Observing protoctista in water using the hanging drop technique
Class practical
This practical has been adapted with permission from information on the Science and Plants for Schools (SAPS) website (see www.saps.org.uk). The original practical is available on the SAPS webpage A world in a hanging drop.
With this technique, your students can observe the variety of microscopic organisms found in pond water or ‘bought-in’ algal and protozoal cultures and develop microscopy skills as well as an appreciation of size and scale. It provides an opportunity to develop appropriate vocabulary to describe and compare these organisms – in terms of their size, shape, structures, characteristics and motility. Observing these organisms could challenge existing ideas about the differences between ‘plants’ and ‘animals’ and reinforce the key ideas about feeding mechanisms and cell structure. It provides a context for initiating discussion about the importance of algae in food chains and as oxygenators for aquatic habitats.
Lesson organisation
This will depend on the number of microscopes you have. Each student, or each pair of students, should set up a slide to view during the course of the lesson. If you have a microscope with a video feed, students could bring their slides in turn to be viewed by the whole class via your projector.
Apparatus and Chemicals
For each group of students:
Access to a microscope with up to 400X magnification
Microscope slide, clean, glass, 2
Film can lid, transparent, 1
Blu-tack
Pipette, 1
For the class – set up by technician/ teacher:
Sources of microscopic organisms:
Pond water (Note 1)
Mixed algal culture, from Sciento (see Suppliers)
Mixed protozoal culture, from Sciento, as above
Acetate with photocopies of graph paper squares (1 cm square of 1 mm squares) (Note 2)
Health & Safety and Technical notes
Take normal hygiene precautions if handling pond water (Note 1).
1 Pond water could contain disease-causing microbes so take hygiene precautions and ensure children cover cuts or abrasions with waterproof plasters or gloves. Students will need to wash their hands with appropriate cleansers before leaving the teaching room. Refer to CLEAPSS Supplementary Risk Assessment SRA 09 09/06 for more details.
2 Cut out some 1 cm square sections of 1 mm grid graph paper. Stick them to a large sheet of paper and photocopy. Cut out so that each 1 cm square is part of a piece of acetate approximately the size and shape of a microscope slide
Procedure
SAFETY:
Apply good hygiene practices when handling pond water or cultures of aquatic organisms.
Preparation
a Ensure that your cultures are healthy and offer a reasonable density of active organisms to view.
Investigation
b Attach the transparent film can lid to a microscope slide with some blu-tack. Put two small pieces of blu-tack on the rim of the film can lid.
c Use a pipette to collect a sample of pond water, algal culture or protozoal culture. If there is green stuff in the sample, make sure you pick some up. If there is some solid material in the sample, make sure you pick up a little bit as the organisms might be feeding on it.
d Take a second microscope slide and put one drop of your sample into the middle of the slide.
e Turn the slide over quickly and stick it to the blue-tack on the film can lid, so that the drop hangs into the space in the middle.
f Place the slides on the stage of the microscope.
g Focus the microscope and describe (or draw) what you see.
h If you want to know how big your tiny organisms are, use a piece of acetate with a 1 cm square of 1 mm grid squares in place of the upper microscope slide. Then you can measure them against the 1 mm grid.
Teaching notes
The hanging drop technique is a well-established method for examining living, unstained, very small organisms. The traditional procedure involves a glass slide with a central circular concavity into which a drop of fluid ‘hangs’ from a coverslip. Cavity slides are relatively expensive, and coverslips are very fragile. The apparatus described in this procedure is cheaper, more robust and less fiddly to work with.
There is a wide variety of (mostly unicellular) algae of different sizes. Some are motile and move across the field of view at remarkable speed. The desmids exhibit interesting and remarkable shapes (with three planes of symmetry) and the individual cells of diatoms demonstrate intricate architecture.
Comparing algae with protozoa should show that most protozoa are not green, they can move by a variety of methods and they can change shape. Relate the changeability of outline to the presence or absence of a tough cellulose cell wall.
Depending on the quality of your microscopes, you may even see organisms feeding and might notice that some have cilia (tiny moving hairs) that create currents in the water to draw in foodstuffs.
Health & Safety checked, May 2009
Web links
SAPS resources This link is to the central page of the Science and Plants for Schools (SAPS) website that connects to all their resources for teaching. The SAPS webpage A world in a hanging drop contains details of this procedure. (Websites accessed October 2011)