When
light falls on a chlorophyll molecule of a certain wavelength it gives one of
the electrons energy, exciting to rise to a higher energy
level + leave the molecule, oxidising the
chlorophyll. As this happens the molecule accepts an electron produced by
splitting water brought to the leaf through the xylem vessel by the roots –
photolysis: H2O -> 1/2 O2 + 2H+ + 2e-
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When
the electron leaves the molecule it enters a series of proteins known as electron
carriers. The electron passes down these, reducing the protein which gains it
and oxidising the protein that passes it on, known as a redox reaction. The
electron loses energy while this happens and this energy is used to drive ATP
synthesis by combining ADP and Pi (inorganic
phosphate) to create ATP.ADP + Pi -> ATP
As
the electron in the electron carriers leave, light is shone onto another
chlorophyll molecule, This electron is excited to a higher energy level and
escapesThe previous electron then replaces the missing one in the new
molecule.
The
new energised electron passes down a new set of electron carriers that doesn’t
cause the electron to lose energy. This electron then combines with NADP+
and the left over hydrogen ions from photolysis to form reduced NADP:NADP+ + H+ +2e- -> NADPH
Pie de foto: : HAPPENS IN THE STROMA - ALSO KNOWN AS THE CALVIN CYCLE
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Step 1
Carbon
dioxide which enters from the bottom of the leaf into the cell is put into the
cycleThis combines with Ribulose biphosphate (RuBP) Known as Carbon fixation.
However this molecule is very unstable and splits into two identical species of
glycerate 3-phosphate (GP).
Then,
using the ATP and reduced NADP from the dependent reaction each molecule of GP
is reduced to a phosphorylated 3-carbon sugar. This step requires the hydrogen
from NADPH2 so requires the utilisation of ATP molecules. This is an
endergonic reaction and it converts 2ATP to 2ADP + Pi and 2NADPH2 to
2NADP. This makes two molecules of Triose Phosphate (TP) and ADP, Pi and NaDP
are sent back to the light dependent reaction.
One
carbon from the TP molecule is used to build up to a glucose molecule. This
cycle must therefore happen 6 times to produce one glucose molecule.
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Step 4
There are five carbons left from the TP and these join to form Ribulose
Phosphate (RuP). For RuP to turn back into RuBP one more phosphate group must
be added, this comes from the utilisation of ATP creating ADP and a phosphate
group for the phosphorylation of RuP.
This
then creates Ribulose biphosphate which restarts the cycle and carbon dioxide
must combine for it to carry on 5 more times to create one glucose molecule.
This stage can happen for a small time without any light energy present; this
is because until the ATP and reduced NADP run out the cycle can continue
creating glucose as all the other factors are available and CO2 will
be at a constant amount. These two stages combined is how photosynthesis occurs.
