The light dependent reaction of photosynthesis

OVERVIEW OF PHOTOSYNTHESIS

Photosynthesis is a process by which green plants and certain animals convert light energy into chemical energy and releases oxygen as a waste product. Photosynthesis occurs not only in eukaryotic organisms such as green plants and green algae but also in prokaryotic organisms such as cyanobacteria and certain groups of bacteria. In higher plants and green algae, the reactions of photosynthesis occur in the chloroplast. The chloroplast traps the radiant energy of sunlight and conserves some of it in a stable chemical form. The reactions that accomplish these energy transformations are identified as the light dependent reaction of photosynthesis. Further, the light reactions of photosynthesis take place on the thylakoids, which is a specialized membrane system that contains four kinds of protein complexes. These complexes function in the capture of light and the synthesis of ATP and NADPH.

Thus, the light reactions harness the light energy to drive the transport of electrons and the pumping of the proton and produce ATP and a usable source of reducing the power NADPH.

Besides CO2, photosynthesis also helps in fixing N2. Some bacteria use the products of the light reactions of photosynthesis to fix N2.

Stages of photosynthesis

Stage 1: capturing solar energy and transferring it to electrons.
Stage 2: using captured solar energy to make ATP and to transfer high-energy electrons to NADP⁺; yields NADPH, which is then used as a high-energy electron carrier molecule.
Stage 3: using energy stored in ATP and high-energy electrons carried by NADPH to form energy-rich organic molecules, such as glucose, from CO2

Definition of Light-dependent Reaction

Light-dependent reactions are the set of reactions of photosynthesis in which light energy excites electrons in chlorophyll molecules, powers ATP synthesis and results in the reduction of NADP⁺ to NADPH.

As mention earlier, the light reactions take place in the thylakoid membranes of chloroplasts. The principal light-absorbing pigments in the thylakoid membranes are chlorophylls a and b and also carotenoids. Chlorophylls a and b absorb violet, blue, and red wavelengths from the visible light spectrum and reflect green. Also, the carotenoid pigments absorb violet-blue-green light and reflect yellow-to-orange light.

Basically, electron transfer from water to NADP⁺ occurs by two successive photochemical reactions in two different types of reaction centres called photosystem II (PSII) and photosystem I (PSI). Moreover, these two photosystems operate in series linked by a third multiprotein aggregate called the cytochrome complex (cyt_b6-f ). As electrons pass through the cyt_b6-f complex, protons are translocated from the stroma to the lumen. Once a proton difference developed, an ATP synthase uses the energy of the proton difference to make ATP. Furthermore, the electrons that result from the photochemical process are used to reduce NADP+ to make NADPH.

Let us first discuss where the light-dependent reaction of photosynthesis takes place. After that, we will discuss the process of the light-dependent reaction of photosynthesis.

Where does the light-dependent reaction take place?

The light-dependent reaction takes place in the thylakoid membranes of chloroplasts that contain four kinds of protein complexes, and these are:

1. Photosystem I (PS I)
2. Photosystem II (PS II)
3. Cytochrome complex
4. ATP Synthase

Photosystem I (PS I)

PS I is a pigment-containing protein complex that contains a core complex and antenna complexes. The core complex, which contains the reaction centre, contains seven polypeptides. The reaction centre contains two chlorophyll molecules, known as P700, which have an absorption maximum of 700 nm.

PS I complexes range in diameter from 10–13nm and appear mostly (85%) on the stromal leaflet of the membrane in the unstacked regions of the thylakoids.

Photosystem II (PS II)

Unlike PS I, the PS II complex has three major functional units:

• the PS II core complex, surrounded by
• an antenna complex, and
• the water-splitting complex.

The core complex contains about six polypeptides that bind four ‘chlorophyll a’ molecules, two of which form the reaction centre that has an absorption maximum of 680nm. These chlorophyll molecules are known as P680. The core also contains two pheophytins, one β-carotene, and one non-heme iron. The β-carotenes function as antioxidants that protect the photosystems from damage caused by the free radicals.

Cytochrome complex

 The cytochrome b₆f complex is a dimer, each monomer composed of eight subunits. These consist of four large subunits and four small subunits.

It accepts electrons from Photosystem II through plastoquinone and contributes to proton transport across the membrane.

ATP Synthase

The fourth complex in the thylakoid membrane is the ATP synthase, which transduces or couples the potential energy inherent in the trans-thylakoid proton difference into the chemical energy of ATP.

Diagram of Light dependent Reaction

The light-dependent reaction steps

1. Absorption of light at PS II

The antenna complex of PS II absorbs light energy and passed it in a somewhat random fashion to P680 by resonance transfer. Once the light is absorbed by P680, the energy of an electron is raised to an excited state and that energy is then passed to pheophytin. Pheophytin, considered the primary electron acceptor, is a form of ‘chlorophyll a’ in which the magnesium ion is replaced by two hydrogens. After that, the electron will pass from pheophytin to a bound form of plastoquinone known as Q_A and then to a freely diffusing form of plastoquinone known as Q_B.

After accepting two electrons, Q_B binds two H+ from the stroma. Pari passu, the oxidized P680 becomes a stronger oxidizing agent and strips an electron from water, which causes the formation of H+ in the lumen and the evolution of ½O2. Please note that one H+ will produce in the lumen from a water molecule for every photon absorbed by P680.

2. Transport of electron from plastoquinone to cytochrome complex

Pheophytin passes the electron to the plastoquinones, which in turn passes it to the cyt_b6f complex. As the electrons pass through the cytochrome chain, protons will pass from the stroma to the lumen. Since the electrons take a zigzag course across the membrane, the biochemical pathway of the light reactions was dubbed Z-scheme of photosynthesis.

3. Synthesis of ATP

After the development of the proton concentration gradient across the thylakoid membrane, the protons will move through the CF₀ portion of the ATP synthase. The energy thus produced will be used for the synthesis of ATP from ADP and Pi. In this way, ATP will form in the light reactions of photosynthesis.

4. Absorption of light at PS I

The antenna complex of PS I will also absorb light. This energy is passed by resonance transfer in a random manner to P700. Once it reaches P700 an electron is raised to a higher energy level where it will reduce ferredoxin.

5. NADPH synthesis

After that, the ferredoxin-NADP⁺ oxidoreductase Oxidises two molecules of ferredoxin and a proton from the stroma to reduce NADP⁺ to NADPH. Thus, the formation of NADPH causes an alkalinization of the stroma and this contributes to the pH difference across the thylakoid membrane.

The ATP and the NADPH generated in the light reactions are then used to fix CO2 into glucose in the dark reactions that take place in the stroma.

End Product of light-dependent Reaction

Indeed, ATP and NADPH requiring reactions in the chloroplast compete with carbon fixation. Nevertheless, it is also possible that the ratio of ATP to NADPH is either regulated or variable.

1. It takes four electrons from PS I and four photons from PS II to reduce two NADP+ molecules and form two NADPH. Thus, it takes eight photons to produce two NADPH.
2. For every four photons absorbed by PS II, one molecule of oxygen will evolve.
3. Also, the ATP molecule will form from the hydrolysis of water.

Summary of the light-dependent reaction

1. In light-dependent reaction, light energy excites electrons in chlorophyll molecules, powers ATP synthesis and results in the reduction of NADP⁺ to NADPH.
2. these reactions take place in the thylakoid membranes of chloroplasts.
3. light will absorb at antenna complex of PS II and passes to P680.
4. after light absorption, an electron raises to an excited state and reduces pheophytin.
5. pheophytin passes the electron to the plastoquinones, which in turn passes it to cytochrome complex.
6. As a result, the proton concentration difference will develop across the thylakoid membrane.
7. ATP synthase utilizes this concentration difference and produces ATP from ADP and Pi.
8. the antenna complex of PS I will also absorb light and passed to P700.
9. after light absorption, an electron raises to an excited state and reduces ferredoxin.
10. finally, ferredoxin-NADP+ oxidoreductase oxidised two molecules of ferredoxin and a proton to reduce NADP+ to NADPH.
11. two molecules of NADPH, one oxygen and ATP molecule are the end product of the light-dependent reaction of photosynthesis

SOURCES AND EXTERNAL LINKS

Plant cell biology from Astronomy to zoology by Randy Wayne, Chloroplast: Organization of the Thylakoid membrane and the light reactions of photosynthesis.

Introduction to Plant physiology: fourth edition by William G. Hopkins and Norman P. A. Huner, chapter no. 7: Energy Conservation in Photosynthesis:
Harvesting Sunlight

Plant Biochemistry edited by P. M. Dey and J.D Harborne, chapter no. 2: photosynthesis

https://en.wikipedia.org/wiki/Cytochrome_b6f_complex