UK GCSE level age ~14-16, ~US grades 9-10 Biology revision notes re-edit 20/05/2023 [SEARCH]

Microscopy: 3. Examples of using a microscope and drawing your observations - scale drawings

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INDEX biology notes: investigations using microscopes

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(3) Examples of using a microscope and drawing your observations - scale drawings

Give your drawing a title e.g. "Onion Cells" and quote the microscope magnification e.g. X50

You should record your observations as neatly and accurately as you can with a sharp pointed black pencil on white paper.

Make sure your sketch is a good size and cell walls with unbroken lines!

Don't use colouring or shading, just show the structures with neat thin lines.

Do you best to get the image features in proportion, either the adjacent cells of the layer or any subcellular structures you can see like the nucleus.

Carefully label the features of your drawing e.g. the cell features such as cell wall, nucleus, cytoplasm or chloroplasts, but not their very fine detailed structure.

Make sure your straight labelling lines match the label exactly!

See magnification calculations

Sketch - diagram of onion cells as seen under a microscope.

A scale bar has been marked on the drawing, allowing the size of a cell to be estimated.

I've just labelled the cell wall, nucleus and cytoplasm.

I've added a scale showing a length of 500 m (500 micrometres, means 10-6 in standard form).

The average size of an onion cell is ~200 m, and the average size of the onion cell nucleus is ~6 m.

The size of each cell and its nucleus do vary a bit from cell to cell.

 

How to work out a scale bar for your drawing by measuring the average real size of a cell

Note: In calculations I've used the symbol to indicate equivalence.

You fix a clear plastic ruler on the microscope slide and clip both of them onto the stage.

Select a magnification of at least X100 and refocus the microscope to obtain another clear image of the cells.

Adjust the slide so that the cells line up with the scale and count the number of cells along a line of 1 mm.

A micrometre is 1 millionth of a metre, 1.0 x 10-6 m (in standard form).

1 millimetre 1000 micrometres 1/1000th of a metre

Therefore 1 mm 10-3/10-6 = 1000 m

1 mm 103m 10-3 m

Suppose the length of 5 onion cells = 1 mm, average length 0.2 mm.

1 mm 1000 m, so the average length of one onion cell = 1000/5 = 200 m

Once you know the average length of one cell you can use it to calculate the length of the scale bar on you diagram using the formula below. Suppose you want a 500 m scale bar on your drawing.

scale bar length (for 500 m) = 500 x drawing length of cell (m) actual length of cell (m)

e.g suppose each cell you have drawn was on average 2.0 cm long on paper.

This is 20 mm, which equals 20 x 1000 = 20,000 m and the actual real average length is 200 m

scale bar length = 500 x 20,000 200 = 50,000 m

50,000 1000 = 50, so the scale bar on my drawing would be 50 mm or 5 cm long.

 

AND you can calculate the actual magnification of your drawing compared to the specimen's real size.

The magnification of your drawing = average length of drawn cell (m) average real length of cell (m)

The drawn cell is 20 mm long 20 x 1000 = 20,000 m, real length of cell is 200 m

magnification of drawing = 20,000 200 = 100X

 

For more on estimating cell size see

More on magnification & measuring the size of a cell using a graticule & stage micrometer

 

Other microscope drawings

e.g. above is a typical plant cell x 200 to x 500 magnification.

Note that is a simple black and white line drawing.

See magnification calculations

See also

Introduction to plant and animal cell structure and function gcse biology revision notes

 

Before examining root tip cells, you need to squash the root tip to break down the cellulose to free the cells and it is possible to observe mitosis in root tip cells where new growth is happening.

 

Other examples of what you can see with a powerful light microscope

Cell division - mitosis

You can stain chromosomes so that they are made visible under a microscope using a 'squashed' layer of cells under the cover slip.

This enables you to actually visually follow the stages of mitosis.

In the school laboratory you can use cells from a plant root tip (e.g. garlic cloves) to observe the various stages of the cell cycle.

A few drops of the stain (dye molecule solution) is added to the plant tissue sample and the mixture squashed on the microscope slide so that you can see the chromosomes more easily.

You can clearly observe the chromosomes being pulled apart in a cell and the subsequent formation of two cells as the mitosis cell division proceeds - fascinating viewing.

Observing the stomata, guard cells, xylem and phloem in the leaves of a plant

It is possible to observe stomata and guard cells on the leaf surface of a plant using a light microscope.

You can also do a comparison of the stomata density of the upper and lower surfaces of a leaf.

Both surfaces of the leaf are coated with a layer of clear nail varnish, allowing the leaf to dry after each coating.

Place a piece of clear cellophane tape over the top of the coated leaf and then slowly and carefully peel away the tape and varnish layer.

An impression of the leaf's surface is left on the tape.

Stick the cellophane tape onto a microscope slide and view under the light microscope.

You can sketch what you see and count the number of stomata and guard cells in a given area from the cellophane impression.

You should find there are more stomata on the underside of the leaf than on the upper side.

To explain this see Transport systems and gas exchange in plants

With a light microscope, it is also possible to observe the structures of xylem and phloem cellular structure in very thin sections of a plant's stem.

You mount a short section of stem vertically in a beaker solution of eosin dye - which diffuses up the stem staining the xylem red.

You carefully a cut a very thin cross-section of the stem, mounted on a slide and viewed under a light microscope.


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