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Photosynthesis in Higher Plants:Photosynthesis is a physico-chemical process by which green plants use light energy to synthesize organic compounds.
Importance of photosynthesis:
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Site of Photosynthesis:
Photosynthesis occurs in the green leaves in a cellular part called chloroplast. Photosynthesis also occurs in others parts of a plant such as green stem.
The membranes of chloroplast trap the light energy and synthesize ATP and NADPH. This set of reactions which depends on light is called light reactions.
In stroma, enzymatic reactions incorporate CO2 into the plant leading to the synthesis of sugar, which in turn forms starch. This set of reactions which are not directly dependent on light but are dependent on the products of light reactions (ATP and NADPH) are called dark reactions. Dark reaction does not mean that they occur in darkness or that they are not light-dependent.
The membranes of chloroplast trap the light energy and synthesize ATP and NADPH. This set of reactions which depends on light is called light reactions.
In stroma, enzymatic reactions incorporate CO2 into the plant leading to the synthesis of sugar, which in turn forms starch. This set of reactions which are not directly dependent on light but are dependent on the products of light reactions (ATP and NADPH) are called dark reactions. Dark reaction does not mean that they occur in darkness or that they are not light-dependent.
Pigments Involved in Photosynthesis:
Four pigments are involved in photosynthesis: chlorophyll a, chlorophyll b, xanthophylls and carotenoids.
- Chlorophyll a is the main pigment associated with photosynthesis.
- Pigments like chlorophyll b, xanthophylls and carotenoids are called accessory pigments.
- Accessory pigments absorb light and transport the energy to chlorophyll a and protect chlorophyll a from photo-oxidation.
Light Reactions (Photochemical Phase):
Light reactions include light absorption, water splitting, oxygen release, and the formation of high-energy chemical intermediates, ATP and NADPH.
Pigment molecules bound to the proteins form LHC (light harvesting complexes).
LHC are located within two photosystems − PSI and PSII.
Each photosystem has two parts: Antennae and Reaction centre.
1. Antennae — consists of accessory pigments, which increase the efficiency of photosynthesis by absorbing different wavelengths of light.
2. Reaction centre — consists of chlorophyll a molecule
Pigment molecules bound to the proteins form LHC (light harvesting complexes).
LHC are located within two photosystems − PSI and PSII.
Each photosystem has two parts: Antennae and Reaction centre.
1. Antennae — consists of accessory pigments, which increase the efficiency of photosynthesis by absorbing different wavelengths of light.
2. Reaction centre — consists of chlorophyll a molecule
- PS I is called P700 since the reaction centre chlorophyll a has an absorption peak at 700 nm
- PS II is called P680 since chlorophyll a has its absorption peak at 680 nm.
The Electron Transport:
In electron transport chain, photosystem I and II absorb light of wavelength 700 nm and 680 nm respectively.
Absorption of light by photosystem II causes the electron to move to excited state. The excited electrons are passed on to cytochromes (electron acceptor proteins). The electrons are then passed on to photosystem I (figure 2). Just like in Photosystem II, absorption of light by photosystem I causes the electrons to move to excited state. The excited electrons are passed on to another cytochrome (electron acceptor proteins) with greater electron affinity. The electrons are then passed on to energy rich molecules called NADP+ , which after receiving the electrons gets reduced to NADPH + H+ (figure 2). |
Z scheme —This whole scheme of transfer of electrons from the PS II to cytochrome to PS I to another cytochrome to NADP+ to form NADPH + H+ is called the Z scheme (figure 2).
Splitting of Water:
Water is split into hydrogen (H+), oxygen (O), and electrons as shown in the reaction:
2H2O → 4H+ + O2 + 4e−
The electron released from splitting of water is used to replace the electrons lost by photosystem II discussed in the previous paragraph.
2H2O → 4H+ + O2 + 4e−
The electron released from splitting of water is used to replace the electrons lost by photosystem II discussed in the previous paragraph.
The process of formation of ATP from ADP and inorganic phosphate(iP) in chloroplast in the presence of sunlight is called Photo-Phosphorylation.
Photo-phosphorylation is of two types:
Difference between Non-cyclic photo-phosphorylation and Cyclic photo-phosphorylation is shown below:
Photo-phosphorylation is of two types:
- Non-cyclic photo-phosphorylation
- Cyclic photo-phosphorylation
Difference between Non-cyclic photo-phosphorylation and Cyclic photo-phosphorylation is shown below:
Chemiosmotic Hypothesis:
- Chemiosmosis requires a membrane, a proton pump, a proton gradient and ATPase.
- Along with the NADPH produced by the movement of electrons, the ATP will be used immediately in the biosynthetic reaction taking place in the stroma, responsible for fixing CO2, and synthesis of sugars.
Dark Reaction (Biosynthetic Phase):
- We now know that the products of light reaction are ATP, NADPH and O2.
- Of these three products, O2 diffuses out of the chloroplast while ATP and NADPH are used to create food (sugar). This process of synthesis of food (sugar) using ATP and NADPH in chloroplast is called the biosynthetic phase or the dark reactions.
- Dark reaction (Biosynthetic phase) occurs in the stroma of chloroplasts.
- It is called dark reactions not because it occurs in dark but because it depends on the products (ATP and NADPH) of light reaction.
- In some plants, the first product of CO2 fixation is a 3-carbon compound called 3-phosphoglyceric acid (PGA). These plants are said to adopt the C3 pathway or the Calvin cycle.
- In other plants, the first CO2 fixation product is a 4-carbon compound called oxaloacetic acid. These plants are said to adopt the C4 pathway.
- Acceptor of CO2 was found to be a 5-carbon ketose sugar called Ribulose bisphosphate (RuBP).
C3 pathway (Calvin cycle):
- Calvin pathway occurs in all photosynthetic plants; it does not matter whether they have C3 or C4 (or any other) pathways.
- Calvin cycle can be described under three stages: 1. carboxylation, 2. reduction and 3. regeneration.
Total ATP and NADPH molecules required to make one molecule of glucose through the Calvin pathway:
ATP required:
For fixing 1 molecule of CO2 = 3 (2 for reduction and 1 for regeneration)
For fixing 6 molecules of CO2 (3 × 6) = 18 ATP
NADPH required:
For fixing 1 molecule of CO2 = 2 (for reduction)
For fixing 6 molecules of CO2 (2 × 6) = 12 NADPH
Therefore, the synthesis of 1 molecule of glucose requires 18 ATP and 12 NADPH.
ATP required:
For fixing 1 molecule of CO2 = 3 (2 for reduction and 1 for regeneration)
For fixing 6 molecules of CO2 (3 × 6) = 18 ATP
NADPH required:
For fixing 1 molecule of CO2 = 2 (for reduction)
For fixing 6 molecules of CO2 (2 × 6) = 12 NADPH
Therefore, the synthesis of 1 molecule of glucose requires 18 ATP and 12 NADPH.
C4 Pathway (Hatch and Slack Pathway):
The primary CO2 acceptor is a 3-carbon molecule phosphoenol pyruvate (PEP) and is present in the mesophyll cells. Enzyme PEP carboxylase is converted to PEP againor PEPcase is responsible for this CO2 fixation and leads to the formation of the C4 compounds OAA (oxaloacetic acid) and then malic acid and aspartic acid in the mesophyll cells. Malic and aspartic acid is then transported to the bundle sheath cells where they are broken down to release 3-carbon molecule and a CO2. The 3-carbon molecule is transported back to the mesophyll where it is converted to PEP again while CO2 enters the C3 or the Calvin pathway.
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- Calvin pathway occurs in all the mesophyll cells of the C3 plants. In the C4 plants it does not take place in the mesophyll cells but does so only in the bundle sheath cells. This is because mesophyll cells of C4 plants lack RuBisCO which is required for the Calvin pathway. To overcome this problem, C4 products are transferred to the bundle sheath cells which are rich in RuBisCO.
Photorespiration:
- In C3 plants RuBP instead of being converted to 2 molecules of PGA binds with O2 to form one molecule and phosphoglycolate in a pathway called photorespiration.
- Photorespiration is considered a wasteful process because sugars, ATP and NADPH are not created. Rather it results in the release of CO2 with the utilisation of ATP.
Differences between C3 and C4 plants:
Factors Affecting Photosynthesis:
Photosynthesis is under the influence of both internal (plant) factors such as number, size, age, etc. and external factors such as availability of sunlight, CO2 concentration, temperature and water.
We will discuss the external factors below:
Light:
CO2 concentration:
Temperature:
Water:
We will discuss the external factors below:
Light:
- light quality, light intensity and the duration of exposure to light are the factors that affect photosynthesis.
- There is a linear relationship between incident light and CO2 fixation rates at low light intensities. At higher light intensities, gradually the rate does not show further increase as other factors become limiting factor.
- Light saturation occurs at 10 per cent of the full sunlight. Hence, except for plants in shade or in dense forests, light is rarely a limiting factor in nature.
- High intensity of light destruct chlorophylls.
CO2 concentration:
- Increase in CO2 concentration of upto 0.05 per cent can cause an increase in CO2 fixation rates; beyond this the levels can become damaging over longer periods.
- C4 plants show saturation at about 360 μlL-1 while C3 responds to increased CO2 concentration and saturation is seen only beyond 450 μlL-1.
Temperature:
- The C4 plants respond to higher temperatures and show higher rate of photosynthesis while C3 plants have a much lower temperature optimum.
- Tropical plants have a higher temperature optimum than the plants adapted to temperate climates.
Water:
- Lack of water (Water stress) causes the stomata to close hence reducing the CO2 availability. Water stress also makes leaves wilt, leading to less photosynthetic area.
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