<P> The proton gradient is also caused by the consumptions of protons in the stroma to make NADPH from NADP+ at the NADP reductase . </P> <P> The molecular mechanism of ATP (Adenosine triphosphate) generation in chloroplasts is similar to that in mitochondria and takes the required energy from the proton motive force (PMF). However, chloroplasts rely more on the chemical potential of the PMF to generate the potential energy required for ATP synthesis . The PMF is the sum of a proton chemical potential (given by the proton concentration gradient) and a transmembrane electrical potential (given by charge separation across the membrane). Compared to the inner membranes of mitochondria, which have a significantly higher membrane potential due to charge separation, thylakoid membranes lack a charge gradient . To compensate for this, the 10,000 fold proton concentration gradient across the thylakoid membrane is much higher compared to a 10 fold gradient across the inner membrane of mitochondria . The resulting chemiosmotic potential between the lumen and stroma is high enough to drive ATP synthesis using the ATP synthase . As the protons travel back down the gradient through channels in ATP synthase, ADP + P are combined into ATP . In this manner, the light - dependent reactions are coupled to the synthesis of ATP via the proton gradient . </P> <P> Cyanobacteria are photosynthetic prokaryotes with highly differentiated membrane systems . Cyanobacteria have an internal system of thylakoid membranes where the fully functional electron transfer chains of photosynthesis and respiration reside . The presence of different membrane systems lends these cells a unique complexity among bacteria . Cyanobacteria must be able to reorganize the membranes, synthesize new membrane lipids, and properly target proteins to the correct membrane system . The outer membrane, plasma membrane, and thylakoid membranes each have specialized roles in the cyanobacterial cell . Understanding the organization, functionality, protein composition and dynamics of the membrane systems remains a great challenge in cyanobacterial cell biology . </P> <P> The thylakoid membranes of the cyanobacteria are not differentiated into granum and stroma regions as observed in plants . They form stacks of parallel sheets close to the cytoplasmic membrane with a low packing density . The relatively large distance between the thylakoids provides space for the external light harvesting antennae, the phycobilisomes . This macrostructure, as in the case of higher plants, shows some flexibility during changes in the physicochemical environment . </P>

What is the function of grana in chloroplast