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TOP BAND/A* AQA A Level Biology Synoptic Essay - Importance of bonds and bonding
marked A* syntoptic essay example -Full response for AQA Level Biology essay in paper 3
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Importance of bonds and bondingDNA acts as a carrier of genetic information. It is a polymer of nucleotides where each
individual nucleotide consists of the pentose sugar deoxyribose, a phosphate group, and one of
four nitrogenous bases: adenine, thymine, cytosine, and guanine. Nucleotides are joined by 3-5
phosphodiester bonds between the phosphate group and deoxyribose sugar. These
phosphodiester bonds are essential in ensuring the stability of the DNA molecule and
establishing the strong phosphodiester backbone of DNA, in which the more chemically reactive
bases are protected. The DNA molecule consists of two strands running antiparallel to each
other, wound up into an a-helix. These two strands are held together by hydrogen bonds that
occur between the complementary base pairs: adenine with thymine and cytosine with guanine.
Three hydrogen bonds form between a cytosine and guanine base pair and two between
adenine and thymine. A key property of these hydrogen bonds is that they can be easily broken
by the enzyme DNA helicase, enabling DNA to replicate semi-conservatively. Therefore, the
hydrogen bonds present in DNA are important as they not only join together the two strands but
also allow them to separate, allowing free nucleotides to line up and form hydrogen bonds
between the nucleotides with their complementary bases. DNA replication is vital, as it occurs
during the interphase stage of the cell cycle. As a result, without hydrogen bonds occurring in
DNA, DNA cannot replicate, and the cell cannot subsequently divide by mitosis.
Hydrogen bonding also occurs between water molecules. Hydrogen bonding in water arises
between the partially negative oxygen atom of one molecule and the partially positive hydrogen
atom of another. Because of these hydrogen bonds, water molecules can ‘stick’ to each other as
well as to other surfaces. Without hydrogen bonding to produce the cohesive and adhesive
properties of water molecules, water cannot be pulled up the xylem of a plant as a continuous,
unbroken column. In cohesion-tension theory, the evaporation or transpiration of water from the
leaves establishes a water potential gradient, which drives the movement of water and inorganic
ions from the soil into the roots and up the xylem into the leaves by transpiration pull. There is a
tension or negative pressure inside the xylem that causes this upward pull of water. Therefore,
hydrogen bonds between water molecules are crucial because, in their absence, water and ions
such as nitrates, which are vital for plant growth, would not be supplied to the cells of a plant.
Water is used by plants in the light-dependent reaction of photosynthesis, where its photolysis
into protons, electrons, and oxygen leads to the subsequent formation of ATP and NADPH,
which are used in the Calvin cycle.
The bonds that occur in proteins are important as they determine their structure and, therefore,
their function. The primary structure of a protein involves a specific sequence of amino acids
joined by peptide bonds in a condensation reaction. Depending on the positions of amino acids
in the primary structure, the folding of the polypeptide in the secondary structure will be either
into an a-helix or a b-pleated sheet and occurs due to the formation of hydrogen bonding
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