Question

Essay Question

Explain clearly the light and dark reactions of photosynthesis? EXPLAIN IN DETAIL

Answer 1

Answer :

Photosynthesis can be dividedin two types of reactions, the light reactions and the dark reactions. In the light reactions,light energy is used to excite a cofactor. Then, an electron is transferred from there to its final acceptor. The excitation and the initial charge separation takes place in reaction centers.The reaction centers of all photosynthetic organisms are similar but differ to some extend in composition and in the redox potentials of the cofactors. Anoxygenic photosynthesis involves only one reaction center, while oxygenic photosynthesis involves two reaction centers. The reaction centers and a membrane-bound cytochrome complex of bc-type generate a transmembrane pH gradient. The ATP-synthase uses this pH gradient to produce ATP from ADP and inorganic phosphate. Furthermore, the light energy is used to reduce NADP to NADPH. The ATP and NADPH produced in the light reactions drive the carbohydrate synthesis in the dark reactions. Carbohydrate synthesis is accomplished by the Calvin cycle, which is a complicated network of biochemical reactions. Also various regulatory processes couple the light and the dark reactions. The membrane proteins involved in the light reactions of photosynthesis are not equally distributed over the thylakoid membrane. Photosystem II and the light harvesting complex II concentrate in the grana thylakoids, while photosystem I and the ATP-synthase concentrate in the stroma thylakoids. The cytochrome b6f complex has nearly the same concentration in both thylakoid regions. The functional reason for the grana stacking is presumably to maintain the separation of photosystem II and photosystem I. Without physical separation of the two photosystems, photosystem I would unbalance the excitation energy within the pigment bed of photosystem II. Furthermore, photosystem I is more efficient in exciton usage.

Figure 1: Light reactions of oxygenic photosynthesis. Electron and proton transfer involves four membrane-spanning proteins (photosystem II, cytochrome b6 f, photosystem I, ATP-Synthase), one protein that is associated to the membrane (Ferredoxin-NADP-Reductase) and two soluble proteins (plastocyanin, ferredoxin). ATP-synthase uses the pH gradient to form an ATP from ADP and inorganic phosphate. The general pathway of the electron flow from the primary donor (water) to the final acceptor(NADPH) is known in detail, while much less is known about the cyclic electron flow. It is not clear whether ferredoxin interacts with cytochrome b6f or not. Dotted lines, thin solid lines, and thick solidlines indicate electron-transfer reactions, proton transfer reactions, and diffusion processes respectively.

The Light Reactions

The light reactions of photosynthesis convert light energy into a transmembrane pH gradient, i. e., into electrochemical energy. The ATP-synthase uses the pH gradient to form an ATP from ADP and inorganic phosphate and thus converts the electrochemical into chemical energy. Figure 1 shows a schematic representation of the energy transducing reactions involved in the light reactions of photosynthesis. Light harvesting complexes absorb light energy and transfer the excitation energy to the special pair, a chlorophyll dimer. In photosystem II, the excited special pair releases one electron. This electron is transferred via chlorophyll, pheophytin, and quinone (QI or QA) to a quinone acceptor (QII or QB). After the quinone received two electrons and two protons, it leaves its binding pocket and enters the membrane. Ferredoxin-NADP reductase is a flavin-adenine dinucleotide containing protein. It is associated to the stromal side of the thylakoid membrane. The protein which mediates the membrane association is not unequivocally known. Probably subunit E of photosystem I is involved in the membrane association of ferredoxin-NADP reductase FerredoxinNADP+ reductase oxidizestwo ferredoxins and usesthe electronsto reduce NADP+ to NADPH,which is needed in the dark reactions of photosynthesis. The crystal structure of ferredoxinNADP+ reductase is known with and without NADP+ associated to the protein. The ATP obtained from this reaction is used in thedark reactions of photosynthesis to synthesize carbohydrates. Because the two photosystems work together in oxygenic photosynthesis, water can be used as primar electron donor for carbon fixation. Beside the electron transfer from water to NADPH, also a cyclic electron transfer occurs in the chloroplasts. Cyclic electron transfer involves photosystem I, cytochrome b6f , plastocyanin, plastoquinones, ferredoxin, and probably also ferredoxin-NADP+ reductase. About the presence of an additional enzyme called ferredoxin-plastoquinone reductase was speculated; such activities may however be also intrinsically be performed by other components of the thylakoid membrane such as photosystem I or ferredoxin-NADP+ reductase.

The light reaction is a light-dependent process which includes a series of events such as light absorption, hydrolysis, the release of oxygen, formation of ATP and NADPH. The light reaction of photosynthesis initiates only when it is supplied with light energy. The photosystem is the arrangement of pigments including chlorophyll within thylakoids.

There are two photosystems in plants:

• Photosystem I (PS-I)
• Photosystem II (PS-II)

Photosystem I absorbs light at a wavelength of 700 nm, whereas Photosystem II absorbs light at a wavelength of 680 nm.

The light reaction occurs in the thylakoids of the chloroplast. When the light hits, chlorophyll a get excited to higher energy state followed by a series of reactions. This energy is converted into energy molecules ATP and NADPH by using PS I and PS II. Also, hydrolysis occurs and releases oxygen.

• This phenomenon occurs in the presence of light.
• The pigment absorbs light and produces energy in the form of ATP.
• The process involves- absorption of light, water splitting, the release of oxygen, and formation of ATP and NADPH.
• The protein-bound pigment molecules form the light-harvesting complexes present within two photosystems- PS-I and PS-II.
• Each photosystem has a reaction centre consisting of chlorophyll a molecule, and antennae containing accessory pigments.
• The reaction centre for PS-I is P-700 because the absorption peak for chlorophyll a is at 700nm while that for PS-II is P-680 because the absorption peak for chlorophyll a is at 680 nm.

Photophosphorylation

The formation of ATP in the presence of sunlight is called photophosphorylation.

It is of two types:

• Non-cyclic photophosphorylation
• Cyclic photophosphorylation

Non-cyclic Photophosphorylation

• PS-II absorbs light at a wavelength of 680 nm and causes excitation in the electrons.
• These excited electrons are accepted by an electron acceptor and transferred to the electron transport system.
• The electrons from the electron transport system are transferred to the PS-I. At the same time, the electrons at PS-I receive a wavelength of 700 nm and get excited.
• An electron from the electron acceptor is added to NADP+, which is then reduced to NADPH+ H+.
• The electrons lost by PS-II do not return to it and hence named non-cyclic photophosphorylation.

In this, both the photosystems were involved.

Cyclic Photophosphorylation

• In cyclic photophosphorylation, only PS-I is involved.
• The electrons circulate within the photosystem which results in a cyclic flow of electrons.
• This only forms ATP and not NADPH+ H+.

The Dark Reactions

The light energy is converted into the chemical energy of ATP during the light reactions of photosynthesis. It is, however, very inefficient to store the energy in the form of ATP and NADPH. Carbohydrates or lipids need much less volume to save the same amount of energy. During the dark reactions of photosynthesis, the chemical energy of ATP is interconverted into the chemical energy of carbohydrates. Furthermore this energy is used to fix carbodioxide in the Calvin cycle. Plants and cyanobacteria are therefore able to use carbodioxide as sole carbon source. The enzymes of the Calvin cycle are located in the stroma of the chloroplasts. Although, none of the dark reactions of photosynthesis was investigated in this work, I briefly summarize the main features of the Calvin cycle for the sake of completeness.

Figure2: Calvin Cycle. 1) Ribulose-1,5-bisphosphate carboxylase cleaves ribulose-1,5-bisphosphate and attaches a CO2 to one of the fragments. Two 3-phosphoglycerate molecules emerge out of one ribulose-1,5-bisphosphate and one CO2.

2) Phosphoglycerate kinase phosphorylates 3- phosphoglycerate to 1,3-bisphosphoglycerate.

3) Glyceraldehyde-3-phosphate dehydrogenase reduces the phosphorylated carboxyl group to an aldehyde group.

4) The resulting glyceraldehyde-3-phosphate is used for the synthesis of fructose-6-phosphate, the product of the Calvin cycle. Ribulose-5-phosphate is regenerated from glyceraldehyde-3-phosphate in a complex reaction scheme which involves several enzymes.

5) Ribulose-5-phosphate is phosphorylated to ribulose-1,5-bisphosphate carboxylase by the enzyme phospho-ribulose kinase. This reaction closes the Cavin cycle.

6) The product fructose-6-phosphate is used to synthesize sugars and polysaccharides such as starch and cellulose.

The Calvin cycle can be divided in two stages. In the first stage ATP and NADPH is used to fix carbodioxide. Two NADPH molecules and three ATP molecules are required to fix one carbodioxide molecule. In the second stage, the carbon atoms are shuffled to enable the release of one sugar molecule. The sugar is then used to synthesize other molecules or stored in the form of polysaccharides such as starch or cellulose. The major steps of the first stage of the Calvin cycle are summarized in Figure 2. The key enzyme of the Calvin cycle is ribulosebisphosphate carboxylase.

Dark reaction is also called carbon-fixing reaction. It is a light-independent process in which sugar molecules are formed from the carbon dioxide and water molecules. The dark reaction occurs in the stroma of the chloroplast where they utilize the products of the light reaction. Plants capture the carbon dioxide from the atmosphere through stomata and proceed to the Calvin cycle. In the Calvin cycle, the ATP and NADPH formed during light reaction drives the reaction and convert 6 molecules.

Calvin Cycle (C3 Cycle)

This cycle involves the following steps:

• Carbon-fixation: Ribulose-1,5-bisphosphate combines with carbon dioxide to fix it to a 3 carbon compound 3-phosphoglyceric acid.
• The enzyme RuBisCO is involved in the process.
• Reduction: 2 molecules of ATP and NADPH fixes one molecule of carbon dioxide to form glyceraldehyde-3-phosphate.
• Regeneration: Some glyceraldehyde-3-phosphate molecules undergo a series of reactions to form glucose while the RuBP regenerates to continue the cycle.

C4 Cycle (Hatch and Slack Pathway)

• It is a cyclic pathway.
• The enzymes involved in the C4 pathway are located in the Mesophyll cells and Bundle Sheath cells.
• In this pathway, the plants convert atmospheric carbon dioxide into a four carboncontaining chemical compound.
• Phosphoenolpyruvate is the primary carbon dioxide acceptor and is located in the mesophyll cells. The reaction is mediated by phosphoenolpyruvate carboxylase.
• After this, aspartic acid and malic acid are formed within the mesophyll cells and transported to the bundle sheath cells. Here, the C4 acids breakdown to release threecarbon molecules and carbon dioxide.
• The three-carbon molecules move back to the mesophyll cells where they get converted into phosphoenolpyruvate and complete the cycle.
• The carbon dioxide enters the bundle sheath cells and completes the Calvin cycle.