An automatic street light switches itself on when the evening gets dark.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
An automatic street light switches itself on when the evening gets dark. A young sunflower plant turns its head to follow the sun across the sky. Both respond to light. Why do we call the sunflower living and the street light not?
Need a reminder? The key ideaShow
Growth and reproduction are common features of living things, but not defining ones. Metabolism, a body made of cells, and a response to the surroundings are found in every living organism and in no non-living object. Every organism senses its surroundings and responds to stimuli: physical (light, temperature, touch, water), chemical (smell, taste, pollutants) or biological (other organisms). This is consciousness. Human beings alone are also self-conscious.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. A response alone. Both respond to light, so the response cannot decide it.
Step 2. The sunflower. It is made of cells, and thousands of metabolic reactions go on in them. It also responds to light.
Step 3. The street light. It has a sensor and a switch, but no cells and no metabolism.
Answer: the sunflower has all three defining features (metabolism, cells and a response); the street light has only a response.
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