Unit 10: Biological Molecules and Metabolic Pathways
A: Understand the structure and function of biological molecules and their importance in maintaining biochemical
processes
Biological molecules and biochemical processes
This report will provide an overview of biochemical processes and the effects of disrupting them, models of relevant
biological molecules and the structure and function of these molecules in living organisms.
Water
Structure of water
Water is a molecule composed of two hydrogen atoms that have a covalent bond to one oxygen atom [Annets, F.
2017]. In water, electrons are not distributed evenly [Annets, F. 2017]. The electrons are drawn towards the oxygen
atom and away from the hydrogen atoms; this occurs because oxygen is more electronegative than hydrogen,
resulting in an uneven distribution throughout the water molecules [Annets, F. 2017]. Water molecules have a slight
negative charge towards oxygen and a slight positive charge towards hydrogens [Annets, F. 2017]. This makes water
a polar molecule [Annets, F. 2017]. When multiple water molecules are bonded together, hydrogen bonds are
established between them due to the opposite attractions between the positive hydrogen and negative oxygen on
nearby water molecules.
Figure 1: Bonding between water molecules [Bartleby, n.d.]
Functions of water
Solvent for chemical reactions
A solvent is a substance that can dissolve other substances [Annets, F. 2017]. Water is a good solvent for chemical
reactions because polar molecules dissolve in water [Annets, F. 2017]. This is because the solute (material to be
dissolved) has an unequal distribution of charges across its molecules [Annets, F. 2017]. The slightly negative ends
of the solute will be drawn to the slightly positive side of the water molecule and vice versa [Annets, F. 2017]. These
interactions between the water molecule and the solute cause the water molecule to gather around the charged
parts of the solute [Annets, F. 2017]. The solute molecules are separated and then dissolved [Annets, F. 2017]. Once
,Unit 10: Biological Molecules and Metabolic Pathways
A: Understand the structure and function of biological molecules and their importance in maintaining biochemical
processes
in solution, molecules can move around and interact with one another [Annets, F. 2017].
Transport medium
Because of its solvent properties, and its ability to stay a liquid over a large temperature range, water is also a good
transport medium for ionic and polar molecules [A Level Biology, 2019]. It is the primary blood transport medium for
glucose, nutrients and gases [A Level Biology, 2019]. It can also be used to transport chemicals both intracellularly
and extracellularly [A Level Biology, 2019].
Specific heat capacity
The heat capacity of a substance refers to its ability to absorb heat while maintaining a constant temperature [A
Level Biology, 2019]. It is measured in terms of specific heat capacity, which is the amount of heat required in
calories to increase the temperature of 1g of a substance by 1 degree celsius [A Level Biology, 2019]. As the
temperature rises, so does the kinetic energy of the molecules, which causes the molecules to move randomly [A
Level Biology, 2019]. Water has a high heat capacity due to the strong hydrogen bonds between its molecules [A
Level Biology, 2019]. Breaking these bonds and releasing the water molecules requires a great amount of energy [A
Level Biology, 2019]. As a result, water can absorb a significant amount of heat while increasing its temperature only
slightly [A Level Biology, 2019].
Cohesion and adhesion
Cohesion is when molecules of pure substances are attracted to each other [University of Hawai‘i, n.d.]. The
cohesiveness of a material is determined by how attracted molecules of the same substance are to one another
[University of Hawai‘i, n.d]. Water is extremely attracted to one another because of hydrogen bonding [University of
Hawai‘i, n.d]. As a result, water is very cohesive [University of Hawai‘i, n.d.]. Adhesion is similar to cohesion, but it
involves different substances attaching together [University of Hawai‘i, n.d.]. Water is incredibly adhesive; it adheres
to a wide range of substances [University of Hawai‘i, n.d.]. Water attaches to other things for the same reason it
sticks to itself: it is polar therefore attracted to charged substances [University of Hawai‘i, n.d.].
Regulation of temperature
Water is capable of withstanding temperature fluctuations since changing temperature requires a lot of energy
[Annets, F. 2017]. This is due to the hydrogen bonds formed between water molecules [Annets, F. 2017]. As the
temperature rises, water molecules gain kinetic energy and vibrate more, forcing the hydrogen molecules in water
to break [Annets, F. 2017]. This means that water molecules can form and break hydrogen bonds more quickly,
increasing the make-break rate [Annets, F. 2017]. Breaking these bonds requires a significant amount of heat energy
[Annets, F. 2017]. When the temperature of water decreases hydrogen bonds can form and water molecules move
less [Annets, F. 2017]. Water is an ideal habitat due to its tolerance to rapid temperature change [Annets, F. 2017].
Living organisms that live in water will not be subjected to potentially harmful changes in temperature [Annets, F.
2017]. Many organisms are composed of water [Annets, F. 2017]. Because water changes temperature slowly, these
creatures can adjust their internal body temperature [Annets, F. 2017]. It is essential that body temperature stays
stable so that optimal enzyme activity in the body is maintained [Annets, F. 2017].
Regulation of pH
Water regulates the pH of living organisms [Annets, F. 2017]. At pH 7, water has an equal amount of H+ and OH- ions
[Annets, F. 2017]. Living organisms are very sensitive to pH [Annets, F. 2017]. They function best when internal
conditions are close to optimal pH [Annets, F. 2017]. Buffer solutions, such as those found in blood, keep pH stable
, Unit 10: Biological Molecules and Metabolic Pathways
A: Understand the structure and function of biological molecules and their importance in maintaining biochemical
processes
when hydrogen or hydroxide ions are introduced [Annets, F. 2017]. Water plays an important role in pH stability
because it has the ability to receive and provide H+ as needed [Annets, F. 2017]. Without water, solutions cannot
maintain their ideal pH [Annets, F. 2017].
Balancing electrolytes
Water also helps to maintain electrolyte balance [Annets, F. 2017]. For example, as the extracellular electrolyte
concentration rises, water diffuses out of the cell by osmosis into the extracellular space, diluting the extracellular
fluid and increasing the intracellular electrolyte concentration [Annets, F. 2017].
Transpiration
Transpiration is the evaporation of water vapour from a plant's surface [The Science Hive, n.d.]. It mostly occurs
through gaps in the leaf called stomata, which need to open throughout the day to facilitate gas exchange [The
Science Hive, n.d.]. Water's polar nature allows for hydrogen bonding, which allows water molecules to cohere and
attach to the plant's cell walls, facilitating the movement of water through the plant [Save My Exams, 2022]. This
process is driven by a water potential gradient from the roots to the leaves, which is maintained by the constant loss
of water by transpiration and absorption via osmosis [Save My Exams, 2022]. The evaporation of water from leaves
causes tension within the xylem, which is sustained by the cohesive and adhesive characteristics of water molecules,
resulting in a stable water column [Save My Exams, 2022]. This entire process is known as the cohesion-tension
theory [Save My Exams, 2022].
Carbohydrates
Structure of carbohydrates
Carbohydrates are macromolecules that provide energy to the body, mostly through glucose, which is a simple sugar
[Nursing Hero, 2024]. Similar to water, they contain hydrogen and oxygen atoms but they also contain carbon atoms
[Nursing Hero, 2024]. They are represented by the general formula Cn(H2O)n, where n is the number of carbon atoms
in the molecule [Nursing Hero, 2024]. In carbohydrates, the ratio of carbon to hydrogen to oxygen is 1:2:1 [Nursing
Hero, 2024]. Carbohydrates are divided into three groups: monosaccharides, disaccharides and polysaccharides
[Nursing Hero, 2024]. Monosaccharides are simple sugars composed of single sugar molecules [Nursing Hero, 2024].
Monosaccharides typically include three to six carbon atoms [Nursing Hero, 2024]. Most monosaccharide names
end with ‘-ose’ [Nursing Hero, 2024]. These carbohydrates are classified as trioses (three carbon atoms), pentoses
(five carbon atoms), or hexoses (six carbon atoms) [Nursing Hero, 2024]. Monosaccharides can exist as linear chains
or as ring-shaped molecules. In aqueous solutions, they are often found in the form of a ring [Nursing Hero, 2024].
Figure 2: Molecular structure of glucose [Nursing Hero, 2024]
