Using Microscopes to Examine Cells (WJEC GCSE Biology: Combined Science)

• Many biological structures are too small to be seen with the naked eye
• Light microscopes allow tissues, cells, and sub-cellular structures to be seen and studied

Light microscope diagram

Light microscopes, also known as optical microscopes, can be used to study cells

Preparing & viewing microscope slides

• Specimens often observed in a school laboratory are cheek cells and onion cells
• Biological stains are used to highlight structures within cells and allow more detail to be seen, e.g.
  • Methylene blue is used to stain cheek cells
  • Iodine can be used to stain onion cells
• Specimens must be prepared on a microscope slide for observation under a light microscope, e.g. to prepare a slide for viewing onion cells the following procedure might be used:
  1. Remove a thin layer of onion cells from an onion; tweezers can be used for this process
  2. Place the layer of cells onto a glass slide and add a drop of water
  3. Add a drop of iodine stain and gently lower a coverslip over the specimen
• The prepared slide can then be placed onto the microscope stage for viewing

Using a light microscope diagram

A light microscope can be used to view cells in a school laboratory

Calculating total magnification

• The magnification at which a light microscope is viewing a specimen can be calculated by multiplying the power of the eyepiece lens by the power of the objective lens:

Total magnification = power of eyepiece lens x power of objective lens

Limitations of light microscopes

• Light microscopes are limited to a maximum magnification of around 1500X, meaning that they can only be used to view whole cells and the largest cellular structures
  • The magnification of light microscopes is limited by their relatively low resolution
• Light microscopes are generally only useful for viewing 2D structures

Comparing light & electron microscopes

• For detailed examination of cells and cellular structures an electron microscope must be used
  • Electron microscopes are capable of much higher magnification than light microscopes

Comparison of electron and light microscopes table

• It is possible to prepare cheek cells and onion cells for viewing under a microscope in the school laboratory

Apparatus

• Light microscope
• 2 × glass slides
• 2 × cover slips
• Freshly cut onion
• Tweezers
• Cotton bud
• 0.1 % methylene blue solution
• Iodine solution
• Paper towel
• Mounted needle

Method

Preparing an onion cell slide

1. Remove a thin layer of onion tissue from an onion
   • Aim for the thin layer of tissue that can be found in between the layers of the onion; it should be possible to pull this layer of tissue away from a piece of onion using tweezers
   • It is essential that the layer is only one cell thick so that light passes easily through the sample and so that individual cells can be clearly seen
2. Add a drop of water to a clean microscope slide and gently place the onion tissue on the slide
3. Add a small drop of iodine stain and gently lower the coverslip over the specimen using a mounted needle
   • Lowering the coverslip slowly reduces the risk of squashing the sample and means that air bubbles are less likely to form
4. Wipe away any excess water and stain using a paper towel

Onion cell slide preparation diagram

Care must be taken to avoid smudging the glass slide or trapping air bubbles under the coverslip while preparing an onion cell slide

Preparing a cheek cell slide

• Human cheek cells are a good choice for examination under the light microscope because they are:
  • Plentiful
  • Easy to obtain safely
  • Relatively undifferentiated and so will display the main cell structures
• A cheek cell slide can be prepared as follows:
  1. Use a cotton bud to swap the inside of your cheek
  2. Swipe the cotton bud across a clean microscope slide
  3. Add a drop of methylene blue solution to the slide
  4. Gently lower a coverslip over the slide using a mounted needle
  5. Use a paper towel to remove excess stain

Cheek cell preparation diagram

Methylene blue stains some cell components a dark blue colour, making them easier to see

Risk assessment

• A hazard in an experiment is something that could potentially harm you
• A risk is the harm that could occur as a result of the risk
• A control measure is the action taken to reduce the chance of the hazard causing a problem

Examining animal & plant cells risk assessment table

Making biological drawings of cells

• To record the observations seen under the microscope a labelled biological drawing is often made
• Biological drawings are line drawings which show the features of the specimen viewed
• There are a number of rules/conventions that are followed when making a biological drawing:
  • The drawing must have a title
  • The magnification under which the observations shown by the drawing are made can be recorded
  • A sharp pencil should be used
  • Drawings should be on plain white paper
  • Lines should be clear, single lines with no sketching
  • No shading
  • The drawing should take up as much of the space on the page as possible
  • Well-defined structures should be drawn with proper proportions
  • Only visible structures should be drawn
  • Label lines should not cross or have arrowheads and should connect directly to the part of the drawing being labelled
  • Label lines should be kept to one side of the drawing (in parallel to the top of the page) and drawn with a ruler

Calculating the magnification of an image

• You may be asked to calculate the total magnification of a microscope image by multiplying the power of the objective lens by the power of the eyepiece (see above)
• You may also be asked to calculate the magnification of your drawing using the equation:

• • • Note that you will need to know the actual size of the cell that you have drawn; this can be provided by your teacher in the classroom, and will always be stated in an exam question
• You can remember the equation using an equation triangle:

An equation triangle for calculating magnification

• Rearranging the equation to find things other than the magnification becomes easy when you remember the triangle:
  • Magnification = image size / actual size
  • Actual size = image size / magnification
  • Image size = magnification x actual size
• Remember magnification does not have any units and is just written as ‘X10’ or ‘5000X’