Cell Biology
Microscopy and Magnification
AQA GCSE Biology
Light and electron microscopes
- An electron microscope has a higher magnification and a higher resolution than a light microscope.
- So it shows more detail: sub-cellular structures such as ribosomes and mitochondria are seen with an electron microscope.
- Cells seen with an electron microscope look more magnified, have more detail and show more sub-cellular structures.
Magnification
magnification =
size of imagesize of real object
- Rearranged for the real size: size of real object = size of imagemagnification
- Rearranged for the image size: size of image = magnification × size of real object.
- Write the equation out in words before substituting.
- Magnification is written as × and a number, with no unit. Never write a unit after a magnification.
- Measure an image with a ruler in mm.
- A very small real size can be written in standard form.
- How many times smaller one cell is than another: divide the larger size by the smaller, both in the same unit.
Method: finding the real size
- Measure the image in mm.
- Write size of real object = size of image ÷ magnification.
- Substitute and divide.
- Convert the real size to µm: × 1000.
- Measure the image and the real size in the same unit before dividing.
- Convert the real size to µm by multiplying by 1000.
Units of length
| Larger unit | Equals |
|---|---|
| 1 cm | 10 mm |
| 1 mm | 1000 µm |
| 1 µm | 1000 nm |
| 1 cm | 10 000 µm |
| 1 mm | 1 000 000 nm |
- Larger unit to smaller unit: multiply by the factor.
- Smaller unit to larger unit: divide by the factor.
Parts of the light microscope
| Part | Function |
|---|---|
| Eyepiece | the lens you look through |
| Objective lens | to magnify the image of a specimen |
| Focus knob | to focus the image |
| Stage | the slide is placed on it |
| Mirror | to direct light into the viewer’s eye |
- Never write the objective lens is the lens you look through.
- Never write that the focus knob changes the magnification.
- No cells seen: not focussed, slide not in the correct position, or looking at an air bubble.
Required practical 1: observing cells with a light microscope
Required practical 1
- Cut an onion into pieces with a sharp knife, on a chopping board.
- Peel off a thin layer of cells with forceps, to allow light to pass through the cells.
- Place the layer on a microscope slide.
- Add drops of iodine solution with a pipette, to stain the parts of the cell so the nucleus, cytoplasm and cell wall can be seen.
- Lower a cover slip at an angle, to prevent air bubbles.
- Place the slide on the stage and adjust the focus knob.
- Draw the cells, label the cell parts, and include the magnification.
- Drawing: continuous lines, do not shade, complete the cell walls, do not draw overlapping cells, label cell parts.
- Iodine: iodine solution is an irritant, so wear gloves.
- Knife: may cut skin, so cut on a chopping board.
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