Photosynthesis
The Light dependent reactions:
A
light-dependent series of reactions which occur in the grana, and require the direct energy of light to make energy-carrier
molecules that are used in the second process:
- light energy is trapped by chlorophyll to make ATP (photophosphorylation)
- at the same
time water is split into oxygen, hydrogen ions and free electrons:
2H2O ---> 4H+ + O2 + 4e- (photolysis) - the electrons
then react with a carrier molecule nicotinamide adenine dinucleotide
phosphate (NADP), changing it from its oxidised state (NADP+) to its reduced state (NADPH):
NADP+ + 2e- + 2H+ --> NADPH + H+
An electron transfer system (a series of chemical
reactions) carries the two electrons to and fro across the thylakoid membrane.
The energy to drive these processes comes from two photosystems:
- Photosystem II (PSII) (P680)
- Photosystem I (PSI) (P700)
In the first photosystem (Photosystem II, PSII):
- photoionisation of chlorophyll transfers excited electrons to an electron acceptor
- photolysis of water (an electron donor) produces oxygen molecules, hydrogen ions and electrons, and the latter are transferred to the positively-charged chlorophyll
- the electron acceptor passes the electrons to the electron transport chain; the final acceptor is photosystem PSI
- further absorbed light energy increases the energy of the electrons, sufficient for the reduction of NADP+ to NADPH
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The oxidised form of nicotinamide adenine
dinucleotide phosphate (NADP+)
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The reduced form of nicotinamide adenine
dinucleotide phosphate (NADPH)
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Light
reactions occur mostly in the thylakoid stacks of the grana. Here, sunlight is
converted to chemical energy in the form of ATP (free energy containing
molecule) and NADPH (high energy electron carrying molecule). Chlorophyll
absorbs light energy and starts a chain of steps that result in the production
of ATP, NADPH, and oxygen (through the splitting of water). Oxygen is released
through the stomata. Both ATP and NADPH are used in the dark reactions to
produce sugar.
Dark
reactions occur in the stroma. Carbon dioxide is converted to sugar using ATP
and NADPH. This process is known as carbon fixation or the Calvin cycle. The
Calvin cycle has three main stages: carbon fixation, reduction, and
regeneration. In carbon fixation, carbon dioxide is combined with a 5-carbon
sugar [ribulose1,5-biphosphate (RuBP)] creating a 6-carbon sugar. In the
reduction stage, ATP and NADPH produced in the light reaction stage are used to
convert the 6-carbon sugar into two molecules of a 3-carbon carbohydrate,
glyceraldehyde 3-phosphate. Glyceraldehyde 3-phosphate is used to make glucose
and fructose. These two molecules (glucose and fructose) combine to make
sucrose or sugar. In the regeneration stage, some molecules of glyceraldehyde 3-phosphate
are combined with ATP and are converted back into the 5-carbon sugar RuBP. With
the cycle complete, RuBP is available to be combined with carbon dioxide to
begin the cycle over again.
The
biochemical conversion of CO2 to carbohydrate is a reduction reaction that
involves the rearrangement of covalent bonds between carbon, hydrogen and
oxygen. The energy for the reduction of carbon is provided by energy rich
molecules that are produced by the light driven electron transfer reactions.
Carbon reduction can occur in the dark and involves a series of biochemical
reactions that were elucidated by Melvin Calvin, Andrew Benson and James
Bassham in the late 1940s and 1950s. Using the radioisotope 14C, most of the
intermediate steps that result in the production of carbohydrate were
identified. Calvin was awarded the Nobel Prize for Chemistry in 1961 for this
work
