Chemicals of Life

20 Grade 10 Biology questions on Chemicals of Life, with worked marking schemes. 20 questions are available on this topic in full.

1. Question 1

   1 mark

   Which element is present in all proteins but absent in both carbohydrates and lipids?

   A. Nitrogen

   B. Carbon

   C. Hydrogen

   D. Oxygen

   (1 mark)

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2. Question 2

   1 mark

   A learner rubbed a crushed groundnut seed on a piece of filter paper, left it to dry and then held it against light. A permanent translucent spot was seen. Which conclusion is correct?

   A. The groundnut seed contains protein, which stains paper

   B. The groundnut seed contains starch, which soaked into the paper

   C. The groundnut seed contains lipid (oil)

   D. The groundnut seed contains only water, which wetted the paper

   (1 mark)

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3. Question 3

   5 marks

   Describe how you would show that a sample of sugarcane juice contains a non-reducing sugar.

   (5 marks)

   Marking scheme

   1 mark each, max 5: boil a portion of the juice with Benedict's solution - it remains blue, showing that a reducing sugar is absent; to a fresh portion of the juice add a few drops of dilute hydrochloric acid; boil the mixture for about two minutes to hydrolyse the sugar; cool the mixture; neutralise the acid by adding sodium hydrogen carbonate until effervescence stops; add Benedict's solution and boil again; an orange-red/brown precipitate confirms that a non-reducing sugar was present in the original juice.

4. Question 4

   4 marks

   Explain why enzymes are described as biological catalysts.

   (4 marks)

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5. Question 5

   1 mark

   Pepsin digests protein in the stomach where the pH is about 2. When the food passes into the duodenum, pepsin stops working. Which statement explains this?

   A. The duodenum is cooler than the stomach, so pepsin becomes inactive

   B. Pepsin is used up during digestion in the stomach, so none of it reaches the duodenum

   C. The alkaline conditions in the duodenum alter the shape of the active site of pepsin

   D. Pepsin can act on a protein molecule only once

   (1 mark)

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6. Question 6

   4 marks

   Describe how you would test a sample of omena (dried sardines) for the presence of protein.

   (4 marks)

   Marking scheme

   Any 4 of the following, 1 mark each: grind/crush the omena and shake it with distilled water to make a suspension; filter to obtain a clear sample solution; place about 2 cm3 of the solution in a clean test tube; add an equal volume of sodium hydroxide solution and shake; add copper(II) sulphate solution drop by drop while shaking after each drop; a purple/violet colour shows that protein is present (no heating is required).

7. Question 7

   2 marks

   Distinguish between condensation and hydrolysis as used in the chemistry of food substances.

   (2 marks)

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8. Question 8

   1 mark

   Maize growing on a farm in Kirinyaga takes in mineral salts such as nitrates from the soil. Which property of water makes this uptake possible?

   A. Water is a good solvent, so the mineral salts dissolve and are taken in while in solution

   B. Water has a high boiling point, so it does not evaporate from the soil during the day

   C. Water reacts chemically with mineral salts to form new nutrients that the root can absorb

   D. Water is denser than mineral salts, so it pushes them into the root hairs

   (1 mark)

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9. Question 9

   3 marks

   Some communities living far from the coast suffer from a swelling at the front of the neck. Identify the mineral element likely to be lacking in their diet, explain how the deficiency causes the swelling, and suggest one way of preventing it.

   (3 marks)

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10. Question 10

    2 marks

    Other than acting as a solvent and as a transport medium, state two roles of water in a living organism.

    (2 marks)

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11. Question 11

    3 marks

    A young child in a home where the main meal is boiled cassava taken three times a day is small for her age, has a swollen abdomen and thin, light-coloured hair. Account for this condition.

    (3 marks)

    Marking scheme

    1 mark each: cassava is almost entirely starch (carbohydrate) and supplies very little protein; proteins supply the amino acids needed to build new cells and tissues for growth and repair, so growth is stunted; a shortage of blood proteins allows tissue fluid to collect in the tissues, causing the swollen abdomen (oedema); protein is also needed to make enzymes, hormones and hair/skin structures, hence the thin discoloured hair. (Max 3; condition is protein-energy malnutrition/kwashiorkor)

12. Question 12

    1 mark

    Starch and cellulose are both polysaccharides built from glucose units. Which statement correctly distinguishes the two in a maize plant?

    A. Starch is made of glucose units while cellulose is made of amino acids

    B. Cellulose dissolves readily in water while starch does not

    C. Starch is a storage carbohydrate while cellulose gives strength to the cell wall

    D. Starch occurs only in animals while cellulose occurs only in plants

    (1 mark)

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13. Question 13

    6 marks

    State the role of each of the following mineral elements and, in each case, describe one effect of its deficiency: (a) calcium in a Friesian cow; (b) phosphorus in a maize plant; (c) magnesium in a maize plant.

    (6 marks)

    Marking scheme

    1 mark for each role and 1 mark for each deficiency effect. (a) Calcium - needed for the formation of strong bones and teeth (accept: for blood clotting and muscle contraction); deficiency causes weak, brittle bones, and in a high-yielding dairy cow it causes milk fever. (b) Phosphorus - needed for healthy root development and for the formation of cell membranes and energy-carrying compounds in the cell; deficiency causes stunted growth with poorly developed roots and purple colouration of the older leaves. (c) Magnesium - it is a component of chlorophyll; deficiency causes yellowing of the leaves (chlorosis) starting with the older leaves, leading to reduced photosynthesis and poor yield.

14. Question 14

    4 marks

    State four functions of lipids in the body of a mammal.

    (4 marks)

    Marking scheme

    1 mark each: they are a store of energy, releasing about twice as much energy per gram as carbohydrates; fat stored under the skin insulates the body and reduces heat loss; fat around organs such as the kidneys and heart cushions and protects them from mechanical injury; lipids are a structural part of cell membranes; oxidation of fat yields metabolic water, useful to animals such as camels in dry areas; fats carry the fat-soluble vitamins A, D, E and K. (Max 4)

15. Question 15

    1 mark

    A learner boiled 2 cm3 of fresh mango juice with Benedict's solution. The mixture changed from blue to orange-red. Which conclusion is correct?

    A. The mango juice contains protein, because Benedict's solution detects nitrogen

    B. The mango juice contains starch

    C. The mango juice contains a reducing sugar

    D. The mango juice contains a non-reducing sugar only

    (1 mark)

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16. Question 16

    3 marks

    A learner boiled a solution of pepsin, cooled it and then added it to egg white at pH 2. The egg white was not digested. Explain this result.

    (3 marks)

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17. Question 17

    1 mark

    Salivary amylase digests the starch in ugali but has no effect on the protein in beans eaten at the same meal. Which statement best explains this observation?

    A. Each enzyme is specific, and the protein molecule does not fit into the active site of amylase

    B. Protein molecules are too large to be broken down by any enzyme

    C. Amylase is made in the mouth, so it never comes into contact with protein

    D. Protein is broken down by acid alone and not by any enzyme

    (1 mark)

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18. Question 18

    1 mark

    During a long dry spell in Turkana, a herder goes for several months without eating any fresh fruits or vegetables. He develops bleeding gums and wounds that take unusually long to heal. Which vitamin is most likely lacking in his diet?

    A. Vitamin K

    B. Vitamin C

    C. Vitamin A

    D. Vitamin D

    (1 mark)

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19. Question 19

    1 mark

    A Grade 10 class was studying the oil stored in the fruit of an avocado tree. Which statement correctly describes the smaller units from which a fat (oil) molecule is built?

    A. One molecule of glycerol joined to three fatty acid molecules

    B. A long chain of glucose units joined end to end

    C. A chain of amino acids joined together by peptide bonds

    D. Three molecules of glycerol joined to one fatty acid molecule

    (1 mark)

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20. Question 20

    1 mark

    Which statement best explains why a person whose diet lacks iron looks pale and tires quickly during games?

    A. Iron is needed to harden bones and teeth, so the skeleton becomes weak

    B. Iron is the main energy-giving nutrient, so the body runs short of energy

    C. Iron is needed to make the hormone that controls the rate of digestion

    D. Iron is needed to make haemoglobin, so less oxygen is carried to the body tissues

    (1 mark)

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