Photosynthesis uses light energy, water and carbon dioxide in two processes to ultimately
produce energy molecules like glucose or building molecules like cellulose.
Oxygen is a byproduct of the light dependent reaction, and the ATP and NADPH that are made
in the light dependent reaction fuel the light-independent reaction, or Calvin cycle.
Let’s take a deeper dive into exactly how these two processes really work, starting
with the light-dependent reactions which occur on the thylakoid discs.
The membrane of the thylakoid discs have proteins embedded in them which carry out the light
reaction.
Outside of the thylakoid disc is the stroma, inside is the thylakoid space.
Light strikes photosystems, which are clusters of chlorophyll and other light harvesting
pigments.
When light excites electrons in the chlorophyll of the photosystem, the chlorophyll is photoactivated.
There are two photosystems in the membrane, but they were discovered out of order.
Photosystem II is actually the first to be activated in the light reaction.
Two photons of light have excited 2 chlorophyll electrons in PS II but they need to be captured
in order to do work.
Plastoquinone will accept the two electrons becoming reduced in the process.
The chlorophyll in the reaction center of PSII is a strong oxidizing agent and will
split a water molecule to replace the missing electrons, leaving two protons and an oxygen
ion it's wake.
This process is called photolysis.
Photolysis occurs rapidly and constantly in the presence of light so another water molecule
will be split and the two oxygen atoms will form molecular oxygen which can then diffuse
out of the plant.
It is a byproduct, or waste product, of photosynthesis since it is not used here.
Let’s go back to our electrons in plastoquinone.
Plastoquinone carries electrons from PSII to the electron transport chain located in
the thylakoid membrane.
As electrons are transferred in the electron transport chain, energy is released and hydrogen
ions are pumped across the thylakoid membrane into the thylakoid space.
The concentration of protons in the thylakoid space is therefore increased by photolysis
and the electron transport chain.
The protons can exit the thylakoid space through ATP synthase.
As they move through ATP synthase, ADP is phosphorylated to make ATP which can be used
in the light independent reactions.
This step in photosynthesis is very similar to the chemiosmosis process in cellular respiration.
Our electrons, however, are still not done with their journey.
At the end of the electron transport chain is photosystem I.
Light strikes photosystem I, and excites electrons which are passed to ferrodoxin.
The electrons are replaced by ones carried from the electron transport chain.
Two molecules of ferredoxin are used with NADP reductase to reduce NADP to NADPH.
This is the final electron acceptor.
This molecule will be used in the light independent reactions.
If NADP runs out, cyclic phosphorylation will occur.
This involves the electron transport chain and photosystem I.
The electrons move from ferredoxin back to plastoquinone, through the electron transport
chain, which brings in hydrogen ions.
These ions will diffuse through ATP synthase and the electrons continue the cycle through
photosystem one again.
This keeps the production of ATP high since more ATP will be needed than NADPH in the
Calvin Cycle.
Now the ATP and NADPH produced from the light reactions can be used to power the light independent
reactions of the Calvin Cycle.
The Calvin Cycle occurs in the stroma which fills the spaces of the chloroplast.
This is where the molecule building actually occurs.
The Calvin Cycle is also known as the light independent reaction, because light is not
directly used in this process.
The purpose here is to produce carbon-based molecules that can be used for energy, storage,
and structural purposes.
The carbon source for all photosynthetic organisms is carbon dioxide.
The first step of the cycle is called carbon fixation because carbon dioxide is converted,
or fixed, into an organic compound.
The carbon dioxide reacts with ribulose bisphosphate, a 5-carbon compound and water.
The enzyme that catalyzes this carboxylation is called ribulose bisphosphate carboxylase,
or RuBisCo for short.
This produces two molecules of glycerate 3-phosphate.
These two molecules are then reduced by 2 NADPH and 2 ATP to make 2 molecules triose
phosphate.
Triose phosphate is really the goal product of the Calvin cycle.
In reality, however, the Calvin cycle will use 3 carbon dioxide molecules at a time to
ultimately produce 6 triose phosphate molecules.
But why all that effort?
The triose phosphate molecule is a simple carbohydrate that can be used by the plant
to build larger molecules.
However, it’s also needed to remake the ribulose bisphosphate that was used in the
beginning of the calvin cycle.
1 triose phosphate will be used by the cell for whatever it needs, and the other 5 triose
phosphate molecules will be recycled back into 3 molecules of RuBP.
This means 3 turns of the calvin cycle make 1 triose phosphate to be used by the cell,
and 3 RuBP are recycled.
It uses 6 NADPH and 9 ATP to form the triose phosphate and reform the RuBP.
To make a hexose sugar like glucose, 6 turns of the calvin cycle will be needed to make
the 6 carbon sugar.
After 6 turns, 1 hexose molecule can be made, which uses 6 carbon dioxide molecules, 12
NADPH, 18 ATP, 6 RuBP and regenerates the 6 RuBP as well.
The spent ADP and NADP molecules can be recharged in thylakoid disks by the light reaction.
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