Why is water essential to life? Water is the universal solvent and is essential to life, where a solvent is a substance in which another substance (known as a solute) dissolves. Most salts and minerals found inside organisms are dissolved and broken into ions by water, creating aqueous solutions. Metabolic reactions occur in solutions composed in water. Water balance inside the body requires constant homeostatic control, collectively termed osmoregulation. Water levels which go outside tolerance limits can result in a number of physiological impacts, affecting enzyme activity and potentially leading to death. The hypothalamus plays a crucial role in regulating water, as it contains osmoreceptors which detect changes to water levels of the blood. The hypothalamus also acts as a modulator, or a coordinating centre, which acts to receive and interpret information, and then create a response.

How do concentrations surrounding cells differ? Osmosis is the process by which water passively moves into and out of cells, and the concentration of a cell's watery surroundings relative to its cell contents may be;

1. Isotonic: when the surroundings are of equal concentration to the cellular contents and there is no net movement of water; water may move in and out via diffusion, however the total input and output is equal, meaning there is a net zero movement of water. The cell maintains its shape and this is considered an optimal state.

2. Hypertonic: when the surroundings are more concentrated than the environment; when solutions which are separated by a semipermeable barrier are of different concentrations, water will move across the barrier via osmosis in order to equalise the concentrations, and so in the case of hypertonic solutions, water moves out of the cell in order to dilute the external concentration which brings it closer to the cellular concentration.

3. Hypotonic: when the surroundings are less concentrated than the cellular contents, causing water to move into the cell via osmosis which means that the cell contents are less concentrated while the external concentrations are more concentrated; the cell volume increases, swells and has the potential to lyse as a consequence.