When would green plants carry out photosynthesis-only during at night, continuously, the day, only or never?

The process of Photosynthesis is composed of two distinct processes. The Light Reaction which takes place when light energy is available, and the Dark Reaction (Calvin Cycle) which takes place continuously.

Here is a simplified explanation of the two processes.

The Light Reaction takes place in the Grana of the Chloroplast when packets of UV light called photons are absorbed by the chlorophyll pigments. This energy is used to split water molecules into Hydrogen atoms that pick up electrons #(H^-)#. These energized hydrogen molecules are picked up by a transport molecule called NADP and become NADPH. Also released from the water are Oxygen #(O_2)# which will be released into the atmosphere. In splitting the water molecule, energy molecules are produced in the form of Adensoine-tri-phosphate #(ATP)#.

The Dark Reaction (aka Calvin Cycle) takes place in the Stroma of the Chloroplast. This reaction takes Carbon Dioxide #(CO_2)# and combines it with the #H^-# from the light reaction to form Glucose #(C_6H_12O_6)#. The energy for this process comes from the ATP produced in the light reaction. During this process excess water is produced which will transpire out of the leaf.

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