Preparing Slides There are 4 different ways of preparing slides – the one you pick is dependent on the specimen and resolution required. The four types of preparation are …. Dry mount Wet mount Squash slides Smear slides

Dry mount Used with whole specimens (e.g. a strand of hair) or when a specimen is ‘sectioned’ (cut into very thin slices – e.g. muscle tissue or leaf tissue) Wet mount For specimens suspended in liquid such as oil or water (e.g. amoeba) It is essential to put a cover slip at an angle and lower it onto the specimen to avoid creating air bubbles which interfere with the view. A needle mount can help with the lowering of the cover slip.

An Air bubble we wish to avoid

Squash slide First made with a wet slide then a lens tissue is used to gently squash the sample down. Cover slips are very delicate so sometimes the same effect is also achieved using two slides together. This technique is used with root tip squashes to observe mitosis. Smear slide The edge of a slide is used to smear the sample across the other slide before a cover slip is put over the top (this is often used with blood samples)

Staining samples When using a light microscope, white light is used to illuminate the sample. Samples tend to absorb little light so (e.g. the cytoplasm is often transparent as it contains a lot of water) Stains can be used to increase the contrast between different parts of the cell and make it easier to observe different forms.

Staining continued… Methylene blue is a positively charged dye which is attracted to negatively charged molecules It is used to stain the nucleus as DNA has negative regions which the dye attaches itself to. Iodine will attach to starch which makes it useful for identifying plant cells.

Differential Staining This is used to distinguish between 2 different types of organisms (very useful when studying bacteria which can be hard to identify) It can also be used to different organelles within an organism Example – Gram Stain Technique Is used to separate bacteria into two different groups (Gram +Ve and Gram –Ve) Crystal violet is used with iodine (helps to fix the dye) and then the bacteria sample on the slide is washed with alcohol. The Gram +Ve bacteria hold onto the dye and stain blue/purple but the Gram –Ve bacteria which have thinner cell walls lose all the stain. Lastly a counterstain is used (safranin dye) which dyes the Gram –Ve bacteria red End result the two different types of bacteria appear different colours and can be identified.