Photosynthesis: Process, steps and reactions

What is photosynthesis?

Photosynthesis is the process in which most of the green plants use carbon dioxide and water to produce sugar (simple form of carbohydrates) in the presence of sunlight. The term photosynthesis means ‘synthesis using sunlight’. But why photosynthesis is so important? If you think it is useful for the production of oxygen and ultimately for the survival of most of the species on the earth, then you might be partly right. With the help of photosynthesis plants produce sugar which is necessary for most of the herbivorous animals. It also helps to maintain the level of carbon dioxide in the atmosphere. Lastly it produces about 170 million tons of dry matter of which 90% is produced in the ocean.

The overall chemical reaction of photosynthesis is represented as follows:

6CO2 + 12 H2O    ————>    sugar (glucose) + 6O2 + 6H2O

Site of photosynthesis

The process of photosynthesis takes place in the chloroplast. But where is this chloroplast present in plants? They are present in the middle layers (mesophyll cells) of leaf. If you carefully observe the structure of leaf, you will see the upper epidermis and below that there is a palisade mesophyll followed by spongy mesophylls. Now most of the chloroplasts are present in the palisade mesophylls. The chloroplast is surrounded by two membranes. It is filled with the fluid inside it known as stroma. There are flattened fluid filled sacs called as thylakoids or lamellae. When these thylakoids are arranged in a stack like pile of coins they are known as grana. Various grana are joined by intergranal or stroma lamellae. One should note that the process of photosynthesis occurs in two phases or two reactions. First is light reaction that occurs in grana region while second is dark reaction which occurs in the stroma of chloroplast.  

Layers of leaf
Mesophyll layer in leaf

Pigments involved in photosynthesis

Different pigments of photosynthesis are present in the chloroplast. The basic function of these pigments is to absorb different wavelengths of light. They belong to two main groups:

  • Chlorophylls

These are green pigments with two principle types as chlorophyll a (bluish green) and chlorophyll b (olive green). Chlorophylls absorb mainly violet-blue and red region of visible light (VIBGYOR region).

  • Carotenoids

These are orange-yellow pigments with two principle types as carotene (beta carotene) that is orange in color and xanthophyll that is yellow in color.

NOTE: Chlorophyll a mainly absorbs violet and red light

             Chlorophyll b mainly absorbs blue and red light

Raw material for photosynthesis

  • As per the overall reaction of photosynthesis you can observe that water and carbon dioxide act as reactants, hence they can be said as raw material for the process.
  • Since carbon dioxide is available in the atmosphere, plants utilize that during photosynthesis and released it during respiration.
  • Similarly when water is used as raw material it undergoes it gives H+ and OH- ions during photolysis. The obtained hydrogen ions are then used to synthesize energy rich molecules.

Mechanism of photosynthesis

As we discussed earlier that the process of photosynthesis occurs in two phases

  • Light reaction or photochemical phase
  • Dark reaction or biosynthetic phase

NOTE: It is important to understand that in the light phase the plant is making certain chemical or energy rich molecules such as NADPH and ATP which can be later used to synthesize the sugar in the bio-synthetic phase. Hence the goal of photo-chemical phase is to prepare energy rich molecule and in bio-synthetic phase it is important to synthesize sugar i.e. glucose.  

  1. Light phase or photo-chemical phase

As the name suggest it is clear that this phase requires light or specifically presence of photons. It occurs in the grana of thylakoid membrane. It involves several steps such as

  • Activation of chlorophyll

When the chlorophyll molecule receives the light energy or photons (smallest unit of light), it gets excited and emits electron. This emitted electron then travel through ETC (electron transport chain). The reaction can be represented as

Chlorophyll a  +  hv (photons of light) ————->  Chlorophyll a  + e (emitted electron).

Light and dark reaction in photosynthesis
  • Photolysis of water

This step involves splitting of water molecule with the help of light energy. The water molecule splits into it H+ and OH- ions.

  • Formation of NADPH

The hydrogen ions released by splitting of water are used to reduce the NADP molecule to NADPH since it is an energy rich molecule. This energy rich molecule is used to reduce carbon dioxide to produce sugars (glucose).

  • Photophosphorylation

This step involves addition of a phosphate group. When ADP is phosphorylated (addition of phosphate/ IP) and convert into ATP (another energy rich molecule) in the presence of sunlight is called as photophosphorylation.

The overall light reaction can be summarized as,

2H2O  +  2NADP  +  3ADP +  3IP  —————>    2NADPH   +    3ATP

  1. Dark reaction or biosynthetic phase

This phase does not require light and occurs in the stroma of chloroplast. Previously synthesized NADPH and ATP molecules are now utilized in this phase to make glucose.

These molecules helps to convert the carbon dioxide (raw material) into carbohydrates (sugar). This process is called as carbon fixation.

This process was discovered by Melvin Calvin and Andy Benson, it is called as Calvin cycle or Calvin Benson cycle.

This process takes place by series of reactions where RuBP (ribulose biphosphate) act as an acceptor for CO2. The hydrogen that is released by NADPH is combined with CO2 by using energy from ATP molecule glucose is formed. It is important to note that Calvin cycle should repeat 6 times to form 1 glucose molecule.  Refer the overall light reaction and calculate the number of NADPH and ATP as multiple of 6.

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