`star` Use of `ATP` and `NADPH`
`star` The Primary Acceptor of `CO_2`


● The `color{violet}"products of light reaction"` are `ATP,` `NADPH` and `O_2`.

● Of these `O_2` `color{violet}"diffuses out"` of the chloroplast while `color{violet}"ATP and NADPH"` are used to drive the processes leading to the `color{violet}"synthesis of food"`, more accurately, `color{violet}"sugars"`.

● This is the `color{Brown}"biosynthetic phase"` of photosynthesis.

● This process does `color{violet}"not directly depend"` on the `color{violet}"presence of light"` but is dependent on the `color{violet}"products of the"` `color{violet}"light reaction"`, i.e., `ATP` and `NADPH`, besides `CO_2` and `H_2O`.

● This could be verified as immediately after `color{violet}"light becomes unavailable"`, the biosynthetic process continues for
some time, and then stops. If then, `color{violet}"light is made available,"` the synthesis starts again.
● We saw earlier that `CO_2` is combined with `H_2O` to produce (`CH_2O`)n or `color{violet}"sugars"`.

● It was of `color{violet}"interest to scientists"` to find out how this reaction proceeded, or rather what was the `color{violet}"first product formed"` when `CO_2` is taken into a `color{violet}"reaction or fixed"`.

● Just after `color{violet}"world war II"`, among the several efforts to put `color{violet}"radioisotopes to beneficial use"`, the work of `color{Brown}"Melvin Calvin"` is exemplary.

● The use of `color{violet}"radioactive"` `C^14` by him in `color{violet}"algal photosynthesis"` studies led to the discovery that the first `CO_2` fixation product was a `color{violet}"3-carbon organic acid."`

● He also contributed to working out the `color{violet}"complete biosynthetic"` pathway; hence it was called `color{Brown}"Calvin cycle"` after him.

● The first product identified was `color{violet}"3-phosphoglyceric acid"`or in short `color{violet}"PGA"`.

● Scientists also tried to know whether `color{violet}"all plants have PGA"` as the first product of `CO_2` `color{violet}"fixation"`, or whether any `color{violet}"other product"` was formed in other plants.

● Experiments conducted over a `color{violet}"wide range of plants"` led to the discovery of `color{violet}"another group of plants,"` where the `color{violet}"first stable product"` of `CO_2` fixation was again an `color{violet}"organic acid"`, but one which had `color{violet}"4 carbon atoms"` in it.

● This acid was identified to be `color{violet}"oxaloacetic acid or OAA"`.

● Since then `CO_2` `color{violet}"assimilation during photosynthesis"` was said to be of `color{violet}"two main types"`: those plants in which the `color{violet}"first product"` of `CO_2` fixation is a `C_3` acid `color{Brown}"(PGA"`), i.e., the `C_3` pathway, and those in which the first product was a `C_4` acid (`color{Brown}"OAA"`), i.e., the `C_4` pathway.


● A question that was asked by `color{violet}"the scientists"` who were struggling to `color{violet}"understand the dark reaction"`

● How many `color{violet}"carbon atoms"` would a molecule have which after `color{violet}"accepting (fixing)"` `CO_2`, would have `color{violet}"3 carbons"` (of PGA)?

● The studies very `color{violet}"unexpectedly showed"` that the accepter molecule was a `color{violet}"5-carbon ketose sugar"` – it was `color{Brown}"ribulose bisphosphate (RuBP)."`

● The scientists also took a `color{violet}"long time"` and `color{violet}"conducted many experiments"` to reach this conclusion.

● They also believed that since the `color{violet}"first product"` was a `C_3` acid, the `color{violet}"primary acceptor"` would be a `color{violet}"2-carbon compound"`; they spent many years trying to identify a 2-carbon compound before they discovered the `color{violet}"5-carbon RuBP."`