Water as the medium for life at Higher Level: The same statement as SL. At HL you will also meet where Earth's water came from and why a planet with liquid water is the first place to look for life, so keep this page's idea in mind: no water, no processes of life.
Practise this as you read
- Explain the idea with a real case, not just a definition.
- Link every answer to what water does: reactions run in it, and it carries dissolved substances.
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The big idea: Life began in the sea and still runs in water. 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: the Pilbara, Australia: In the Pilbara region of Western Australia, rocks 3.48 billion years old hold stromatolites, the oldest clear traces of life. They were built by microbes in a shallow sea: life's first address was water.
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Brine shrimp (Artemia) lay eggs that can dry out completely and sit in a packet for years. Tip them into salt water and they hatch within a day.
The eggs were alive the whole time. Their chemistry had simply stopped, because the water was gone.
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.
Even in a human body, the tissues that do most are the wettest:
Real example: blood plasma: Blood plasma is about 92 % water. That is why it can carry dissolved glucose, oxygen, hormones and waste to and from every cell.
How this could be tested: In May 2025 one Paper 2 question opened with the line "Water is the medium in which most processes of life occur", then asked about water's physical properties. No question so far has asked about this idea on its own; if one does, expect a short part like the one here, with two clear points for two marks.
Dried brine shrimp eggs survive in a sealed packet for years, but hatch within a day of being added to salt water. Outline why water is described as the medium for life.
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