SHORT ANSWER QUESTIONS
3. Write a note on “Hanging drop preparation procedure”.
Answers
Answer:The hanging drop technique is a well-established method for examining living, unstained, very small organisms. The traditional procedure employs a glass slide with a circular concavity in the centre into which a drop of fluid, containing the ‘microorganisms’, hangs from a coverslip. Cavity slides are expensive and coverslips are fragile so some pupils can find them fiddly to work with. Here we offer an alternative technique that is easy to use in the classroom. The simple substitute of a transparent film can lid, blu-tak and two microscope slides gives a cheap and practical option which allows pupils to look at living cultures easily and effectively. Algae and protozoa are of sufficient size for pupils to view them successfully using a standard school microscope. Examination of such ‘hanging drops’ can lead to useful discussions of size, variety, characteristics and importance of microorganisms and to consideration of differences between ‘plant’ and ‘animal’ cells.
Explanation:
The aim of this activity is to allow pupils to experience the magnificence of the microworld that can exist in a hanging drop and to observe some of the microorganisms that populate fresh water. When viewing a correctly focussed hanging drop preparation of the recommended mixed algae, using lenses to give x 100, x 200 or x 400 magnification, pupils are able to watch a wide variety of (mostly unicellular) algae of different sizes. Some are motile and swim across the field of view with amazing rapidity. Others, such as the desmids, which possess three perfect planes of symmetry, exhibit interesting and remarkable shapes, as do diatoms whose individual cells demonstrate astonishing intricate architecture.
You can use this method to study microorganisms in a ‘bought-in’ preparation, in pond water or in ‘home-grown’ cultures. We recommend mixed algal and mixed protozoal cultures obtained from Sciento. (We have found these Sciento cultures to be of consistently high quality both in variety and numbers of organisms.) In the case of the algae, make sure that the drop contains some obvious green material to allow observation of a variety of different cells and non-motile organisms (although a drop of ‘clear’ liquid may contain a limited variety of motile organisms such as Chlorella). When removing a sample of protozoa from the jar, it is important to select some of the solid material as the protozoa are likely to be feeding there. All algae can carry out photosynthesis. This activity can, therefore, lead into discussions of the importance of the algae in carbon fixation in rivers, lakes (or lochs), seas and oceans and also of plant plankton as the producers at the start of food webs in water environments. Diatoms are thought to be an important constituent in the formation of oil deposits.