Thylakoid

Interior to plant cells that carry out photosynthesis, tiny, bacteria-sized organelles, called chloroplasts, contain, within the inner membrane of their dual membrane structure, the early photosynthesizing apparatus, an extensive system of membrane-bound sacs called thylakoids, their membranes housing the pigment molecules that absorb energy of photons of particular energy emitted by the sun, thereby initiating the physico-chemical sequence of steps in the photosynthesizing process.

In general, in the familiar example of green plants, the photosynthetic process enables the energy of photons radiated from the sun to energize electrons in special photon-absorbing pigment molecules embedded in the thylakoid membranes, electrons ultimately supplied by the splitting of water molecules in a reaction that also converts water's oxygen atoms to molecular oxygen for release into the atmosphere and for use by the plant. The energized electrons subsequently transfer their energy to energy-carrier molecules, the energy therein used to synthesize organic compounds using the inorganic carbon compound, carbon dioxide, as the carbon source starting material.