Sylvia Earle, an American marine biologist, once said ‘there’s plenty of water in the universe without life, but nowhere is there life without water’. What did she mean by this? She’s referring to water being the most important biological molecule on Earth. Losing just 4% of the water in our body leads to dehydration, and a loss of 15% could be fatal. A person could survive a month without food but wouldn’t survive even 3 days without water. Furthermore, it provides an environment for aquatic organisms (3/4 of the planet is covered in water). Clearly water is vital for survival, but what is it about water that makes every organism totally dependent on it?
To start we must understand what gives water its unique properties. Water is a simple molecule composed of two small hydrogen atoms with a slight positive charge and one larger, negatively charged oxygen atom. When the hydrogens bind to the oxygen, an asymmetrical molecule is created, with a positive charge on one side and a negative charge on the other side. This uneven distribution of charge is called polarity and dictates how water interacts with other molecules. Due to water’s polarity, adjacent molecules are attracted to each other and a hydrogen bond forms between them. Individually, the hydrogen bonds are weak, but collectively they make water very stable.
Hydrogen bonding means molecules stick together – a property called cohesion. Cohesion is what makes transpiration possible: the movement of water up the roots and up through the xylem occurs as a result of them sticking together, moving in a continuous column known as the transpiration stream. Adhesion is another consequence of hydrogen bonding. It facilitates attraction between water and lignocellulose molecules in the sides of the xylem so that molecules travel up the sides of the vessels efficiently.
Water requires a substantial amount of heat energy to raise its temperature (it has a high specific heat capacity of 4184J/kg°C). This is crucial as it permits living organisms to maintain particular temperatures in order to optimise enzymes activity. Xerophytes are a type of plant that thrive in drought conditions - perhaps the most famous example is cacti. When these plants are fully hydrated they become over 90% water. Water’s high specific heat capacity becomes essential when thinking about how organisms like this are able to survive in desert environments. During the daytime, when temperatures are high, heat is absorbed into the plant’s tissue, increasing its internal temperature. However, its thermal inertia prevents the plant overheating. At night, this heat gradually radiates back out, keeping the plant tissue from freezing.
Additionally, water has a high latent heat of vaporisation (it requires a lot of heat energy to convert water from its liquid state into its gaseous state). This property makes water useful when involved in cooling mechanisms. For example, when humans sweat, a relatively large amount of heat energy is used to evaporate water from our skin’s surface and in doing so heat energy leaves our bodies. Dogs pant and in a similar manner water evaporates off their tongues (that have evolved to have a large surface area), removing heat energy from their bodies on hot days. There is little risk of dehydration as small amounts of water require large amounts of energy to evaporate.
Furthermore, water is an excellent solvent for ions and polar molecules (such as sugars and glycerol) because of its polarity.
This property of water is very important as all of the substances which are essential for the functioning of cells and whole organisms (such as glucose, amino acids, vitamins, fats, respiratory gases) are transported in solution. Similarly, all metabolic reactions, catalysed by enzymes, occur in solution. For example, in phagocytosis, phagocytes (a type of white blood cell) bind to pathogens and engulf them in a phagosome. Pathogens are broken down by hydrolytic enzymes stored in lysosomes. This process is essential to the functioning of the immune system in order to prevent infections.
Water also contributes to the formation of phospholipid membranes that surround the majority of cells as well as certain organelles. The phospholipids have two distinct components: a phosphate hydrophilic ‘head’ (polar) and a hydrophobic (non-polar) ‘tail’. Due to this, the polar heads interact with water, while the non-polar tails try to avoid water and interact with each other instead. Seeking these favourable interactions, phospholipids spontaneously form bilayers with the heads facing outward towards the surrounding water and the tails facing inward. Membranes are clearly vital as they give the cell structure and keep important molecules inside the cell and harmful molecules outside.
Water also buffers cells from extreme pHs. Highly acidic or basic substances, like bleach or hydrochloric acid are corrosive to even the most durable materials. Proteins require a specific structure to function properly, so it’s important to protect them from acids and bases. Although the chemical bonds within a water molecule are very stable, it’s possible for a water molecule to give up a hydrogen and become a hydroxide ion, thus acting as a base, or accept another hydrogen and become a hydronium ion (H₃O⁺), thus acting as an acid. This adaptability allows water to combat drastic changes of pH due to acidic or basic substances in the body in a process called buffering. Ultimately, this protects proteins and other molecules in the cell.
Furthermore, water provides support for animals with hydrostatic skeletons like jellyfish. They use a cavity filled with water at a high pressure as they do not have a real skeleton. As water is incompressible, the organism can use it to apply force or change shape. In plants, turgor pressure pushes the cell surface membrane against the cell wall, due to the osmotic flow of water from the outside of the cell to the inside of the vacuole and expanding the volume of the cytoplasm. This is important because it prevents the plant from wilting and supports the leaves in order to maximise light absorption for photosynthesis.
Finally, when water freezes into ice, the molecules form hydrogen bonds between each other and arrange themselves into an open lattice structure. This causes the molecules in ice to be further apart from each other compared to the molecules in liquid water, meaning ice can float on water. This unique property allows aquatic organisms to be able to survive in water below the surface of ice.
Are there any organisms that can survive without water? Yes, there are a handful of species, for instance anhydrobionts like tardigrades which can survive 30 years without water. However, the vast majority of life on earth can only exist because of water. So, having only named a handful of the uses that water has in biological organisms I hope you have reached the same conclusion as me and Sylvia Earle: that water is in fact the most important substance in accommodating life on our planet.
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