Notes on Photosynthesis Prepared by Arvind Kumar

Why is diffusion insufficient to meet the oxygen requirements of multicellular organisms like humans? Ans : In multicellular organisms all the cells of body are not in direct contact with the surrounding environment. Moreover the demand of oxygen is more as compared to unicellular organisms. Therefore diffusion alone is insufficient to meet the requirement.

2. What criteria do we use to decide whether something is alive? Ans : The movement of molecules at micro level is main criteria to decide whether something is alive. It is necessary to replace worn out parts and also to produce energy to work. Other characteristics are nutrition, respiration, excretion, transportation, reproduction, response to stimuli.

3. What are outside raw materials used for by an organism? Ans : All living organisms need food for which carbon based molecules are taken as raw material from outside. Besides this oxygen is required for oxidation of food.

4. What processes would you consider essential for maintaining life? Ans : Nutrition, Respiration, Transportation and excretion are essential for sustenance of life.

5.List the important events in the process of photosynthesis. (i) Absorption of light energy by chlorophyll. (ii) Conversion of light energy to chemical energy and splitting of water molecules into hydrogen and oxygen. (iii) Reduction of carbon dioxide to carbohydrates.

Ans : Autotrophic Nutrition : 1. What are the differences between autotrophic nutrition and heterotrophic nutrition? Ans : Autotrophic Nutrition : 1. In such nutrition carbon dioxide is made to combine with water to form carbohydrates by using sunlight. 2. It is found in green plants and in some chemosynthetic bacteria. 3. Solar energy is trapped and changed into storable form.

Heterotrophic Nutrition : 1. In such nutrition food in the form of complex organic molecules taken from the plants or other animals which is then broken down into simpler molecules by digestion. 2. It is found in animals and non green plants. 3. Solar energy is not trapped in such nutrition.

3. Light is obtained from the sun. 2. Where do plants get each of the raw materials required for photosynthesis? Ans : Raw materials for photosynthesis are obtained in the following way 1. CO2 is absorbed from the air by stomata. In aquatic plants CO2 in dissolved form is taken from water. 2. Water is taken from the soil which is absorbed by the roots. Aquatic plants absorb it directly from the surroundings. 3. Light is obtained from the sun.

Ans : The necessary conditions for photosynthesis are 7. What are the necessary conditions for autotrophic nutrition and what are its byproducts? Ans : The necessary conditions for photosynthesis are 1. Carbon dioxide. 2. Water. 3. Chlorophyll 4. Sunlight. Its byproducts are oxygen and water. Glucose produced in this process is used as food. It is converted to starch for its storage.

Describe the structure of chloroplast Chloroplast are cell organallae found in plant cells. Each chloroplast is disc shaped bounded with double membrane. Inside the lumen (cavity) of chloroplast bag like structures called thylakoids are found which are arranged one over the other like the stacks of coins. Each stack is called granum. The wall of thylakoids contain chlorophyll and other photosynthetic pigments in which light reaction occurs. A semi liquid material called stroma remains filled around the granum in the remaining cavity of the chloroplast. Dark reaction occurs in stroma.

Structure of chlorophyll molecule

Q. Name the different types of photosynthetic pigments. Following are the different types of photosynthetic pigments most commonly found in all the leaf cells Chlorophyll A Chlorophyll B Carotenoids Xanthophylls Besides these the other pigments found are Chlorophyll C, D and E

In which type of light is the rate of photosynthesis maximum? Rate of photosynthesis is maximum in blue and orange red colour light. Since green colour is reflected the rate of photosynthesis is minimum in green colour.

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Describe the process of opening and closing of stomata Opening and closing of stomata is regulated by guard cells. Stomata opens during day and it closes during the night. The process involves following steps During day time guard cells perform photosynthesis and produce glucose. It makes the surrounding medium hypotonic causing endosmosis. Due to swelling of guard cells these curve apart causing opening of stomata. During night time sugar gets converted to starch. It reduces the osmotic pressure and causes exosmosis. As a result the guard cells shrink causing the closure of stomata

Light Reaction or Hill Reaction

Q. Describe light reaction Light reaction or Hill reaction occurs in presence of light. The two important products obtained by this reaction are ATP and NADPH. It causes photolysis of water molecule. It involves following main steps Light energy is absorbed by Photosystem II (P680) and the molecule gets excited and releases an electron. The electron passes through electron transport system. The energy passes into ADP molecule which gets converted to ATP The electron reaches Photosystem I (P700) which further enhances its energy. The electron is ultimately picked up by NADP. The electron deficient Photosystem I pulls out electron from water molecule. It splits the water molecule into Hydrogen and Oxygen. Oxygen is released into the atmosphere. Hydrogen ion is picked up by NADP which changes into NADPH

Dark Reaction or Calvin Cycle

Dark Reaction or Calvin Cycle

Q. Describe dark reaction ? Dark reaction is light independent reaction of photosynthesis. It occurs in stroma part of the chloroplast. ATP and NADPH produced during light reaction are used up in this process. CO2 is combined with RuBP (Ribulose 1,5- bi phosphate) and get converted to 3-PGA (Phosphoglyceric Acid). Further in a series of cyclic steps it is reduced by NADPH thus converting it into glucose molecule. ATP is used to provide energy to carry out this reaction. Since these reactions occur in cyclic manner and RuBP is regenerated it is also known as calvin cycle. 12 NADPH molecules and 18 ATP molecules are used up in this process. NADP = Nicotineamide Adenine Dinucleotide Phosphate