Observation

This was a popular question among candidates. Some of the candidates that lost some marks in this question lost them to spelling errors. In (a), many candidates could (b), some candidates interchanged the substrates the enzymes act on. In (c), some candidates were able to state one way in which chlorophyll is important to plants and name two macroelements that are required for the formation of chlorophyll in plants. Some candidates were also able to state the deficiency symptom of each macroelement named.
In (d), some candidates could not outline the procedure for testing for starch sequentially. Candidates could not describe the technicalities involved in the process. Majority of candidates also wrote that the leaf should be boiled for several minutes instead of few seconds. Candidates also wrote ‘boiled’ alcohol instead of ‘warm’ alcohol
The expected answers are:

1. (i) Characteristics of enzymes
   

   - They are biological catalysts/alter/speed up the rate of biological reactions;

   - remain chemically unchanged at the end of a reaction;

   - their actions are reversible;

   - they are specific in action/act on specific substrates;

   - enzymes are required in small amount;

   - are protein in nature;

   - they act best at optimum temperatures/high temperature denatures them/low temperature inactivates them;

   - specific/sensitive to pH;

   - actions retarded by poisons/inhibitors;

   - some require co-enzymes to function.

    

2. (i)Digestive enzymes produced in the duodenum of humans

   - Amylase;

   - Trypsin/chymotrypsin;

   - Lipase.

   Note: Spelling must be correct to score.

   (ii)Substrates for each enzyme

   Amylase - starch

   Trypsin/chymotrypsin - protein

   Lipase – fats and oil

   Note: Spelling must be correct to score.

   (iii) Product of enzyme activity

   Amylase - maltose

   Trypsin/chymotrypsin - peptones/polypeptides/peptide

   Lipase - glycerol and fatty acid

   Note: Spelling must be correct to score.

3. (i) Importance of chlorophyll in plants
   Absorbs/traps sunlight; for photosynthesis

   (ii) Macro-elements required for the formation of chlorophyll
   Nitrogen; Magnesium; Iron Note: Spelling must be correct to score.

    

   (iii) Deficiency symptoms of each macro-element
   Nitrogen

   - Chlorosis/yellowing of leaves;

   - stunted growth;

   - poor flower/fruit development.

   Magnesium

   - Poor growth;

   - chlorosis/yellowing of leaves;

   Iron

   - Poor/stunted growth;

   - chlorosis/yellowing of leaves

4. Procedure for test for starch

   - Detach/pluck off/take leaf to be tested from plant after 4 to 6 hours of exposure to sun to enable it to form starch;

   - Put leaf in boiling water for about 30 seconds to kill the cells;

   - Put boiled leaf in 70% warm alcohol/ethylated spirit in water bath to decolorize the leaf;

   - Rinse decolorized/brittle leaf in running cold water to soften the leaf tissues/remove alcohol;

   - Spread out on white tiles/watch glass and add a few drops of iodine solution;

   - Allow to stand for a few minutes;

   - It turns blue-black showing/indicating presence of starch.

   Note: Marking ends at the inception of wrong procedure.