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Photosynthesis is the main autotrophic process and is performed by chlorophyllate beings, represented by plants, some protists, photosynthetic bacteria and cyanobacteria.

In photosynthesis performed by photosynthetic beings, except for bacteria, carbon dioxide (CO2) and water (H20) are used for carbohydrate synthesis, usually glucose. In this process, there is the formation of oxygen (O2), which is released to the middle.

Photosynthesis performed by photosynthetic bacteria differs in many respects from that performed by other photosynthetic organisms, as we shall see below.

The general formula for glucose production by photosynthesis of eukaryotes and cyanobacteria is:

6 CO2 + 12H2O Ç6H12O6 + 6 O2 + 6H2O

This equation shows that, in the presence of light and chlorophyll, carbon dioxide and water are converted to a hexose - in this example, glucose - with oxygen release.

Photosynthetic beings are fundamental to the maintenance of life on our planet, as they are the basis of most food chains and produce oxygen, gas kept in the atmosphere in adequate concentrations thanks mainly to photosynthetic activity.

Oxygen origin and bacterial photosynthesis

The oxygen released by photosynthesis by eukaryotes and cyanobacteria comes from water, not carbon dioxide, as previously thought.

The first researcher to propose this was Cornelius Van Niel, in the 1930s, when he was studying photosynthetic bacteria. He found that sulphurous red bacteria (or purple thiobacteria) performed a particular form of photosynthesis in which there was no need for water or oxygen formation. These bacteria use carbon dioxide and hydrogen sulfide (H2S) and produce carbohydrate and sulfur.
Van Niel then wrote the general formula for photosynthesis performed by these bacteria:

Bacterial Photosynthesis
6 CO2+ 2 H2s CH2O + H2O + 2 S

It was the understanding of this photosynthesis process that led the researcher to propose the general equation of photosynthesis:

6 CO2+ 2 H2THE CH2O + H2O + 2 A

This equation shows that H2A may be water (H2O) or hydrogen sulfide (H2S) and shows that if it is water it is the source of oxygen in photosynthesis.
This interpretation was later confirmed in the 1940s by experiments in which researchers supplied plants with water whose oxygen was mass 18.18, heavy oxygen isotope) instead of 16 (O16), like ordinary water oxygen. They found that the oxygen released by photosynthesis was O18, corroborating Van Niel's interpretation.

It was proved, then, that the oxygen released during the photosynthesis of eukaryotes and cyanobacteria comes from water and not from carbon dioxide.

Where does photosynthesis occur?

In the simplest organisms, such as cyanobacteria, photosynthesis occurs in hyaloplasma, which is where several chlorophyll molecules are associated with an internal membrane network, which are extensions of the plasma membrane. Remember that cyanobacteria are prokaryotes and do not have membrane-bound organelles. On the other hand, in eukaryotic organisms photosynthesis occurs entirely within the chloroplast.