IB Biology A1.1.1 Water as life's medium
Water sheltered early life and remains the main cellular medium, enabling dissolved substances to move, react and support temperature control in organisms.
- Syllabus
- First assessment 2025
- Objective
- ib_null_biology_001_001_001
- Level
- All levels
Quick summary
The earliest cells originated in water, where the surrounding ocean provided a stable liquid environment and reduced exposure to damaging ultraviolet radiation. Water still forms much of cell mass and is the medium in which most cellular reactions occur. Its polarity allows ions and many polar molecules to dissolve, so reactants can move, collide and be transported in cytoplasm, blood and plant vascular tissues. Hydrogen bonding keeps water liquid across biologically useful temperatures and gives it a high heat capacity, helping organisms and habitats resist rapid temperature change. Evaporation also requires substantial energy, so sweating and transpiration can remove heat. In exam answers, connect each biological role to the relevant molecular property rather than listing uses of water without explaining why they are possible.
Concept visual
One diagram for the relationship students need to remember.
Common mistakes
Describing water only as a habitat without explaining its role as a cellular reaction medium.
Listing transport, metabolism and cooling without linking each role to a relevant property of water.
Claiming that water creates energy during evaporation rather than absorbing thermal energy.
Treating UV shielding as the only reason early life originated in water.
Key takeaways
- Water provided early life with a stable liquid environment and some protection from ultraviolet radiation.
- Most cellular reactions occur in aqueous solution because dissolved reactants can move and interact.
- Water transports dissolved substances through cytoplasm, blood and plant vascular tissues.
- Hydrogen bonding supports thermal stability and makes evaporation an effective cooling process.
