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The reactions of photosynthesis take place in the presence of sunlight. The intensity of moonlight is several thousand times less than that of direct sunlight which is insufficient for the light dependent phase of photosynthesis. As the sun sets, rate of photosynthesis also decreases. Therefore, moonlight does not support photosynthesis.

The chemical substance used to test the presence of starch in the cell of a leaf is Iodine. 

1. Life on earth is dependent on solar energy directly or indirectly. 

2. Plants by carrying out photosynthesis converts solar energy into chemical energy by producing carbohydrates. 

3. Humans and animals depend on plants for food. Basically, life on earth depends totally on photosynthesis for food and energy. 

4. Therefore, all life on earth is bottled solar energy.

a. Sun is the main source of energy. 

b. Plants utilize sunlight, carbon dioxide and water for the process called photosynthesis to produce sugars. 

c. Animals make use of these sugars provided by the plants in their own cellular energy factories called mitochondria. Thus, energy is obtained.

Photosynthesis

a. Energy is the basic requirement of life. 

b. Without energy no work can be done. 

c. All living organisms need energy for reproduction and survival. 

d. Sun is the main source of energy, and that energy should be transformed into the usable forms for living organisms to carry out life processes.

Therefore, energy is essential in different life processes.

External factors which affect photosynthesis are as follows: 

1. Light: 

a. It is an essential factor as it supplies the energy necessary for photosynthesis. 

b. Quality and intensity of light affects the photosynthesis. 

c. Highest rate of photosynthesis takes place in red light followed by blue light. 

d. The rate of photosynthesis considerably decreases in plants which are growing under a forest canopy. 

e. In most of the plants, photosynthesis is maximum in bright diffused sunlight.

f. Uninterrupted and continuous photosynthesis for a very long period of time may be sustained without any visible damage to the plant. 

2. Carbon dioxide: 

The main source of CO₂ in land plants is the atmosphere, which contains only 0.3% of the gas. 

b. Under normal conditions of temperature and light, carbon dioxide acts as a limiting factor in photosynthesis. 

c. Increase in concentration of CO₂ increases the photosynthesis. 

d. Increase in CO₂ to about 1% is advantageous to most of the plants. 

e. Higher concentration of the gas has an inhibitory effect on photosynthesis.

3. Temperature: 

a. Like all other physiological processes, photosynthesis also needs a suitable temperature.

b. The optimum temperature at which the photosynthesis is maximum is 25-30 °C. Except in plants like Opuntia, photosynthesis takes place at as high as 55 °C. 

c. This is the maximum temperature. Minimum temperature is temperature at which photosynthesis process just starts. 

d. In the presence of sufficient light and CO₂ , photosynthesis increases with the rise of temperature till it becomes maximum. After that there is a decrease or fall in the rate of the process.

4. Water: 

a. Water is necessary for photosynthetic process. 

b. An increase in water content of the leaf results in the corresponding increase in the rate of photosynthesis. 

c. Thus, the limiting effect of water is not direct but indirect. 

d. It is mainly due to the fact that it helps in maintaining the turgidity of the assimilatory cells and the proper hydration of their protoplasm.

1. Leaves of C₄ plants show some structural peculiarities called Kranz anatomy. 

2. The chloroplast of mesophyll cells contain enzyme PEP Carboxylase, which can fix CO₂ at low concentration. 

3. Thus, light reaction and evolution of O₂ occurs in mesophyll cells. 

4. Decarboxylation of malate occurs in bundle sheath cells, which results in release of CO₂, due to which concentration of CO₂ in bundle sheath cells increases.

5. Enzyme RuBisCO present in bundle sheath cells acts as carboxylase in presence of high CO₂ concentraion and catalyses carboxylation of RuBP. 

6. Thus, possibility of oxygenation of RuBP is avoided and photorespiration does not take place.

Cyclic photophosphorylation: 

a. Illumination of photosystem-I causes electrons to move continuously out of the reaction center of photosystem-I and back to it. 

b. The cyclic electron-flow is accompanied by the photophosphorylation of ADP to yield ATP. This is termed as Cyclic photophosphorylation. 

c. Since this process involves only pigment system I, photolysis of water and consequent evolution of oxygen does not take place.

Non-cyclic photophosphorylation:: 

a. It involves both photosystems- PS-I and PSII. 

b. In this case, electron transport chain starts with the release of electrons from PS-II. 

c. In this chain high energy electrons released from PS-II do not return to PS-II but, after passing through an electron transport chain, reach PS-I, which in turn donates it to reduce NADP to NADPH. 

d. The reduced NADP⁺ (NADPH) is utilized for the reduction of CO₂ in the dark reaction. 

e. Electron-deficient PS-II brings about oxidation of water-molecule. Due to this, protons, electrons and oxygen atom are released.

f. Electrons are taken up by PS-II itself to return to reduced state, protons are accepted by NADP⁺ whereas oxygen is released. 

g. As in this process, high energy electrons released from PS-II do not return to PS-II and it is accompanied with ATP formation, this is called Non-cyclic photophosphorylation.

Interdependence of light and dark reactions: 

1. The light reaction gives rise to two important products, a reducing agent NADPH₂ and an energy rich compound ATP. Both these are utilized in the dark phase of photosynthesis. 

2. ATP and NADPH₂ molecules function as vehicles for transfer of energy of sunlight into dark reaction leaving to carbon fixation. In this reaction CO₂ is reduced to carbohydrate. 

3. During dark reaction, ATP and NADPH₂ are transformed into ADP, iP and NADP which are transferred to the grana in which light reaction takes place.

Calvin cycle: 

The entire process of dark reaction was traced by Dr. Melvin Calvin along with his co-worker, Dr. Benson. Hence, the process is called as Calvin cycle or Calvin Benson cycle. Since the first stable product formed is a 3-carbon compound, it is also called as C₃ pathway and the plants are called C₁₄ plants. 

Calvin carried out experiments on unicellular green algae (Chlorella), using radioactive isotope of carbon, C₁₄ as a tracer. It is also called synthesis phase or second phase of photosynthesis. 

The cycle is divided into the following phases: 

1. Carboxylation phase: 

a. Carbon dioxide reduction starts with a fivecarbon sugar ribulose-l,5-bisphosphate (RuBP). It is a 5- carbon sugar with two phosphate groups attached to it.

b. RuBP reacts with CO₂ to produce an unstable 6 carbon intermediate in the presence of Rubisco. 

c. It immediately splits into 3 carbon compounds called 3-phosphoglyceric acid. 

d. RuBisCO is a large protein molecule and comprises 16% of the chloroplast proteins.

2. Glycolytic reversal: 

a. 3-phosphoglyceric acid form 1,3- diphosphoglyceric acid by utilizing ATP molecule. 

b. These are then reduced to glyceraldehyde3-phosphate (3-PGA) by NADPH supplied by the light reactions of photosynthesis. 

c. In order to keep Calvin cycle continuously running there must be sufficient number of RuBP and regular supply of ATP and NADPH. 

d. Out of 12 molecules of 3- phosphoglyceraldehyde, two molecules are used for synthesis of one glucose molecule. 

3. Regeneration of RuBP: 

a. 10 molecules of 3-phosphoglyceraldehyde are used for the regeneration of 6 molecules of RuBP at the cost of 6 ATP. 

b. Therefore, six turns of Calvin cycle are needed to get one molecule of glucose.

Photorespiration: Mechanism: 

1. Photorespiration involves three organelles chloroplast, peroxisomes and mitochondria and occurs in a series of cyclic reactions which is also called PCO cycle. (Photosynthetic Carbon Cycle) 

2. Enzyme Rubisco acts as oxygenase at higher concentration of O₂ and photorespiration begins. 

3. When RuBP reacts with O₂ rather than CO₂ to form a 3-carbon compound (PGA) and 2- carbon compound phosphoglycolate. 

4. Phosphoglycolate is then converted to glycolate which is shuttled out of the chloroplast into the peroxisomes. 

5. In Peroxisomes, glycolate is converted into glyoxylate by enzyme glycolate oxidase. 

6. Glyoxylate is further converted into amino acid glycine by transamination. 

7. In mitochondria, two molecules of glycine are converted into serine (amino acid) and CO₂ is given out. 

8. Thus, it loses 25% of photosynthetically fixed carbon. 

9. Serine is transported back to peroxisomes and converted into glycerate. 

10. It is shuttled back to chloroplast to undergo phosphorylation and utilized in formation of 3-PGA, which get utilized in C₃ pathway. 

Hatch-Slack pathway:

M. D. Hatch and C. R. Slack while working on sugarcane found four carbon compounds (dicarboxylic acid) as the first stable product of photosynthesis. It occurs in tropical and sub-tropical grasses and some dicotyledons. The first product of this cycle is a 4-carbon compound oxaloacetic acid. Hence it is also called as C₄ pathway and plants are called C₄ plants. 

Mechanism:

1. CO₂ taken from atmosphere is accepted by a 3-carbon compound, phosphoenolpyruvic acid in the chloroplasts of mesophyll cells, leading to the formation of 4-C compound, oxaloacetic acid with the help of enzyme phosphoenolpyruvate carboxylase. 

2. It is converted to another 4-C compound, malic acid. 

3. It is transported to the chloroplasts of bundle sheath cells. 

4. Malic acid (4-C) is converted to pyruvic acid (3-C) with the release of CO₂ in the cytoplasm. 

5. Thus, concentration of CO₂ increases in the bundle sheath cells. 

6. Chloroplasts of these cells contain enzymes of Calvin cycle. 

7. Because of high concentration of CO₂ , RuBP carboxylase participates in Calvin cycle and not photorespiration.

8. Sugar formed in Calvin cycle is transported into the phloem. 

9. Pyruvic acid generated in the bundle sheath cells re-enter mesophyll cells and regenerates phosphoenolpyruvic acid by consuming one ATP. 

10. Since this conversion results in the formation of AMP (not ADP), two ATP are required to regenerate ATP from AMP. 

11. xi. Thus, C₄ pathway needs 12 additional ATP. 

12. The C₃ pathway requires 18 ATP for the synthesis of one glucose molecule, whereas C₄ pathway requires 30 ATP. Thus, C₄ plants are better photo synthesizers as compared to C₃ plants as there is no photorespiration in these plants.

13. CAM Pathway:

In CAM plants, malic acid accumulates during night, which is formed from Oxaloacetic acid in presence of the enzyme malate dehydrogenase.

Photorespiration would occur if C₄ plants did not have Kranz anatomy.

Photosynthesis occurs in the presence of sunlight. During photosynthesis, plants give out oxygen and take in carbon dioxide. The plants present underwater carryout photosynthesis and release oxygen. Hence, large number of gas bubbles are evolved during day time in a pond.

Red colour pigment present in tomatoes, carrots and chillies is lycopene.

1. The Blackman’s law of limiting factors states that when a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is controlled by the pace of the “slowest factor”. 

2. The slowest factor is that factor which is present in the lowest or minimum concentration in relation to others. 

3. The law of limiting factor can be explained by taking two external factors such as carbon dioxide and light. 

4. For example, a plant photosynthesizing at a fixed light intensity sufficient to utilize 10mg of C₂ per hour only.

5. Photosynthetic rate goes on increasing when concentration of CO₂ increases. 

6. Further increase in CO₂ concentration will not increase the rate of photosynthesis. In this case light becomes the limiting factor. Therefore, under such circumstances rate of photosynthesis can be increased only by increasing the light intensity. 

7. This proves that the rate of photosynthesis responds to one factor alone at a time and there would be a sharp break in the curve and a plateau formed exactly at the point where another factor becomes limiting. 

8. If any one of the other factors which is kept constant (e.g. Light) is increased, the photosynthetic rate increases again reaching the optimum where again another factor becomes limiting.

1. The Blackman’s law of limiting factors states that when a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is controlled by the pace of the “slowest factor”. 

2. The slowest factor is that factor which is present in the lowest or minimum concentration in relation to others. 

3. The law of limiting factor can be explained by taking two external factors such as carbon dioxide and light. 

4. For example, a plant photosynthesizing at a fixed light intensity sufficient to utilize 10mg of C₂ per hour only. 

5. Photosynthetic rate goes on increasing when concentration of CO₂ increases.

6. Further increase in CO₂ concentration will not increase the rate of photosynthesis. In this case light becomes the limiting factor. Therefore, under such circumstances rate of photosynthesis can be increased only by increasing the light intensity. 

7. This proves that the rate of photosynthesis responds to one factor alone at a time and there would be a sharp break in the curve and a plateau formed exactly at the point where another factor becomes limiting. 

8. If any one of the other factors which is kept constant (e.g. Light) is increased, the photosynthetic rate increases again reaching the optimum where again another factor becomes limiting.

External factors which affect photosynthesis are as follows: 

1. Light: 

a. It is an essential factor as it supplies the energy necessary for photosynthesis. 

b. Quality and intensity of light affects the photosynthesis. 

c. Highest rate of photosynthesis takes place in red light followed by blue light. 

d. The rate of photosynthesis considerably decreases in plants which are growing under a forest canopy. 

e. In most of the plants, photosynthesis is maximum in bright diffused sunlight. 

f. Uninterrupted and continuous photosynthesis for a very long period of time may be sustained without any visible damage to the plant. 

2. Carbon dioxide: 

a. The main source of CO₂ in land plants is the atmosphere, which contains only 0.3% of the gas. 

b. Under normal conditions of temperature and light, carbon dioxide acts as a limiting factor in photosynthesis. 

c. Increase in concentration of CO₂ increases the photosynthesis. 

d. Increase in CO₂ to about 1% is advantageous to most of the plants. 

e. Higher concentration of the gas has an inhibitory effect on photosynthesis.

3. Temperature: 

a. Like all other physiological processes, photosynthesis also needs a suitable temperature. 

b. The optimum temperature at which the photosynthesis is maximum is 25-30 °C. Except in plants like Opuntia, photosynthesis takes place at as high as 55 °C. 

c. This is the maximum temperature. Minimum temperature is temperature at which photosynthesis process just starts. 

d. In the presence of sufficient light and CO₂ , photosynthesis increases with the rise of temperature till it becomes maximum. After that there is a decrease or fall in the rate of the process. 

4. Water: 

a. Water is necessary for photosynthetic process. 

b. An increase in water content of the leaf results in the corresponding increase in the rate of photosynthesis. 

c. Thus, the limiting effect of water is not direct but indirect. 

d. It is mainly due to the fact that it helps in maintaining the turgidity of the assimilatory cells and the proper hydration of their protoplasm.

Internal factors which affects photosynthesis are as follows: 

1. Chlorophyll: 

a. Though presence of chlorophyll is essential for photosynthesis but rate of photosynthesis is proportional to the quantity of chlorophyll present. 

b. It is because of the fact that chlorophyll merely acts as a biocatalyst and hence a small quantity is quite enough to maintain the large bulk of the reacting substances. 

2. Sugar: 

The final product in the photosynthesis reaction is sugar and its accumulation in the cells slow down the process of photosynthesis.

3. Internal structures: 

The thickness of cuticle and epidermis of the leaf, the size and distribution of intercellular spaces and the distribution of the stomata and the development of chlorenchyma and other tissues also affects the rate of photosynthesis.

The correct answer is (C) C₃ plants responds to higher temperatures with enhanced photosynthesis while C₄ plants have much lower temperature optimum.

The correct answer is (A) Phosphoenol pyruvic Acid

(d) PGA and phosphoglycolate