Edexcel IGCSE Biology Experimental Method Notes

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Edexcel IGCSE Biology Experimental Method Notes

  1. 1. Exexel IG Bio – Experimental Methods https://bioknowledgy.wikispaces.com/ (Chris Paine) Edexcel Experimental Methods Learning outcomes that refer to experiments: Topic code Topic name Code Outcome 2c Biological molecules 2.7 describe the tests for glucose and starch 2c Biological molecules 2.11 describe experiments to investigate how enzyme activity can be affected by changes in temperature. 2d Movement of substances into and out of cells 2.16 describe experiments to investigate diffusion and osmosis using living and non-living systems. 2e Nutrition - Flowering plants 2.22 describe experiments to investigate photosynthesis, showing the evolution of oxygen from a water plant, the production of starch and the requirements of light, carbon dioxide and chlorophyll 2e Nutrition - Humans 2.32 describe an experiment to investigate the energy content in a food sample. 2f Respiration 2.37 describe experiments to investigate the evolution of carbon dioxide and heat from respiring seeds or other suitable living organisms. 2g Gas exchange - Flowering plants 2.43 describe experiments to investigate the effect of light on net gas exchange from a leaf, using hydrogen-carbonate indicator 2g Gas exchange - Humans 2.48 describe experiments to investigate the effect of exercise on breathing in humans. 2h Transport - Flowering plants 2.56 describe experiments to investigate the role of environmental factors in determining the rate of transpiration from a leafy shoot 4a Organisms and their environment 4.2 & 4.3 explain how quadrats can be used to estimate the population size of an organism in two different areas AND explain how quadrats can be used to sample the distribution of organisms in their habitats. 5a Food production - Micro-organisms 5.6 describe a simple experiment to investigate carbon dioxide production by yeast, in different conditions Additional recommended investigations (students are not expected to describe the methodology, but might well be asked to analyse data in examinations) Topic code Topic name Code Outcome 2j Coordination and Response - plants 2.81, 2.82 Controlled experiments to demonstrate phototropic and geotropic plant growth responses 2j Coordination and Response - humans n/a A simple experiment to show how the sensitivity of the skin differs on finger tips, back of hand, wrist and forearm 3a Reproduction - plants 3.3 A practical exercise comparing floral structure in insect- pollinated and wind-pollinated flowers
  2. 2. Exexel IG Bio – Experimental Methods https://bioknowledgy.wikispaces.com/ (Chris Paine) 2.7 describe the tests for glucose and starch Test for glucose (reducing sugars) 1. If the food is solid grind in a pestle and mortar and add a small quantity of water. 2. Add a depth of 1cm of the mixture/substance (to be tested) to a test tube. 3. Add a similar volume (1 cm depth) of Benedict’s reagent (original colour is blue). 4. Heat to at least 70oC for 2 minutes 5. If glucose (or another reducing sugar) is present the colour will change to yellow/orange/red (the closer to red the colour the higher the concentration of sugar) Test for starch 1. If the food is solid grind in a pestle and mortar and add a small quantity of water. 2. Add a depth of 1cm of the mixture/substance (to be tested) to a test tube. 3. Add a similar volume (1 cm depth) of Iodine solution (original colour is orange/brown). 4. If starch is present the colour will change to blue/black (depending on the concentration of the iodine solution) n.b. 1. Iodine solution is not pure iodine, but actually potassium iodide. 2. Once iodine has changed colour to blue/black to indicate the present of starch this colour change cannot be reversed. If amylase is added to digest the starch the iodine remains a blue/black colour.
  3. 3. Exexel IG Bio – Experimental Methods https://bioknowledgy.wikispaces.com/ (Chris Paine) 2.11 describe experiments to investigate how enzyme activity can be affected by changes in temperature. The effect of temperature upon the amylase digestion of starch Method:  Select temperatures in range from 0°C to 100°C (controlled by waterbaths and ice-water mixtures in troughs). Remember the optimum temperature will be between 30°C and 50°C.  Place amylase solution and starch suspension separately in the water bath and wait until they reach the desired temperature.  Mix together amylase solution and the starch suspension. The mixture is then kept at the same temperature.  Immediately start the stopclock.  To measure the rate of reaction, drops of the mixture can be collected at intervals of one minute (starting at time zero) and added to individual iodine drops on a white tile.  The time taken for the starch to disappear, i.e. the iodine remains orange/brown (remember a blue/black colour indicates the presence of starch) is recorded.  Repeat the procedure at each temperature. Weaknesses:  The rate of reaction can only be measured to an accuracy of one minute.  Small concentrations of starch may persist making it difficult to decide when all the starch has been digested. The effect of temperature upon the action of catalase Method:  Select temperatures in range from 0°C to 100°C (controlled by waterbaths and ice-water mixtures in troughs). Remember the optimum temperature will be between 30°C and 50°C.  Measure 10 cm3 hydrogen peroxide solution into a large test tube and place in a waterbath  Add 3 drops of detergent.  Wait until the temperature is at the desired temperature.  Prepare a cube of potato with each side measuring 1 cm in length.  Add the cube to the boiling tube (try not to spill any hydrogen peroxide) and immediately start the timer.  Measure the height (h), of the foam every minute (from time zero) for 5 minutes.  Repeat the procedure at each temperature. n.b. Hydrogen peroxide is toxic and so great care needs to be taken with its use. Weaknesses:  The detergent may vary in it’s efficiency to capture the oxygen. Variance in the relative rates of reaction can be measured, but not the actual volumes generated.  Difficult to keep the cubes evenly shaped
  4. 4. Exexel IG Bio – Experimental Methods https://bioknowledgy.wikispaces.com/ (Chris Paine) 2.16 describe experiments to investigate diffusion and osmosis using living and non-living systems. Models of cells Visking (dialysis) tubing 1. Tie the visking tubing at one end 2. Half fill with sucrose solution 3. Tie the other end 4. Measure the mass 5. Place in water for 30 mins 6. Dry the outside of the tubing and measure the mass again 7. The change in mass indicates the direction of osmosis and how quickly it has happened Cubes of agar jelly 1. Cut three cubes out of the agar jelly. Each cube should measure 2cm x 2cm x 2cm 2. Place the agar cube in a beaker of potassium permanganate solution (purple dye) 3. After 10 mins remove one cube 4. Dry the surface 5. Cut the cube in half 6. Measure the distance (from the edge) that the potassium permanganate has reached by diffusion. 7. After 20 mins and 30 mins remove the second and third cubes and similarly measure the diffusion distance (repeat steps 4 – 6) Observing changes in cells using a microscope  Onion epidermis can be peeled away, cut into squares and mounted on slides  If the Onion epidermis is mounted in a strong sucrose solution plasmolysis, the cytoplasm and vacuole will decrease in volume as water leaves the cell by osmosis and the cell membrane will pull away from the cell wall.  If the sucrose solution is removed (e.g. using a paper towel) and replaced with water then the water, by osmosis, will return to the cell, the volume of the cytoplasm and vacuole will increase the the cell membrane will again press up against the cell wall.  Red blood cells in blood obtained from a butcher may be mounted on slides in hypotonic, isotonic and hypertonic saline, and observed under a microscope to show the effects of osmosis.  Hypotonic solution should cause the red blood cells to burst whereas the hypertonic solutions should cause them to decrease in volume, i.e. shrink.
  5. 5. Exexel IG Bio – Experimental Methods https://bioknowledgy.wikispaces.com/ (Chris Paine) Plant tissues  A number of strips/cores/cubes of potato(or carrot etc.) of the same size are cut  Dry the pieces of potato with a paper towel and measure their starting mass  Place the pieces of potato into differents concentration of salt (or sucrose) solution  Wait for a specified time (at least one hour)  Remove the pieces of potato with a paper towel and measure their ending mass  Calculate the % change in mass: % change = [ending mass (g) – starting mass (g)] x 100 starting mass (g) n.b. 1. Length can be measured instead of mass. 2. The method can be changed/extended to investigate the effect of surface area, temperature, pH and various other factors.
  6. 6. Exexel IG Bio – Experimental Methods https://bioknowledgy.wikispaces.com/ (Chris Paine) 2.22 describe experiments to investigate photosynthesis, showing the evolution of oxygen from a water plant, the production of starch and the requirements of light, carbon dioxide and chlorophyll Photosynthesis (and hence the rate at which it happens) can be measured in a number of ways. Mostly commonly the concentration or the volume of the photosynthetic products are measured. It is straightforward to measure the volume of oxygen evolved and to test for present of starch (a polymer of glucose, which is the other product of photosynthesis) in leaves. Oxygen evolution Method 1: 1. A piece of water plant (typically elodea, cambomba or a similar species) is placed in a large beaker (or trough) and covered with water 2. Cover the water plant with an inverted glass funnel. The depth of water should be sufficient so that the tip of the funnel is several centimetres under the level of the water. 3. Completely fill a test tube with water. 4. Seal the end of the tube with a thumb (or rubber bung) and invert. Place the sealed end of the tube under the level of the water in the beaker and uncover it, the test tube should remain full of water. 5. Whilst keeping the now open end of the test tube under water carefully place the test tube on top of the funnel’s neck. 6. Bubbles of oxygen that appear on the surface of the water plant should rise to the top of the test tube. As they collect they will displace the water. 7. After 24 to 48 hours there should be sufficient oxygen that it can be tested for. Oxygen will relight a glowing splint. Weaknesses:  Temperature and light are very important for photosynthesis. If not controlled the levels of oxygen that build up will vary greatly.  Certain water plants photosynthesise too slowly and sufficient oxygen for testing will a longer period of time to be produced. Image source: http://www.elateafrica.org/elate/biology/biologyindex.html
  7. 7. Exexel IG Bio – Experimental Methods https://bioknowledgy.wikispaces.com/ (Chris Paine) Method 2: 1. Place a short piece of a water plant (typically elodea, cambomba or a similar species) in a solution of sodium hydrogen carbonate (this provides a good source of carbon dioxide). 2. Invert the water plant and keep it submerged by weighing it down (plasticine and paper clips both work well). 3. Whilst submerged cut the stem (near to the top), this enables oxygen bubbles to escape. 4. Place an a strong artificial light source, e.g. a desk lamp close to the beaker containing the water weed 5. Wait for (at least) 5 minutes as the plant equilibrates with the conditions. 6. Measure the (oxygen) bubble production by either: a. counting the number of bubbles evolved in a minute b. measuring how much time it takes for 10 bubbles to be given off. 7. Calculate the rate of photosynthesis use the equation below: Rate of photosynthesis (bubble/min) = bubbles generated (bubbles) Time taken (min) Independent variables – what can be investigated:  Light intensity – can be varied by moving the lamp closer/further from the water plant  Colour/wavelength of light - coloured filters can be placed between the plant and the light source.  Carbon dioxide availability - the plant is immersed in solutions of different concentration of sodium hydrogen carbonate.  Temperature – the water the plant is placed in is maintained at different temperatures using a waterbath. Weaknesses:  Temperature and light are very important for photosynthesis. If not controlled the levels of oxygen that build up will vary greatly.  The age/freshness of the water plant affect how fast growth and photosynthesis occurs  Ambient light levels will affect the rate of photosynthesis
  8. 8. Exexel IG Bio – Experimental Methods https://bioknowledgy.wikispaces.com/ (Chris Paine) Starch Testing Method: 1. Placing a plant in the dark for 24 hours to de- starch the leaves (the starch will be converted to glucose and used for respiration and without light it cannot be replaced by photosynthesis). 2. Cover part of a leaf using aluminium foil. 3. Place the plant in bright sunshine for 12 hours 4. Remove a leaf from the plant 5. Put the leaf in boiling water for 1 minute (to destroy the cell membranes so that chlorophyll molecules can pass through) 6. Heat the leaf in boiling ethanol (to remove/dissolve the green chlorophyll from leaf) 7. Place leaf into room temperature water (to rehydrate and soften the leaf so that it can be spread out) 8. Drop iodine solution onto the leaf (test for starch) – blue- black colour will show the presence of starch. Expected results: The un-covered parts of the leaf should be blue-black indicating the presence of starch and hence photosynthesis occurred in this region of the leaf. This indicates that light is necessary for photosynthesis. Alternatives:  A starch test on a variegated leaf can be used to demonstrate that chlorophyll is needed for photosynthesis.  To show that carbon dioxide is needed for photosynthesis, a leaf on a plant may be surrounded by air with no carbon dioxide. To remove the carbon dioxide from the air insert a leaf into a conical flask (or plastic bag, ziplock bags work well) containing a small amount of potassium or sodium hydroxide. Weaknesses:  A good experiment always includes a control. An untreated leaf should always be tested along with the treated leaves to provide evidence that the factor investigated is responsible for the observed changes.  Temperature and light are very important for photosynthesis. If not controlled the levels of starch that build up will vary greatly.  Certain plants photosynthesise too slowly and longer in sunshine to develop starch reserves  Certain plants are very efficient at relocating the glucose from photosynthesis and hence the starch reserves are distributed more evenly through the plant and don’t show clear differences between covered and uncovered regions.  If the plant is exposed too long then, like above, there is no clear difference between treated areas/leaves as the glucose has been relocated and starch reserves are present throughout the plant. Image source: http://www.elateafrica.org/elate/biology/biologyindex.html Image source: http://www.elateafrica.org/elate/biology/biologyindex.html
  9. 9. Exexel IG Bio – Experimental Methods https://bioknowledgy.wikispaces.com/ (Chris Paine) 2.32 describe an experiment to investigate the energy content in a food sample. A Calorimeter is a simple device used to determine the energy content of food. It takes 4.2 joules of energy to heat 1g/1ml of water by 1OC. Using this information, food can be burnt and the heat energy used to heat water. The increase in temperature of the water can be calculated to determine how much energy was contained in the food. Method: 1. A boiling tube is mounted at a near vertical angle in a clamp stand. 2. The water temperature is recorded. 3. 25cm3 of water is added to the boiling tube. 4. A small quantity mass of the food sample (e.g. potato chips/crisps) is weighed and the mass noted. 5. The food sample is put in a crucible or burning spoon and ignited (for example in the flame of a Bunsen burner). 6. The crucible/burning spoon containing the food sample is quickly placed directly under the boiling tube. 7. When the food sample has completely burnt note the maximum water temperature (the temperature may continue to rise for a short time after the completion of combustion) 8. The energy content of the food is calculated using the equation: Energy content of food sample (joules per gram) = 
 mass of water heated (g) x temperature rise (oC) x 4.2 x 1 / mass of food sample (g) Weaknesses:  Not all foods burn easily  The food does not burn completely  The glass of the test tube is heated not just the water  Heat is dissipated to the surrounding air
  10. 10. Exexel IG Bio – Experimental Methods https://bioknowledgy.wikispaces.com/ (Chris Paine) 2.37 describe experiments to investigate the evolution of carbon dioxide and heat from respiring seeds or other suitable living organisms. Carbon dioxide is released during aerobic respiration a build up either in the volume or here the concentration of carbon dioxide can be used to measure the rate of respiration. Similarly when aerobic respiration occurs a small quantity of heat is released as a waste product. Again the increase in temperature caused by this release in of heat energy can be used to indicate the relative rate of respiration. Method: 1. Place a number of germinating seeds are put in a large test tube 2. Use a clamp to hold the large test tube in position 3. Use the rubber bung to seal the large test tube 4. Connect an angled glass tube to the rubber bung 5. Half fill a second test tube with hydrogen carbonate indicator. 6. Use a second clamp to position the test tube containing the hydrogen carbonate indicator so that the end of the angled glass tube is immersed in the solution. 7. Wait several hours, preferably a day then observe and record the colour changes in the indicator solution. Why use hydrogen carbonate indicator, why not limewater?  Limewater only measures increases in carbon dioxide, hydrogen carbonate indicator can show decreases as well as increases  Changes in hydrogen carbonate indicator are reversible and colour changes constantly reflect changes in carbon dioxide concentration whereas once limewater has become cloudy it will not clear if carbon dioxide levels drop, the change is irreversible.  hydrogen carbonate indicator is more sensitive and will respond to smaller changes in carbon dioxide than limewater When cellular respiration occurs it gives off heat. The changes below to the equipment setup allow for heat to be detected and measured:  Wrap the large test tube with an insulating material (or use a vacuum flask).  Use a rubber bung with two holes.  Place a thermometer in the second hole to measure temperature changes in the large test tube. Weaknesses:  The system is closed and oxygen is used up. After a certain point there will be no oxygen left for seeds to use for respiration  Temperature should be controlled, if too low then respiration will not happen  No additional water is provided. Seeds need water for germination and growth  Volumes of solution, numbers of seeds not controlled  Colour changes in the indicate show a change in carbon dioxide levels, but cannot indicate how large the change is.
  11. 11. Exexel IG Bio – Experimental Methods https://bioknowledgy.wikispaces.com/ (Chris Paine) 2.43 describe experiments to investigate the effect of light on net gas exchange from a leaf, using hydrogen-carbonate indicator A leaf/plant can be placed in a sealed tube of air-equilibrated hydrogen carbonate solution (red in colour) and placed in the light or in the dark. Method: Setup the apparatus below according to the diagram. The tubes should be left for several hours, ideally at least 24 hours. Expected results:  Tube A – The hydrogen carbonate indicator turns yellow due to increased levels of carbon dioxide. The leaf can only respire due to a lack of light energy. This results in a production of carbon dioxide.  Tube B – No colour change. This tube is a control to check that all the colour changes that occur are due the presence of the leaf.  Tube C – The hydrogen carbonate indicator turns purple due to decreased levels of carbon dioxide. The leaf is respiring at the same rate as the leaf in tube A, but due to higher levels of photosynthesis more carbon dioxide is being used as a reactant (in photosynthesis) than is being produced (by respiration).  Tube D?? – Ideally a second control should be setup identically to tube B, but additionally wrapped in foil so that like tube A it receives no light. Variants and extensions:  Small invertebrates (on a gauze platform) can be used to show the effect of respiration relative to photosynthesis  Waterweeds can be used instead of leaves  Similarly to the small invertebrates water snails can be used with the waterweed to show the effect of respiration relative to photosynthesis n.b. if using animals they must not be harmed during the experiment and the conditions they are subjected too must ones normally experienced in their natural environments. Weaknesses:  Health and size of plants varies. Therefore the rate of respiration and photosynthesis will also vary. Ideally repeats should be used to make the investigation more reliable.  The effect of night-time respiration may be larger than the net reduction of carbon dioxide during the day meaning tube C as well as tube A contains yellow hydrogen carbonate indicator solution. An artificial light should be used to make sure that this does not happen.  Due to use of the waterbath and placement in the room tube C may receive minimal light levels and hence photosynthesis may not be greater than respiration. As previously an artificial light should be used.
  12. 12. Exexel IG Bio – Experimental Methods https://bioknowledgy.wikispaces.com/ (Chris Paine) 2.48 describe experiments to investigate the effect of exercise on breathing in humans. n.b. this experiment is best done in pairs. Be careful not to exercise to hard and also make sure you talk to your teacher before participating if you have asthma or any other condition that affects your breathing. Method: 1. Fill a large plastic soft drink bottle (1l) completely with water and screw the cap in place. 2. Invert the drinks bottle and place just the neck of the bottle under water in the sink (or trough). 3. Carefully unscrew the cap. n.b. whilst the neck of the bottle remains under water the bottle should stay full of water. 4. Breathe a normal breath out. The air breathed out should displace some of the water in the bottle. 5. Inset the end of a plastic tube into the neck of the bottle. 6. Mark a line to show the level of the water. 7. Empty the water bottle. 8. Fill the water bottle up to the marked line. 9. Pour the water from the water bottle into a large (1l) measuring cylinder. volume (cm3) of water in the measuring cylinder = volume (cm3) of a normal breath. 10.Count and record breaths per minute at rest. 11.Calculate breathing rate after exercise (use the formula below). 12.Exercise for 1 minute 13.Count and record breaths per minute immediately after the exercise. 14.Calculate breathing rate after exercise (use the formula below). Breathing rate at rest (cm3 per minute) = breaths per minute x volume of each breath Weaknesses:  Breath volume changes and during exercise and afterward it should be larger than at rest.  Exercise intensity and duration affect the breathing rate.  Breaths per minute is likely to decrease during the minute of measurement after exercise as the oxygen debt is repaid
  13. 13. Exexel IG Bio – Experimental Methods https://bioknowledgy.wikispaces.com/ (Chris Paine) 2.56 describe experiments to investigate the role of environmental factors in determining the rate of transpiration from a leafy shoot A bubble potometer can be used to illustrate the effects of light, wind, temperature and air humidity. Potometers come in a variety of designs, but all follow the same basic principles and usually consist of:  A length of capillary tube A bubble is introduced to the capillary. As water is taken up by the plant, the bubble moves. The distance the bubble travels in a given time is determined by the rate of water uptake by the plant.  A reservoir. By turning the tap on the reservoir, the position of the bubble can be set at the start of the experiment. Some designs of potometer use a syringe instead of a funnel with a tap.  A tube for holding the shoot. In the diagram the shoot is held in place by inserting a rubber bung in the tube. The hole in the bung through which the shoot passes must be thoroughly greased with petroleum jelly to keep it airtight. Method: 1. set up the apparatus as in the diagram above. 2. Leave undisturbed for 5 minutes or until the shoot equilibrates to the conditions. 3. Introduce a bubble into the capillary tubing by lifting the whole potometer upwards. To do this, loosen the screw on the boss and slide the boss up the clamp stand so that the capillary tube comes out of the water in the beaker. Retighten the screw on the boss. 4. Gently blot the end of the capillary tube with a piece of paper towel and an air bubble should appear in the capillary tube. 5. Loosen the screw on the boss and lower the potometer, so that the capillary tube just goes back into the water in the beaker. Retighten the screw on the boss. 6. There are two ways of taking measurements: a. Measure the distance the bubble travels in a fixed amount of time, e.g. 5 mins b. Measure the time taken for the bubble to travel a fixed distance, e.g. 3 cm. 7. Calculate the rate of movement of the air bubble (and hence rate of transpiration) using the formula given here. SI units are cm/s. Rate of transpiration (cm/s) = Distance moved by the air bubble (cm) Time taken (s)
  14. 14. Exexel IG Bio – Experimental Methods https://bioknowledgy.wikispaces.com/ (Chris Paine) Independent variables – what can be investigated:  Light - Plants covered with dark polythene bags simulate darkness and can be compared with plants covered with transparent polythene bags.  Wind - Hairdryers can simulate wind.  Humidity - Plant in a sealed polythene bag (versus one not in a bag) can be used to simulate humid conditions  Temperature - Controlling temperature is more difficult, but can be done by controlling the room temperature. n.b. use of potted plants is acceptable. The pot and the soil needs to be sealed with polythene during the investigation. The mass of the potted plant before and after the investigation is measured. The larger the loss in mass the greater the rate of transpiration. Weaknesses:  If the plant shoot is not well sealed then the air bubble will either not move or will move less  Leakages will also cause false readings due to too little or too much movement.  The plant shoot needs to be fresh if the same shoot is used over a number of days the rate of transpiration is likely to decrease as the cut shoot is slowly dying
  15. 15. Exexel IG Bio – Experimental Methods https://bioknowledgy.wikispaces.com/ (Chris Paine) 4.2 explain how quadrats can be used to estimate the population size of an organism in two different areas 4.3 explain how quadrats can be used to sample the distribution of organisms in their habitats. Method: 1. Count the number of organisms in a single quadrat 2. Count the number of quadrats that fit in the given area. 3. Use the formula below to calculate the population size: Population size = (number of organisms in a quadrat) x (number of quadrats in the area) Worked example:  the quadrat size is 50 cm x 50 cm  the field (area) is 100m x 100m  the number of daisies per quadrat is 10 Daisies in the field = (10) x (number of quadrats in the area) (population size) = (10) x (area / size of a quadrat) = (10) x (100 x 100 m2 / 0.5 x 0.5 m2) = (10) x (10,000 m2 / 0.25m2) = (10) x (40,000) = 400,000 Important considerations:  The more quadrats placed the more reliable the estimate population size  Quadrats need to be placed randomly to avoid human bias Examples and comparing sampling to actual counts: Scatter a number of beads on the floor and compare estimated the population size to the actual count. Discuss the usefulness of sampling compared with counting after completing the activity.
  16. 16. Exexel IG Bio – Experimental Methods https://bioknowledgy.wikispaces.com/ (Chris Paine) 5.6 describe a simple experiment to investigate carbon dioxide production by yeast, in different conditions This investigation provides evidence that carbon dioxide is released during anaerobic respiration (fermentation). The rate at which (carbon dioxide) bubbles are released can be used as a measure of the rate of anaerobic respiration Method: 1. Mount a side-arm test tube (or flask) using a clamp stand. 2. Add glucose solution to a side-arm test tube (or flask). The level of glucose solution should be below the level of the side-arm. 3. Add a small amount of dried yeast to the test tube and stir gently to mix. 4. Add a drop or two of oil, enough to cover the surface of the glucose/yeast mixture. This prevents oxygen exchange and hence the conditions are anaerobic. 5. Use a rubber tube to attached a glass pipette to the side arm of the test tube. 6. Place water in a second test tube. 7. Mount a the water filled test tube (or flask) at an angle using a second clamp stand. 8. Place the tip of the pipette under the water level in test tube. 9. Wait 10-20 mins for the yeast to fully activate. 10.Count the number of carbon dioxide bubbles generated per minute. The more bubbles generated the faster the rate of anaerobic respiration. Independent variables – what can be investigated:  Temperature - place the side-arm test tube in a water bath. The end of the glass pipette simply needs to be submerged in the bath.  Glucose concentration - make up different concentration of glucose solution.  pH - add the same volumes of different pH solutions (made using, acids, alkali, water) Weaknesses:  An increase in temperature causes gases to expand. If temperature is not controlled this will cause the number of bubbles generated to change dramatically.  If too much yeast/glucose solution is added, or if respiration is too fast then the mixture will flow out the the pipette along with the carbon dioxide gas.  If respiration is too fast then the oil layer maybe disrupted and respiration will become aerobic.
  17. 17. Exexel IG Bio – Experimental Methods https://bioknowledgy.wikispaces.com/ (Chris Paine) 2.81 & 2.82 Controlled experiments to demonstrate phototropic and geotropic plant growth responses n.b. students are not expected to describe the methodology, but might well be asked to analyse data in examinations Method:  Wheat, maize, oat or cress seedlings can be used to demonstrate phototropism.  Place petri dishes containing moist cotton wool and the plant material into light-proof boxes (cardboard boxes work well)  Cut a hole to create a small slit in one side of the box to allow light to be shone into the box from a single direction.  Control seedlings can either:  have aluminium caps put on their tips  kept in a shoe box without a slit for light. 2j A simple experiment to show how the sensitivity of the skin differs on finger tips, back of hand, wrist and forearm n.b. students are not expected to describe the methodology, but might well be asked to analyse data in examinations Method:  Fix the two prongs of a hairpin/toothpick/pin 5 mm apart.  Touch one or both prongs against the fingertips of a student who is looking away.  The student then has to judge whether one or two points were used and their response recorded as correct or incorrect.  Repeat using pins/prongs 1 cm apart and 2 cm apart.  Repeat the whole process on the back of the hand, wrist and forearm. Conclusion and analysis:  Use the data to identify which body area is the most sensitive.  Conclusions can be related to the number of sensory nerve endings, receptive field size and the thickness of skin. 3a A practical exercise comparing floral structure in insect- pollinated and wind-pollinated flowers n.b. students are not expected to describe the methodology, but might well be asked to analyse data in examinations  Collect suitable specimens and using teacher guidance identify the different reproductive structures.  Though insect-pollinated flowers are often available year round due to florists.  Wind-pollinated flowers are often more difficult to find at certain times of the year.

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