The microscopy, osmosis and culturing microorganisms practicals, plus the magnification and percentage change calculations that come with them.
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1.In the microscopy practical, why is a stain such as iodine added to the slide?
Cells are largely transparent. The stain increases contrast so the structures can be seen.
2.Why is the coverslip lowered at an angle?
To avoid trapping air bubbles, which would obscure the view of the cells.
3.Which objective lens should you start with and why?
The lowest power lens, because it is easier to find the specimen in a wider field of view before increasing magnification.
4.How do you focus a light microscope safely?
Use the coarse focus to move the stage and lens apart while watching from the side, then focus using the fine adjustment while looking down the eyepiece.
5.What should a scientific drawing of cells include?
Clear unbroken pencil lines with no shading, labels with straight ruled lines that do not cross, a title, and the magnification used.
6.How do you calculate the total magnification of a microscope?
Multiply the magnification of the eyepiece lens by the magnification of the objective lens.
7.An eyepiece is x10 and the objective is x40. What is the total magnification?
10 x 40 = x400.
8.In the osmosis practical, what is the independent variable?
The concentration of the sugar or salt solution the potato cylinders are placed in.
9.In the osmosis practical, what is the dependent variable?
The percentage change in mass of the potato cylinders.
10.Give three control variables for the osmosis practical.
The temperature, the length of time in the solution, the volume of solution, and the size of the potato cylinders.
11.Why is percentage change in mass calculated rather than simply change in mass?
The cylinders do not all start at exactly the same mass, so percentage change allows a fair comparison between them.
12.State the formula for percentage change in mass.
percentage change = (change in mass divided by original mass) x 100
13.A potato cylinder changes from 5.0 g to 5.5 g. Calculate the percentage change.
Change = 0.5 g. 0.5 divided by 5.0 = 0.1. 0.1 x 100 = an increase of 10 per cent.
14.Why are potato cylinders blotted dry before being weighed?
To remove surface water, which would add mass and make the results inaccurate.
15.What does it mean if a potato cylinder shows no change in mass?
The solution has the same concentration as the cell contents, so there is no net movement of water in either direction.
16.Why does a potato cylinder gain mass in pure water?
The cell contents are more concentrated than the water, so water moves into the cells by osmosis.
17.In the culturing practical, what is agar jelly used for?
It is a nutrient medium that provides carbohydrates, minerals and water so colonies of microorganisms can grow.
18.Why is the inoculating loop sterilised before use?
To kill any unwanted microorganisms on it that would otherwise contaminate the culture.
19.How is an inoculating loop sterilised?
Pass it through a Bunsen burner flame until it glows red, then allow it to cool before touching the culture.
20.Why is the lid of a Petri dish taped down but not sealed all the way round?
The tape stops microorganisms from the air getting in, while the gap allows oxygen in so that harmful anaerobic bacteria do not grow.
21.Why are cultures in school laboratories incubated at 25 degrees Celsius rather than body temperature?
To reduce the risk of growing pathogens that are harmful to humans, which grow best at body temperature.
22.Why are Petri dishes stored upside down?
So that condensation collects in the lid rather than dripping onto the agar surface and disturbing the colonies.
23.How do you calculate the area of a zone of inhibition?
area = pi x radius squared. Remember to halve the diameter to get the radius first.
24.A zone of inhibition has a diameter of 12 mm. Calculate its area.
Radius = 6 mm. Area = pi x 6 x 6 = 113 mm squared to three significant figures.
25.What does a larger zone of inhibition tell you?
That the antibiotic or antiseptic is more effective at killing or inhibiting the growth of that type of bacteria.
26.Why must aseptic technique be used throughout?
To prevent contamination by unwanted microorganisms, which would make the results invalid and could be harmful.
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