Microorganisms – a world in a hanging drop

Do all microorganisms look the same? Can they move and change shape? What do they feed on? How small are they . . . and are they actually alive?

One way to find answers to these questions is to use a microscope to look at some microorganisms. In this practical activity, you will make a ‘hanging drop’ preparation then use a microscope to look at what is inside the hanging drop.

Teachers’ Notes

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.

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. 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.

How can you find out the actual size of the organisms?

You could ask pupils what they think is the actual size of the algae. Then see what ideas they have as to how to measure the cells. They may suggest comparing the microorganisms to the width of a hair and examining them side by side under the microscope. To do this, remove the apparatus from the stage of the microscope and quickly invert it. Then remove the slide with the film can lid. Place a hair across the drop and carefully lower a coverslip over the drop and the hair. Pupils can then compare the width of the hair with that of the microorganisms. They can estimate the width of the hair by laying it on a ruler and judging size on the millimetre scale. A reasonable approximation can be made using a clear ruler under the microscope. Another idea is to photocopy graph paper (with mm squares) onto thick acetate and cut it to make graduated ‘slides’. Then just hang the drop from these acetate graduated slides.

Suggestions for further activities

Examination of algae provides only a partial picture of the protoctista. This can be made more complete by preparing and examining hanging drops of cultures of protozoa. Pupils will see that most of the protozoa are not green, that they move by a variety of methods and that they can change shape. Pupils should be able to relate the ability to change shape to the presence or absence of a cell wall and then to their knowledge of animal and plant cell structure. Depending upon the microscope, they may be able to watch the organisms feeding and even see that some have cilia, which create ‘currents’ to ‘draw in’ foodstuffs. Pupils can measure the size of protozoa by one of the methods suggested above. These activities are suitable for use with a videocam.

Discussion from pupil questions (see Pupil guide – Questions and More questions)

1. Pupils may mention different sizes / shapes / symmetry / green colour
2. Hopefully they will answer “Yes” (because some can move) … / possibly “No” (because they do not move)
3. Green colour
4. Hopefully – colour and ability to change shape
5. Hopefully they may refer to ability to change shape linked to presence / absence of cell wall; colour linked to   ability to carry out photosynthesis)

Pupil guide (Student Sheet 25)

At first it may be difficult for you to find the microorganisms under the microscope. You may need to take a little time to practise focussing the microscope until the image becomes sharp. Gently turn the focussing knob backwards and forwards so that you can focus up and down through the hanging drop. When you do this, you may see the microorganisms in three dimensions. Look carefully at the microorganisms and draw some of them, then answer the following questions.

Questions

• How would you describe in words the size and shape of the microorganisms that you see down the microscope?
• Do you think the microorganisms are alive? Say why you think that what you do.
• What feature of the microorganisms suggests that they could carry out photosynthesis?
• When you have looked at the algae and answered the questions above, make a hanging drop of protozoa and look at them under the microscope. Draw some of them and then answer the next group of questions.

More questions

• Look for two differences between algae and protozoa and write down your answers.
• Can you link the differences in appearance or other features you can see in the algae and protozoa with what you know about the structure or function of plant and animal cells?
• Try to think of something you could do to measure the actual size of the algae and protozoa. (Clue – can you think of something very thin that you could look at with a microscope for comparison?)

Developed by Kath Crawford
SAPS Biotechnology Scotland Project & SSERC