As you learned in the tutorial on cellular respiration, an electron transport chainis a series of redox reactions. There are two electron transport chains used in photosynthesis and they are energetically linked to one another as electrons travel through both.
This figureillustrates the movement of electrons through the two electron transport chains. Both photosystems are absorbing sunlight at the same time but let's start our analysis by looking at the activity in photosystem II. Light is absorbed by photosynthetic pigments and the absorbed energy excites an electron to a higher energy state. The excited electron moves around the photosystem until it is transferred to the primary electron acceptor molecule of the associated electron transport chain. The electron travels through the electron transport chain (via a series of redox reactions) until it is passed to a chlorophyll molecule inphotosystem I. Sunlight is absorbed by the pigment molecules in photosystem I and the electron is once again excited and transferred along the electron transport chain associated with photosystem I. When this process is diagrammed, it looks like the letter Z on its side. An electron is excited to a higher energy state, falls back down in energy, is excited again, and falls back down again. Thus, the two electron transport chains are called the Z-scheme .
Water is the source of the electrons that are used in photosynthesis. As shown in this figure, electrons are removed from water in photosystem II, and consequently, these electrons become energized into a more energetic state. Protons and oxygen are released as by-products of this reaction.
Remember, the electron transport chain of respiration occurs in the mitochondria, while the electron transport chains of photosynthesis are located in the chloroplasts.