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 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.
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 photosynthetic process consists of two kinds of reactions: light dependent reaction and light independent reaction.
Following is an overview of the light dependent part of the reactions in photosynthesis:
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:
There are two parts of the light dependent reactions in photosynthesis. They are the z scheme and water photolysis.
The z scheme is a scheme used to describe the light reactions that take place in photosynthesis.
There are two types of light reactions:
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:
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.
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.
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.
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