A Level Biology Paper 3 Questions And Answers 2024/2025 All Answers Correct Verified Best, Exams of Nursing

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RP01 - Rate of an Enzyme Controlled Reaction - CORRECT ANSWERS What are the factors that affect enzyme activity (4)? - CORRECT ANSWERS 1. Enzyme concentration

2. Substrate concentration
3. Temperature
4. pH How is a control set up in a practical measuring enzyme activity? - CORRECT ANSWERS Replace the enzyme solution with distilled water or boiled enzyme solution. How can the results of the practical measuring enzyme activity be used to find the initial rate of reaction? - CORRECT ANSWERS Plot your results on a graph of 'rate of reaction' against 'time'. Draw a tangent at time = 0 to find the initial rate. Outline the practical procedure used to measure the effect of temperature on enzyme activity, using trypsin and milk. - CORRECT ANSWERS Immerse equal volumes of trypsin and milk, stored in different test tubes, in a water-bath for 5 minutes for the temperature to equilibrate. Mix together and immediately start timing, record the time taken for the milk to be completely hydrolysed (become colourless/ same as the control standard set up). Test at least 5 temperatures, with at least 3 repeats at each temperature. How is the rate of reaction calculated from time? - CORRECT ANSWERS Rate of reaction = 1/time What is the effect of temperature on enzyme activity? - CORRECT ANSWERS As temperature increases, kinetic energy increases so more ES complexes form. The rate of reaction increases up to the optimum temperature. Beyond that, bonds in the enzyme tertiary structure break, which changes the shape of the active site. The substrate and enzyme are no longer complementary, so rate of reaction decreases.

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What is the risk and level of risk associated with handling enzymes? - CORRECT ANSWERS Students may have allergic reactions to enzymes, so avoid contact with skin and eyes, wear eye protection. Low risk. RP02 Calculating Mitotic Index using Plant Cells - CORRECT ANSWERS Where in plants can cells undergoing mitosis be found? - CORRECT ANSWERS Meristem tissue at shoot and root tips. What is the mitotic index? - CORRECT ANSWERS The ratio of cells undergoing mitosis to the total number of cells in a sample. Outline the procedure to prepare a root tip slide. - CORRECT ANSWERS Warm 1M HCl to 60°C in a water bath. Cut a root tip using a scalpel and add to the HCl. Leave for 5 minutes. Remove from HCl and wash with distilled water. Cut the tip of the root tip sample and place on a slide. Add a few drops of stain to make chromosomes visible. State the formula for the mitotic index. - CORRECT ANSWERS Mitotic index = Number of cells with visible chromosomes /Number of cells in sample State the hazards and precautions for reagents used in this procedure. - CORRECT ANSWERS HCl - corrosive, avoid contact with skin, wear eye protection Toluidine Blue O stain - irritant, avoid contact with skin, wear eye protection Scalpel - cut away from fingers

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RP03 - Investigating Water Potential - CORRECT ANSWERS What is the purpose of calibration curves? - CORRECT ANSWERS They are used to determine the concentration of an unknown sample by comparing it to a set of standard values with known concentrations. How is a calibration curve used to find the concentration of plant tissue? - CORRECT ANSWERS Plot a calibration curve of percentage change in mass against concentration. Find the x-intercept where the plant tissue is isotonic to the sucrose solution. What occurs when plant tissue is placed in a hypotonic solution? - CORRECT ANSWERS Water moves into the plant tissue by osmosis, plant tissue increases in mass. What occurs when plant tissue is placed in a hypertonic solution? - CORRECT ANSWERS Water moves out of the plant tissue by osmosis, plant tissue decreases in mass. hy are the potato discs left in solution for 20 minutes? - CORRECT ANSWERS To allow time for osmosis until the plant tissue reaches equilibrium with its surrounding solution. What is water potential determined by? - CORRECT ANSWERS The concentration of solutes. The higher the solute concentration then the lower the water potential. Outline the procedure of investigating osmosis using potato tissue. - CORRECT ANSWERS Make a simple dilution of 1M sucrose to produce 5 concentrations. Add 5 cm3 to 5 different test tubes. Cut a potato into equal sized chips and weigh. Place a chip in each test tube and leave for 20 minutes. Take out, dab the excess water and weigh them again. (using a 2dp balance)

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Calculate the percentage change in mass. Why is the percentage change used rather than the actual change in mass? - CORRECT ANSWERS Potato chips may not all have same starting mass. Percentage change allows comparison. What is indicated by the x-intercept of the calibration curve? - CORRECT ANSWERS The concentration that is isotonic to the solution tested. Explain the change in mass in the potato chips. - CORRECT ANSWERS The potato chips with concentration lower than the sucrose solution (higher water potential) lose mass as there is a net movement of water out of the cells. The potato chips with concentration higher than the sucrose solution (lower water potential) gain mass as there is a net movement of water into the cells. Why are the potato chips dabbed dry after removing from the sucrose solution? - CORRECT ANSWERS To remove any excess water clinging to its surface that may add mass to the potato. What are the controlled variables of this practical? - CORRECT ANSWERS Volume of sucrose solution Size of potato chips Length of time left in solution Dab each potato disc with paper towels RP04 - Investigating Cell Membrane Permeability - CORRECT ANSWERS State 2 factors that affect the permeability of cell membranes. - CORRECT ANSWERS Temperature Concentration of solvents (ethanol)

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How is beetroot used to measure the permeability of cell membranes? - CORRECT ANSWERS The higher the permeability, the more red pigment that leaks out into the surrounding solution within a given time. A colorimeter can be used to determine the absorbance, hence concentration of pigment. Outline the procedure to investigate the effect of temperature on permeability of cell membrane. - CORRECT ANSWERS Cut beetroot into 6 identical cubes with a scalpel. Place each cube in a different test tube with equal volumes of distilled water. Place each test tube into water baths ranging from 30-80° C. Leave for 20 minutes. Filter each solution out into a cuvette and measure the absorbance using a colorimeter. What are the safety hazards involved in testing the effect of ethanol concentration on membrane permeability? - CORRECT ANSWERS Ethanol is an irritant and is flammable, keep away from naked flames, wear eye protection. Keep sharp scalpel away from fingers. Handle hot liquid with care. What is the effect of temperature on membrane permeability? - CORRECT ANSWERS Increasing temperature results in increase membrane permeability. What is the effect of ethanol concentration on membrane permeability? - CORRECT ANSWERS Increasing ethanol concentration leads to increased membrane permeability. Alcohol destroys membrane. RP05 - Dissection - CORRECT ANSWERS How should label lines in a diagram be drawn? - CORRECT ANSWERS With a ruler, no arrows, without crossing other label lines, in pencil

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How should a diagram be drawn? - CORRECT ANSWERS Large diagram - at least half the space given No shading, single continuous lines (no sketching) with pencil RP06 - Aseptic Techniques - CORRECT ANSWERS State 6 aseptic techniques. - CORRECT ANSWERS ● Wipe down surfaces with antibacterial cleaner, before and after experiment ● Use a Bunsen burner in the work space so that convection currents draw microbes away from the culture. ● Flame the wire hoop before using it to transfer bacteria. ● Flame the neck of any bottles before using them to prevent any bacteria entering the vessel. ● Keep all vessels containing bacteria open for the minimum amount of time. ● Close windows and doors to limit air currents. Why is bacteria incubated at 25°C? - CORRECT ANSWERS To prevent the growth of pathogens (harmful bacteria), which occurs at higher temperatures. How can you compare the effectiveness of different antibiotics applied to the same bacteria?

• CORRECT ANSWERS Measure the diameter and calculate the area of the zone of inhibition (clear zone) on the agar. What does the zone of inhibition indicate? - CORRECT ANSWERS It indicates the bacteria killed by the antibiotic. The larger the zone the more effective the antibiotic. If an antibiotic has very little or no zone of inhibition, the bacteria is likely resistant to the antibiotic. State the hazards and precautions of this practical. - CORRECT ANSWERS Naked flame: keep away from flammable materials, tie hair up, wear goggles

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Bacteria is a biohazard, use disinfect and wash hands, dispose of bacteria safely Disinfectant is flammable, keep away from naked flame. Why should the lid not be completely taped to the petri dish? - CORRECT ANSWERS To allow oxygen to enter the petri dish, preventing the growth of harmful anaerobic bacteria. Describe the graph that can be plotted from the results of this practical. - CORRECT ANSWERS A bar chart of zone of inhibition against antibiotic. RP07- Chromatography of photosynthetic pigments - CORRECT ANSWERS What is the purpose of chromatography? - CORRECT ANSWERS To separate different components in a sample. State the factors affecting the rate of migration of different pigments. - CORRECT ANSWERS Solubility Mass Affinity to the paper What is the formula of the RF value? - CORRECT ANSWERS Distance moved by pigment / Distance moved by solvent What is the purpose of finding the RF value of a pigment? - CORRECT ANSWERS Experimental RF value can be compared to a standard value in a database to identify the pigment. The standard value should be measured using the same paper and solvent. Outline the procedure of using chromatography to separate photosynthetic pigments. - CORRECT ANSWERS Draw a horizontal pencil line 1cm above the bottom of the filter paper. Add some acetone and use the mortar and pestle to grind up the leaf sample

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and release the pigments. Use a capillary tube to transfer the pigment onto the pencil line. Suspend the paper in the solvent so that the level of the liquid does not lie above the pencil line and leave the paper until the solvent has run up the paper to near the top. Remove the paper from the solvent and draw a pencil line marking where the solvent moved up to. Calculate the Rf value for each spot. State the hazards and precautions in this practical. - CORRECT ANSWERS Solvents are irritant and flammable. Keep away from naked flames, wear eye protection and avoid contact with skin. Leaf extract may be a biohazard. Wash hands after use. RP08 - Dehydrogenase Activity in Chloroplasts - CORRECT ANSWERS What is the function of dehydrogenase in chloroplasts? - CORRECT ANSWERS It catalyses the acceptance of electrons by NADP in the light dependent reactions. What is the purpose of DCPIP? - CORRECT ANSWERS It is a redox indicator dye and acts as an alternate electron acceptor instead of NADP. It turns from blue to colourless when reduced. Why is the plant extract chilled in an ice-water bath? - CORRECT ANSWERS To lower the activity of enzymes to prevent them from breaking down the chloroplasts. How is the control set up? - CORRECT ANSWERS Fill a cuvette with chloroplast extract and distilled water. How is light intensity controlled? - CORRECT ANSWERS Adjust the distance of the lamp from the set up. Perform the practical in a dark room so that the only light source is the lamp.

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What is the function of the muslin cloth? - CORRECT ANSWERS To filter out any debris in the ground leaf mixture but allowing chloroplasts to pass through. Why are the stalks of leaves removed before grinding? - CORRECT ANSWERS The stalks do not contain many chloroplasts. Outline the procedure of investigating the effect of light intensity, after chloroplast extract has been obtained. - CORRECT ANSWERS Set the colorimeter to the red filter. Zero using a cuvette containing chloroplast extract and distilled water. Place test tube in the rack 30cm from light source and add DCPIP. Immediately take a sample and add to cuvette. Measure the absorbance of the sample. Take a sample and measure its absorbance every 2 minutes for 10 minutes. Repeat for different distances from lamp up to 100 cm. RP09 - Respiration in Single Celled Organisms - CORRECT ANSWERS What is the function of methylene blue in this practical? - CORRECT ANSWERS It is a redox dye and acts as an alternate electron acceptor of the electrons transferred during ATP synthesis. It turns from blue to colourless, indicating the end point. Outline the procedure to investigate the effect of temperature on the rate of respiration of yeast. - CORRECT ANSWERS Set up a water bath at 35°C. Add equal volumes of the yeast and glucose solution to three test tubes.

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Place test tubes in the water bath and leave them to equilibrate for 10 minutes. Add 2 cm3 of methylene blue to the test tubes and start the timer. Shake for 10 seconds and place test tube back in water bath. Record how long it takes for the methylene blue to turn colourless for each test tube. Repeat the experiment using temperatures of 40°C, 50°C, 60°C and 70°C. How are the results used to calculate the rate of respiration at each temperature? - CORRECT ANSWERS Rate = 1 / time taken for methylene blue to decolourise Why does the yeast solution need to be buffered? - CORRECT ANSWERS To maintain a constant pH so that the enzymes are functioning at their optimum pH. What is the effect of temperature on the rate of respiration? - CORRECT ANSWERS As temperature increases, the rate of respiration increases to an optimum. This is because the rate of enzyme activity increases. Beyond the optimum, enzyme activity decreases as enzymes denature with high temperature. RP10 - Investigating Simple Animal Responses - CORRECT ANSWERS How can a choice chamber be used to measure the favourable environment of a small organism? - CORRECT ANSWERS By setting up chambers in different quadrants with different environmental conditions: dark + dry, dark + damp, light + dry, light + damp Organisms will move to the quadrant they find favourable What factors must be controlled when repeating the experiment? - CORRECT ANSWERS Number of animals Environmental conditions

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Time allowed for animals to choose Which statistical test is used to analyse the results of this practical and why? - CORRECT ANSWERS Chi squared test. It compares the expected and observed values, and tests if there is a significant difference. What is the conclusion drawn if the calculated value is greater than the critical value? - CORRECT ANSWERS Null hypothesis is rejected. Less than 5% probability that the difference is due to chance alone. There is a statistically significant difference between the expected and observed values. What do animals do when they are in unfavourable environments? - CORRECT ANSWERS They move faster and change direction more frequently to increase their chances of survival. State the hazard and precaution involved in this practical. - CORRECT ANSWERS The live organisms used are a biohazard. Wash hands after handling. RP11 - Measuring the Concentration of Glucose using a Calibration Curve - CORRECT ANSWERS How can Benedict's solution be used to measure the concentration of glucose in a solution?

• CORRECT ANSWERS Use a colorimeter to measure the absorbance of a series of solutions of known concentrations to create a calibration curve. Compare the absorbance of an unknown sample to the calibration curve. What is a serial dilution? - CORRECT ANSWERS A dilution where successive concentrations increase/decrease in a logarithmic fashion Outline the procedure of this practical. - CORRECT ANSWERS Make a serial dilution of glucose, ranging from 0 to 10 mmol dm-. Place 2 cm3 of each of the unknown samples in separate boiling tubes. Add 2 cm3 of Benedict's solution to all boiling tubes. Place boiling tubes in a water bath at 90°C for four minutes.

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Zero the colorimeter using a cuvette with distilled water and set to red filter. Place known samples into cuvette and measure the absorbance of each using the colorimeter. Make a calibration curve. Measure the absorbance of the unknown samples using the colorimeter. Use the calibration curve to determine glucose concentrations. What are the axes in a calibration curve? - CORRECT ANSWERS Absorbance against glucose concentration. What would a high glucose concentration in urine suggest? - CORRECT ANSWERS It may suggest diabetes. Lack of insulin leads to high blood glucose concentration, hence high concentration in the glomerular filtrate, so not all glucose can be reabsorbed in the proximal convoluted tubule. State the hazards and precautions of this practical. - CORRECT ANSWERS Benedict's solution is an irritant, wear eye protection, avoid contact with skin. Handle the hot water bath with care. How can you increase the accuracy of the estimate of the unknown glucose solution? - CORRECT ANSWERS Increase the number of concentrations (at smaller intervals) for the calibration curve within the range of concentrations that the unknown solution belongs in. RP12 - Effect of Different Variables on Species Distribution - CORRECT ANSWERS list 3 abiotic factors - CORRECT ANSWERS Water supply Nutrient supply Humidity Wind speed Day length Rainfall Light intensity Temperature

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List 3 biotic factors - CORRECT ANSWERS Competition for resources Predation Disease How is percentage cover calculated? - CORRECT ANSWERS Use a quadrat with squares. Count how many squares the required species is present in. Only count a square if more than half of the square is covered. Calculate the percentage of squares the species is present in. Outline the procedure to this practical. - CORRECT ANSWERS Outline the procedure to this practical. Choose an area to take samples from. Use a random number generator to generate 10 sets of random coordinates. Use two tape measures to create a set of axes off which coordinates can be read. Place the quadrat at each of the coordinates, placing the bottom left corner on the coordinate every time. Record the percentage cover for the chosen species. At each coordinate, a measure of the independent variable should be taken. Eg. record light intensity using a photometer at each location How can the results be used to determine the relationship between the chosen factor and the percentage cover? - CORRECT ANSWERS Spearman's rank - correlation Why should a random number generator be used? - CORRECT ANSWERS To avoid bias in random sampling.