Living things require water to carry out life's functions, such as moving materials into and out of cells and keeping cool.
All living things require water in order to survive. Water is important for life because of the ways it behaves, which arise from its structure. Water consists of two hydrogen atoms bonded to one oxygen atom. Water is a polar molecule, meaning it has oppositely charged sides. One side of the water molecule is positively charged while the other side is negatively charged. Hydrogen forms weak bonds readily, especially because of its charge. These weak bonds allow water to dissolve a large number of other molecules. Water's ability to dissolve many substances has led it to be called the universal solvent. Further, hydrogen bonding leads to cohesion (the tendency of water to stick to itself) as well as adhesion (the tendency of water to stick to other materials). These properties allow for water to be drawn up great distances through thin tubes. This property is vital for plants, which draw water up through their xylem, the tubes in the plant that transport water. Materials dissolved in water can easily move within and between cells due to water's adhesion and cohesion. This allows cells to obtain nutrients, such as sugars, and eliminate wastes, such as carbon dioxide. The movement of water molecules across a semipermeable membrane from an area of lower concentration of solute to higher concentration of solute is called osmosis. Water also gives many cells their shape. For example, plant cells remain rigid because water fills a large space within them, known as a vacuole. Water's interactions with lipids also allow cell membranes to form. Water can be found in three states (solid, liquid, and gas) at ambient temperature (the temperature of the environment) on Earth. One unique characteristic of water is that solid water (ice) is less dense than liquid water. That is, molecules in cold liquid water are more tightly packed than those in ice. This means that ice floats, which provides an insulating layer on top of lakes and rivers. Organisms can survive winters under this layer of ice without being harmed. Water also has a high heat of vaporization, the amount of energy that must be added to a liquid in order to transform it to a gas. This property means that it takes large amounts of thermal energy for water to change from a liquid to a gas. For example, it takes 40,660 Joules/mole to convert liquid water to steam. This is more than five times the amount of energy required to heat water from zero degrees C to 100 degrees C (7,530 J/mol). Although this change happens as water evaporates, the process occurs much faster at higher temperatures. Furthermore, the surface from which the water evaporates becomes cooler due to this change in state. Many organisms, such as humans, depend on this special property of water by producing sweat in order to stay cool. Others, such as dogs, pant to allow the evaporation of water in saliva to cool them.