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Biology Required Practicals Paper 1 (Effect of pH on Enzyme Activity…
Biology Required Practicals Paper 1
Microscopy
Add a drop of water to the middle of a clean slide
Use tweezers to remove a layer of epidermal tissue from an onion
Place the epidermal tissue onto the drop of water
Add a drop of iodine solution - iodine is a stain which highlights objects in cells by adding colour to them
Place a cover slip on top - stand the cover slip upright on the slide, then carefully tilt and lower it onto the slide, avoiding any air bubbles as this will obstruct the view
Using the Light Microscope
Clip the prepared slide onto the stage
Select the lowest powered objective lens and use the coarse adjustment knob to move the stage just below the objective lens
Look down the eyepiece and use the coarse adjustment knob to move the stage down until the object is roughly in focus
Adjust the focus with the fine adjustment knob until the image becomes clear
If it won't focus, swap to a higher powered objective lens and refocus
Preparing the slide
Culturing microorganisms
Bacteria and some other organisms are grown in a 'culture medium' which contains the carbohydrates, minerals, proteins and vitamins they need to grow
The culture medium used can be a nutrient broth solution or solid agar jelly
Bacteria grown on agar plates will form visible colonies on the surface of the jelly, or will spread out to give an even covering of bacteria
How to make an agar plate
Hot agar jelly is poured into shallow round plastic dishes - petri dishes
Inoculating loops can be used to transfer microorganisms to the culture medium, or a sterile dropping pipette and spreader can be used to get an even covering
Microorganisms then multiply
The cultures aren't kept at above 25 degrees as harmful pathogens are more likely to grow at this temp
Investigating the effect of antibiotics on bacterial growth
Place paper disks soaked in different types or concs of antibiotics on an agar plate with an even covering of bacteria - leave space between disks
The antibiotic should diffuse into the jelly, so non-resistant antibiotic strains will die, leaving a clear area - an inhibition zone
A control should be used - a paper disk with no antibiotic - instead sterile water, so that there are unknown variables influencing the inhibition of bacterial growth
Use uncontaminated cultures
Unwanted microorganisms will affect the results and potentially result in the growth of pathogens
Petri dishes and culture medium to be sterilised before use to kill any unwanted microorganisms
Inoculating loop to be passed through a hot flame
Lid of petri dish to be lightly taped on to stop microorganisms from the air entering the dish
Petri dish stored upside down to prevent drops of condensation falling onto the agar surface
Calculating the inhibition zone
Look at the sizes of the zones - the larger the zone the more effective the antibiotic
Use the diameter to compare sizes
Can also be used to look at the area of a colony of bacteria
Osmosis
Cut up a potato into cylinders and pour different sugar solutions into them - one should be water and another very concentrated, and a few inbetween
Measure the mass of the cylinders, then place them in the beakers
After 24 hours, take them out, dry them, and measure their masses again
If they have taken in water by osmosis (the pure water) then they will have increased in mass
If they weigh less (the very concentrated one) then they have lost water by osmosis
Dependent variable
Mass of the potato
Independent variable
Concentration of sugar solution
Control variable
All other variables eg. temperature
Effect of pH on Enzyme Activity
Amylase catalyses breakdown of starch to maltose - easy to test for starch using iodine solution
Add a drop of iodine solution into every well of a spotting tile
Place a Bunsen on a heat proof mat with a tripod and gauze, and heat a beaker of water until it is 35 degrees (use a thermometer
It is best to keep the temp of the water constant throughout the experiment
Use a syringe to add a set amount (1cm^3) of amylase solution and a set amount (1cm^3) of buffer solution with pH 5 to a boiling tube
Put the tube into the beaker and wait for 5 mins
Use a different syringe to add 5cm^3 of a starch solution to the boiling tube
Mix the contents and start a stopwatch
Record how long it takes using continuous sampling for the amylase to breakdown all of the starch
Use a dropping pipette to take a fresh sample very 30 seconds and put a drop into a well
When the iodine remains browny-orange, starch is no longer present
Do this with different pH values of the buffer solution
Rate of photosynthesis
The rate at which the pond wed produces oxygen corresponds to the rate of photosynthesis
A source of white light is placed at a set distance away from the pondweed (use a ruler to measure)
Leave the pondweed to photosynthesise for a certain amount of time, and as it photosynthesises oxygen released is collected in a capillary tube
At the end, the syringe is used to draw the gas bubble in the tube alongside a ruler and the length of the gas bubble is measured - this is proportional to the amount of O2 produced
Repeat the experiment twice at the same distance to get a mean result, then change the distance
As the light is further away, the rate of photosynthesis should decrease
Food tests not included