Unit 10: Biological Molecules and Metabolic Pathways
Learning Aim: To understand the structure and function of biological
molecules and their importance in maintaining biochemical
processes.
1
, Water
Importance of water
serves as an exceptional transport medium in living organisms, primarily due to its ability to
remain in a liquid state over a wide temperature range (0-100 degrees Celsius). This property is
attributed to the hydrogen bonding between water molecules, enabling organisms to maintain
their metabolic processes efficiently. All metabolic processes within organisms require chemical
substances to react within a solution, making water an indispensable component for these
interactions. The effectiveness of water as a solvent in chemical reactions can be attributed to
its capacity to dissolve polar and some nonpolar molecules, which is essential for the majority of
biochemical reactions to occur.
The solute's (a substance to be dissolved)
uneven charge distribution throughout its
molecular structure plays a pivotal role in
water's solvent properties. Water, being a
polar molecule due to the difference in
electronegativity between hydrogen and
oxygen atoms, results in slightly negative
portions (oxygen) of the solute being
attracted to the slightly positive components
(hydrogen) of the water molecule and vice
versa. These interactions, called ion-dipole
interactions, drive water molecules to cluster
around the charged areas of the solute,
effectively separating and dissolving the solute molecules through a process called solvation or
hydration. Once dissolved, these molecules can move throughout the solution and interact with
other molecules, facilitating vital biological processes such as nutrient transport, waste removal,
and chemical signalling, including neurotransmission.
The hydrogen bonds that link water molecules give it the ability to resist temperature
fluctuations. A significant amount of energy, approximately 4.18 J/g°C, must be applied to
change the water's temperature due to its high specific heat capacity. This quality allows water
to absorb or release large amounts of heat while undergoing minimal temperature changes,
making it an excellent temperature buffer for organisms. When temperatures increase, water
molecules gain more kinetic energy, resulting in intensified vibrations and disruption of hydrogen
bonds. This occurrence leads to a more rapid formation and breakage of hydrogen bonds,
increasing the make-break rate. Conversely, as the water cools, hydrogen bonds can form,
limiting the mobility of the water molecules, reaching their maximum density at 4 degrees
Celsius.
2
, Water's ability to stabilize pH and electrolyte balance within living organisms is indispensable to
maintaining their health. At a pH of 7, water exhibits equal concentrations of H+ and OH- ions
(approximately 10^-7 M). Living organisms are particularly sensitive to pH values and function
optimally when their internal conditions fall within their ideal pH range, generally near neutral.
Hence, water is vital in maintaining pH balance in living organisms. Buffer systems, including
bicarbonate and phosphate systems found in blood, help stabilize pH by preventing drastic
fluctuations when hydrogen or hydroxide ions are introduced into the system. As water can
accept and donate H+ ions as needed through its auto-ionization property, it plays a critical role
in maintaining stable pH levels.
Furthermore, water has an essential function in managing electrolyte balance, which is critical
for various physiological processes like nerve impulse conduction and muscle contraction. For
example, when the concentration of extracellular electrolytes rises, water moves out of the cell
through osmosis into the extracellular space. This process dilutes the extracellular fluid,
consequently increasing the intracellular electrolyte concentration level. This homeostatic
mechanism highlights the essential role of water in regulating key physiological processes
crucial to the survival and well-being of living organisms.
Water's unique properties, such as high surface tension and heat capacity, significantly
influence various biological processes. High surface tension, caused by hydrogen bonding, is
essential for capillary action, enabling plants to efficiently transport water and nutrients from
roots to leaves against gravity. Moreover, water's high heat capacity ensures that organisms'
internal environment remains stable, allowing for optimum metabolic processes and enzyme
activities, as enzymes typically have a narrow range of optimal temperatures.
In conclusion, water is a fundamental element in sustaining life due to its unique physical and
chemical attributes, such as polarity, hydrogen bonding, high surface tension, and high heat
capacity, which enable it to support critical biological processes.
Structure and Bonding of Water
Water is comprised of two hydrogen atoms and one oxygen
atom, denoted by the molecular formula H2O. Atoms bond
when they share electrons to reach a stable state, but in
water's case, these electrons are not shared equally due to
their varying levels of electronegativity. Electronegativity
gauges an atom's capability to draw bonding electrons
towards itself; as oxygen has higher electronegativity scores
than hydrogen, the electron density is unequally distributed
throughout the molecule.
3
