Life ProcessesIntroduction
It is a lazy Sunday afternoon at Aman's house. Moti, the family dog, is fast asleep under the neem tree in the courtyard. Next to him stands Nani's croton plant, its leaves splashed with yellow and red, and beside the pot lie a few smooth stones that Aman brought back from the river.
Aman's little sister Riya asks a simple question: "How do we know Moti is alive? He isn't doing anything."
Which clue tells you that a sleeping dog is alive?
Right. We usually decide that something is alive because it moves in some way — a dog runs or breathes, and a plant like the croton grows over time. But that test has holes. A plant that is not visibly growing is still alive, and some animals can breathe without any movement you could see. So visible movement alone is not enough to define life.
Movement Too Small to See
Look closer — much closer. Every living thing is a highly organised structure: an organism has tissues, tissues are made of cells, and cells have still smaller parts inside them. Left to itself, this neat order keeps breaking down because of the effects of the environment, and if the order breaks down, the organism is no longer alive.
So a living thing must keep repairing and maintaining its structures, all the time. And since all those structures are built from molecules, it has to keep moving molecules around — movement far too small to see with the naked eye. Ask a biologist, and they will tell you that this invisible molecular movement is necessary for life.
Here is a puzzle. A virus shows no molecular movement at all until it gets inside a living cell. What does that mean?
Exactly — that is a big part of the controversy about whether viruses are alive. So here is the question for this chapter: what are the maintenance jobs that every living body keeps doing, and how does it do them? Let us explore.