A* A level biology essay on importance of shapes fitting together in organisms. Completed to a very high standard. Four different topics covered in great detail, plus evaluation and links to the question. Real life application also covered in this essay. Teacher corrections further applied.
The Importance of Shapes fitting together in Cells and Organisms
Enzymes are one of the major processes in biology which require complementary shapes. Enzymes
are proteins that catalyse many different reactions in the body, e.g., in the mouth during chemical
digestion. Enzymes have a specific tertiary structure, to ensure their active site is specifically
complementary to the substrate. This ensures an enzyme-substrate complex can form. Active site
and substrate are not identical, but active site Molds using the induced fit model, its highly
important so the substrate’s bonds can be broken with help of the enzyme. For example, Maltase
catalyses the hydrolysis of Maltose with the product of glucose. If the induced fit model cannot
happen, I.e., shapes are no longer complementary due to active site change, vital chemical reactions
cannot occur in the body. A change in the environment could cause this, I.e., temperature change
which causes the enzyme to denature as its active site changes. This is bad because no more
enzyme-substrate complexes can form, which slows down the chemical reactions within the body,
which can lead to slowed metabolism, bad digestion of food. This can also be caused by mutations in
the DNA sequence, coding for the wrong amino acids leading to defective enzymes. This highlights
how important induced fit model is, as the effects of an active site change causes life threatening
side effects on organisms.
A further example is when molecules are attempting to cross the phospholipid membrane. If
molecules are too large to diffuse across the membrane alone, they use facilitated diffusion via an
intrinsic protein. As these proteins only allow certain molecules to use them, their cavities in their
channels are substrate specific to prevent just any molecule passing. The specific molecules will
enter the protein, once all cavities are full the entrance will close and the exit will open allowing the
molecules to pass into the cell, aided by a phosphate molecule from the hydrolysis of ATP. If a
molecule not suitable to that protein enters the channel, the protein won’t be able to close and
transfer it across the membrane. This is essential as molecules like glucose enters the cell this way,
and this is our energy to respire and survive.
Another vital process which involves shapes fitting is the production of mRNA molecules. MRNA is
used in making proteins, e.g., enzymes, by transporting certain required information from DNA in
the nucleus, to the ribosomes in the cytoplasm using complimentary base pairing. Transcription is
the formation of (pre)mRNA. For this to happen, the DNA stand must temporarily split to allow base
pairing to happen, this is where RNA polymerase comes into play. This enzyme bonds to the DNA
strand specifically and breaks the hydrogen bonds between the nucleotide0 bases to allow a
template strand to be exposed. The RNA polymerase moves up the DNA strand, breaking the
hydrogen bonds and connecting base paired nucleotides to form the pre-mRNA molecule, then
reforming the hydrogen bonds to ensure DNA template strand isn’t exposed for longer than needed.
This continues until RNA polymerase reaches a triplet called a ‘Stop Codon’ where it detaches, and
the pre-mRNA is formed ready to be spliced to remove its introns, as introns would prevent the
synthesis of a polypeptide. This is important to human life because without this precise formation of
mRNA important proteins won’t be formed in the ribosomes as the wrong triplet codes and
information would be transmitted, or worse, if the enzyme RNA polymerase didn’t fit so exactly it
could leave the highly reactive nucleotides exposed with improper hydrogen bonding leaving them
open to damage.
Further examples of the importance of shapes fitting together is haemoglobin and red blood cells
bonding to oxygen molecules. In the lungs, haemoglobin binds to oxygen in a process called ‘loading’
or ‘association’. The haemoglobin (in the red blood cells) carries oxygen through the body in the
blood stream and ‘unloads’ at different tissues that require it. The shape of the red blood cells is
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