Biology Practical Revision

Biology Practical Revision Precision Instrument Values can be recorded to the nearest … Examples Burette 0.05 cm3 25.00 cm3, 10.05 cm3 Measuring cylinder 0.5 cm3 14.0 cm3, 14.5 cm3 Electronic balance (Copy completely) 0.1 g 150.0 g, 151.8 g 0.01 g 131.00 g, 189.30 g Ruler 0.1 cm 11.0 cm, 15.8 cm Thermometer 0.5 oC 40.0 oC , 40.5 oC Stopwatch 1s 23 s, 50s Units • Density: gcm-3 • Rate: s-1 Graph answering • x à y axis Other important things 1. Describe 2 ways in which your method of using the syringe ensured the dilutions were made accurately. + WHAT DID YOU DO / HOW? • Remove the bubbles in the syringe by inverting and tapping the syringe. • Use a syringe that is only slightly larger than the volume (e.g. use a 5cm3 syringe to measure 2 cm3, 10cm3 syringe to measure 9cm3) • Do not cross-contaminate the syringe. (Dedicate one syringe to each solution) • Wash the syringe with distilled water and dry thoroughly with paper towel to prevent dilution of the solutions. 2. Why was iodine added to the onion epidermis? • To stain the onion epidermis for easy viewing under the microscope. 3. Why calculate the percentage in length? • Different cells have different initial lengths • Percentage change = can accurately reflect how much the cell contents have shrunk / increased in size. à can compare to what they were like in the beginning. 4. Why take more than 1 readings? • Calculate average • Identify anomalies

5. The comparison of the diameters would be ideal if the three slices of eggplant came from one section of the same fruit. Why? • Same starting water potential of the cell sap of the plant tissues from the same fruit. • Hence more reliable comparison can be made. 6. Why rinse the vising tubing? • Remove the starch / amylase / named solution on the exterior of the bag Variables • TLVTCM Food tests Test for starch: Iodine test 1. Add 2-3 drops of iodine onto the food substance. 2. If iodine solution turns from brown to blue-black, starch is present. If iodine remains brown, there is no starch present. Test for reducing sugar: Benedict’s test 1. Add 2cm3 of food sample into a test tube containing 2cm3 of Benedict’s solution. 2. Shake well. 3. Place the test tube into a boiling water bath for 15 minutes. 4. A positive result ranges from green to brick-red. A green precipitate shows a small amount of reducing sugar. An orange precipitate shows a moderate amount of reducing sugar. A brick-red precipitate shows a large amount of reducing sugar. Test for protein: Biuret test 1. Add 2cm3 of sodium hydroxide to 2cm3 of food sample. Shake well. 2. Add 1% copper (ll) sulfate solution, shaking after each drop. 3. A violet colouration shows the presence of protein. A blue solution shows the absence of protein. Test for fats: Ethanol emulsion test 1. Add 2cm3 of ethanol to a test tube containing a mixture. Shake well. 2. Add 2cm3 of water to the mixture. Shake well. 3. The presence of a white emulsion shows the presence of fats. The clear and colourless solution shows the absence of fats.

Planning Type Requirements Effect of temperature on enzyme activity 1. Constant + constant pH 2. Dependent 3. Independent 4. Reliability 5. Control 6. Analysis Effect of pH on enzyme activity 1. Constant + constant temperature 2. Dependent 3. Independent 4. Reliability 5. Control 6. Analysis Effect of concentration on potato 1. Constant 2. Initial length / weight – final length / weight 3. Dependent 4. Independent 5. Reliability 6. Control 7. Graph 8. Analysis Sources of error Type Source of error How does it affect? Improvement Majority of the experiments (enzyme, osmosis) Time lapse in adding iodine solution in the mixture Time lapse in reading the volume Time lapse in reading the length Inaccurate readings from expected **More/less? Conduct experiments with staggered start times Osmosis Varied volume of water adhered to surface of Viking tubing Inaccurate readings from expected **More/less? Evaporation of water from salt solution Concentration of salt solution will increase, hence water potential of salt solution decreases. Length of Cover petri dish with a lid

potato shorter than expected. Bubbling of gas into limewater (calcium hydroxide) Different volume of gas dissolved in the solution Lesser gas volume measured than expected Collect gas with a gas syringe Yeast Insufficient oxygen Aerobic respiration may occur and hence the rate of reaction will be slower. Longer time taken than expected. Stirring to allow aeration Calorie count Food sample is not completely burnt Change in temperature is lower than expected hence calculated value is lower than expected. Loss of heat to the surroundings Change in temperature is lower than expected hance calculated energy value is lower than expected. Capillary tube used When apparatus is moved, the liquid level in the capillary tube changes Liquid level in the capillary wall will shift inaccurate readings from expected Clamp more tightly colours Subjective judging of colour Inaccurate readings from expected Use a colourimeter connected to a data logger Slow down the reaction by using less enzyme/substrate, to increase the time taken to reach the end-point Variation experiment Small sample size The results is not representative of the whole population Increased sample size Homeostasis Loss of heat to the surroundings Change in temperature is higher than expected Use styrofoam cups for insulation Effect of varying chloroplast concentration Temperature of the set-up not consistent/heat from the lamp causes temperature to be not consistent Time taken for decolourisation is lesser than expected as the temperature of the environment increases Carry out experiment in a thermostatically controlled water bath

on the rate of photosynthesis Difficulty in gauging the exact amount at which the decolourisation occurs/subjective colour interpretation from the heating of the lamp May increase or decrease the time taken Use a colourimeter connected to a data logger Effect of catalyse in hydrogen peroxide (measure the time taken for the plant tissue to float) Potato tissues are of different lengths, hence travelling distance is lesser Shorter travelling distance for longer potato tissues. Hence time taken is shorter than expected Use a taller container so that the difference in length becomes insignificant Measure the volume of oxygen using a gas syringe