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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Water is essential for life because it is an excellent solvent (dissolves many substances for transport and reactions), has a high specific heat capacity (resists temperature changes), and is the medium for most metabolic reactions.
Water makes up about 70% of the human body and is the medium in which virtually all biological reactions occur. Understanding water's properties and how they support life is important for linking together topics from transport to excretion to photosynthesis.
Key Properties of Water
Excellent solvent: many substances dissolve in water (glucose, mineral ions, amino acids, urea, oxygen, carbon dioxide). This allows them to be transported in blood plasma and to react in solution in the cytoplasm.
High specific heat capacity: water requires a lot of energy to change temperature. This helps organisms maintain a stable internal temperature and prevents large fluctuations in aquatic environments.
High latent heat of evaporation: evaporating water absorbs a lot of heat, making sweating an effective cooling mechanism.
Cohesion: water molecules are attracted to each other (hydrogen bonding), allowing water to be pulled up xylem vessels in a continuous column.
Biological Roles of Water
Role
Example
Solvent for metabolic reactions
Enzymes work in aqueous solution in the cytoplasm
Transport medium
Blood plasma transports dissolved glucose, hormones, urea and CO₂
Reactant in chemical reactions
Hydrolysis reactions (breaking down molecules); photosynthesis (water is split to release O₂)
Temperature regulation
Sweating uses evaporation of water to cool the body
Turgor pressure
Water in plant cell vacuoles keeps cells turgid, supporting herbaceous stems
Habitat
Aquatic organisms live in water; the stable temperature supports life
Water in Key Syllabus Reactions
Water appears as a reactant or product in several important reactions:
Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (water is a reactant: it is split in the light-dependent stage).
Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (water is a product).
Condensation reactions: Water is released when monomers join to form polymers (e.g. Amino acids → protein, glucose → starch).
Hydrolysis reactions: Water is used to break polymers back into monomers (e.g. During digestion).
Try a focused question (3 marks)
Explain why water is important as a transport medium in the blood. (3 marks)
Show the answer and marking guidance
Water is an excellent solvent: many substances dissolve in it.
Dissolved substances (glucose, amino acids, urea, hormones, CO₂) can be transported in the blood plasma.
Water remains liquid over a wide temperature range, maintaining flow at body temperature.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Listing water's properties without linking them to biological examples: always state the property AND its importance.
Forgetting that water is a reactant in photosynthesis, not just a medium.
Confusing specific heat capacity with latent heat: specific heat capacity resists temperature change; latent heat of evaporation enables cooling.