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AS core practicals (Effect of garlic and mint on bacterial growth (IV:…
AS core practicals
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Observing mitosis
Method: cut 1cm from the tip of a growing root. It needs to be the tip as this is the growth region. Prepare a boiling tube of 1M hydrochloric acid and but it in a water bath at 60 degrees. Transfer the root tip into the boiling tube and leave it for 5 mins. Once removed use a pipette to rinse the root tip with cold water and leave to dry on a paper towel. Place the root tip on a microscope slide and cut 2mm from the very end of it. Use a mounted needle to break apart the root tip and spread the cells out thinly. Add a small drop of ethanol-orcein stain and place a cover slip over the cells and push down firmly. This makes the tissue thinner and allows light to pass through it more easily. Then look at the stages of mitosis under the microscope.
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Dissecting plant stems
Method: use a scalpel to cut a cross-section of the stem and cut the sections as thin as possible. Use tweezers to place the cut sections in water until you use them to stop them drying out. Transfer each section onto a dish containing a stain e.g TBO and leave for one minute. TBO stains lignin a blue-green color so you will see the positions of the xylem and sclerenchyma fibre. The phloem should appear a pinkish purple. Rinse off each section and place onto a slide. Place the prepared slide under a microscope and adjust the microscope until you get a clear image and then make a labelled drawing that shows the position of the xylem, phloem and sclerenchyma.
Method: Add a set volume of hydrogen peroxide to a boiling tube. To keep the pH constant, add a set amount of buffer solution to the tube. Set up a bowl of water with an upside down measuring cylinder. Then get a boiling tube and then attach a bung with a delivery tube attached and put the delivery tube in the upside down measuring cylinder. Add a set volume of one concentration of catalase to the boiling tube and quickly attach the bung and delivery tube. Record the amount of oxygen produced every 10s for the first minute using a stopwatch. Repeat this for each concentration.
Controls: Temperature, volume of enzyme, volume of substrate, concentration of substrate, pH.
Controls: length of fibre, individual mass, temperature, humidity, age of fibre.
Method: Attach the fibre to a clamp stand and hang a weight from the other end. Keep adding weights, one at a time until the fibre breaks. Record the mass needed to break each fibre - the more mass, the higher the tensile strength. Repeat the experiment with different samples of the same fibre and calculate a mean.
Method: Make up three nutrient broths containing all the essential minerals but vary the concentration of one mineral, in this case calcium ions. Make a broth of high concentration, medium concentration and low concentration. Split 9 test tubes into 3 groups and fill them with the different broths. Take 9 seedlings from the same plant and measure its mass and then put it into the top of the test tubes so that the root is suspended and put cotton wool in the opening of the tube to support it. Cover the outside of the tube with foil so no light can get to it and place the tubes near a light source and leave them for 2 weeks. Remove each plant from test tube and blot it dry and then record the new mass and calculate the mean change in mass.