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Adenosine triphosphate (ATP)
· ATP is a nucleotide with three phosphate groups attached.
The chemical energy stored in the phosphate bonds are made
available to cells to use in synthesising or breaking bonds.
· The third phosphate bond is usually broken by a hydrolysis
reaction, catalysed by the enzyme ATPase and resulting in
ADP and a free inorganic phosphate group (Pi) and energy.
· ATP + H20 + ATPase = ADP + Pi + energy (used in cell).
· This is a reversible reaction. Energy to synthesise ATP comes
from redox reactions, providing an immediate supply of
energy when needed.…read more

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Making ATP
· Formed from ADP and inorganic phosphate
· Two hydrogen atoms are removed from a compound and are
picked up by a hydrogen carrier/acceptor, making it reduced.
· Electrons from the hydrogen atoms are then passed along an
electron transport chain through redox reactions, which each
release a small amount of energy used to drive the synthesis
of ATP.…read more

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Chloroplasts
· The membranes within a chloroplast
are arranged in stacks called grana. The
individual membrane disks within this
are known as thylakoids. The pigment
molecules are arranged on the membrane.
· The light-dependent reaction occurs in the thylakoids.
· The light-independent reaction occurs in the stroma. The
stroma contains all the enzymes needed for photosynthesis.…read more

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The light-dependent reaction
· Two main functions ­ to produce ATP, and to split water
molecules in photochemical reactions (photolysis).
· Photons of light hit a chlorophyll molecule which excites the
electrons, causing them to leave the chlorophyll. The election
is then picked up by an electron acceptor , which passes down
an electron transport chain to produce energy to synthesise
ATP. Electrons return to the chlorophyll molecule and can be
excited again. This involves only PSI. (cyclic
photophosphorylation)
· Water always dissociates spontaneously, so there are plenty
H+ and OH- ions in the cell. In non-cyclic
photophosphorylation, an electron from PSI is picked up by
NADP. The NADP also picks up a free hydrogen ion to form
reduced NADP.…read more

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The light-dependent reaction
· At the same time, an electron from PSII is picked up by
another electron acceptor and passes down an electron
transport chain, creating energy to make ATP. When it
reaches PSI, it replaced the previously lost electron.
· Now PSII is short of one electron, which must be replaced
through photolysis (splitting of water through light).
· There are many hydroxide ions left in the cell (hydrogen ions
have been removed by NADP), which react together to form
this water that can then be split with light to create free
electrons.
· This process can then be repeated, but it requires the
presence of light in order for it to take place.…read more

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