● `color{Brown}"T.W Engelmann"` (1843 – 1909), using a `color{violet}"prism split light"` into its spectral components and then illuminated a `color{Brown}"green alga, 𝘊𝘭𝘢𝘥𝘰𝘱𝘩𝘰𝘳𝘢"`, placed in a `color{violet}"suspension of aerobic bacteria"`.

● The bacteria were used to `color{violet}"detect the sites"` of `O_2` `color{violet}"evolution"`.

● He observed that the `color{violet}"bacteria accumulated"` mainly in the region of `color{violet}"blue and red light"` of the split spectrum.

● A `color{Brown}"first action spectrum"` of photosynthesis was thus described.

● It resembles `color{violet}"roughly the absorption spectra"` of `color{violet}"chlorophyll a and b"`.

● By the middle of the `color{violet}"nineteenth century"` the key features of `color{Brown}"plant photosynthesis"` were known, namely, that plants could use `color{violet}"light energy"` to make `color{violet}"carbohydrates"` from `CO_2` and `color{violet}"water"`.

● The `color{Brown}"empirical equation"` representing the total process of `color{violet}"photosynthesis"` for `color{violet}"oxygen evolving organisms"` was then understood as:



where `CH_2O` represented a `color{violet}"carbohydrate"` (e.g., glucose, a six-carbon sugar).

● A `color{violet}"milestone contribution"` to the understanding of photosynthesis was that made by a microbiologist, `color{brown}"Cornelius van Niel"` (1897-1985), who, based on his studies of `color{violet}"purple and green bacteria"`.

● He demonstrated that `color{violet}"photosynthesis"` is essentially a `color{violet}"light-dependent reaction"` in which `color{violet}"hydrogen"` from a suitable `color{violet}"oxidisable compound"` reduces `color{violet}"carbon dioxide to carbohydrates"`.

`color{violet}"This can be expressed by:"`



● In `color{violet}"green plants"` `H_2O` is the `color{violet}"hydrogen donor"` and is oxidised to `O_2`.

● Some organisms `color{violet}"do not release"` `O_2` during photosynthesis.

● When `H_2S`, instead is the `color{violet}"hydrogen donor"` for `color{violet}"purple and green sulphur"` bacteria, the ‘oxidation’ product is `color{violet}"sulphur or sulphate"` depending on the organism and not `O_2`.

● Hence, he `color{violet}"inferred"` that the `O_2` evolved by the `color{violet}"green plant"` comes from `H_2O,` `color{violet}"not from carbon dioxide"`.

● This was later proved by using `color{violet}"radioisotopic techniques"`.

● The `color{Brown}"correct equation"`, that would represent the `color{violet}"overall process of photosynthesis"` is therefore:



where `C_6H_(12)O_6` represents `color{violet}"glucose"`.

● The `O_2` released is from `color{violet}"water"`; this was proved using `color{violet}"radio isotope techniques"`.

● Note that this is `color{violet}"not a single reaction"` but description of a `color{violet}"multistep process"` called `color{Brown}"photosynthesis"`.