let's see how quickly we can cover the required practicals for Ed Exel pson GCSE biology first some tips that you should always keep in mind when answering a question on a practical in your exams remember that in many of these investigations there's an independent variable the thing you change a dependent variable the other thing that changes as a result which you measure and controls variables that could change but we keep them the same throughout in order to ensure the results are accurate always say what piece of equipment you use for each measurement don't just say measure the length of the object also add with a ruler or whatever you're using that's a mark in itself when it comes to safety we always use goggles and off and gloves when working with chemicals State the fliping obvious if you think surely they don't want me to put that put it down anyway you never know what marks you might pick up talk about the accuracy of measurements how will you reduce errors and uncertainties for example you get your eye in line with the measurement when using a ruler or measuring cylinder to reduce Parallax error another classic thing you should put down is multiple or repeat measurements or readings to calculate a mean from finally it's okay to write your answers in bullet point format in fact I recommend it as it helps you and the examiner keep track of how many different points are being made because I'm trying to fit lows in here you might see me write abbreviated points for the sake of brevity but when you write a point do it in full make sure you use proper English don't start going all Tarzan like saying heat liquid with fire more like heat the water gently on a gauze on a tripod over a buns and bur a flame also don't forget that you can see me and others from mansbury science doing these practicals for realsies on mansbury education link is in the description let's go Biology one cells and microscopes usually you'll get a thin layer of onion skin using a scalpel and tweezers place it on the microscope slide add a drop of iodine to stain the cells so they're more visible and place a cover slip on top place the slide on the stage turn the microscope's light on or if it's a mirror instead tilt it so it reflects up the condenser to the slide and make sure you start with the shortest objective lens the smallest magnification use the cause Focus knob then the fine focus knob to move the stage until the specimen is in Focus then change to a higher magnific objective lens and refocus if needed you could have a tiny graticule a tiny ruler that sits on the slide as well that lets you measure the size of cells in micrometers micro is time 10- 6 in standard form so a cell length of 2.5 micrometers is 2.5 * 10 Theus 6 M biology 2 enzymes the aim is to determine the optimum temperature or pH for an enzyme usually done with the enzyme amalay and substrate starch but it could be something else instead the independent variable is either temperature or pH which we change using water bath or buffer Solutions respectively the dependent variable is the time taken for all of the starch the substrate to be broken down measure out a set volume of the amalay and starch Solutions using a syringe or measuring cylinder then mix together and start your timer every 10 seconds remove a bit of the mixture and put a drop in the spotting tile dimple that has iodine in it will turn black initially showing there's still starch present it hasn't been completely broken down yet repeat this every 10 seconds until there is no color change at all that's the end point record this time repeat these steps using different temperatur thanks to the water bath and measure the temperature with thermometer in the test tube itself of course or we change to a different pH buffer solution plot the time for each against temperature or pH and draw a line of best fit which will be a curve we say the optimum temperature or pH is between the two lowest points biology 3 food tests finding out what nutrients are in different foods for solid food we grind using a pesin water then add distilled water to create a solution we've already seen the test for starch we just add iodine solution if it turns black or a dark purple there is starch present to test for glucose and simple sugars we add bentic solution and heat using a water bath it's a semi-quantitative test as the color can go from blue to green yellow to Orange depending on how much sugar is in the food biret reagent will turn from Blue to purple in the presence of protein to test for fats that's lipids we add cold ethanol and leave it for a minute then add this to a test tube of water if the solution goes cloudy that's a positive test for lipids biology four osmosis the aim is to find the concentration of sugar in potato cells but it could be another veggie instead cut equal size cylinders from the same vegetable using a COR chop off the ends so there's no non-permeable skin left down the excess water off the surface way using a top B balance then placing test tubes filled with different concentrations of sugar solution this is our independent variable after a set time say a day we remove them dab off the excess water again and reway calculate the percentage difference in Mass for each cylinder this is our dependent variable some will have a positive change some negative plot these against solution concentration and you should get get a straight line where the line of best fit meets the x-axis is the concentration at which no osmosis occurs no water moves in or out of the cells so that must be the same as the concentration of glucose in the T cells themselves biology five microbiology or microbial cultures we can either put spots of different bacteria cultures on AAR in a pet tradition observe how they grow over time or spread a culture all over the agar to make a lawn on which we can put drops of antibiotics or paper discs soaked in them we use aseptic technique that is piece of equipment must be sterile we can ensure this by putting the glass pet spreader or Rod Etc through a bunson flame before using we also open the disc just a little bit to work on it and also towards the Bunton Flame the heat will kill microbes and also the updraft from the flame will stop microbes from moving into the dish we only put a couple of bits of tape on the dish to secure the lid as we want air to get in to allow aerobic respiration in the bacteria if it's anerobic some real nasty stuff could be made instead we then leave them incubate for a number of days then we measure the diameters of the colonies on the AAR using a ruler or the areas in which there were no bacteria if we used antibiotics we then calculate the areas using p Pi r^ s or P pi d^2 over 4 and compare them biology 6 photosynthesis the aim is to determine the relationship between light intensity and rate of photosynthesis technically the independent variable is the distance of the plant from the light source the dependent variable is either the volume of gas made in a certain time or the number of bubbles released say in a minute we use Pond weed that's submerged in water in an inverted test tube or measuring cylinder once it's in there we want to get our little scissors and cut the stem at an angle and add sodium hydrogen carbonate in the water to promote oxygen release and of course we want to do this in a dark room we measur the distance between the light source and pondweed using amiter rule turn on the light and wait say a minute for the pondweed to acclimatize for the photosynthesis to reach a constant rate then we start counting bubbles or we measure the volume of oxygen made we repeat this at different distances then plot bubbles or volume of oxygen made against distance you should end up with a curve that looks something like this this is because light intensity follows an inverse Square Rel ation ship with distance that is if you double the distance the light intensity quarters and therefore the rate of photosynthesis should two biology 7 respiration we use a respirometer to measure the rate at which oxygen is absorbed by germinating P's the tube has a small opening at the end to allow the pressure to equilibrate a drop of food coloring is inserted in the end which will move down the tube as the pressure decreases inside alternatively the tubes could be submerged in water either way the liquid will move down the graduated tube due to oxygen being absorbed by the peas and the carbon dioxide produced being absorbed into the soda line you can also use cotton wool dipped in potassium hydroxide solution it's best to put these in a water bath at 25° C for say 30 minutes as this is the optimum temperature for respiration to take place in the peas you can then compare the end result against two other respirometers under the same conditions one with boiled peas in and another with just glass beads in as a control biology 8 field workk and using quadrats we can use a quadrat to help us estimate the population of an organism in an area use a random number generator to choose grid positions in your area to place the 1 M squ quadrat over and count the number of the chosen organism inside each you should aim to sample 10% of the total area to give an accurate estimate calculate the mean number per me squ then multiply this by the total area in me squar to give an estimate for the total population you can also combine the quadrant with a transect aign to see how the population density varies with distance in a certain direction say along a beach moving the quadrat up the transect 1 M at a time you can plot population density against distance then this could also be a kite graph don't forget the factors that affect population density caused by other living things are biotic factors say Predators if it's a non-biological Factor say the surface that the organism is on this is an abiotic factor leave a like and a comment if you found this helpful click on the cards to go 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