This is an example of a synoptic essay which was written by an a level candidate who was scored in the top band (A*).
THE CONTENTS OF THIS DOCUMENT ARE TO BE USED AS AN EXAMPLE AND SHOULD NOT BE REPLICATED DURING AN EXAM.
Write an essay on the importance of bonds and bonding in organisms
Deoxyribonucleic acid (DNA) is an essential molecule in maintaining the life of organisms. This polynucleotide
is made up of a single DNA nucleotide consisting of a nitrogenous organic base, deoxyribose sugar and a
phosphate group. Through condensation reactions, the complementary bases (A,T and C,G) are joined via
weak hydrogen bonds and the nucleotides are joined by phosphodiester bonds. The weak hydrogen bonds
present in the double stranded molecule is significant because it can be broken down (by helicase) and forms a
template strand for the production of mRNA. Free RNA nucleotides are attracted to the complementary bases
and form hydrogen bonds. The enzyme RNA polymerase synthesises the mRNA strand by forming
phosphodiester bonds between the ribose sugar and phosphate. This is important in organisms as mRNA is
required for making proteins. Enzymes are types of proteins and are essential in many metabolic reactions. For
example, Rubisco, the most abundant enzyme, is an important biological catalyst for the light independent
reaction (Calvin cycle) during photosynthesis in plants. Rubisco catalyses the reaction of CO2 with ribulose
bisphosphate to produce 2 Glycerate-3-phosphate molecules. This is important because the G3P is then
oxidised by NADPH and ATP to form triose phosphate. Triose phosphate is then converted into organic
molecules such as carbohydrates and lipids. The sugars made from this, such as glucose, are important for
growth of plants and is also an energy source which is moved across trophic levels.
Enzymes are biological catalysts which reduce the activation energy of metabolic reactions. During protein
synthesis, the polypeptide it transported to the Golgi apparatus and is modified. The primary structure of the
protein consists of peptide bonds joining the amino acids, the secondary structure (alpha helix/beta pleated
sheet) contains hydrogen bonds, and the tertiary structure consists of hydrogen bonds, ionic bonds, and
disulfide bridges. Interaction between these bonds form a specific active site for each enzyme. Enzymes can
bind to complementary molecules through the induced fit model. This is when an enzyme-substrate complex is
formed, both the structure of enzyme and substrate have a conformational change which enables catalysis.
Digestion enzymes such as the membrane bound disaccharidase maltose, are involved in the digestion and
absorption of essential molecules. Maltase catalyses the breakdown of maltose into alpha glucose, this is
important because maltose is too large to diffuse through the phospholipid bilayer. The glucose can be
absorbed by co-transport with sodium ions, and act as an energy store in animals. Glucose is the primary
source of energy in almost all metabolic reactions and are essential in maintaining cellular processes.
In Adenosine Triphosphate (ATP) there are unstable phosphoric anhydride bonds between the phosphate
groups. These bonds have a low activation energy, which makes them important for allowing coupled
reactions to occur. ATP hydrolysis uses the enzyme ATP hydrolase to break down the ATP into ADP and an
inorganic phosphate. The phosphate attaches to other molecules and lowers their activation energy (this is
known as phosphorylation). An example of where phosphorylation is important is during glycolysis in
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