4.9: Photosynthesis in detail: the energy of sunlight is captured as chemical energy.

Photosynthesis is a complex process, but our understanding can be greatly aided by remembering one phrase: FOLLOW THE ELECTRONS. Where are they coming from? What are they passing through? Where are they going? What will happen to them when they get there?

In the first part of photosynthesis, the “photo” part, sunlight hits the chloroplasts of a plant’s leaves and some of the energy in this sunlight is captured and stored in ATP and in another molecule, called NADPH, which stores energy by accepting high-energy electrons (FIGURE 4-17). After these transformations, the captured energy is ready to be used to make sugar molecules in the “synthesis” part of photosynthesis.

The energy-capturing process occurs in two photosystems. Embedded in the thylakoid membranes in the chloroplast, these photosystems are structures composed of light-catching pigments (including chlorophyll) and protein. As the pigments absorb photons, electrons in the pigments gain energy and become excited, and then return to their resting state, releasing energy. The released energy (but not the electrons) is transferred to neighboring pigment molecules. This process continues until the energy transferred among many pigment molecules makes its way to a chlorophyll a molecule at the center of the photosystem, and excites an electron there (FIGURE 4-18). This is where the electron journey begins.

The chlorophyll a molecule at the center of the photosystem is special, differing from the other pigment molecules in one key feature. When its electrons are boosted to an excited state, they do not return to their resting, unexcited state. Instead, the special chlorophyll a continually loses its excited electrons to a nearby molecule, called the primary electron acceptor, which acts like an electron vacuum.

The electrons taken away from the special chlorophyll a molecule must be replaced. The replacement electrons come from water. As long as photosynthesis is occurring, a constant supply of replacement electrons is required. Molecules of water, near the special chlorophyll a molecule in the thylakoid membrane, are continuously split. This split—in which four photons of light split two molecules of water into four electrons, four protons (H⁺), and a molecule of O₂—provides the electrons necessary to replenish chlorophyll a’s electron supply. A convenient and life-sustaining by-product of the splitting of water in photosynthesis is the oxygen that is released from the cell, a by-product essential for much of the life on earth, including all animal life.

Once the primary electron acceptor gets hold of the high-energy electron from chlorophyll a, it passes it along like a hot potato to another molecule, which passes it to another, which passes it to yet another, in what is called an electron transport chain (FIGURE 4-19). The photosynthetic electron transport chain consists of two photosystems and several protein complexes that hand off electrons from one to the next. At each step in the electron transport chain’s sequence of electron handoffs, the electrons fall to a lower energy state, and a little bit of energy is released. These bits of energy are harnessed to power pumps in the thylakoid membrane that move protons (which are also referred to as hydrogen ions or H⁺ ions) from the stroma to the inside of the thylakoid. The pumps pack the protons inside the thylakoid sac at higher and higher concentrations. Think of a pump pushing water into an elevated tank, creating a store of potential energy that can gush out of the tank with great force and kinetic energy. Similarly, the protons eventually rush out of the thylakoid sacs with great force—and the force of the protons moving down their concentration gradient is harnessed to build energy-storing ATP molecules, one of the two products of the “photo” portion of photosynthesis.

Recall that the energy-capturing and energy-transforming processes of the “photo” reactions occur in two photosystems (arrangements of chlorophyll and other light-catching molecules). The electron transport chain physically links the first photosystem to the second. As the traveling electrons continue their journey, they fill electron vacancies in the reaction center of the second photosystem, right next to the first photosystem (FIGURE 4-20). Like the first photosystem, the second photosystem also has numerous pigments that harness photons from the sun and pass the light energy to another special chlorophyll a molecule. The special chlorophyll a molecule at the center of this second photosystem has electron vacancies because, as in the first photosystem, when electrons in the special chlorophyll a molecule are boosted to an excited state, they are whisked away from the chlorophyll molecule by another primary electron acceptor. This electron acceptor then passes the electrons to a second electron transport chain. At the end of this second electron transport chain, the electrons are passed to a molecule called NADP⁺, creating NADPH, a high-energy electron carrier. NADPH is the second important product of the “photo” portion of photosynthesis.

With the electrons’ passage through the second photosystem and arrival in NADPH, we now have the final products of the “photo” part of photosynthesis (which is also called the “light-dependent reactions”): we’ve captured light energy from the sun and converted it to the chemical energy of ATP and the high-energy electron carrier NADPH (see Figure 20). But we haven’t made any food yet. In the next section, we cover the “synthesis” part of photosynthesis and see how plants use the energy in ATP and NADPH to produce sugar from carbon dioxide.