Dichotomous key works on the principle of asking yes-or-no questions about the organism. To identify it, the answers for that organism are then compared to existing organisms to identify it. Prepare a list of four to five questions about each of the organisms listed below to help you create your own key. For each question, make sure the answer isn’t the same for all four organisms. For example, Does the organism have cells? would be an inappropriate question, because all the organisms have cells.

brewer’s yeast (Saccharomyces cerevisiae)
E. coli bacteria (Escherichia coli)
green hydra (Hydra viridissima)
house cat (Felis catus)

Here's the list of questions, the yes-no response for each has been provided under each organism's heading in the same order:

Is it a unicellular organism?

Is it found inside the human body?

Is it visible without a microscope?

Can it produce its own food?

Can it be used in making food?

Brewer’s yeast (Saccharomyces cerevisiae)

Yes

Yes

Yes

No

Yes

E. coli bacteria (Escherichia coli)

Yes

Yes

No

No

No

Green hydra (Hydra viridissima)

No

No

Yes

Yes

No

House cat (Felis catus)

No

No

Yes

No

No

Explanation:

volcanoes (an event in the geosphere) release a large amount of particulate matter into the atmosphere. these particles serve as nuclei for the formation of water droplets (hydrosphere). rainfall (hydrosphere) often increases following an eruption, stimulating plant growth (biosphere).

organ system-an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.