Water is everywhere on our planet. It is essential to us for almost everything we do, eating, drinking, washing and more. Despite having no nutrients or calories on its own, it is also important for the maintenance of life. That’s why scientists get so excited when they find water on other planets: this indicates signs of life!
It exists in all three basic states of matter, solid (ice), liquid, and gas (vapour); but liquid water is the most relevant in biology, as we will discuss later. A single water molecule is made up of an oxygen atom covalently bonded to two hydrogen atoms, as shown in Figure 1. A covalent bond is formed by a pair of electrons shared between two atoms in a molecule.
Figure 1: Structural representation of a water molecule. The red sphere represents the oxygen atom, whilst the white spheres represent the hydrogen atoms.
A polar molecule has an asymmetric distribution of charge at different places in the molecule. With that in mind, water is polar because the electronegative oxygen attracts electrons towards it resulting in a slightly negative charge. In the meantime, the hydrogen atoms would thus have a slightly positive charge.
Meanwhile, hydrogen bonding is a type of intermolecular bond (i.e. a bond between two molecules rather than between two atoms in a molecule). It is special because it can only occur between a highly electronegative atom and a hydrogen atom covalently bonded to another electronegative atom. On that note, oxygen, nitrogen and fluorine are the three electronegative elements that have the ability to form hydrogen bonds as they have a very strong affinity for electrons, thus can acquire a partial negative charge. Besides that, hydrogen is only able to participate in a hydrogen bond if it is also covalently bonded with to of the three highly electronegative atoms listed above. This is because the electronegative atom is needed to pull the electrons away from the hydrogen atom, thereby allowing it to have a partial positive charge. Hence, the highly electronegative atom and the hydrogen atom are able to form attractive interactions.
Hydrogen bonding is also what gives water its high boiling point (100ºC) since a lot of energy is required to break these bonds between water molecules. It also causes the somewhat unique phenomenon where water’s solid phase, ice, is less dense than its liquid phase. When water freezes, the hydrogen bonds change their organisation so they are fixed and each water molecule forms hydrogen bonds to three others. This forms a tetrahedral lattice whereby the molecules are fixed in position. However, in its liquid phase, water molecules are mobile and are free to form hydrogen bonds with different water molecules. And so, the water molecules could be closer together on average.
Figure 2: The structure of water. (a) An individual water molecule showing the bond angle of 104.5 degrees and (b) hydrogen bonding between three water molecules. It is possible for a single water molecule to form a hydrogen bond with four other water molecules. δ+ and δ- are used to represent the partial positive and the partial negative charges, respectively, on the atoms in the water molecules. Hydrogen bonds are shown in blue dotted lines. Note: the bond angles and lengths are not to scale.
The ability for water to form hydrogen bonds underpin important properties such as high melting point, cohesiveness, and expansion when it freezes (which is why you should not put a glass filled with liquid in the freezer). Cohesiveness describes the ability of a substance, in this case water molecules, to stick to itself. Unsurprisingly, this is controlled by the intermolecular forces that exist between water molecules, including hydrogen bonds, which are relatively difficult to break. Water's cohesive properties also give rise to other properties, such as high surface tension and viscosity. In addition, water has a high specific heat capacity, meaning it can store a lot of energy before increasing increasing in temperature. This is also because of the strength of hydrogen bonds between water molecules, so a lot of energy can be absorbed to break them.
So, how are these properties relevant to life?
The water cycle
Let’s begin by talking about the environment as water is essential for the climate, agriculture, and more.
The water cycle refers to the global circulation of water from water sources on the ground to clouds in the sky. It begins when water evaporates from the surface of the Earth from water sources such as oceans, groundwater, and soil water.
Evaporation requires energy in the form of heat to break the intermolecular bonds between water molecules. Thus, as it takes this energy from the surface of the Earth, the surface will be cooled down. Not to mention, the efficiency of evaporation depends on the temperature, the wind which can increase the rate of evaporation, and the humidity. If the surrounding air is very humid (i.e. more saturated with water), evaporation will be slower.
Figure 3: The water cycle. Liquid water evaporates from water sources and the ground on Earth, turning into water vapour. Water vapour is incredibly light so it travels up into the sky. As it cools down, water condenses into tiny droplets to form clouds. More water is collected in the clouds until they get heavy and the water falls back to Earth in the form of rain, snow or hail. This figure was adapted from Arnell (2012).
The water cycle strongly influences climate and different ecosystems, creating different habitats such as rainforests, snowy mountains and deserts. It also affects the accessibility of freshwater, which determines whether a place is habitable or not (for humans, animals and other lifeforms).
The majority of human water use is for agriculture to grow plants to feed both humans and the livestock that are farmed, but it is also used for power generation, public use, and waste disposal. It is especially important that everyone in the world has access to clean water, which remains a huge issue, especially as the population grows.
Furthermore, the continual issue of global warming will greatly affect water. Firstly, precipitation patterns will continue to shift resulting in unpredictable weather and more frequent heat waves. In these instances, flooding or drought can occur, disrupting agriculture and more. More precipitation can also lead to an increase in pollutants in water sources, such as pesticides, waste and pathogens, disrupting, once again, the access to clean, safe water.
Why is water so important for life?
Water is important to every life form known to us for a huge variety of reasons. It defines the structure of cells and other compartments in an organism; it acts as a solvent to transport and store molecules; it can even act as a coolant. There are so many more uses for water, which will be discussed in this section.
Firstly, the structure of organelles, cells and tissues is controlled by the amount of water in them. Water makes up the majority of the protoplasm, which is defined as the living parts of the cell within the plasma membrane. In most organisms, there are also dedicated transporter proteins, known as aquaporins, which maintain the correct level of water within the cell under normal conditions. Aquaporins are transmembrane proteins, they are embedded in the plasma membrane, spanning from the cytoplasm to the extracellular space. This allows them to transport water in and out of the cell.
So what happens when this balance of water is disrupted?
Well, it depends on the organism. Firstly, dehydration is never a good thing. But it is also bad for an organism to have too much water. If this happens to humans, it is known as water intoxication and causes a disruption in electrolyte balance. This leads to excess water entering the cells, causing them to swell. Water intoxication is so serious it can even be fatal, but don’t worry too much as it only happens under very extreme circumstances.
Moreover, as mentioned before, water is a solvent, which is a substance with the ability to dissolve solutes to form a solution. Water’s polarity is what makes it an excellent solvent for polar and charged molecules. The slight negative charge on oxygen enables it to favourably interact with positively charged molecules (such as sodium ions, Na+), or the partially positive regions of another polar molecule (such as ammonia, NH3). On the other hand, the slight positive charge of the hydrogens in water means they can favourably interact with negatively charged molecules (such as phosphate ions, PO43-). Once dissolved in water, small molecules can easily diffuse through the aqueous solution to their required location.
Water does not just solvate small molecules, it can also dissolve large biological molecules, such as proteins or nucleic acids (for example, DNA). It is able to interact with charged regions of the molecule to prevent their interaction with other charged regions of the molecule or nearby molecules. On the other hand, water can force the aggregation of non-polar molecules if they do not favourably interact with water, causing the molecules to come together, forming a precipitate.
The ability of water to solvate large biological molecules is important for protein folding. Proteins consist of many amino acids covalently bonded together, forming a polypeptide chain. The polypeptide chains are folded into higher-order structures allowing proteins to carry out their functions. Many proteins are enzymes, which catalyse biochemical reactions. Water is able to hydrate the peptide backbone and force non-polar parts of the protein to aggregate, thereby directing protein folding. Water’s interaction with large molecules even dictates the structure of DNA and the ability of proteins to interact with DNA.
Biological reactions require water. This may be to correctly orientate the molecules, mediate interactions, control enzyme movement or because water is one of the reagents. As you may know, formations of bonds between proteins or other biological substances often release water in a condensation reaction. On the other hand, reactions that break these bonds require the addition of water, so are known as hydrolysis (from the Greek ‘hydro’ meaning ‘water’ and ‘lysis’ meaning ‘to unbind’).
Figure 4: Condensation, specifically dehydration, and hydrolysis reactions involving the release and incorporation of water, respectively. Condensation reactions occur when two molecules become covalently bound together, whilst hydrolysis reactions involve the addition of water to break one molecule into two.
Additionally, some organisms have requirements for water that do not fit into the general reasons discussed so far.
Plants are primary producers, meaning they produce their own food which they consume during respiration. They do this by photosynthesis, where light is converted into chemical energy. During this process, plants consume carbon dioxide and water to produce glucose and oxygen. In addition to photosynthesis, plants require water to take up nutrients and ions from the soil as they can only take up these components in their soluble forms.
Algae are another group of eukaryotic primary producers, which mostly live in aquatic environments. They can be split into three broad types known as brown, green and red algae; these are characterised based on a range of characteristics including the pigments they contain. Most algae live and thrive in water, they produce oxygen and take in a huge amount of carbon dioxide as they grow.
Figure 5: Giant kelp (Macrocystis pyrifera), a type of brown algae, grows in kelp forests and can reach about 60m in height. The picture was taken by the National Oceanic and Atmospheric Administration.
Although you may not consciously think about it, our brain is constantly letting you know if you need water. After all, humans need water for so many processes, ranging from protecting our joints to getting rid of waste products. Three uses of water will be discussed.
Excretion of waste products
Firstly, water is used to get rid of waste products. Ammonia and other nitrogenous waste products are produced during nitrogen metabolism and must be disposed of as they can be toxic. These products are particularly dangerous as they can alter interactions at the blood-brain barrier. The blood-brain barrier describes the endothelial cells that line the capillaries in the brain; these are specialised in order to prevent unwanted movement of substances, such as immune cells, into the brain. The presence of too much ammonia in the central nervous system can lead to an altered mental state and a variety of other symptoms.
Generally, nitrogenous waste products are processed into urea, a soluble waste product, that is taken to the kidneys and converted into urine to be secreted from the bladder. For mammals and birds, the concentration of solutes in the urine can be altered depending on the hydration state of the animal and the amount of waste product.
Next, saliva aids a great number of processes and contributes to human health. Saliva is first produced in a number of dedicated glands and then modified as it is transported through ducts. As a matter of fact, approximately 99% of saliva is water. Some of the components in saliva help taste, such as the concentration of sodium and chloride ions; whilst others help with chewing and digesting food. It is also important to protect soft and hard surfaces of the mouth.
Sweat as a coolant
Finally, due to its high specific heat capacity and latent heat of evaporation (or vaporisation), humans and many other organisms use water as a coolant, although in very different ways. The latent heat of evaporation describes the amount of heat absorbed when water transforms from liquid to gas. The ability of humans to sweat is important in thermoregulation (i.e. maintaining a good temperature), particularly when carrying out intense physical activity or in hot environments. As humans sweat, water evaporates from the skin and, as discussed before, takes energy (in the form of heat) with it. Therefore, this cools the person down. Sweat, much like urine, also removes waste products.
Water is everywhere, linking to every area of human life for one way or the other. Not only this, water is required for all life, albeit in different ways. Water impacts structure and organisation of cells and mediates biological reactions in all organisms. So in other words, the central role of water is definitely underplayed!
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Imperial College London