The big idea: Life started in water, and it never left. Water is the medium for life.
The points to remember
- The first cells formed in water, in the early sea, more than 3.5 billion years ago.
- Water is the medium for life: the substance in which the processes of life happen.
- Life has never left water: every living cell is still mostly water inside.
Remember it as: Life began in water and still runs in water.
Real example: Shark Bay, Australia: At Shark Bay in Western Australia, mats of microbes still build rocky mounds (stromatolites) in shallow sea water, the way their ancestors did 3.5 billion years ago. Life's oldest habit is living in water.
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Think of a packet of dried beans. In the jar they sit for years and nothing happens. Soak them overnight and by morning they have swollen and started to grow.
The beans were alive all along. What was missing was water: without it, the chemistry of life cannot run.
The points to remember
- Inside every cell, the reactions of life happen in the watery cytoplasm.
- Water also moves things around the body: sugar, salts and waste dissolve in it and travel with it.
- In animals, blood plasma is about 90 % water. It carries food, oxygen and waste around the body.
- In plants, sap carries water and minerals from the roots to the leaves.
- No water, no life processes: a dry seed does nothing until it takes in water.
How much of a living thing is water? Almost all of it, except when it is lying dormant:
Real example: dry seeds: A dry bean seed is about 12 % water, so it can be stored for years. Soaked, it takes in water, its reactions start, and it grows.
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How this could be tested: No exam so far has asked about this idea on its own: it is the starting point for the rest of the topic. If it does come up, expect a short written part like the one here: outline why water is the medium for life. Two marks, two clear points.
A packet of dried beans can be stored for years. Soaked in water, the beans swell and begin to grow within a day. Outline why water is described as the medium for life.
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