The light dependent reactions of photosynthesis take place in the thylakiods.
Light energy powers the formation of two types of high energy compounds.
One compound is NADPH and the other is ATP.
The plant cell uses the energy in these compounds during the second phase of photosynthesis called the Calvin cycle.
The Calvin cycle takes place in the fluid stroma surrounding the thylakoids.
Each turn of the Calvin cycle builds one small subunit that leaves the lumen.
These small subunits are the building blocks for sugars and starches.
The starting compound for the Calvin cycle is a five carbon compound called RuBP.
During the first step of the cycle, one molecule of carbon dioxide is added to each of three RuBP molecules.
The plant absorbs carbon dioxide from the surrounding air.
The three RuBP's become six carbon molecules.
Each of these molecules splits in half, forming six three carbon molecules called PGA.
Six ATP's must be spent to energize the six PGA's.
Then six NADPH's must be spent to energize the PGA still further.
These three carbon molecules are now called PGAL.
Of the six PGAL's formed, five remain in the Calvin cycle while one exits.
As the cycle continues, the five PGAL's rearrange back into three molecules of 5 carbon RuBP.
This requires three more ATP's
In each turn of the Calvin cycle, the five carbon RuBP's accept carbon dioxide molecules.
Six new PGAL's form at the cost of ATP's and NADPH's.
A PGAL molecules leaves the cycle and the five carbon starting material is regenerated.
The the PGAL that left the Calvin cycle moves out of the chloroplast and into the plant cell cytoplasm.
Once in the cytoplasm, the PGAL can be combine with another PGAL into a six carbon glucose molecule or other sugar.
