AQA GCSE Combined Science Foundation (8464), Foundation tier · Biology › Cell biology › Cell structure
Practise Microscopy. 11 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How light and electron microscopes compare, how the electron microscope improved our understanding of cells, and how to use magnification = size of image ÷ size of real object. It includes the microscopy required practical: preparing a slide, using a light microscope and drawing cells with a magnification or scale.
Key facts
magnification = size of image ÷ size of real object
real size = size of image ÷ magnification
total magnification = eyepiece lens × objective lens
1 mm = 1000 µm; 1 µm = 1000 nm
Resolution: the ability to distinguish between two points that are close together
Light microscope: up to about ×2000; can view living cells in colour; cheap
Electron microscope: much higher magnification and resolution; shows ribosomes and fine detail; specimens dead
A stain (e.g. iodine solution) makes cell structures easier to see
Notes
Light and electron microscopes
A light microscope uses light and glass lenses. It magnifies up to about ×2000 and shows detail down to about 0.2 µm. It is cheap and can show living cells in colour.
An electron microscope uses a beam of electrons. It has a much higher magnification (up to about ×2 000 000) and a much higher resolving power (detail smaller than 1 nm).
Resolution is the ability to distinguish between two points that are close together. Higher resolution means more detail can be seen.
Electron microscopes are large and expensive, and specimens must be dead because they are viewed in a vacuum.
How microscopy developed
Light microscopes were first used in the 1600s and showed that living things are made of cells. Better light microscopes later showed structures such as the nucleus and chloroplasts.
Electron microscopes were developed in the 1930s, so cells could be studied in much finer detail.
Magnification calculations
diagram
magnification = size of image ÷ size of real object
Rearranged: real size = image size ÷ magnification, and image size = magnification × real size.
Cover the one you want: magnification = image ÷ real size; real size = image ÷ magnification; image = magnification × real size.
Both sizes must be in the same unit (1 mm = 1000 µm). Magnification has no unit: write it as, e.g., ×400.
Total magnification of a light microscope = eyepiece lens magnification × objective lens magnification.
Required practical: looking at cells
diagram
Place a thin layer of tissue (e.g. onion epidermis) on a slide, so light can pass through it. Add a drop of stain (e.g. iodine solution) to make structures such as the nucleus easier to see.
Lower a coverslip at an angle to avoid trapping air bubbles.
Start with the lowest-power objective lens. Focus with the coarse focus, then the fine focus. Then switch to a higher power and refocus using only the fine focus.
A light microscope. Focus with the lowest-power objective lens first.
Draw with a sharp pencil: clear continuous lines, no shading, ruled label lines, and a magnification or scale bar.
How to answer each type of question
Calculate the real size of a cell
2 to 3 marksGrade 5
Write the equation, rearranged: real size = image size ÷ magnification.
Convert the image size into the unit the answer needs (usually µm).
Substitute and calculate, and give the unit.
Example. A student draws a cheek cell. The drawing of the cell is 45 mm wide. The magnification of the drawing is ×750. Calculate the real width of the cheek cell in micrometres (µm).
Add a stain, then a coverslip lowered at an angle.
Describe focusing: lowest power first, coarse then fine focus, then a higher power.
Example. Describe how a student could prepare a slide of onion cells and view the cells with a light microscope.
Show the model answerHide the model answer
Any four from: peel a thin layer of onion epidermis and place it flat on a slide (1) add a drop of iodine solution / a stain (1) lower a coverslip at an angle to avoid air bubbles (1) start with the lowest-power objective lens (1) focus with the coarse focus, then the fine focus (1) then switch to a higher power and refocus with the fine focus (1)
Don’t lose marks
Forgetting to convert mm to µm before dividing, giving an answer 1000 times too big or too small.
Putting a unit on magnification, or leaving the unit off a size.
Confusing resolution with magnification. Resolution is about telling two close points apart, not about size.
Using the coarse focus on high power, which can crush the slide. On high power use only the fine focus.
Shading a drawing, using sketchy lines, or drawing label lines that do not touch the structure.
More tips
Memory tricks
'I AM': Image size = Actual size × Magnification. Cover the quantity you want to find.
Convert everything to µm before you divide: it avoids most slips.
Sense check: the real size of a cell should usually come out between about 1 µm and 100 µm.
Magnification is about size; resolution is about detail. Magnifying a blurred image does not show more detail.
Exam technique
Write the equation first: it is often worth a mark and helps you rearrange it correctly.
Show the unit conversion as a separate line, e.g. 45 mm = 45 000 µm.
If the question asks for standard form or a number of significant figures, do it in your final answer.
In 'explain' questions about electron microscopes, use the words 'resolution' and 'magnification'.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State what magnification meansHow many times bigger the image is than the real object.
Grade 4
Calculate the total magnification of a microscopeMultiply the eyepiece lens magnification by the objective lens magnification, e.g. ×10 and ×40 give ×400.
Grade 4
Compare light and electron microscopesElectron microscopes have much higher magnification and resolution; light microscopes can show living cells in colour.
Grade 5
Use magnification = image size ÷ real sizeRearrange the equation to find the magnification, the image size or the real size.
Grade 5
Prepare a slide and focus a microscopeThin sample, stain, coverslip; start on the lowest power and focus with the coarse, then the fine, focus.
Required practical:Microscopy (method, variables and exam tips)
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Microscopes are used to look at cells.
(a) Which equation is correct? Tick (✓) one box.[1]
magnification = size of image ÷ size of real object
magnification = size of real object ÷ size of image
magnification = size of image × size of real object
magnification = size of image − size of real object
(b) A light microscope has an eyepiece lens with a magnification of ×10 and an objective lens with a magnification of ×40. Calculate the total magnification.[1]
(c) The image of a cell is 20 mm wide. The magnification is ×400. Calculate the real width of the cell in mm.[2]
(d) Give the real width of the cell in micrometres (µm). 1 mm = 1000 µm[1]
Show the answer and mark scheme
(a)Answer: magnification = size of image ÷ size of real object
(b)Answer: ×400
(10 × 40 =) 400 (×)
(c)Answer: 0.05 mm
20 ÷ 400
0.05 (mm)
(d)Answer: 50 µm
50 (µm)
Question 2Medium8 marks
A student used a light microscope to observe onion epidermis cells.
(a) Describe how the student should prepare a slide of onion epidermis cells.[3]
(b) Give one reason why the student added a stain to the cells.[1]
(c) The student made a drawing of some of the cells. Give two rules the student should follow when making a scientific drawing.[2]
(d) The student also looked at human cheek cells. Give two structures the student would see in the onion cells that would not be seen in the cheek cells.[2]
Show the answer and mark scheme
(a)
peel off a thin layer of epidermis (using forceps)
place it flat on a (drop of water on a) microscope slide
add a drop of iodine solution / stain
lower a coverslip at an angle / carefully to avoid trapping air bubbles
(b)
to make the cells / nucleus easier to see / to increase contrast
(c)
use a sharp pencil
use clear, continuous lines (no sketching)
no shading / colouring
draw only what is seen / keep structures in proportion
draw label lines with a ruler / label lines should not cross