A LEVEL BIOLOGY: QUESTIONS WITH MARK SCHEMED ANSWERS(PAPER 1 ,2 AND 3 )

A LEVEL BIOLOGY: QUESTIONS WITH MARK SCHEMED ANSWERS(PAPER 1 ,2 AND 3 ) Describe how you would test a liquid sample for the presence of lipid and how you would recognise a positive result. - ANSWER-1. (Mix / shake sample) with ethanol, then water; Sequence is important 2. White / milky (emulsion); Explain why the student: 1. used only the first 5 mm from the tip of an onion root. 2. pressed down firmly on the cover slip. - ANSWER-1. Where dividing cells are found / mitosis occurs 2. Single / thin layer of cells / spread out cells so light passes through (making cells / nuclei visible); Describe and explain what the student should have done when counting cells to make sure that the mitotic index he obtained for this root tip was accurate. - ANSWER-1. Examine large number of fields of view / many cells; Mark as pairs only Accept large number / 20 or more for many 2. To ensure representative sample; Accept typical / reliable OR 3. Repeat count; 4. To ensure figures are correct; OR 5. Method to deal with part cells shown at edge /count only whole cells; 2 | Page 6. To standardise counting; Describe how he made a 1 in 10 dilution and then used this to make a 1 in 1000 dilution of the original liquid culture of bacteria. - ANSWER-1. Add 1 part (bacteria) culture to 9 parts (sterile) liquid (to make 10-1 dilution); Accept water / nutrient / broth for liquid 2. Mix (well); Accept stir 3. Repeat using 9 parts fresh (sterile) liquid and 1 part of 10-1 and 10-2 dilutions to make 10-3 dilution; OR Add 1 part 10-1 (suspension) to 99 parts (sterile) liquid (to make 10-3 dilution); Describe the method the student would have used to obtain the results in the graph. Start after all of the cubes of potato have been cut. Also consider variables he should have controlled. (osmosis) - ANSWER-1. Method to ensure all cut surfaces of the eight cubes are exposed to the sucrose solution; Credit valid method descriptions to fulfil mp1, 2 and 3 (no explanation is required). 2. Method of controlling temperature; Accept 'at room temperature' for method 3. Method of drying cubes before measuring; 4. Measure mass of cubes at stated time intervals; The scientists collected bees using a method that was ethical and allowed them to identify accurately the species to which each belonged. In each case, suggest one consideration the scientists had taken into account to make sure their method 1. was ethical. 3 | Page 2. allowed them to identify accurately the species to which each belonged. - ANSWER-1. Must not harm the bees 2. use photographs/specimens (to identify species) Suggest and explain two ways in which the scientists could have improved the method used for data collection in this investigation. - ANSWER-1. Collect at more times of the year so more points on graph/better line (of best fit) on graph; Both suggestion and explanation is required for each mark point. The explanation must relate to the graph. 2. Counted number of individuals in each species so that they could calculate index of diversity; 3. Collected from more sites/more years to increase accuracy of (mean) data; Suggest one reason why it was important that the student left the apparatus for one hour after the yeast culture reached a constant temperature. - ANSWER-(So the) oxygen is used/absorbed/respired; Explain why a log scale is used to record the number of cells. - ANSWER-Large range/difference/increase in numbers; Explain the purpose of: 1. boiling the agar 2. transferring the same volume of liquid culture onto each agar plate. - ANSWER-1. So no contamination / other bacteria; 2. So same number of bacteria transferred to allow comparison; Describe how you would use cell fractionation techniques to obtain a sample of chloroplasts from leaf tissue. Do not include in your answer information about any solutions - ANSWER-1. Macerate / homogenise / blend / break tissues / cells (in solution);