Water

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Summary

  • Water is made up of two hydrogens covalently bonded to oxygen.
  • Water molecules are polar, and are attracted to one another by hydrogen bonding
  • Water is an excellent solvent, breaking up compounds so that the components can take part in biological processes
  • It is also important in regulating temperature fluctuations in living organisms
  • Water provides a liquid environment for chemical reactions to take place, and a good transporter of chemicals in the cell

Water (H2O) is a very important molecule to all living things, forming between 70-95% of the cytoplasm in cells, and on average about 70% of the mass of the entire organism. Water is liquid at most temperatures we encounter, but freezes at 0° and boils at 100°C. Its presence as a solid, liquid and gas on earth makes it an extremely unique substance, and this is exploited by living things. Much of water’s important properties rely on its chemical structure.

Chemical properties

Water contains two hydrogen atoms and one oxygen atom. These atoms are joined by covalent bonds. Water is a polar molecule, this means that it has one side (oxygen) that is slightly negatively charged, and the hydrogens are slightly positively charged. This happens because the oxygen has a larger pull on the shared electrons (which are negatively charged).

When two water molecules are close together, the polar forces mean they are attracted to one another; the oxygen atom (negatively charged) from one molecule will bond with several hydrogen atoms (positively charged) of other molecules. This type of bonding is called hydrogen bonding, which is weaker than covalent bonds, which are stable. Hydrogen bonds break and reform constantly. Even though hydrogen bonds are relatively weak, they strongly contribute to the stability of water, and are the reason for many of its properties and biological functions.

 

The properties of water

Solvent
Water is often described as the general or universal solvent, this means it is capable of dissolving many substances (solutes). Water is a good solvent for ions and other polar molecules, because the water molecules are attracted to the uneven charges that are present. Water molecules cluster around the charged regions, separating the chemicals and freeing them up to move around and participate in other reactions. Water cannot dissolve non-polar substances such as lipids, which tend to collect together in water.

Heat capacity
Water requires a large amount of energy to increase its temperature, this is because of its hydrogen bonding. This means water is highly stable, providing a stable environment for cells to maintain temperature. That is, water can act as a buffer for any fluctuating temperature changes, allowing cells to better maintain temperatures required for important biological processes. This is also why large bodies of water  such as lakes and oceans are able to maintain a stable temperature, providing a constant environment for aquatic animals to thrive.

Secondly, evaporation of water from animals and plants (through sweating and transpiration, respectively) acts to decrease their body temperature and cool the organism down in warm temperatures. This is possible because large amounts of energy are required to turn water into vapour, owing to the hydrogen bonds that are present. The energy taken up by the process turning water into vapour removes the heat, and cools down the environment, regulating body temperature. Therefore, water is important in maintaining a stable body temperature.

Because water is both an excellent solvent and liquid at most temperatures, it can act as a transport vehicle inside the cell as well as providing a medium for (and sometimes participating in) metabolic reactions. Metabolic processes rely on chemicals being able to react together in solutions and move through the cell dissolved in water. For example, glucose, salt and amino acids enter the cells dissolved in water. Water also actively participates in biochemical processes in the cell such as condensation, hydrolysis and photosynthesis.

 

Further reading and references:

[1]. https://www.ck12.org/c/physical-science/hydrogen-bond/lesson/Hydrogen-Bonding-MS-PS/ Image water bonding

[2]. https://courses.lumenlearning.com/boundless-biology/chapter/water/

[3]. https://www.youtube.com/watch?v=A88ih2PQDNs

[4]. https://www.britannica.com/science/water