# The Light Reactions

The light reactions, also known as photolysis reactions, convert energy from the sun into chemical energy in the form of NADPH and ATP. These reactions must take place in the light and in chloroplasts of plants.

### Introduction

Chlorophyll, which is a light-absorbing organelle in plant cells, is a very complex molecule that works in conjunction with the metal magnesium. There are primarily two types of chlorophyll: chlorophyll a and chlorophyll b. These chlorophyll molecules absorb light in the red and blue wavelengths, making the plants in which they are stored look green.

### Photosystem I and II

Photosystems are light-absorbing complexes in the thylakoid membranes that are present in photosynthetic organisms. There are two types of photosystems: Photosystem I and Photosystem II. Each has one primary photochemical reaction center (either chlorophyll P700 or P680) and a set of accessory pigments to absorb additional light.

There is a special molecule called chlorophyll a P700, located in photosystem I, which absorbs light best at 700 nanometers (nm). It also contains other accessory pigments. Another special chlorophyll a molecule in photosystem II is called chlorophyll a P680 because it absorbs best at 680 nm. Photosystem II also contains chlorophyll b and other accessory pigments.

Figure 1. The structures of chlorophyll a and chlorophyll b, respectively. Notice that the only difference between the two is the additional carbonyl group that chlorophyll b has. Used with permission from Wikipedia Commons.

All photosythetic cells and photosynthetic bacteria contain photosystem I but only higher organisms such as plants, algae, and cyanobacteria have both photosystem I and II. To start the reactions, a photon of light must be absorbed by a chlorophyll molecule. Light will hit a chlorophyll a P700, located in photosystem I, causing the electrons to become excited and move to a higher energy level. These high energy electrons can then flow along one of two pathways giving cyclic electron flow or noncyclic electron flow.

### Noncyclic Electron Flow

Figure 2. The Z scheme of electron transport links the two photosystems. This is schematic diagram showing flow of electrons from water to NADP+. It is called the Z scheme because it links the two photosystems in a way that resembles the letter "Z". Can be found at http://commons.wikimedia.org/wiki/File:Z-scheme.png.

In this pathway, the electrons that were excited by the P700 in photosystem I are transferred to the electron acceptor NADP+, which is very much like NAD+ , used in cellular respiration. Chlorophyll a P700 becomes a strong oxidizing agent because it lost electrons. The electrons moved to a higher energy level because they were excited by a photon of light. The energized electrons are passed through a chain of electron carriers beginning with A0, which is a form of chlorophyll.

The first electron-transfer step is

$P700^* + A_0 \rightarrow P700^+ + A_0^- \tag{1}$

There is a spontaneous flow of electrons because the excited electrons are at such a high energy state. A0 transfers its electrons to A1 (phylloquinone) to an iron-sulfur protein (Fe-S) complex to ferrodoxin (another Fe-S complex). Lastly, the electrons are transfered to NADP+.

$2 Fd_{(reduced)} + NADP^+ +2H^+ \rightarrow 2Fd_{(oxidized)} + NADPH + H^+ \tag{2}$

The transfer of electrons from P700 creates an unstable P700+. The "electron gap" is then filled by chlorophyll P680.  When light hits P680 in photosystem II, electrons become excited and travel down another electron carrier chain until it reaches the chlorophyll a P700. The energized electrons travels to Pheophytin (Ph) to plastoquinone QA to plastoquinone QBto a cytochrome bf complex to plastocyanin to P700+Those electrons are accepted by the chlorophyll a P700 molecule and ATP is produced through noncyclic phosphorylation.

Keep in mind that Ph, QA, QB, cytochrome bf complex, and plastocyanin are all electron carriers. Cytochrome bf is a cluster of membrane proteins that use heme groups and iron-sulfur clusters. Plastocyanin is a blue copper protein.

Unfortunately, chlorophyll a P680 is now electron deficient. But, it can oxidize water, through the oxygen evolving complex (OEC), to gain the same number of electrons it lost. Water is oxidized this way:

$2 H_2O \rightarrow 4 H^+ + 4e^- + O_2 \tag{3}$

The OEC contains a cofactor that has a cluster of 4 manganese (Mn) ions. This Mn complex is highly oxidized so it is able to transfer the electrons in water to P680+ (oxidized):

$[Mn_4]^{4+} + 2H_2O \rightarrow [Mn_4]^0 + 4H^+ + O_2 \tag{4}$

The electrons are taken in by the Mn complex and are given to P680+ one by one. The net result of this reaction is the production of 2 ATP and 9 NADPH and the photolysis of water. The ATP and NADPH will be used in the Calvin-Benson-Bassham cycle of the dark reactions.

#### Reactions

The electron acceptor in chloroplasts of green plants is NADP+ and the donor is H2O. The electrons are flowing in an uphill direction which means energy from light is required. To split each water molecule, 4 electrons are necessary. The energy is provided by 8 photons of light. The net reaction of the reduction of NADP+ and oxidation of water is:

$2 H_2O + 2 NADP^+ \xrightarrow[]{light} 2 NADPH + 2 H^+ + O_2 \tag{5}$

#### Photophosphorylation

The Z scheme is accompanied by the phosphorylation of ADP. This process which converts light energy into chemical energy is called photophosphorylation. Light induced electron transfer from H2O to NADP+ pumps protons through the thylakoid membrane to the inner compartment. On the outer surface of thylakoid membranes, there are two protein complexes, CF0and CF1, that make up an enzyme called ATP synthase. CF0 acts as a proton channel across the membrane whereas CF1acts as a binding site for the joining of ADP and Pi to make ATP.

Figure 3. Diagram of the thylakoid membrane showing electron transfer through a number of electron carriers. It also shows the enzyme ATP synthase pumping protons. The unequal proton concentrations cause the H+ to move across the membrane and provide energy for ADP phosphorylation. Used with permission from Wikipedia Commons.

### Cyclic Electron Flow

Cyclic electron flow is also observed in the chloroplasts of green plants. It results in the production of ATP but not O2 or NADPH. Only photosystem I is present in this reaction. In cyclic electron flow, the electrons that were excited by P700 move along a chain of electron carriers. However, they never reach NADP+. Once they reach ferrodoxin, they are tansferred to the cytochrome bf complex. Then, in the process of being transferred to plastocyanin, an ATP molecule is made from ADP and Pi. From the plastocyanin, they flow back to the P700+. These reactions are meant to produce ATP from ADP and inorganic phosphate in a process called cyclic photophosphorylation by pumping protons across the thylakoid membrane.

This cyclic pathway may be used when a plant has enough NADPH but requires synthesis of ATP.

### References

1. Zubay, Geoffrey. Biochemistry. New York: Macmillan Publishing Company, 1988.

### Problems

1. Select the true statement:

In the cyclic pathway of electron flow:

a) water is oxidized

b) O2 evolution occurs

d) ADP is phosphorylated to ATP

2. What is the electron acceptor in the light reactions?

3. What is the driving force of production of ATP?

4. True or False: Photosynthetic bacteria contain both photosystem I and II.

5. What are the two protein complexes that make up ATP synthase?