6.3. Organizing Pigments into Photosystems

The light-dependent reactions of photosynthesis occur on membranes. In most photosynthetic bacteria, the proteins involved in the light-dependent reactions are embedded within the plasma membrane. In algae, intracellular membranes contain the proteins that drive the light-dependent reactions. In plants, photosynthesis occurs in specialized organelles called chloroplasts. The chlorophyll molecules and proteins involved in the light-dependent reactions are embedded in the thylakoid membranes inside the chloroplasts. A portion of a thylakoid membrane is enlarged in figure 6.5. The chlorophyll molecules can be seen as the green spheres embedded along with accessory pigment molecules within a matrix of proteins (the purple area) within the thylakoid membrane. This complex of protein and pigment makes up the photosystem.

Figure 6.5. Chlorophyll embedded in a membrane.

Chlorophyll molecules are embedded in a network of proteins that hold the pigment molecules in place. The proteins are embedded within the membranes of thylakoids.

The light-dependent reactions take place in five stages, illustrated in figure 6.6. Each stage will be discussed in detail later in this chapter:

1. Capturing light. In stage 1, a photon of light of the appropriate wavelength is captured by a pigment molecule, and the excitation energy is passed from one chlorophyll molecule to another.

2. Exciting an electron. In stage 2, the excitation energy is funneled to a key chlorophyll a molecule called the reaction center. The excitation energy causes the transfer of an excited electron from the reaction center to another molecule that is an electron acceptor. The reaction center replaces this “lost” electron with an electron from the breakdown of a water molecule. Oxygen is produced as a by-product of this reaction.

3. Electron transport. In stage 3, the excited electron is then shuttled along a series of electron-carrier molecules embedded in the membrane. This is called the electron transport system (ETS). As the electron passes along the electron transport system, the energy from the electron is “siphoned” out in small amounts. This energy is used to pump hydrogen ions (protons) across the membrane, indicated by the blue arrow, eventually building up a high concentration of protons inside the thylakoid.

4. Making ATP. In stage 4, the high concentration of protons can be used as an energy source to make ATP. Protons are only able to move back across the membrane via special channels, the protons flooding through them like water through a dam. The kinetic energy that is released by the movement of protons is transferred to potential energy in the building of ATP molecules from ADP. This process, called chemiosmosis, makes the ATP that will be used in the Calvin cycle to make carbohydrates.

5. Making NADPH. The electron leaves the electron transport system and enters another photosystem where it is “reenergized” by the absorption of another photon of light. In 5, this energized electron enters another electron transport system, where it is again shuttled along a series of electron-carrier molecules. The result of this electron transport system is not the synthesis of ATP, but rather the formation of NADPH. The electron is transferred to a molecule, NADP⁺, and a hydrogen ion that forms NADPH. This molecule is important in the synthesis of carbohydrates in the Calvin cycle.

Figure 6.6. Plants use two photosystems.

In stage 1, a photon excites pigment molecules in photosystem II. In stage 2, a high-energy electron from photosystem II is transferred to the electron transport system. In stage 3, the excited electron is used to pump a proton across the membrane. In stage 4, the concentration gradient of protons is used to produce a molecule of ATP. In stage 5, the ejected electron then passes to photosystem I, which uses it, with a photon of light energy, to drive the formation of NADPH.

Architecture of a Photosystem

In all but the most primitive bacteria, light is captured by photosystems. Like a magnifying glass focusing light on a precise point, a photosystem channels the excitation energy gathered by any one of its pigment molecules to a specific chlorophyll a molecule, the reaction center chlorophyll. For example, in figure 6.7, a chlorophyll molecule on the outer edge of the photosystem is excited by the photon, and this energy passes from one chlorophyll molecule to another, indicated by the yellow zig-zag arrow, until it reaches the reaction center molecule. This molecule then passes the energy, in the form of an excited electron, out of the photosystem to drive the synthesis of ATP and organic molecules.

Figure 6.7. How a photosystem works.

When light of the proper wavelength strikes any pigment molecule within a photosystem, the light is absorbed and its excitation energy is then transferred from one molecule to another within the cluster of pigment molecules until it encounters the reaction center, which exports the energy as high-energy electrons to an acceptor molecule.

Using Two Photosystems

Plants and algae use two photosystems, photosystems I and II, indicated by the two purple cylinders in figure 6.6. Photosystem II captures the energy that is used to produce the ATP needed to build sugar molecules. The light energy that it captures is used to transfer the energy of a photon of light 1 to an excited electron 2; the energy of this electron is then used by the electron transport system 3 to produce ATP 4.

Photosystem I powers the production of the hydrogen atoms needed to build sugars and other organic molecules from CO₂ (which has no hydrogen atoms). Photosystem I is used to energize an electron that, carried by a hydrogen ion (a proton), forms NADPH from NADP⁺ 5. NADPH shuttles hydrogens to the Calvin cycle where sugars are made.

The photosystems are not numbered in the order in which they are used. Photosystem II actually acts first in the series, and photosystem I acts second. The confusion arises because the photosystems were named in the order in which they were discovered, and photosystem I was discovered before photosystem II.

Key Learning Outcome 6.3. Photon energy is captured by pigments that employ it to excite electrons that are channeled away to do the chemical work of producing ATP and NADPH.