Photosynthesis in Higher Plants: Light & Dark Reactions

Photosynthesis in higher plants is the same as in smaller plants, except for some additional steps.

• Photosynthesis is the process through which plants make their food with the help of sunlight, water, and carbon dioxide.
• The by-product of photosynthesis is oxygen, which is released into the atmosphere.
• In higher plants, the main method and outcome of photosynthesis remain the same but complexity increases.
• Photosynthesis in higher plants takes place in two phases namely light reaction and dark reaction.

Key Terms: Photosynthesis, Light Reaction, Dark Reaction, Photophosphorylation, C3 Cycle, C4 Cycle, Photorespiration, Sunlight, Chloroplast

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Photosynthesis

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Photosynthesis is a physicochemical process in which plants use sunlight to produce glucose.

• Along with light energy or sunlight, Carbon dioxide and Water are the main ingredients.
• Excess glucose formed is stored as starch in the stroma of the chloroplast.

The basic reaction of photosynthesis is:

Photosynthesis Reaction

Photosynthesis in Higher Plants occurs in two main phases:

• Light Reaction
• Dark Reaction

This does not denote that dark reactions don’t require sunlight, rather they require the products formed by trapping sunlight in the light reaction. Photosynthesis takes place in the chloroplast of the mesophyll cells. 

• Within the chloroplast, the inner and outer membrane, stroma lamellae, grana, and matrix stroma collect the sunlight.
• It is here that the ATP and NADPH are generated.
• In the next step, ATP and NADPH are used to produce glucose from carbon dioxide (CO₂) that takes place in the stroma, also known as the kitchen of the cell. 

Diagrammatic Representation of a Chloroplast

Pigments in Photosynthesis

It is often assumed that only chlorophyll traps the required sunlight for photosynthesis but in reality, there are a total of 4 pigments responsible for trapping sunlight. 

1. Chlorophyll a 
2. Chlorophyll b
3. Xanthophylls
4. Carotenoids

Out of these Chlorophyll a is the main pigment that attracts the majority of the sunlight.

• Chlorophyll b, Xanthophylls, and Carotenoids are called accessory pigments.
• These pigments trap light energy and complement the absorption by chlorophyll a.
• These pigments are located in the thylakoid membrane in the chloroplast.
• Each pigment traps light of different wavelengths.
• Thus, four pigments allow a wide range of lights to get trapped for photosynthesis. 

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Light Reaction

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Light reaction is the process in which pigments absorb sunlight to produce energy molecules, i.e., ADP and NADPH.

• Light reaction occurs in four steps, trapping of light energy, splitting of water molecules, release of oxygen, and production of ATP and NADPH. 
• The protein complexes and pigments form the LHC (Light Harvesting Complex) within Photosystem I (PS I) and Photosystem II (PS II).
• Each photosystem comprises two sections, the antennae which consist of accessory pigment and protein complexes, and the reaction center which contains a single molecule of chlorophyll a. 
• In PS I the reaction center has an absorption peak at 700 nm, therefore, known as P700.
• In PS II, the reaction center has an absorption peak at 680 nm, therefore known as P680. 

Photosystem/ Light Harvesting Complex

Absorption of Light

When sunlight falls on PS II, the electrons get charged up and are thrown away from the nucleus after which they are received by an electron acceptor.

• The electron acceptor transfers the received electron to the electron transport system which ultimately gives it to PS I.
• In the meantime, electrons of PS I also get charged on absorbing sunlight.
• These electrons are also transferred to a molecule of NADP+.
• On reacting with the electron NADP+ is reduced to NADPH + H+. 

Diagrammatic Representation of Light Absorption

This method is also known as photophosphorylation, in which ATP is produced from ADP with the help of sunlight.

There are two types of photophosphorylation, cyclic and non-cyclic. 

• In cyclic photophosphorylation, only PS I is functional and the excited electron comes back into the system.
• Here, only the production of ATP takes place. 
• In the non-cyclic photophosphorylation, both photosystems are functional.
• The excited electron of PS II does not come back.
• Here, production of both ATP and NADPH + H+ occurs.
• The reaction is an example of non-cyclic photophosphorylation. 

Read More: Receptors

Splitting of Water

Water is split to release oxygen, H+ ions, and electrons. These electrons are accessed by PS II to provide a continuous supply of electrons. 

2H₂O → 4H⁺ + O₂ + 4e^-

When PS II receives sunlight below 680nm water molecules are split, hence, splitting of water is related to PS II. This process of splitting water in the presence of sunlight is known as photolysis. 

Read More: Respiration and Combustion

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Dark Reaction

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In Dark Reaction, ATP and NADPH are used to produce sugar which ultimately forms glucose or starch.

• Dark Reaction occurs in the stroma of the chloroplast.
• This is the process where the actual food of the plant i.e., starch is prepared.
• Hence, stroma is also known as the Kitchen of the Cell. 

Calvin Cycle (C3 plants)

Carboxylation: Carbon dioxide reacts with ribulose-1,5-bisphosphate in the presence of RuBP carboxylase, also known as RuBisCO to form three molecules of 3-phosphoglyceric acid (3-PGA). 

Reduction: Here, 2 molecules of ATP and 2 molecules of NADPH are utilized to form one molecule of glucose. ATP undergoes photophosphorylation and NADPH reduces carbon dioxide molecules to form glyceraldehyde-3-phosphate. 

Regeneration: Here, RuBP or ribulose-1,5-bisphosphate is formed again so that it reacts with CO2 molecules again and the cycle continues. 6 full cycles are needed to make one molecule of glucose.

Diagrammatic Representation of Regeneration Process

Calvin Cycle (C4 Plants)

The Calvin cycle in C4 plants is also known as the Hatch and Slack Pathway. C4 plants are those plants that are habituated to live in dry tropical areas.

• Phosphoenolpyruvate (PEP) in the mesophyll cells receives carbon dioxide in the presence of PEP carboxylase or PEPcase. This results in the formation of aspartic acid and malic acid in the mesophyll cells. 
• From mesophyll cells aspartic acid and malic are transferred to bundle sheath cells. In the sheath cells, these acids are broken into carbon dioxide and a 3-carbon molecule.
• The 3-carbon molecule formed is sent back to mesophyll cells where it is converted into PEP to complete the cycle.

Calvin Cycle, C4

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Differentiate between Light and Dark Reaction

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Plants go through two sorts of sequential processes during photosynthesis: light reaction and dark reaction.

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Photorespiration

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Photorespiration is a wasteful process in plants in which plants intake oxygen, and sunlight and release carbon dioxide and no glucose is formed.

• Photorespiration generally takes place when the concentration of oxygen in the leaves is very high and that of carbon dioxide is very low.
• Hence, the carbon dioxide fixation does not take place in the Calvin cycle.
• Hence, instead of carbon dioxide oxygen reacts with RuBP.

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Things to Remember

• Photosynthesis in higher plants involves two main steps, Light Reaction and Dark Reaction. 
• In light reaction light energy is collected to form ATP and NADPH whereas in dark reaction these two energy molecules are utilized to form glucose or starch.
• Four pigments responsible for trapping sunlight are Chlorophyll a, Chlorophyll b, Xanthophylls, and Carotenoids. 
• The main pigment is Chlorophyll known as the reactor center and other pigments are known as accessory pigments.
• The C3 Cycle takes place in plants living in dry and sandy areas.
• Whereas the C4 Cycle is for all other photosynthetic plants. But the process in both cycles is the same.
• Calvin Cycle is named after Dr. Melvin Calvin, an American Biochemist.