Microscopy

• Microscopes let us see things that we can’t see with the naked eye. The microscopy techniques we can use have developed over the years as technology and knowledge have improved.
• Light microscopes use light and lenses to form an image of a specimen and magnify it (make it look bigger). They let us see individual cells and large subcellular structures, like nuclei.
• Electron microscopes use electrons instead of light to form an image. They have a much higher magnification than light microscopes.
• Electron microscopes have a higher resolution than light microscopes. Resolution is the ability to distinguish between two points, so a higher resolution gives a sharper image.
• Electron microscopes let us see much smaller things in more detail, like the internal structure of mitochondria and chloroplasts. They even let us see tinier things like ribosomes and plasmids.
• You can calculate magnification by using the formula: magnification = image size/ real size.
• You can calculate the image size if you magnify object by using the formula: image size = magnification x real size
• You can calculate the real size of a magnified object by using the formula: real size = image size/ magnification
• A specimen is 50 micrometres wide. If you calculate the width of the image of the specimen under a magnification of x 100, the answer is 5000 micrometres. This is 5 in mm.
• Because microscopes can see such tiny objects, sometimes it’s useful to write numbers in standard form.
• Standard form is where you change very big or small numbers with lots of zeros into something more manegeable, e.g. 0.017 can be written 1.7 x 10^-2.
• In standard form, the number of places the decimal point moves is then represented by a power of 10 - this is positive if the decimal point‘s moved to the left, and negative if it’s moved to the right.
• A mitochrondrion is approximately 0.0025 mm long. If you write this in standard form, the answer is 2.5 x 10^-3.
• If you want to look at a specimen (e.g. plant or animal cells) under a light microscope, you need to put it on a microscope slide first. A slide is a strip of clear glass or plastic onto which the specimen is mounted.
• To prepare a slide for a microscope to view onion cells the first three steps are:

  1. Add a drop of water to the middle of a clean slide.
  2. Cut up an onion and seperate it out into layers. Use tweezers to peel off some epidermal tissue from the bottom of one of the layers.
  3. Using the tweezers, place the epidermal tissue into the water on the slide.
• When preparing a slide for a microscope to view onion cells, once you’ve placed the epidermal tissue into the water on the slide:

  1. Add a drop of iodine solution. Iodine solution is a stain. Stains are used to highlight objects in a cell by adding colour to them.
  2. Place a cover slip (a square of thin, transparent plastic or glass) on top. To do this, stand the cover slip upright on the slide, next to the water droplet. Then carefully tilt and lower it so it covers the specimen. Try not to get any air bubbles under there - they’ll obstruct your view of the specimen.
• Once you've prepared a slide for a microscope to view onion cells, the first three steps would be:

  1. Clip the slide you’ve prepared onto the stage.
  2. Select the lowest-powered objective lens (i.e. the one that produces the lowest magnification).
  3. Use the coarse adjustment knob to move the stage up to just below the objective lens.
• When using a light microscope to look at a slide, once you’ve used the coarse adjustment knob to move the stage up to just below the objective lens:

  1. Look down the eyepiece. Use the coarse adjustment knob to move the stage downwards until the image is roughly in focus.
  2. Adjust the focus with the fine adjustment knob, until you get a clear image of what’s on the slide. If you need to see the slide with greater magnification, swap to a higher-powered objective lens and refocus.
• When drawing your observations from a light microscope, you use a pencil with a sharp point. You make sure it takes up at least half of the space available and that it is drawn with clear, unbroken lines. It should not include any colouring or shading. If you are drawing cells, the subcellular structures should be drawn in proportion. Remember to include a title of what you are were observing and write down the magnification that it was observed under. Label the important features of your drawing (e.g. nucleus, chloroplasts), using straight, uncrossed lines.