CONCLUSIONS 1. Why can\'t you tell if there are pathogenic organisms in water by

Question

CONCLUSIONS 1. Why can't you tell if there are pathogenic organisms in water by looking at it with the naked eye? 2. Using a microscope on a sample of water, can you tell if the cells you find are pathogenic or not? Explain your answer. 3. If you put a drop of water from a lake onto a microscope slide and don't see any microorganisms of any kind, would that guarantee that the lake had no pathogens? Explain. 4. What substance is the test (or shell) of a diatom? Why would calcium carbonate (like clam shells) be a poor choice for the covering of a photosynthetic organism? 5. How does water or a moving stream? Why? Spirogyra move? Where do you think Siprogyra is likely to spread more easily, in still

Explanation / Answer

1. The size of pathogenic organisms such as bacteria and viruses is too small ( 0.5 - 5 micrometers) to be seen by the naked eye. The human eye can see till a size of 0.1 mm and no smaller. Hence, just by looking at the water, we cannot tell if there are pathogens in it.

2. You can observe some bacteria, fungi, and parasites under a microscope but not viruses. We can use different staining techniques to find out the nature of the organism. (Gram staining: positive or negative). However, just by looking at this organism, we cannot be sure of its pathogenicity.

3. No. The organisms in the water could be smaller than the scope of the microscope and may require higher magnification. So, the lake could possibly have any number of pathogens even if no microorganisms are visible under the microscope.

4. The shell of a diatom is made of silica. It is called frustule and has two overlapping thecae. In the presence of water, carbon dioxide reacts with calcium carbonate to give bicarbonate ions. Hence, calcium carbonate would make a poor covering for photosynthetic organisms since they need carbon dioxide as a starting material for photosynthesis.

5. Spirogyra is free floating. They have flagella but don't use them for movement. Since they go with the flow of a stream, they will spread more easily in a moving stream as compared to still water.

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