located in special bodies (later called chloroplasts) within plant cells. He found that the green parts in plants is where glucose is made, and that the glucose is usually stored as starch. Now consider the interesting experiments done by T.W Engelmann ( – ). Using a prism he split light into its spectral components and then illuminated a green alga, Cladophora , placed in a suspension of aerobic bacteria.
The bacteria were used to detect the sites of O evolution. He observed that the bacteria accumulated mainly in the region of blue and red light of the split spectrum. A first action spectrum of photosynthesis was thus described. It resembles roughly the absorption spectra of chlorophyll a and b (discussed in section .
). By the middle of the nineteenth century the key features of plant photosynthesis were known, namely, that plants could use light energy to make carbohydrates from CO and water. The empirical equation representing the total process of photosynthesis for oxygen evolving organisms was then understood as: CO H O CH O O Light → [ ] where [CH O] represented a carbohydrate (e.g., glucose, a six-carbon sugar). A milestone contribution to the understanding of photosynthesis was that made by a microbiologist, Cornelius van Niel ( - ), who, based on his studies of purple and green bacteria, demonstrated that photosynthesis is essentially a light-dependent reaction in which hydrogen from a suitable oxidisable compound reduces carbon dioxide to carbohydrates.
This can be expressed by: H A CO A CH O H O Light → In green plants H O is the hydrogen donor and is oxidised to O . Some organisms do not release O during photosynthesis. When H S, instead is the hydrogen donor for purple and green sulphur bacteria, the ‘oxidation’ product is sulphur or sulphate depending on the organism and not O . Hence, he inferred that the O evolved by the green plant comes from H O, not from carbon dioxide.
This was later proved by using radioisotopic techniques. The correct equation,