Light Reaction

Introduction

 Majority of autotrophs (organisms that are not dependent on external sources for their nutrition) are dependent on light reactions for energy production. Such organisms are called photoautotrophs. Light reactions are chemical reactions that take place in the presence of light. In plants the process that involves a light reaction is called photosynthesis. This is the process of converting light energy into chemical energy. In this process the light energy obtained from the sun is converted into ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).

The Photosynthetic Process

The process of photosynthesis involves the conversion of water and carbon dioxide in the presence of sunlight into oxygen and glucose. 

 6CO2 + 6H2O → C6H12O6 + 6O2

 Plant cells contain chloroplasts, an organelle which is one of the major factors that differentiate plant cells from animal cells. Photosynthesis takes place inside chloroplasts. There are two stages involved in photosynthesis.

1. Stage 1 where water is used and oxygen is produced in the presence of sunlight.
2. Stage 2 where glucose is produced and carbon dioxide is used.

Chloroplasts

There are disclike flattened stacks of membranes contained within chloroplasts called thylakoids. These contain the green pigment chlorophyll which absorbs sunlight. Chlorophyll is found in the membrane of thylakoids. The space within the chloroplast is divided into two sites where photosynthesis takes place:

•     The first phase of photosynthesis which involves the absorption of sunlight and uses water and the resultant is oxygen takes place in the lumen of the thylakoids. The lumen is the space inside the thylakoids
•     The second phase takes place outside the thylakoids but inside the chloroplasts. The space that surrounds the thylakoids inside the chloroplasts is filled with a liquid called stroma. Here carbon dioxide is used and glucose is produced

The photosynthetic process consists of two kinds of reactions: light dependent reaction and light independent reaction.

Light Dependent Reactions in Photosynthesis

Following is an overview of the light dependent part of the reactions in photosynthesis:

• Molecule of chlorophyll – loses one electron as it absorbs one photon
• This released electron is taken up by pheophytin(primary electron acceptor) which is a modified form of chlorophyll
• Pheophytin then passes the electron onto a quinone molecule
• This process starts the electron transport chain or a flow of electrons
• The electron transport chain ultimately causes the reduction of NADP to NADPH

The above process causes a gradient of energy which helps ATP synthase to produce ATP. The chlorophyll molecule regains its lost electron when water splits to give an electron. Following is the reaction for the light dependent reaction taking place in plants in the presence of non-cyclic electron flow:

 2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + O2

 So, during light reaction in photosynthesis the following are formed:

• NADPH
• Proton cations
• ATP
• O2

 There are two parts of the light dependent reactions in photosynthesis. They are the z scheme and water photolysis.

Z Scheme of Photosynthesis

The z scheme is a scheme used to describe the light reactions that take place in photosynthesis.

There are two types of light reactions:

• Cyclic
• Non-cyclic

 The cyclic reaction is called so because the electron is emitted from photosystem I, passes onto electron acceptor molecules and finally returns to photosystem I. Cyclic reaction is the same as the non-cyclic process. Here too ATP is created but no NADPH.

 The non-cyclic reactions (Z scheme) follow the undermentioned steps:

1. The antenna complexes such as photosystem II, chlorophyll, and accessory pigments absorb a photon and release an electron. This antenna system is found in the photosystem II’s chlorophyll molecule.
2. This releasing of an electron is known as photoinduced charge separation.
3. This released electron is taken up by pheophytin which is the main electron acceptor molecule. 
4. The electrons are now in a flow called the electron transport chain.
5. This causes the proton cations to be shuttled across the thylakoid space making an energy gradient across the chloroplast. This gradient is also known as a chemiosmotic potential.
6. The ATP synthase uses this chemiosmotic potential to create ATP by photophosphorylation.
7. Now the electron is absorbed by photosystem I.
8. In the photosystem I the electron is further agitated by the absorption of more light. 
9. This energised electron passes along other electron acceptors transferring its energy along the way.
10. This energy in the electron acceptors is used to transport hydrogen ions across the thylakoid membrane. 
11. The electron is finally used to reduce NADP to NADPH. This is the part where the journey of the electron ends.

 Water Photolysis

In green plants and cyanobacteria the source of electrons is water. Four successive reactions of photosystem II separate the charge from two water molecules. This yields two four hydrogen ions and a diatomic molecule of oxygen. A redox active tyrosine residue gets the released electrons. It is then oxidised by using the energy from photosystem II. When this energy from photosystem II is used up it regains its ability to reabsorb another photon and release another electron which is photodissociation. Photosystem II is the only known enzyme in nature that oxidises water. The hydrogen ions from the oxidation of water are released into the lumen of the thylakoid. Thus a chemiosmotic potential is created which is used by ATP synthase to produce ATP.

 Light Independent Reactions of Photosynthesis

There are two main parts of the light independent reactions of photosynthesis.

 The Calvin cycle: This is the sequence in which the plants use the carbon dioxide from the air to make sugar, the nutrition that plants use for their metabolic activities. Carbon dioxide enters the leaves of plants through the stomata on the lamina of the leaves. It then goes into the stroma inside the chloroplasts where the reactions of the Calvin cycle take place. There are three main stages in the Calvin cycle:

○ Carbon fixation

○ Reduction

○ Regeneration

 Along with the carbon dioxide, the stroma also has the enzyme RuBisCo and ribulose biphosphate (RuBP – has 5 carbon atoms and two phosphate groups at each end). The reaction between RuBP and carbon dioxide ( 6-carbon compound), catalysed by RuBisCo which is converted into two 3-carbon sugars immediately. This process is the “fixation” of carbon into organic forms from its inorganic forms. 

 The reduction happens when ATP reduces the 3-carbon compounds into another type of 3-carbon compounds called G3P. One of these G3P molecules forms a part of the glucose molecule. And since the glucose molecule has 6 carbon atoms, it takes 6 turns of the Calvin cycle to form one molecule of glucose. The regeneration part of the Calvin cycle happens when the remaining G3P regenerates RuBP.

 Carbon concentrating mechanisms: This type of mechanism increases the efficiency of photosynthesis by increasing the concentration of carbon dioxide near RuBP. On land when the weather conditions are hot and dry plants close their stomata to prevent water loss. This makes the concentration of oxygen high and the restriction of the inflow of air makes carbon dioxide less concentrated. 

 One mechanism is the fixation of four-carbon acids. When the four-carbon acids undergo decarboxylation they release carbon dioxide which is then used in the Calvin cycle. Some plants accumulate crystal oxalate. This compound acts like the storage of carbon dioxide which can be used by the plant in photosynthesis.

 Conclusion

Photosynthesis is an important process in the nutrition cycle. It occurs in the chloroplasts of plant cells. These are the organelles that house the chlorophyll-containing thylakoids. There are various complex reactions and processes which form a part of photosynthesis. However, studying these processes can be fascinating because they display how delicate and fine the mechanisms of nature are where transport of substances takes place at a molecular level and atomic activities fuel some of the biggest organisms on the planet.