Essay
Importance of ATP in organisms [AQA A-Level Biology]
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Essay on 'Explain the uses and importance of ATP in organisms' [25] for AQA A-Level Biology [Paper 3]
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Explain the uses and importance of ATP in organismsAdenosine triphosphate [ATP] is a nucleotide which is made up of adenine which is a
nitrogen-containing base, ribose sugar and a chain of three phosphate groups. The bonds
between the phosphate group are unstable and so have low activation energy which means
they’re easily broken. However, when they do break, they release large amounts of energy.
The hydrolysis of ATP into ADP, Pi and energy is catalysed by ATP hydrolase and since it is
a reversible reaction, ATP can be reformed by ATP synthase via a condensation reaction.
ATP is an immediate energy source of a cell because ATP releases energy in small,
manageable amounts and only one bond is hydrolysed in a single reaction to release
energy. ATP is important because it is used in muscle contraction. In the sliding filament
theory of muscle contraction, action potential/depolarisation causes release of calcium ions
from the sarcoplasmic reticulum which diffuses through the sarcoplasm to the myofibril. The
calcium ions bind to troponin which is attached to tropomyosin which causes the
tropomyosin to move and as it changes shape, it exposes the myosin binding sites on the
actin. So the myosin heads [with ADP attached] attach to the binding site forming an
actinomyosin crossbridge and this requires ATP. The myosin heads move and pulls/slides
actin along the myosin [ADP released - requires ATP] = powerstroke. ATP binds to the
myosin head causing it to detach from the actin binding site [break crossbridge]. The
hydrolysis of ATP by ATPase [which is activated by calcium ions] releases energy for the
myosin heads to move back to its original position [recovery stroke]. The myosin then
reattaches to a different binding site further along the actin filament.
ATP is also important in active transport which is the net movement of molecules against a
concentration gradient using carrier proteins and using energy from the hydrolysis of ATP via
ATPase. The molecule binds to the receptor complementary in shape in the protein. ATP
binds to the carrier protein and it is hydrolysed into ADP and Pi [inorganic phosphate] which
causes the carrier protein to change shape and release the molecule to the other side. The
phosphate ion is then released and the protein returns to its original shape. Active transport
is important in aerobic respiration as after glycolysis if oxygen is present then pyruvate and
reduced NAD are actively transported into the mitochondrial matrix. If this doesn’t occur then
link reaction wouldn’t occur where pyruvate is oxidised & decarboxylated to form acetate so
carbon dioxide & reduced NAD is produced. Acetate combines with coenzyme A to form
acetyl coenzyme A. Link reaction occurs twice for every glucose molecule which therefore
creates 2 molecules of acetyl coenzyme A, 2 molecules of carbon dioxide released and 2
NAD produced. If link reaction doesn’t occur then Krebs cycle wouldn’t take place. Krebs
cycle occurs in the mitochondrial matrix where acetyl coenzyme A [2 carbon molecule]
reacts with a 4 carbon molecule [oxaloacetate], producing a 6 carbon molecule [citrate] that
enters Krebs cycle. Acetyl Coenzyme A is then released which is recycled back into the link
reaction. 4 carbon molecules are generated through a series of oxidation-reduction
reactions. Decarboxylation & dehydrogenation occurs so 4 carbon dioxide molecules are
removed & coenzymes NAD & FAD are reduced [8 reduced coenzymes]. 2 ATP molecules
are also produced by substrate level phosphorylation. If Krebs cycle doesn’t take place then
oxidative phosphorylation doesn’t occur. Oxidative phosphorylation occurs on the cristae of
the mitochondria. Reduced NAD & FAD is oxidised to release hydrogen atoms which split
into protons & electrons. Electrons are transferred down the electron transport chain [ETC]
by redox reactions. Energy released is used by electrons carriers to actively transport
protons from the matrix to the intermembrane space. Protons then diffuse down the
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