Biology Paper 3 Questions and Answers - KCSE 2021 Past Papers

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    1. You are provided with plant specimens labelled E, F, G, H, and J. Use the specimens to develop a dichotomous key that can be used to identify the plants from which they were obtained based on the following characteristics in the order they are given: (6 marks)
      1. Leaf form
      2. Leaf venation
      3. Leaf colour
    2. Account for the likely observation if fresh specimen E was exposed to light and tested for starch. (3 marks)
    3. Explain one observable feature that adapts plants from which specimen G and H were obtained to a dry environment. (2 marks)
    4. Besides leaf characteristics, state one other observable characteristic on the plant from which specimen F was obtained that enables it to be placed in its Class. (1 mark)
  2. You are provided with solution M which is a food substance.
    1. Using the reagents provided, test for the food substance present in substance M and complete the table below. (12 marks)
      Food Test Procedure Observation Conclusion
    2. State two precautions one should observe while conducting the experiment in 2(a) (2 marks)
  3. You are provided with specimen N and P which are plants of the same species grown under different conditions.
    1. State two observable differences between the two specimens. (2 marks)
      1. Name the phenomenon observed in specimen N. (1 mark)
      2. Explain how the knowledge on the phenomenon named in bi) is applied in agriculture(2 marks)
    3. Account for the appearance of specimen N. (3 marks)
    4. State two other environmental factors necessary for seed germination apart from light. (2 marks)
    5. State two observable features on the specimens that make them be placed in the same Class. (2 mark)


      1. Leaf simple ....go to 2
        Leaf compund....G
      2. Leaves/leaf network veined ....go to 3
        LeavEs/leaf parallel veined...F
      3. Leaves/leaf green/non variegated ...G/H or go to 4
        Leaves/leaf non-green/ variegated ....E
      4. Leaf margin serrated.....G
        Leaf margin smooth.....H
    2. Green parts/ parts with chlorophyll will turn blue black (with iodine solution/iodine) due to presence of starch since photosynthesis has occurred; white parts turn to brown/retain iodine solution; due to absence of starch since photosynthesis has not occured
      1. Rough/ hairy leaf surface; to reduce  transpiration.
        Leaves fold; to reduce transpiration
      2. Shiny/ glossy leaf surface; to reduce transpiration folding of leaves; to reduce water loss
    4. Fibrous root system;
      Floral parts in threes/ multiples of three
      Food Test Procedure Observation Conclusion

      To about 2ml of food substance (in a test tube). Add 2 drops of iodince solution

      Color of iodine retained/ yellow brown;
      Reject; no change/ no observable change
      Accept; no color change
      Starch absent

      Vitamin C(Absorbic acid) To about 2ml of DCPip in a test tube. Add a drop of the  food substance /M DCPip is decolorised or DCPip becomes colorless Vitamin C present

      Put/rub a drop of M onto the filter paper/ plain paper(allows the drop to drip)
      Hold against a source of light
      No translucent
      Mark left (on the filter paper)

      Lipids absent

      • Avoid contamination of reagants/ aparatus/ avoid mixing of droppers/use clean apparatus
      • Avoid burning of filter paper/ plain paper when dripping
      • Avoid spilling/ misusing of regants/food substance
      Yellow leaves/ white green/ yellow stem
      small leaves
      Long stem/tall stem/long internodes
      thin stem
      Weak/ fragile/feeble/frain stem 
      Green leaves/green stem
      large/big leaves
      Sort stem/short internodes
      thick stem
      strong/firm stem 
      1. Etiolation
      2. Proper spacing/thinning/pruning/pricking out/weeding/using a transparent material/polythene on a green house; to enable adequatee penetration of light for the crop
    3. The specimen is weak/tall/long/thin; because they have grown in a darkness;/ absence of light/ insufficient light; in darkness there is high concentration of auxin in shoot tip that stimulate faster elongation;
      The specimen has small/ yellow leaves/white stem/ lack of chlorophyll; because they were grown in darkness; hence couldn't carry out photosynthesis/ synthesis of chlorophyll
    4. Oxygen
      Optimal/optimum temperature/warmth;
    5. Tap root system
      Reticulate/network venation/branched venation/net-veined leaves;
      Broad leaves
      Compact petiole
      Accept presence of two cotyledon
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