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AQA Biology A level Enzymes Essay
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The functions of enzymes and their importance in organismsEnzymes are biological catalysts - they speed up the rate of chemical reactions in organisms
by providing an alternate reaction pathway with a lower activation energy. Enzymes are
proteins whose 3D shape is determined by the protein tertiary structure. Their 3D shape is
particularly important as their active sites are complementary in shape to a substrate with
which they form enzyme-substrate complexes via the induced fit model. A specific substrate
binds to the active site of the enzyme and the active site changes shape slightly to form the
best fit for catalysis. Enzymes catalyse reactions in different ways: they can put pressure on
the bonds of the substrate, allowing it to break into products more easily, or bring substrates
together in a favourable orientation to react.
The enzyme ATP synthase is used in respiration to produce ATP from ADP and Pi. During
oxidative phosphorylation, NADH and FADH are oxidised as they go down an electron
transport chain. The electron transport proteins pump protons from the inner mitochondrial
membrane to the intermembrane space creating an electrochemical gradient where there is
a higher proton concentration in the intermembrane space. This causes protons to diffuse
down their concentration gradient back into the inner mitochondrial membrane via ATP
synthase. The energy from the proton gradient allows ATP synthase to synthesise ATP from
ADP and Pi. ATP is important in organisms as it can be hydrolysed back into ADP and Pi via
ATP hydrolase providing a supply of energy as a high energy phosphate bond is broken. For
example at muscle cells, when an action potential is received from a motor neurone, Ca2+
ions are released into the sarcoplasm where they activate ATP hydrolase to hydrolyse ATP
into ADP and Pi - releasing energy to bend the myosin head and move the actin filaments for
muscle contractions. Another ATP is needed to provide energy to break the actin-myosin
cross bridge and allow the myosin head to return to its resting state in order to attach to a
new actin binding site.
Digestive enzymes are vital in an organism to allow them to absorb nutrients from their food
into their bloodstream to be transported to body cells where they are needed. Different types
of aminopeptidases in the small intestine further hydrolyse proteins into amino acids to be
absorbed into the bloodstream through cotransport in the ileum. Exopeptidases hydrolyse
specific peptide bonds at the ends of protein chains to produce dipeptides or singular amino
acids while endopeptidases hydrolyse specific peptide bonds within protein chains to form
shorter polypeptides. Endopeptidase pepsin is also located in the stomach. Dipeptidases
hydrolyse peptide bonds between 2 amino acids in dipeptides. Amino acids are the final
products of protein digestion and once absorbed into the bloodstream can be transported to
cells for DNA translation for synthesis of new proteins. Amino acids are carried by tRNA to
ribosomes on the endoplasmic reticulum where the anticodon on the tRNA binds to the
complementary codon on the mRNA sequence. Peptide bonds form between adjacent
amino acids carried by tRNA to form polypeptide chains coded for by the mRNA sequence.
Lysozymes are enzymes important in the process of phagocytosis of the cellular response.
Phagocytes engulf and ingest the pathogen (e.g. bacteria, virus, foreign cells) and the
pathogen is contained within a phagosome. A lysosome constraining lysozymes fuses with
the phagosome to form a phagolysosome and lysozymes break down the pathogen.
Pathogen antigens are displayed on the plasma membrane of the phagocyte and undigested
material is expelled from the cell via exocytosis. T helper cells can bind to antigens on the
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