- The theory which explains how ATP is synthesized in the chloroplast is chemiosmotic hypothesis.
- Like in respiration, in photosynthesis too, ATP synthesis is linked to development of a proton gradient across the membranes of the thylakoid.
- In phototsynthesis, proton accumulation is towards the lumen, whereas in respiration, protons accumulate in the intermembrane space of the mitochondria when electrons move through the ETS.
- The processes that take place during the activation of electrons and their transport to determine the steps that cause a proton gradient to develop are the following:
- Since splitting of the water molecule takes place on the inner side of the membrane, the hydrogen ions that are produced accumulate within the lumen of the thylakoids.
- As electrons move through the photosystems, protons are transported across the membrane because the primary accepter of electron transfers its electron to an H carrier; hence, this molecule removes a proton from the stroma while transporting an electron.
- Protons are necessary for the reduction of NADP+ to NADPH+ H+, these protons are also removed from the stroma.
- Within the chloroplast, protons in the stroma decrease in number, whereas in the lumen there is accumulation of protons, which creates a proton gradient across the thylakoid membrane as well as a decrease in pH in the lumen.
- The gradient is broken down due to the movement of protons across the membrane to the stroma through the transmembrane channel of the F0 of the ATPase.
- The ATPase enzyme consists of two parts
- one is F0 embedded in the membrane and forms a transmembrane channel that carries out facilitated diffusion of protons across the membrane
- other portion is called F1 and protrudes on the outer surface of the thylakoid membrane on the side that faces the stroma.
- The break down of the gradient provides enough energy to cause a conformational change in the F1 particle of the ATPase, which makes the enzyme synthesise several molecules of energy-packed ATP.
- Chemiosmosis requires a membrane, a proton pump, a proton gradient and ATPase.
Fig. ATP synthesis through chemiosmosis