Microscopy Practicle

• Many biological structures are too small to be seen by the naked eye.
• Optical microscopes are an invaluable tool for scientists as they allow for tissues, cells and organelles to be seen and studied.
• Light is directed through a thin layer of biological material (containing the tissue(s), cell(s) or organelle(s) to be observed) that is supported on a glass slide.
• This light is focused through several lenses so that an image is visible through the eyepiece.
• Specimens must be prepared on a microscope slide to be observed under a light microscope.
• This must be done carefully to avoid damaging the biological specimen and the structures within it.
• The most common specimens to observe under a light microscope are cheek cells (animal cells) and onion cells (plant cells).
• Preparing a slide using a liquid specimen involves adding a few drops of the sample to the slide using a pipette, covering the liquid/smear with a coverslip and gently pressing down to remove air bubbles, and wearing gloves to ensure there is no cross-contamination of foreign cells.
• Preparing a slide using a solid specimen involves cutting a small sample of the tissue, peeling away or cutting a very thin layer of cells from the tissue sample to be placed on the slide (using a scalpel or forceps), and treating the tissue sample with chemicals to kill/make the tissue rigid.
• When using an optical microscope, it is easier to find what you are looking for in the field of view by starting with the low power objective lens.
• The thin layers of material placed on slides can dry up rapidly, so adding a drop of water to the specimen (beneath the coverslip) can prevent the cells from being damaged by dehydration.
• Care must be taken to avoid smudging the glass slide or trapping air bubbles.
• An eyepiece graticule and stage micrometer are used to measure the size of the object when viewed under a microscope.
• Producing biological drawings of what you see under the microscope is a key skill, and the key is not to try to be too artistic with your drawings – they are supposed to be scientific so make sure you follow the rules.
• Biological drawings should be as large as possible – aim to take up at least half of the space available on the page with your drawings.
• The size of cells or structures of tissues may appear inconsistent in different specimen slides, as cell structures are 3D and the different tissue samples will have been cut at different planes resulting in inconsistencies when viewed on a 2D slide.
• Optical microscopes do not have the same magnification power as other types of microscopes and so there are some structures that cannot be seen.
• The treatment of specimens when preparing slides could alter the structure of cells.