A-level paper 3 mistakes
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Explain how the method of setting up a potometer could be modified to produce accurate readings.
Don’t put the leaves under water make sure they’re dry. Wet leaves reduce diffusion
Explain how a potometer could be modified to obtain repeat readings (when the bubble keeps moving)
Attach a syringe To return bubble to scale
Describe how you would use chromotography to separate pigments in a plant leaf.
Add extract to start line dry and repeat to concentrate the spot Place paper in a solvent such as propanone Obtain solvent front
Describe how to prepare a microscope slide of root tissue to show stages of mitosis.
Use warm acid Remove acid using water Add a stain such as toluidine blue Tease with a needle Add a coverslip and squash
Devise a method the student should use to investigate the effect of waterlogged soil on mitosis in root cells.
Place plant in water logged and non-water logged soil Use the same species and age of plant leave for 2 days Control temperature and pH Take cells from the same part of the root tip.
Explain why a lack of oxygen in waterlogged soil could reduce the growth of a plant.
Aerobic respiration will be inhibited. Therefore oxidative phosphorylation So less ATP synthesised Less active transport of mineral ions Less GP
Describe hoe a microscope should be used to observe the stomata using the high power lens.
Locate the specimen using a low power lens Focus using low power objective lens before high power Only use fine focus with higher power lens
Explain how fewer stomata might affect the growth of coffee plants.
Less carbon dioxide will be absorbed So less GP produced Less transpiration so less mineral ions So less cellulose produced
Devise a method to compare the effectiveness of 4 antibiotics on the growth of bacteria.
Same species of bacteria Same volume of antibiotic Use of agar growth medium Incubate between 20-25 degrees for 2 days Measure zone of inhibition Work near a bunsen flame
In the experiment where potato cubes are put in different concentrations of sucrose, what are 3 ways this experiment can be improved to get a more accurate value for water potential of these potato cells?
Use small intervals of sucrose concentration. Blot cubes dry before weighing to remove extra mass. Repeat to identify anomalies. Control temperature as this affects membrane permeability.
Describe the steps that should be taken for the student to be able to see pollen grains using a light microscope.
Place pollen grains in water Use of a coverslip Use lowest power lens first then increase magnification
Explain why standard deviation may be plotted on a graph.
Shows variation around the mean Therefore gives an indication of significant difference.
stat the difference between the action spectrum and the absorption spectrum.
The absorption spectrum shows absorption of light of different wavelengths. The action spectrum shows rate of photosynthesis at each wavelength.
State how an action spectrum and an absorption spectrum show that chlorophyll is used in photosynthesis.
Spectra are similar/they overlap
Explain why smaller mammals have a larger oxygen consumption per kg of body mass compared to larger animals.
Mammals need to maintain body temperature. Smaller mammals have a larger surface area to volume ratio. Therefore lose more heat. Heat is produced by respiration so more oxygen required.
Explain how gills are adapted for gas exchange.
Large surface area to volume ratio for diffusion. Thin walls for a shorter diffusion distance. Countercurrent system maintains concentration gradient.
Explain why a low oxygen concentration in water will Lower the pH of the tadpole’s blood.
Anaerobic respiration will take place. Therefore lactic acid will be produced. Because the pyruvate is reduced.
Justify 3 methods for ensuring aseptic conditions.
Clean surfaces with disinfectant to kill bacteria. Work near a bunsen flame to create updraft. Flame apparatus to kill bacteria.
Compare and contrast the structure of the cell walls of gram positive and gram negative bacteria.
Both contain peptidoglycan. Gram positive bacteria have a thicker layer of peptidoglycan in their cell walls. positive has teichoic acid, negative does not
Explain how phloem tissue is adapted to its function.
companion cells for ATP production Energy needed for the active transport of sugars into the phloem. sieve tubes have no organelles, so less barriers to transport
Give 2 differences between gram negative and gram positive bacteria.
Gram positive bacteria has a thicker peptidoglycan layer in their cell wall whereas gram negative has a thinner layer of peptidocglycan. Gram positive contains teichoic acid, gram negative does not.
Explain the advantage to the ecosystem of fungal spores being spread around.
Fungi can cause decomposition. So mineral ions are recylced within the ecosystem.
Devise and experiment to show whether the presence of clover plants affects the concentration of the green pigment chlorophyll in grass plants.
grass taken from in or near clover and from an area without clover. Same mass of grass plants used. Crush using a mortar and pestle. measure absorbance in a colorimeter. colorimeter zeroed with solvent replicates taken and means calculated t test used to look for significant difference
Why does mitotic index decrease as you move further from the root tip?
cells differentiate further from the root tip.