KCSE 2012 Biology Paper 2 with Marking Scheme

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SECTION A (40 marks)
Answer ALL the questions in this section in the spaces provided

  1. In a certain plant, species which is normally green, a recessive gene for colour (n) causes the plants to be white in colour. Such plants die at an early age. In heterozygous state; the plants are pale in green colour grown to maturity.
    1. Give a reason for the early death of the plants with the homozygous recessive gene. (2 marks)
    2. If a normal green plant was crossed with the pale green plant, what would be the genotype of the first filial generation (F1 generation)? Show your working. (4 marks)
    3. If heterozygous plants were self-pollinated, and the resulting seeds planted, work out the proportion of their offspring that would grow to maturity.
  2. The diagram below illustrates the structure of the kidney nephron.

               kidney nephron kcse 2012
    1. Name the part labeled E. (1 mark)
    2. How is the part labeled F adapted to its function? (4 marks)
    3. State three physiological mechanisms of controlling the human body temperature during a cold day. (3 marks)
    1. In an investigation, equal amounts of water was placed in three test tubes labeled G, H and J. Pondweeds of equal length were dropped in each test tube. The test tubes were then placed in identical conditions of light and carbon (IV) oxide at different temperatures for five minutes. After five minutes, the bubbles produced in each test tube were counted for one minute. The results were as shown in the table below.
       Test tube   Temperature (°C)   Number of bubbles 
      1. Name one requirement for this process that is not mentioned in the investigation. (1 mark)
      2. Name the gas produced in the investigation. (1 mark)
      3. Account for the results in test tubes H and J. (2 marks)
    2. State two ways in which intestinal villus is adapted to its function. (4 marks)
    1. The diagram below illustrates the arrangement of bones and muscles in the human arm.

      bones and muscles on human arm

      1. Name the bones labeled K and L. (2 marks)
      2. Explain how the upward movement of the lower arm is brought about by the bones and muscles shown in the diagram above. (3 marks)
    2. State three ways in which support is brought about in a leaf. (3 marks)
    1. Describe the process of inhalation. (4 marks)
    2. Explain the mechanism of stomatal opening. (4 marks)

SECTION B (40 marks)
Answer question 6 (compulsory) and either 7 or 8 in the spaces provided after question 8.

  1. The data provided below represents populations of a predator and its prey over a fifty years period.
    5 24500 17000
    10 30000 20500
    15 33500 26000
    20 33500 30000
    25 31000 33000
    30 27000 32000
    35 25000 30000
    40 29000 27500
    45 32500 28000
    50 34000 28500
      1. Using the same axes, draw graphs of the relative populations of P and Q against time.
        (7 marks)

      2. With a reason, identify the curve that represents the prey. (2 marks)
      3. Account for the populations between 25 and 32 years. (2 marks)
      4. Which years were the two populations equal? (2 marks)
      5. Apart from predation, state three biotic factors that may have led to the decline of the prey population. (3 marks)
    2. Describe the hazards of air pollution by Sulphur (IV) Oxide. (4 marks)
  2. Using a relevant example in each case, describe simple and conditional reflex actions. (20 marks)
    1. Using a relevant example, describe how the allergic reaction occurs in human beings. (10 marks)
    2. Describe how environmental factors increase the rate of transpiration in terrestrial plants.
      (10 marks)


    1. Lack of chlorophyll, the plants do not manufacture food photosynthesize; plants die as soon as the stored food reserves get depleted;     (2 marks)
      KCSE 2012 Bio PP2 Ans1b  (4 marks)
      KCSE 2012 Bio PP2 Ans1c
    1. E -glomerulus;  (1 mark)
      F - loop of henle;   (1 mark)
    2. It is long; to increase the surface area for re-absorption of water; It is U-shaped; to bring about counter - current flow/multiplier effect to enhance, water absorption.
      It is lined with a network of blood capillaries; to enhance re-absorption of water;  (4 marks)
    3. vasoconstriction; hair rises, metabolic rate increases; shivering (3 marks)
      First correct 3
      1. chlorophyll;   (1 mark)
      2. oxygen;  (1 mark)
      3. Test tube H is at optimum temperature for enzyme activity; hence high rate of photosynthesis/more bubbles. In test tube J most enzymes have been denatured by the high temperature; hence low rate of photosynthesis/fewer bubbles.   (2 marks)
      • The villus epithelium is thin; for faster diffusion of dissolved food substances;
      • The epithelium has goblet cells, which produce mucus to lubricate food passage;
      • They have microvilli; which further increase their surface area for absorption;
      • Have lacteal; for absorption of fatty acid & glycerol/transportation of lipids;
      • Highly vascularised; for absorption of digested food.
        (4 marks)
        First correct 2
      1. K-ulna;  (1 mark)
        L-humerus;  (1 mark)
      2. movement of the lower arm upwards takes place at the elbow/olecranon process which is between the ulna and the humerus; biceps/flexor muscles contract; while the triceps/extensor muscles relax; bringing about the movement of the lower arm upwards.   (3 marks)
      • The rigid midrib holds leaf out away from the stem;
      • Profuse network of veins have lignified cells which support leaf to stay spread out;
      • Turgidity in spongy mesophyll and palisade cells support the leaf to remain open;
        (3 marks)
    1. The external intercostal muscles contract while internal intercostal muscles relax; the rib cage is pulled upwards and outwards; the diaphragm muscles contract and the diaphragm flattens; the volume of the thoracic cavity increases/the pressure in the thoracic cavity decreases; air rushes into the lungs; from the atmosphere through the nose
      (4 marks)
    2. The osmotic pressure of guard cells increase when sugar is manufactured during photosynthesis/starch is converted to sugar in low acidity/potassium moves into guard cells during the day, water enters guard cells from the surrounding cells by osmosis; because the guard cells are bean shaped with thin outer walls and thick inner walls, the thin outer walls expand faster as the cell becomes turgid; thus the thick inner wall curves, causing the stomatal aperture to open.
      (4 marks)
        title - Graph of Prey-predator relationship: (OWTE)
        Scales X axis; Graph should cover more than half of the grid provided.
        Y axis; Graph should cover more than half of the grid provided. 
        correct plotting   P;   Q;
        smooth curves    P;   Q;
        labelling axes;
        (7 marks)
        PP3 KCSE Bio Ans 6a 2012
      2. P represents the prey;
        Prey population is initially higher/ prey population usually starts falling earlier,  (1 mark)
      3. Both populations decrease; (1 mark)
        because prey is not enough to sustain predator/population environmental stress limit population of prey;  (1 mark)
      4. at 23 ± 0.5years; and at 39 ± 0.5 (2 marks)
        • Less food for the prey/ intra specific competition;
        • emigration of the prey;
        • diseases causing death of prey;
        • parasitism;
        • human activities;
          (3 marks)
    2. sulphur dioxide in the air-causes respiratory diseases, poisons plants; forms acid rain which increase soil pH; corrodes metals in buildings: 
      (4 marks)
      Total (20)
    • Simple reflex action 
      withdrawal of finger from a sharp object. Is an automatic response to a specific stimulus; When the finger touches a sharp object, pain receptors in the skin; are stimulated and trigger off a nerve impulse; The nerve impulse is transmitted via the sensory neuron; to the grey matter of the spinal cord; The impulse is then transmitted via a synapse; to the relay neuron; and then through another synapse; to the motor neuron; The impulse is then transmitted to the effector muscles in the hand; These effector muscles contract; and the finger is withdrawn from the hot object;
      (Accept use of other relevant examples)
    • Conditioned reflex action
      Is an automatic response evoked from an animal by unrelated stimulus; substituted for the one which normally elicits the response; It develops from past experience; and involves modification of behaviour through learning: It weakens with time, and must be reinforced by repeating the unrelated stimulus; Students salivate when the bell for lunch rings; because they have learned to associate the ringing of the bell at lunchtime with food; from experience; every time it rings, they are offered food;
      (Accept use of other relevant examples)
      Maximum 20 marks
    1. An allergic reaction is a hypersensitive response; to an antigen by the body immune system; The body immune system responds by overproducing antibodies; against harmless antigens; The antigen-antibody reaction occurs on the surface of body cells; which burst open; and release histamines; Histamines cause inflammation/itching swelling/pain, etc; which damage the body; Allergic people are hypersensitive to materials like dust/pollen grains/some foods/some drugs/some pollutants, etc;  
      (10 marks)
      • In bright light; stomata are fully/wide open; increasing contact between the atmosphere and air spaces in the leaf; This in turn increases water loss by evaporation through the open stomata
      • High environmental temperatures, increase the rate of evaporation from the leaf surface thus more water leaves cells due to the increased diffusion gradient;
      • In a windy day; air around the leaf is carried away reducing water vapour around the leaf, more water moves into the atmosphere from the leaf air spaces;
      • In low humidity/when the atmosphere is less saturated with water vapour; more water will move from leaf air spaces into the atmosphere; leading to increased rate of transpiration;
      • Low atmosphere pressure; increases diffusion gradient between atmospheric and leaf increased rate of evaporation;
      • Availability of water, causes turgidity of guard cells hence stomata open, increasing rate of transpiration.
        (10 marks)
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