Water and Inorganic Ions

Water molecules are polar and form hydrogen bonds with one another. They interact with many ions and polar solutes, allowing reactions to occur in an aqueous medium and substances to be transported in fluids such as blood plasma. Water does not dissolve all biological molecules equally; non-polar lipids are an important counterexample.

Water is also a metabolite. It is used in hydrolysis and produced in condensation; it participates in other pathways including the light-dependent reactions of photosynthesis. “It is where reactions happen” describes a solvent role, whereas “it takes part in the reaction” describes a metabolite role.

Water’s high specific heat capacity means a substantial energy transfer changes its temperature relatively little. This buffers temperature fluctuations in organisms and aquatic habitats, helping enzyme-controlled reactions remain within usable ranges. A high latent heat of vaporisation means evaporating a relatively small amount of water can remove substantial thermal energy in sweating or transpiration.

Cohesion arises from attraction between water molecules and helps maintain continuous water columns under tension in xylem. Surface tension also contributes to the properties of the air–water boundary. Cohesion is not the same as water being actively pumped by a plant or every hydrogen bond remaining permanently intact.

Hydrogen-ion concentration affects pH and can alter charges and interactions in proteins, affecting enzyme activity. Iron ions form part of haem groups in haemoglobin and are involved in oxygen binding. Sodium ions contribute to the electrochemical gradient that drives glucose co-transport in the intestine.

Phosphate ions occur in ATP and nucleotide backbones, linking energy transfer and information molecules. Ions may occur at very different concentrations, but low abundance does not imply unimportance. A useful answer explains the relevant molecule or gradient instead of giving a list of ion names without their functions.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.