Photosynthesis in Higher Plants Class 11 Notes Chapter 11
Last Updated : 27 Dec, 2023
Photosynthesis in higher plants is an important biological process for energy production. Chlorophyll pigments capture sunlight, initiating the light-dependent reactions in chloroplasts. Water molecules are split, release oxygen, and generate ATP and NADPH. In the subsequent Calvin Cycle, carbon dioxide is assimilated, producing glucose as the end product essential for plant growth and metabolism.
C3 plants use the Calvin Cycle for photosynthesis in mesophyll cells, while C4 plants separate carbon fixation and the Calvin Cycle between mesophyll and bundle sheath cells, reducing photorespiration. C4 plants are adapted to hot and arid conditions with improved water-use efficiency.
Photosynthesis in Higher Plants
Photosynthesis is a physicochemical process by which plants use light energy for the synthesis of organic compounds. Photosynthesis is important as it is the primary source of all food on Earth and is also responsible for the release of oxygen into the atmosphere by green plants. Photosynthesis takes place only in the green parts of the plants, mainly the leaves.
Within the leaves, the mesophyll cells consist of a large number of chloroplasts that carry out photosynthesis. Photosynthesis has two stages: the light reaction and the carbon-fixing reaction. Within the chloroplasts, the light reaction takes place in the membranes, while the chemosynthetic pathway occurs in the stroma. There are 4 pigments involved in photosynthesis: Chlorophyll a, Chlorophyll b, Xanthophylls, and Carotenoids.
Also Read: Photosynthetic Pigments
Processes Of Photosynthesis in Higher Plants
In higher plants, photosynthesis involves light absorption by chlorophyll, leading to the generation of ATP and NADPH through light-dependent reactions followed by carbon dioxide fixation and glucose production in the Calvin Cycle. Photosynthesis in higher plants involves the following processes:
- Light Reaction
- Dark Reaction
Light Reaction
Light reactions also known as the ‘Photochemical’ phase include light absorption, water splitting, oxygen release, and the formation of high-energy chemical intermediates, ATP and NADPH. Various protein complexes participate in the process. Features of light reaction are as follows:
- Two different photochemical light harvesting complexes (LHC) are found in Photosystem I (PS I) and Photosystem II (PS II).
- LHCs consist of hundreds of pigment molecules bound to proteins.
- Each photosystem, including all pigments except one chlorophyll a, forms an antennae for light absorption.
- Pigments in LHCs optimize photosynthesis efficiency by absorbing light of different wavelengths.
- The reaction center of each photosystem is a single chlorophyll a molecule.
- PS I's reaction center chlorophyll a (P700) absorbs light at 700 nm, while PS II's (P680) absorbs at 680 nm, distinguishing their roles in the light reactions.
Photophosphorylation
Photophosphorylation is the process by which light energy is used to add a phosphate group to ADP (adenosine diphosphate), converting it into ATP (adenosine triphosphate). This process occurs during the light-dependent reactions of photosynthesis in the thylakoid membrane of chloroplasts. There are two main types of photophosphorylation: non-cyclic and cyclic photophosphorylation.
Non-Cyclic Photophosphorylation
- Light energy is absorbed by pigments in Photosystem II (PS II), leading to the excitation of electrons in the reaction center chlorophyll (P680).
- Excited electrons are transferred to the primary electron acceptor, leaving P680 in an oxidized state.
- Water molecules are split through photolysis, releasing electrons, protons, and oxygen. The electrons replace those lost from P680.
- As electrons move through the chain, energy is released which is used to pump protons (H+) across the thylakoid membrane into the thylakoid space.
- Electrons, now at a lower energy state, reach Photosystem I (PS I) where light energy is again absorbed by pigments in PS I. It excites electrons in the reaction center chlorophyll (P700) which are then transferred to another primary electron acceptor.
- Electrons from PS I, along with protons from the thylakoid space, reduce NADP+ to form NADPH. This reduction reaction stores energy in the form of NADPH.
- Protons flow back into the stroma through ATP synthase, a protein complex that utilizes this flow to synthesize ATP from ADP and inorganic phosphate (photophosphorylation).
- The electrons that leave PS I combine with the protons and electrons from water photolysis, forming molecular oxygen (O2), which is released as a byproduct.
Cyclic Photophosphorylation
- In cyclic photophosphorylation, electrons follow a cyclic pathway, return to the reaction center of Photosystem I (PS I) after being transferred to the electron transport chain. This cyclic flow occurs when there is a high demand for ATP and a limited need for NADPH.
- The cyclic flow of electrons generates ATP through the phosphorylation of ADP. Electrons are cycled back to PS I, and the process continues, leading to the continuous production of ATP without reduction of NADP+.
Also Read: Cyclic and Non-Cyclic Phosphorlylation
Water Splitting
Water splitting, or photolysis, is an important step in the light-dependent reactions of photosynthesis. It take place in the thylakoid membrane, mainly in Photosystem II (PS II). It absorbs light energy, particularly in the P680 reaction center chlorophyll a that leads to the excitation of electrons in P680 and causes its release. Released electrons replace those lost in PS II and enter the electron transport chain, contributing to ATP production. Simultaneously, water molecules are split into oxygen, protons, and electrons, with oxygen being released as a byproduct into the atmosphere.
Dark Reaction
The dark reactions, also known as the Calvin Cycle or light-independent reactions, are the second stage of photosynthesis. It take place in the stroma of the chloroplasts. It does not depend on the light. The dark reactions convert carbon dioxide into glucose, using ATP and NADPH produced during the light-dependent reactions. The dark reaction take place in the following steps:
Calvin Cycle (C3 Cycle)
- Carbon Fixation:The reaction starts with the fixation of carbon dioxide. Ribulose bisphosphate (RuBP) a five carbon moelecule, with the help of the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) reacts with carbon dioxide. It produces two molecules of 3-phosphoglycerate (3-PGA), each containing three carbon atoms.
- Reduction Phase: 3-PGA is converted into glyceraldehyde-3-phosphate (G3P) through ATP and NADPH produced during light reaction. This reaction involves phosphorylation (using ATP) and reduction (using NADPH).
- To maintain the continuity of the Calvin Cycle, some of the G3P molecules produced are used to regenerate RuBP.
- Formation of Glucose: For every three molecules of carbon dioxide entering the Calvin Cycle, one molecule of G3P is produced. Two G3P molecules combine to form one molecule of glucose.
- ATP/NADPH Regeneration: The remaining G3P molecules continue through additional steps and results in the regeneration of ATP and NADPH, which are necessary for the continuation of the Calvin Cycle.
- The Calvin Cycle helps in maintaining the carbon balance of the plant by fixing carbon dioxide from the atmosphere into organic compounds.
The dark reactions of photosynthesis involve the Calvin Cycle, where carbon dioxide is fixed and converted into glucose and other carbohydrates with the help of ATP and NADPH produced in the light-dependent reactions. The cycle ensures the synthesis of energy-rich molecules and the continuation of the plant's metabolic processes.
C4 Cycle (Hatch and Slack Pathway)
The C4 pathway, also known as the Hatch-Slack pathway, is an alternative carbon fixation pathway in photosynthesis that evolved as an adaptation to hot and dry conditions. The steps of the C4 cycle are as follows:
- PEP carboxylase in mesophyll cells fixes CO2 to phosphoenolpyruvate (PEP), forming oxaloacetate.
- Oxaloacetate is converted to malate or aspartate, both four-carbon compounds.
- Four-carbon compounds transport to bundle sheath cells where they release CO2.
- Rubisco in bundle sheath cells fixes released CO2 in the Calvin Cycle.
- PEP is regenerated in bundle sheath cells and transported back to mesophyll cells.
- C4 pathway reduces photorespiration, increases photosynthetic efficiency in high-temperature environments and improves water use efficiency.
- Examples: Maize, sugarcane, sorghum, and certain grasses are common C4 plant.
Also Read: Difference Between Light reaction and Dark reaction
Photorespiration
Photorespiration is a process in plants where oxygen, instead of carbon dioxide, is fixed by the enzyme Rubisco during photosynthesis.
- It take place in C3 plants in high-temperature regions when the stomata close to prevent water loss. Under water, stress conditions, the rate of photorespiration is high. When the level of carbon dioxide is low and oxygen is high, the rate of photorespiration increases.
- Rubisco catalyzes the binding of oxygen to ribulose-1,5-bisphosphate (RuBP) instead of carbon dioxide, forming a two-carbon compound.
- This process leads to the release of carbon dioxide and the consumption of ATP. It reduces power and overall photosynthetic efficiency.
Differences Between C3 and C4 plants
The differences between C3 and C4 plants are as follows:
|
| Photosynthesis | Photosynthesis occur in mesophyll cells. | Photosynthesis occur in mesophyll and bundle sheath cells. |
Kranz anatomy | Do not show Kranz anatomy | Shows Kranz anatomy |
| Initial Fixation Enzyme | Rubisco | PEP Carboxylase in mesophyll cells |
| First Stable Product | 3-phosphoglycerate (3-PGA) | 4-carbon compounds (e.g., oxaloacetate) |
| Location of Calvin Cycle | Mesophyll cells | Bundle sheath cells |
| CO2 Concentration | Lower affinity for CO2; susceptible to photorespiration. | Higher affinity for CO2; less susceptible to photorespiration. |
| Environmental Adaptation | Suited to moderate temperatures and normal conditions. | Suited to hot and arid conditions; better water-use efficiency. |
| Examples | Wheat, rice, and most plants. | Maize, sugarcane, and certain grasses |
Also Read: NCERT Solutions Chapter 11 of Class 11 Biology – Photosynthesis in Higher Plants