The minimum number of molecules of pigment and other associated compounds which are capable of converting a quantum is known as photosynthetic unit. When the electron microscopy revealed quantasomes on the lamellae of the chloroplast, most of the scientists believed that they had discovered a photosynthetic unit.
However, Anderson and Boardman (1966) first of all raised the question whether the granules which are known as quantasomes morphologically represented a photosynthesis unit or these are PSI subunits which remain in contact with the membrane embedded PSII.
Recent studies have shown that the so called quantasomes do not represent a photosynthetic unit, possibly these are one half of a unit. A diagrammatic representation of photosynthesis unit has been shown in Fig. 13-9.
Following is the approximate number of pigment and other molecules in one photosynthesis unit:
(i) Total chlorophyll: approximately 2500 molecules.
(ii) The oxidizing agent of second system Z; condensation centre Q and plastoquinone-A in equal number (approximately 8 per unit).
(iii) Cytochrome f, plastocyanin and P 700 (a special form of chlorophyll on each per 400-600 chlorophyll molecules (approximately 8 per unit).
(iv) Cytochrome b: this is twice the number of plastocyanin P 700 and cyt. f and one eighth of Z, Q and A (approximately 16 per unit).
On the basis of the above findings M. Avron (1967) presented a hypothetical model. Half part of this model is shown in Fig. 13-9. This model is based on the assumption that electron transport path has an important constituent E, which is present in the average ratio of 1:250 with chlorophyll.
