Atoms of a 'lighter' element collide and 'stick' together to make a heavier element. The most notable reaction, takes place in stars. Where Hydrogen fuses to make Helium, in the process energy is produced. Further fusion then takes place, to make other elements
There are many sorts of reactions that take place in stars, in the sun for instance the main nuclear chain reaction isP-P ( Proton-Proton ), in general the hydrogen consumed and turns into helium and by the way it produces a lot of energy.
Chemical reaction takes place because of the reaction of the ionic charge of a particular element to another. How fast the reaction would depend on the number of ions the element can liberate ( the more the faster)
Photosynthesis takes place in the cells of green plants. The process of photosynthesis is carried out in the chloroplasts. More specifically, the light reaction takes place in the thylakoid membrane, while the dark reaction (light independent) takes place in the stroma.
ATP and NADPH are produced by reactions in the thylakoids and are consumed by reactions in the stroma. The reason is because the reaction that takes place in the thylakoid is the light reaction and the one that takes place in the stroma is the light-independent reaction.