AQA Biology Essay 07/12/2021
The ways in which water and the regulation of water content are important in organisms
One way in which water can be considered important in organisms is through its role in hydrolysis
reactions. Hydrolysis is the chemical reaction where a water molecule is used to break a bond. One
example of where this process plays a vital role in many organisms is in the hydrolysis of a
phosphoric anhydride bond between the phosphate groups of adenosine triphosphate (ATP) to form
adenosine diphosphate (ADP) and an inorganic phosphate. In this reaction, the Gibbs free energy of
the products is (much) less than the Gibbs free energy of the reactants, meaning the change in Gibbs
free energy is excessively negative (i.e. the reaction ‘releases energy’). If water was not present in
organisms, this reaction would not occur and, as a result, many biosynthesis reactions would also
not occur. An example of a biosynthesis reaction that is dependent upon the hydrolysis of ATP is the
construction of a polypeptide (polymer) from many amino acids (monomers) via condensation
reactions that form peptide bonds. In biosynthesis reactions, the change in Gibbs free energy is
positive – meaning that it is thermodynamically unfavourable and will not occur spontaneously.
Nonetheless, in a process termed coupling, it is possible to turn this thermodynamically
unfavourable reaction into a thermodynamically favourable reaction; the two reactions (ATP
hydrolysis and biosynthesis) can be coupled and the excessively negative change in Gibbs free
energy from ATP à ADP can counteract the smaller, positive, change in Gibbs free energy from the
monomers to the polymers. Ultimately, this implies that water is essential to not only the hydrolysis
of polymers to monomers, but also to the condensation reactions of monomers to polymers. These
anabolic and catabolic reactions are essentially what make up an organism’s metabolism and,
without water, neither of these processes could occur.
Water also plays a key role in plants through its involvement in photosynthesis. In the light-
dependent reaction, photosynthetic pigments will absorb light energy from a light source with an
appropriate wavelength and 4 photons will be transferred to the reaction centre of PSII – the P680
special pair. Upon excitation of the P680 complex, it will enter a high-energy state where it becomes
a good electron donor and subsequently oxidises, transferring an electron to the primary electron
receptor (pheophytin) in a photoact. Once the P680 special pair loses its electron, it gains a positive
charge and immediately replaces it by splitting water molecules (in ‘photolysis’) using the tetra-
manganese penta-oxygen calcium (Mn4O5Ca) cluster. The Mn4O5Ca cluster will simultaneously bind
to two water molecules and extract 4 electrons, releasing 4 protons, and an oxygen molecule (that
proceeds to be used in the respiration of mammals). Thus, water is essential in this process as it is
required to replace the missing electrons in the P680 special pair. If water was not present, the high-
energy electrons at pheophytin would not be able to travel through plastoquinone à cytochrome
b6f complex à plastocyanin (the electron transport chain). Through that process, the electrons
would lose energy which would be used to pump H+ ions from the stroma to the thylakoid lumen.
The electron would then reach PSI, where the P700 special pair in the reaction centre would become
excited in a similar way to P680 in PSII but, instead, transfers its high-energy electron to ferredoxin
and replenishes it with the electron from the prior electron transport chain. Ferredoxin proceeds to
move the electrons onto NADP+ reductase which transfers them onto the electron carrier of NADP+,
to produce NADPH. The protons that are needed for this reaction are provided via chemiosmosis
from the movement of H+ ions in the thylakoid lumen à ATP synthase à stroma. Therefore,
without water, NADPH will not be produced. This prevents the flow of the Calvin Cycle and the
production of glyceraldehyde-3-phosphate (G3P). Without G3P, the plants will not be able to
