Photosynthesis In Higher Plants-Quick Revision

1

Recall the history of photosynthesis

Many scientists have contributed to understanding how plants carry out photosynthesis. These are as follows:
1. Van Helmont (1648) concluded that all food of the plant is derived from water and not from the soil.
2. Stephen Hales (Father of Plant Physiology) (1727) reported that plants obtain a part of their nutrition from air and light may also play a role in this process.
3. Joseph Priestley (1772) demonstrated that green plants purify the foul air (i.e., Phlogiston), produced by burning of the candle, and convert it into the pure air (i.e., Dephlogiston).
4. Jan Ingen-Housz (1779) concluded by his experiment that purification of air was done by green parts of the plant only and that too in the presence of sunlight. Green leaves and stalks liberate dephlogisticated air during sunlight and phlogisticated air during dark.
5. Jean Senebier (1782) proved that plants absorb CO2​ and release O2​ in presence of light. He also showed that the rate of O2​ evolution depends upon the rate of CO2​ consumption.
6. Lavoisier (1783) identified the pure air (i.e., phlogiston) as oxygen (O2​) and noxious air (i.e., Phlogiston) produced by the burning of the candle as carbon dioxide (CO2​).
7. Nicolas de Saussure (1804) showed the importance of water in the process of photosynthesis. He further showed that the amount of CO2​ absorbed is equal to the amount of O2​ released.
8. Pelletier and Caventou (1818) discovered chlorophyll. It could be separated from the leaf by boiling in alcohol.
9. Sachs (1864) discovered the carbohydrate as a product of the photosynthesis.
10. Emerson and Arnold (1932) described the occurrence of light and dark reaction during the process of photosynthesis.
11. Robert Hill (1937) demonstrated the photolysis of water in light reaction of photosynthesis.

2

Photosynthetic pigment – Chlorophyll

Chlorophylls are greenish pigments present in green plants.
1. It contains a porphyrin ring. It contains a magnesium atom in the center and an attached phytol side chain as we can see in the given image.
2. The structure of chlorophyll is related to the function such as long phytol chain is lipid-soluble and anchored in the thylakoid membrane.
3. This is a stable ring-shaped molecule around which electrons are free to migrate. Because the electrons move freely, the ring has the potential to gain or lose electrons easily, and thus the potential to provide energized electrons to other molecules. 
4. It absorbs the blue and red wavelength of light. The green wavelength is reflected and gives a green colour to the plant.
5. There are two types of chlorophyll, chlorophyll a and chlorophyll b.
6. Chlorophyll b along with xanthophyll and carotenoid are called accessory pigments.
7. Chlorophyll a and b shows maximum absorption in blue-violet and orange-red region of the visible light.
8. They absorb sunlight and transfer the light energy to chlorophyll a. 
9. Accessory pigments protect the chlorophyll a from photo-oxidation.

3

Light dependent phase or photochemical phase

The process of photosynthesis has two phases light-dependent and light-independent.

Light-dependent or photochemical phase 1. The light-dependent reaction takes place in the presence of light inside the thylakoid of the chloroplast.
2. It occurs in 400-700 nm wavelength of visible light.
3. It is maximum in blue- red light and least in green light.

It occurs in two steps:1. Activation of chlorophyll: The chlorophyll becomes activated by absorbing light energy.
2. Splitting of water: The water molecule splits into hydrogen, oxygen and releases electron. This process occurs due to the absorbed energy. The hydrogen ion from the photolysis of water reduced the NADP to NADPH. Electrons from photolysis convert ADP to ATP.

4

Z scheme of electron transport

Z-scheme is the graphical representation of the electron transport pathway according to the redox potential pathway. It occurs in non-cyclic phosphorylation.
1. In this process, electron flows from water to reduce NADP to NADPH.
2. The energy required to move electrons is provided by the absorbed sunlight.
3. First of all, PSII absorbs light and promotes to the excited state.
4. In an excited state, electrons acquire a large amount of energy and move primary acceptor Q.
5. Electrons from Q pass into the PQ. The reduced form of PQ donates electron to cytochrome where ATP is formed.
6. The electron then passes to the plastocyanin and then to PS I.
7. The electron from PS I expelled to the excited state, moves to the ferredoxin protein.
8. Ferredoxin protein transfers the electron to the NADP that reduces to NADPH.

5

Cyclic photophosphorylation

1. Cyclic Photophosphorylation is a process of photophosphorylation in which an electron expelled by the excited photocentre is returned back to it after passing through a series of electron carriers. ATP is formed when electrons pass Ferredoxin to PQ and from PQ to the Cytochrome system.
2. Phosphorylation is the process of formation ATP from ADP in the light reaction of photosynthesis. It can be done in two ways cyclic and non-cyclic. 
3. Cyclic photophosphorylation occurs in both aerobic and anaerobic conditions.
4. It is a process of producing carbohydrates by green plants using carbon dioxide and water in the presence of sunlight.

6

Chemiosmotic hypothesis of ATP formation

The chemiosmotic hypothesis was proposed by Peter Mitchell. It is the generation of ATP by ATP synthase in an electron transport chain.
1. ETC is an oxidative phosphorylation reaction that takes place in the inner membrane of the mitochondria.
2. It begins with NADH and FADH2​ that loses electrons. These electrons are transferred along the chain. 
3. As each complex accepts electrons and passes them on, energy is released. 
4. This energy is used to pump the protons against the concentration gradient from the matrix of the mitochondria to the inner membrane space. 
5. Eventually, there is a high concentration of protons build up in the membrane space and the protons try to move back into the matrix. However, the inner membrane is impermeable to protons. 
6. A proton motive force (PMF) is set up and ATP synthase undergoes a conformational change and uses the PMF to make ATP from ADP. 
7. ATP synthase allows the proton to diffuse back down their gradient.      ATP + Pi -> ATP

7

C_4 pathway

C4​ pathway occurs by the cooperation between bundle sheath cells and mesophyll cells. 
1. The first step takes place in the mesophyll cell where phosphoenolpyruvate carboxylase enzyme catalyses the formation oxaloacetate from phosphoenolpyruvate and carbon dioxide.
2. Oxaloacetate reduces to malic acid (C4​ compound) in the presence of malate dehydrogenase enzyme. 
3. The malic acid then transferred to the bundle sheath cell through plasmodesmata.
4. In bundle sheath cell, malic acid is decarboxylated in the presence of the malic enzyme and releases carbon dioxide. 
5. By the decarboxylation of malic acid 3 carbon compound i.e., pyruvate is formed. Carbon dioxide enters into the Calvin cycle and pyruvate returns back to the mesophyll cells. 
6. In mesophyll cell, pyruvate in the presence of pyruvate phosphate dikinase enzyme converts into phosphoenolpyruvate, and the cycle repeats.

8

Phases of Calvin cycle

Calvin cycle was given by the Melvin Calvin, James Bassham and Andrew Benson. In this cycle, phosphoglyceric acid (3 carbon compound) is produced as a first product and thus called as C3​ cycle. From the given diagram we can understand the stages of the Calvin cycle.
Calvin cycle can be divided into three stage:
1. Carboxylation: It is the first stage where CO2​ is fixed from an atmosphere in the presence of the rubisco enzyme. One molecule of RuBP joins with the molecule of CO2​ in the presence of enzyme and produces phosphoglyceric acid.
2. Reduction: In this stage, glucose is formed.  ATP and NADPH are used to reduce 3-PGA into G3P; then ATP and NADPH are converted to ADP and NADP+, respectively.
3. Regeneration of RuBP: It is the series of enzymatic reactions. In this stage, RuBP is regenerated by the use of ATP.  It again enables the system to fix CO2​.
The fixation and reduction of one molecule of CO2​ requires 2 molecules of ATP and 3 molecules of NADPH.

9

Photorespiration

Photorespiration is the process where the enzyme RuBisCO oxygenates RuBP with the release of carbon dioxide. It is also known as a wasteful process because it prevents the plant from using ATP and NADPH to synthesize carbohydrate. 
1. It occurs in C3​ plants and not in C4​ plants.
2. In C3​ pathway, RUBP binds with oxygen and form phosphoglycolate and phosphoglycerate. This reaction decreases the CO2​ fixation as we can see in the given cycle. 
3. It occurs when the level of CO2​ is low inside the leaf.
4. It occurs on a hot dry day to close stomata and prevent excess loss of water.
5. It limits the damaging product of the light reaction of photosynthesis.
6. It reduces the photosynthetic efficiency in C3​ plant by 25%.

10

Blackman’s law of limiting factors

F.F. Blackman in 1905 proposed the principle of limiting factor. According to this principle, when a process depends on number of factors its rate is limited by the pace of the slowest factor.
1. It determines the rate of photosynthesis.
2. It explains that if all other factors are kept constant the factor under consideration will affect the rate of photosynthesis