecules before maltase hydrolyses maltose to give glucose molecules.
Hydrolysis of fats:
- Ester bonds between fatty acids and glycerol in the lipid molecule are broken down
Hydrolysis of proteins:
- Peptide bond between amino acid residues in the protein are broken down by enzyme-catalysed hydrolytic reactions