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1. Chlorophyll is a light absorbing pigment. It absorbs light in red and blue regions of the visible light spectrum. Absorption spectrum of chlorophyll for red light is maximum so chlorophyll appears red in transmitted light. Green light is not absorbed but reflected so chlorophyll appear green in reflected light. 

2. Chlorophyll predominantly absorbs red and violet-blue light and it allows plants to use this light as a form of energy for photosynthesis process. 

3. It is most effective wavelength of light in photosynthesis as it has exactly right amount of energy to excite electrons of chlorophyll and boost them out of their orbits to higher energy level.

Advantages of having more pigment molecules in a photocenter are as follows: 

1. Having more than one pigment molecule in photocenter means more sunlight being captured and thus facilitating more effective light reaction. 

2. It will provide protection to chlorophyll molecule against photo-oxidation. 

3. More pigments will capture more energy to start the initial reactions, which is not possible by single pigment.

1. C₄ plants are favoured in tropical regions as they require 30 ATP to produce 1 molecule of glucose. 

2. High temperature in tropical regions leads to closure of stomata to reduce rate of transpiration. Due to this availability of CO₂ decreases.

3. PEP carboxylase present in mesophyll cells can fix CO₂ even at low concentration. This helps the plant in efficient assimilation of atmospheric carbon dioxide. 

4. C₄ plants contain a special leaf anatomy called Kranz anatomy which minimizes the losses due to photorespiration. 

5. It helps C₄ plants to survive in conditions of high daytime temperatures, intense sunlight and low moisture.

1. C₄ plants are favoured in tropical regions as they require 30 ATP to produce 1 molecule of glucose. 

2. High temperature in tropical regions leads to closure of stomata to reduce rate of transpiration. Due to this availability of CO₂ decreases. 

3. PEP carboxylase present in mesophyll cells can fix CO₂ even at low concentration. This helps the plant in efficient assimilation of atmospheric carbon dioxide. 

4. C₄ plants contain a special leaf anatomy called Kranz anatomy which minimizes the losses due to photorespiration. 

5. It helps C₄ plants to survive in conditions of high daytime temperatures, intense sunlight and low moisture.

1. Photorespiration is considered to be a wasteful process in plants. It is an energy consuming process in plants which ultimately leads to reduction in final yield of plants. 

2. During C₃ photosynthesis, 25% of the carbon dioxide fixed has to pass through photorespiratory process. 

3. This decreases the photosynthetic productivity. 

4. In C₄ plants, photorespiration is absent and hence they have better productivity.

1. Xerophytic plants are those that have adapted to dry environments. 

2. They have adapted to arid conditions by storing water in stems. 

3. Stomata of these plants remain closed during day time to reduce the rate of transpiration to bare minimum. 

4. Leaves are modified into spines or are reduced in size to check the loss of water due to transpiration. 

5. The waxy surfaces of xerophytic plants prevent the loss of moisture. 

6. Thus, they are able to survive in high temperature.

(a) increases the rate of photosynthesis

1. Carotenoids are lipid compound present universally in almost all the higher plants and several microorganisms. 

2. They are usually red, orange, yellow, brown, and are associated with chlorophyll. They are of two types – the carotenes and xanthophylls. 

3. The carotenes (C₄₀H₅₆ ) are orange red and xanthophylls contain oxygen. 

4. The light energy absorbed by the carotenoids is transferred to chlorophyll-a to be utilized in photosynthesis. 

5. All photosynthetic plants have these pigments that absorb light between the red and blue region of the spectrum. 

6. Carotenoids found mainly in higher plants, absorb primarily in the violet to blue regions of the spectrum. 

7. They not only absorb light energy and transfer it to chlorophyll but also protect the chlorophyll molecule from photo-oxidation.

Photochemical phase of photosynthesis is light dependent. A series of chemical reactions occur in quick succession initiated by light, therefore the phase is called photochemical phase. It takes place in chlorophyll-containing thylakoids of chloroplast.

It occurs in following steps :

1. The chlorophyll on exposure to light energy becomes activated by absorbing photons.

2. This energy is used in splitting the water molecules (photolysis) as below :-

H₂O ——> H⁺ + OH^–

3. OH^– ions through a series of steps produce water (H₂O) and oxygen (O₂). Water may be used inside the plant but O₂ released into the atmosphere.

4. H⁺ combines with CO₂ in dark reaction to produce glucose.

Chloroplasts are the main site of photosynthesis and both energy yielding process (Light reaction) and fixation of carbon dioxide (Dark reaction) that takes place in chloroplast.

(a) 1700 million tonnes

Formation of ATP in the chloroplast in the presence of light is called photophosphorylation.

(c) Carboxylase oxygenase

(b) Kortschak, Hart and Burr