All types of stimuli that trigger certain responses from cells are broadly classified into two categories:
Self/ Endogenous Ligands
A ligand is a carrier molecule that can binds to a stimulus, thus, producing a signaling molecule. Ligands bind to specific receptors in or the surface of the cell.
Self/endogenous signals are signaling molecules produced by the body itself. They are part of normal cellular communication.
2. Non-self/Exogenous Ligands:
These are signals produced by a foreign source such as a pathogen or a drug administered into the body. Basically, any signal that is foreign to the body and not produced by the body itself are exogenous ligands. Exogenous ligands elicit an immune or allergic response in the body. When identified, these ligands and their source is attacked by the immune system.
Answer: Photosynthesis uses the energy obtained from light to free electrons from the chlorophyll pigments that collect the light. In cellular respiration the electron transport chain occurs after glucose has already been broken down.
