Changes in microscope technologySpec 1.3
Early light microscopes could show cells, but not much detail inside them. Two ideas matter here. Magnification is how many times bigger the image is than the real object. Resolution (resolving power) is how well a microscope can show two points that are close together as separate points.
| Light microscope | Electron microscope | |
|---|---|---|
| Uses | Light and glass lenses | A beam of electrons |
| Magnification | Lower | Much higher |
| Resolution | Lower | Much higher |
| What can be seen | Cells, nucleus, and some larger structures such as chloroplasts | Fine detail, such as the internal structure of mitochondria and chloroplasts, and ribosomes |
Because electron microscopes have a higher magnification and higher resolution, we can see sub-cellular structures in much more detail and with more clarity than in the past. This has increased our understanding of what each structure does, because we can now see how its structure is related to its function.
Number, size and scale: estimationsSpec 1.4
Biology deals with very big and very small numbers, from the number of cells in a body to the size of a ribosome. You need to understand number, size and scale so you can compare sizes sensibly.
An estimation is an approximate answer, usually found by rounding numbers to one significant figure. Estimations should be used:
- to check that the answer to a calculation is sensible, for example that a cell is not 5 metres long
- when an exact value is not needed, or is impossible or too slow to find, for example estimating the number of cells in a sample by counting a few areas and scaling up
- to make quick comparisons between sizes, for example roughly how many times bigger one cell is than another
Units: milli, micro, nano and picoSpec 1.5
Cells are far too small to measure in metres, so we use smaller units. Each unit below is 1000 times smaller than the one before it.
| Prefix | Symbol | Power of ten | Unit of length |
|---|---|---|---|
| milli | m | 10⁻³ | millimetre (mm) |
| micro | µ | 10⁻⁶ | micrometre (µm) |
| nano | n | 10⁻⁹ | nanometre (nm) |
| pico | p | 10⁻¹² | picometre (pm) |
To go to a smaller unit, multiply by 1000: mm to µm, µm to nm, nm to pm. To go to a larger unit, divide by 1000. So 1 mm = 1000 µm, 1 µm = 1000 nm and 1 nm = 1000 pm.
Standard form
Standard form writes a number as a number between 1 and 10 multiplied by a power of ten. For example, 0.000 045 m is 4.5 × 10⁻⁵ m, and 3 200 000 is 3.2 × 10⁶. A negative power means a small number. You must be able to do calculations with numbers written in standard form.
Calculating with standard form
A cell is 2.5 × 10⁻⁵ m long. It is viewed at a magnification of ×400. Calculate the length of the image in mm.
- image size = real size × magnification
- 2.5 × 10⁻⁵ × 400 = 1.0 × 10⁻² m
- 1.0 × 10⁻² m = 0.01 m
- 0.01 m × 1000 = 10 mm
Answer: 10 mm
Core practical: using a microscopeSpec 1.6
In this core practical you investigate biological specimens with a light microscope, calculate magnification and make a labelled scientific drawing. A common specimen is onion epidermis, which is a thin, single layer of plant cells.
Method
- Put a drop of water or stain (for example iodine solution) on a clean slide.
- Peel a thin layer of epidermis from the onion and lay it flat on the drop with forceps.
- Lower a coverslip on at an angle using a mounted needle, so air bubbles are not trapped.
- Clip the slide on the stage. Select the lowest power objective lens and use the coarse focus to bring the cells into focus, then the fine focus to make them sharp.
- Move to a higher power objective lens and focus again with the fine focus only.
- Draw what you see and label it.
Scientific drawings
- use a sharp pencil and make clear, continuous lines, with no shading or colouring
- draw large, filling most of the space, and only draw what you can see
- label with straight lines drawn with a ruler that touch the structure they name, and do not cross the labelling lines
- give the drawing a title and write the magnification
Total magnification of a microscope is the eyepiece lens magnification × the objective lens magnification. For a drawing, measure the image with a ruler and use the equation above. Rearranged: size of real object = size of image ÷ magnification.
Finding the real size
A student draws a cell. The drawing is 54 mm wide and the magnification is ×300. Calculate the real width of the cell in µm.
- real size = image size ÷ magnification
- 54 ÷ 300 = 0.18 mm
- 0.18 mm × 1000 = 180 µm
Answer: 180 µm
Finding the magnification
A cell is 90 µm long in real life. In a drawing it is 45 mm long. Calculate the magnification.
- Convert to the same unit: 45 mm × 1000 = 45 000 µm
- magnification = image size ÷ real size
- 45 000 ÷ 90 = 500
Answer: ×500
Quick check
Why can electron microscopes show more detail than light microscopes?
Show answer
They have a higher magnification and higher resolution.
How many nanometres are there in 1 µm?
Show answer
1000 nm.
Convert 0.05 mm into µm.
Show answer
50 µm (0.05 × 1000).
State the equation linking magnification, image size and real size.
Show answer
magnification = size of image ÷ size of real object.
Why is a coverslip lowered at an angle?
Show answer
To avoid trapping air bubbles.