BIOLOGY 0180/03 ALTERNATION TO PRACTICAL QUESTION 1

Transcription

1 BIOLOGY 0180/03 ALTERNATION TO PRACTICAL QUESTION 1 This question was about a student carrying out an investigation into the best conditions for optimum enzyme activity on starch. A mixture of starch solution and enzyme was placed in each of four test tubes A, B, C and D with different conditions of temperature and PH. After 30 minutes, the contents of each test tube were tested with iodine solution. In question 1(a), the candidates were asked to name an enzyme suitable for this investigation. This question was well performed. However some candidates who missed the correct answer wrote enzymes such as lipase or protease. The other common mistake was to state carbohydrate or ptyalin. These were not accepted because carbohydrate generalizes all enzymes whose substrate is a carbohydrate, not necessarily starch. Ptyalin has been phased out of the syllabus. In question 1 (b) candidates were expected to complete table by suggesting the possible observations made in each test-tube from this investigation. Very few candidates scored all four marks; with most scoring one or two. Candidates suggested conclusions such as no reaction instead of giving specific colour (for test tube C). enzyme denatured for test tube D; instead of writing blue- black; ; yellow- brown, blue- black, and blue-black in test tubes A,B,C & D respectively. QUESTION 1 Question 1 (c) required candidate to explain the observations in test tube D. The question was done well by most candidates; obtaining two marks. For candidates who gave wrong responses, they had written answers such as starch/temperature/solution denatured. The expected response was to explain that the high temperature had denatured the enzyme; therefore there would be no enzyme action. In question 1 (d) candidates were to suggest and explain the observations if the temperature in test tube A was increased to 35 C. The question was even guiding the candidates by separating an answer space for observations from that of explanation. This question was poorly done by most candidates. Instead of writing observations in its answer space, candidates stated explanations or conclusions such as starch will be digested or the

2 temperature is optimum. Those who did not get the explanation correct gave responses such as 35 C would either denature or inactivate enzymes. Some candidates were able to perceive that 35 C was suitable for the optimum enzyme activity, but still gave blue-black as a positive result; indicating that they failed to understand that starch was digested; and therefore a starch Iodine complex not possible. The expected response for observation was clear yellow-brown/brown. For an explanation; expected response was that kinetic energy would be increased, leading to increased enzyme activity as the temperature was suitable for digestion of starch. Generally this question was not well performed. Candidates showed lack of experimental exposure. Therefore it is recommended that pupils be exposed to more hands on practical work and follow it up with a laboratory report in order to be able to differentiate between observations and conclusions. QUESTION 2 A student was asked to investigate if chlorophyll is necessary for photosynthesis. a) Describe and explain the steps the students would follow to carry out the investigation. It was poorly done. No learner got the total marks and many candidates got zeros and there are other students who did not attempt the question. The expected responses for the question were Use of variegated leaf/ variegated leaf plant Has areas with chlorophyll and areas without chlorophyll Destarching a plant/place in darkness To remove existing starch Place plant in light For photosynthesis to occur Variegated leaf tested for starch For comparison Most candidates gave responses such as use of a leaf (separated) / variegated leaf instead of the plant as a whole.

3 Investigating light as a factor instead of absence or presence of chlorophyll, e.g some of their responses were; covering a leaf or plant with a black material. Candidates lacked correct sequence when carrying out an investigation e.g. destarching then placing in light followed by testing for stach and comparing the results giving an explanation (account) for every step taken. They didn t understand that a variegated plant/leaf is a genetic make up of that particular plant and is not due to lack of magnesium ion. Advice to the teachers Teachers should do more practical work with learners on investigations. After investigations learners should be made to do worksheets and write the laboratory reports in order to acquire the required experimental skills. b) Name the part of the leaf that can be used as a control in this investigation. It was poorly done; very few learners got the correct responses. The expected response was; the white part of the leaf/part lacking chlorophyll. Most candidates response gave the following; green part of the leaf/part containing chlorophyll. Advice to teachers Teachers are encouraged to explain to learners for every investigation carried out, what a control is. It was observed that this response was a common misconception. QUESTION 3 The question had the graph as fig 3.1 which had time after germination in days on x-axis and amount of starch in mg on y-axis. Candidates were asked to; a) Determine the amount of starch the seed would have 7 days after germination. The general performance on this question was good, as most of the candidates got it right. The expected responses ranged from 42.0 to 42.5 mg, but some of the candidates could not read the scale appropriately. b) Explain what was happening to the amount of starch in the first 14days. The question was fairly done as most of the amount of starch was decreasing, used or converted to other forms of sugars; Only a few explained further that glucose was then used for energy release or generation which amounted to scoring full marks.

4 Some misconceptions and errors included starch being denatured, lack of suitable conditions for germination; starch used for respiration, starch produce energy c) Explain why the amount of starch was increasing after 17 days. It was fairly done as a good number of them got all the two marks. Expected responses were seedling, plant or leaves developed and carried out photosynthesis. A few could not get marks as they responded that seeds photosynthesized. d) Seed do not usually germinate in winter. Candidates were asked what the cause of that could be. The question was generally well done even though there were some who still included sunlight as one condition for germination to occur; they also referred to enzyme denaturing. Teachers are therefore advised to give more work on graphs and pay more attention to the terms germination, photosynthesis and respiration. Explanation should be given when putting set up near windows in the laboratory for warmth or for light such that they do not get confused. Respiration should not be considered as energy production but as generation or release of energy. Starch is an insoluble form of carbohydrate that is not usable but has to be converted into transportable or usable forms like sucrose and glucose respectively. QUESTION 4 This question shows a photomicrograph of a sample of blood showing different blood cell. Generally, the question was well done. (a) Candidates were asked to name the cells labeled R and T. Most candidates gave the correct responses. The expected responses were: R- Red blood cell. T- White blood cell/ phagocyte. However, some candidates swapped the names. The use of photomicrographs is highly recommended. (b) Candidates were asked to give three visible differences between cells R and T. The expected responses were: R T No nucleus Nucleus present Biconcave shape Irregular shape smaller larger more fewer

5 The weakness was that, candidates gave differences which were not visible. E.g. Red blood cells contain hemoglobin while white blood cells do not. Surprisingly some candidates said white blood cells have cell wall while red blood cells do not. This question was fairly done. The candidates should take into consideration the words written in bolds in the stem of the question because they give a guide to them. (c) The candidates were asked to explain how cell R is adapted to its function. The expected responses were: No nucleus: To increase surface area for absorption of oxygen. Presence of haemoglobin: For attachment of oxygen Spongy cytoplasm: To squeeze through blood vessels. Disc like in shape/biconcave: for easy movement through blood vessels. The question was fairly done however, some candidates failed to understand the word adapted that let them give on the function. (d) (I) The candidates were asked to draw a large diagram of cell T. Here the candidates were expected to: outline in a single clear line (and no shading anywhere) - Detail at least of nucleus and cell membrane. -The weakness here was that candidates drew small and shaded diagrams. Some candidates drew nucleus only. For d (ii) and (iii) candidates were asked to measure the lengths across cell T and their diagrams in d (i). They were expected to measure in mm instead but instead they measure in cm. D (IV) This question was ow calculating the magnification of their drawings. The question was poorly done. The correct calculation was: image x magnification given Object Accurate answer The common error in this part was to give the calculation other way round that is

6 Object x magnification or the same expression without magnification factors. Image