The basics of photosynthesis are known to all. Photosynthesis in higher plants involves additional processes, but fundamentally it remains the same. It is a physicochemical process that uses sunlight for the synthesis of organic compounds. In this process, oxygen is released in the atmosphere.
Photosynthesis occurs in the chloroplast, found in the mesophyll cells of the leaves. There are 4 pigments involved in photosynthesis:
- Chlorophyll a
- Chlorophyll b
We are all aware of the process of photosynthesis in small plants. Let us have a detailed view of the photosynthesis in higher plants.
Processes Of Photosynthesis in Higher Plants
Photosynthesis in higher plants involves the following processes:
- Light Reaction
- Dark Reaction
- This phenomenon occurs in the presence of light.
- The pigment absorbs light and produces energy in the form of ATP.
- The process involves- absorption of light, water splitting, the release of oxygen, and formation of ATP and NADPH.
- The protein-bound pigment molecules form the light-harvesting complexes present within two photosystems- PS-I and PS-II. Each photosystem has a reaction centre consisting of chlorophyll a molecule, and antennae containing accessory pigments.
- The reaction centre for PS-I is P-700 because the absorption peak for chlorophyll a is at 700nm while that for PS-II is P-680 because the absorption peak for chlorophyll a is at 680 nm.
Explore more about – Light Reaction and Dark Reaction
The formation of ATP in the presence of sunlight is called photophosphorylation. It is of two types:
- Non-cyclic photophosphorylation
- Cyclic photophosphorylation
- PS-II absorbs light at a wavelength of 680 nm and causes excitation in the electrons. These excited electrons are accepted by an electron acceptor and transferred to the electron transport system.
- The electrons from the electron transport system are transferred to the PS-I. At the same time, the electrons at PS-I receive a wavelength of 700 nm and get excited.
- An electron from the electron acceptor is added to NADP+, which is then reduced to NADPH+ H+.
- The electrons lost by PS-II do not return to it and hence named non-cyclic photophosphorylation.
- In this, both the photosystems were involved.
- In cyclic photophosphorylation, only PS-I is involved.
- The electrons circulate within the photosystem which results in a cyclic flow of electrons.
- This only forms ATP and not NADPH+ H+.
Also Refer: Cyclic and Non-cyclic photophosphorylation
The light-dependent splitting of water is called photolysis. This process is associated with PS-II in which manganese and chlorine play an important role. The electrons lost from P680 are replaced by the electrons formed in this process. A molecule of water splits to releases oxygen on the absorption of light by P680.
This process occurs in the absence of light in the stroma of the chloroplasts. The following cycles are involved in the process:
Calvin Cycle (C3 Cycle)
This cycle involves the following steps:
- Carbon-fixation: Ribulose-1,5-bisphosphate combines with carbon dioxide to fix it to a 3 carbon compound 3-phosphoglyceric acid. The enzyme RuBisCO is involved in the process.
- Reduction: 2 molecules of ATP and NADPH fixes one molecule of carbon dioxide to form glyceraldehyde-3-phosphate.
- Regeneration: Some glyceraldehyde-3-phosphate molecules undergo a series of reactions to form glucose while the RuBP regenerates to continue the cycle.
Also, read Calvin Cycle
C4 Cycle (Hatch and Slack Pathway)
- It is a cyclic pathway.
- The enzymes involved in the C4 pathway are located in the Mesophyll cells and Bundle Sheath cells.
- In this pathway, the plants convert atmospheric carbon dioxide into a four carbon-containing chemical compound.
- Phosphoenolpyruvate is the primary carbon dioxide acceptor and is located in the mesophyll cells. The reaction is mediated by phosphoenolpyruvate carboxylase.
- After this, aspartic acid and malic acid are formed within the mesophyll cells and transported to the bundle sheath cells. Here, the C4 acids breakdown to release three-carbon molecules and carbon dioxide.
- The three-carbon molecules move back to the mesophyll cells where they get converted into phosphoenolpyruvate and complete the cycle.
- The carbon dioxide enters the bundle sheath cells and completes the Calvin cycle.
In this process, there is no formation of ATP or NADPH. In this, plants take in oxygen and release carbon dioxide, in the presence of light.
Factors affecting photorespiration
- When the level of carbon dioxide is low and oxygen is high, the rate of photorespiration increases.
- Underwater stress conditions, the rate of photorespiration is higher.
Also Refer: Photorespiration in C3 and C4 plants
Differences between C3 and C4 plants
|C3 Plants||C4 plants|
|Do not represent Kranz anatomy.||Represent Kranz anatomy.|
|Photosynthesis occurs in mesophyll cells.||Photosynthesis occurs mesophyll and bundle sheath cells.|
|RuBisCO is carbon dioxide acceptor.||PEP carboxylase is the carbon dioxide acceptor.|
|The first stable compound is a three-carbon compound 3-phosphoglyceric acid.||The first stable compound is a 4-carbon compound oxaloacetic acid.|
|The photorespiratory loss is high.||The photorespiratory loss if low.|
|The optimum temperature is 20-25°C.||The optimum temperature is 35-44℃.|
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