Identification of Food Components
Senior 2 · Biology
Identification of Food Components
Lesson 1 of 3
18 min
Let's begin
A plate of ubugari with beans and isombe, a cup of ikivuguto, a handful of groundnuts and a ripe avocado look nothing alike. Yet a chemist would find the same few kinds of substance in all of them, in different amounts.
Knowing what is in a food lets you plan a balanced diet, understand deficiency diseases, and design the laboratory tests in the next two lessons.
A nutrient is a substance in food that the body uses for energy, growth, repair or keeping healthy. Food contains seven main components:
| Component | Main use in the body | Rwandan sources |
|---|---|---|
| Carbohydrates | main source of energy | cassava, sweet potato, maize, sorghum, rice, bananas (ibitoki), honey |
| Lipids (fats and oils) | stored energy, insulation, cell membranes | groundnuts, avocado, palm and sunflower oil, milk, butter |
| Proteins | growth, repair, enzymes, antibodies | beans, peas, soya, isambaza, eggs, milk, meat |
| Vitamins | small amounts needed to keep the body healthy | passion fruit, oranges and tomatoes (vitamin C); carrots, pumpkin and dark green leaves (vitamin A) |
| Minerals | bones and teeth (calcium), blood (iron), thyroid (iodine) | milk and small whole fish (calcium); beans and dark leaves such as isombe (iron); iodised salt (iodine) |
| Dietary fibre | helps food move through the gut; prevents constipation | whole grains, beans, vegetables, fruit skins |
| Water | the solvent for all reactions; transport; cooling by sweat | drinking water, fruit, soups, milk |
Carbohydrates contain only the elements carbon, hydrogen and oxygen, usually in the ratio 1 C : 2 H : 1 O, so their general formula is (CH₂O)ₙ. There are three kinds:
Glucose, fructose, maltose and lactose are reducing sugars: they can reduce (give electrons to) copper ions in Benedict's reagent. Sucrose is a non-reducing sugar. This difference is the basis of a food test.
Lipids also contain carbon, hydrogen and oxygen, but much less oxygen than carbohydrates. Most fats and oils are made of one glycerol joined to three fatty acids. Lipids do not dissolve in water, but they do dissolve in ethanol. Fats are solid at room temperature (butter, animal fat) and oils are liquid (sunflower oil). A gram of lipid releases about twice as much energy as a gram of carbohydrate.
Proteins contain carbon, hydrogen, oxygen and nitrogen, and often sulfur. They are long chains of about 20 kinds of amino acid, joined by peptide bonds. The order of amino acids decides how each protein folds, and its shape decides its job. Enzymes, antibodies, haemoglobin and the keratin in hair are all proteins.
Question: For each food, name its main nutrient: (a) cassava flour, (b) beans, (c) avocado, (d) passion fruit.
Solution: (a) carbohydrate, as starch; (b) protein, although beans also provide starch, fibre and iron; (c) lipid, since avocado flesh is rich in oil; (d) vitamin C, together with sugars and water. Most foods contain several nutrients, so "main nutrient" means the one present in the largest useful amount.
Tip
Memory hook: carbohydrates and lipids are made of C, H and O; proteins add N for Nitrogen, as in "N for New tissue".
Exam technique: sucrose is the odd one out among the sugars: it is a disaccharide but non-reducing. This is a favourite trick question.
Name the three chemical elements found in all carbohydrates.
Solution: carbon, hydrogen and oxygen.
Which element is found in proteins but not in carbohydrates or lipids?
Solution: nitrogen (and often sulfur).
Name the polysaccharide that stores energy (a) in plants and (b) in animals.
Solution: (a) starch; (b) glycogen.
Name the two kinds of molecule that make up a typical fat.
Solution: glycerol and fatty acids (one glycerol joined to three fatty acids).
A child who eats mostly cassava and very little else becomes weak and stops growing well. Which food component is most likely lacking? Suggest two Rwandan foods to add.
Solution: Protein, which is needed for growth and repair. Good additions are beans, peas, soya, isambaza, eggs or milk.
Summary
Sources and credits
The table of food components, the Rwandan food examples, the worked example and the practice questions are written by Ganzaa.
