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Important Questions Class 11 Biology Chapter 13 Photosynthesis Higher Plants

2 H 2 O → 4 H + + 4 e − + O 2 ↑ 2 H 2 O → 4 H + + 4 e − + O 2 ↑

N A D P + 2 H + → N A D P H 2 N A D P + 2 H + → N A D P H 2

Green plants synthesize their own food which is utilized by them and other organisms dependant on them, through a process known as photosynthesis. Hence plants are known as autotrophs. Photosynthesis is a physio-chemical process which uses light energy to produce organic compounds. Photosynthesis is essential as it not only is the primary source of food for living entities but it also releases oxygen into the atmosphere which is required to breathe. Through this chapter, we understand the structure of photosynthetic machinery and the numerous reactions that take place to convert light energy to chemical energy.

a) Is this composition present in a plant cell or animal cell?

b) Can it be inherited by the offspring? How?

c) Write the metabolic processes that are occurring at the places marked as (1) and (2) in the figure

A.1. a)  It is a chloroplast, green in colour, found in plant cells. b) Because of the presence of extranuclear DNA, it has the potential to self-replicate. c) At site 1 – Dark reactions of photosynthesis takes place at the stroma of the chloroplasts. At 2 – It is the structure of extracellular DNA that is responsible for replication of chloroplasts when necessary in the photosynthesizing cells.

Q.2. Answer the following questions based on the equation given below:

a) Where in plants does this reaction occur?

b) What is the importance of this reaction?

a)  In the reaction centre, PS II, situated in the inner surface of the thylakoid membrane. Here electrons are extracted from water and are known as water splitting centre, and is catalyzed by the Mn +  and Cl –  ions.

b)  The process releases molecular oxygen as a by-product of photosynthesis and is an important source of oxygen in the air, for living entities on earth. The ions of hydrogen hence produced are used in reducing NADP to NADPH which is a strong reducing agent. It creates a gradient for the synthesis of ATR when electrons released are transferred from PS II to PS I through electron carriers.

Q.3. How do photosynthetic bacteria such as Cyanobacteria conduct photosynthesis in the absence of chloroplasts?

A.3.  They are prokaryotes containing photosynthetic pigments in a membranous form that are primitive in nature, that can trap and use solar energy. Hence they can carry out photosynthesis.

Q.4. a) Where is the enzyme NADP reductase located?

b) What does the breakdown of proton gradient cause the release of?

A.4. a)  It is located on the outer side of the thylakoid membrane. b) ATP molecules. In the presence of enzyme ATP synthase, it is coupled with the movement of ions across the membrane and formation of ATP synthesis.

Q.5. Can we perform girdling experiments on monocots? State why if yes or no.

A.5.  No, we cannot perform. We cannot reach a specific band of the phloem tissues as the monocot stem has vascular bundles scattered all over the width of the stem.

Q.6. Does moonlight promote photosynthesis?

A.6.  No, it does not as it does not carry enough energy to excite chlorophyll molecules – reaction centres PS I and PS II .  Hence light-dependent reactions are not initiated.

Q.7. Where in the chloroplast is the NADP reductase enzyme located? State its role in the proton gradient development.

A.7.  On the outer side of lamella or thylakoid of the  chloroplast . It breaks down the proton gradient to release energy i.e. NADPH.

Q.8. Name the two parts in ATPase enzyme. State their arrangement in the thylakoid membrane. Which part of the enzyme shows conformational changes?

A.8.  The two parts are a headpiece and a stalk and are located all across the inner mitochondrial membrane. The conformational changes occur in the headpiece (F 1 ) of the enzyme.

Q.9. Name the products used to drive the dark reaction that is formed during the light reaction of photosynthesis.

A.9.  ATP and NADPH. They are used to fix CO 2  and to form a glucose molecule.

Q.10. On what basis is the C3 and C4 pathways of photosynthesis selected?

A.10.  On the basis of carbon atoms in the first stable product of carbon dioxide fixation.

Q.1. How are succulents able to meet their photosynthetic CO 2  requirements as they are known to keep their stomata closed during the day to check transpiration?

A.1.  During day time, the succulent plants have to shut their stomata as they grow in dry and xeric conditions, for prevention of water loss during  transpiration , the gaseous exchange does not take place. Hence plants have adapted to fix CO 2  in the form of malic acid during night time. Malic acid is a four carbon compounds that stores CO 2  which is liberated during the day within the photosynthetic cells.

Q.2. Define accessory pigments. State their significance in photosynthesis.

A.2.  They are chlorophyll-b, xanthophyll, and carotenoids and are also referred to as photosynthetic pigments. They aid in accumulating solar radiations and passing them onto chlorophyll-a. They are not directly involved in the emission of excited electrons. While doing so, this pigment itself absorbs maximum radiations at regions – blue and green. Hence chlorophyll-a is the main pigment and other pigments are accessory pigments (chlorophyll-Ai, xanthophyll, and carotenoid)

Q.3. Do photosynthetic reactions such as dark reactions require light? Explain.

A.3.  Dark reactions are light-independent reactions. Through various processes, carbon dioxide is reduced to glucose which does not require light. However, they rely upon the products formed during the light reactions (NADPH2 and ATR).

Q.4. What is the relationship between photosynthesis and respiration?

A.4.  They are related. In both mechanisms, plants gain energy in photosynthesis from solar radiations while in respiration, the breakdown of glucose molecule takes place to get energy in the form of ATP molecules. Both processes are dependant on each other. Respiration uses the end product of the photosynthesis(glucose in the form of food). To produce ATP for energy. In the process, carbon dioxide and water are released which photosynthesis uses to produce more sugars.

Q.5. Can green plants kept in dark with proper ventilation carry out photosynthesis? Can they be administered with supplements to maintain survival or growth?

A.5.  No, they cannot carry out photosynthesis as light is essential for green plants to make their own food. For survival, plants need to be watered properly.

Q.6. How are short plants growing below thick canopies and receiving filtered light able to carry out photosynthesis in tropical rain forests?

A.6.  In the presence of light, plants carry out photosynthesis. The rate of photosynthesis is the only factor that gets affected by the quality and intensity of light. Hence only the rate could vary amongst plants growing in different canopy regions.

Q.7. The rate of photosynthesis decreases at higher temperatures. Why?

A.7.  The process of photosynthesis is an enzyme specific process. All the enzymes act at an optimum temperature. If the temperature increases beyond 35 °C, enzymes denature, which affects the rate of photosynthesis.

Q.8. In a C3 plant, a light dependent cyclic process is occurring that requires oxygen. Instead of producing, it consumes energy.

c) Write the end products of this process.

b) Not necessary for the survival of a C3 plant.

d) Three cell organelles are involved – chloroplast, peroxisome, mitochondria

Q.9. In a certain ‘X’ organism, a process is occurring throughout the day in which cells are participating. Water, ATP and carbon dioxide are evolved during the process and is not a light-dependent process.

a) Which process is discussed above?

b) Is the process a catabolic or anabolic process?

c) Write the material of this process.

b) It involves the breakdown of glucose and is a catabolic process.

c) Raw material – oxygen and glucose molecule, which are products of photosynthesis.

Q.10. Name the pigment that causes the red colour in tomato and chillies. Is the pigment a photosynthetic pigment?

A.10.  Pigment lycopene imparts the red colour to them. Because of the presence of chromoplasts in plants, it gives rise to different colours. It is a kind of plastid that develops different coloured pigments that are not photosynthetic.

Q.11. Explain why is the colour of a leaf in the dark changes?   Which pigment do you think is more stable? A.11.  The colour of leaf stored in dark changes from green to pale green or yellow in colour. This is mainly because of the absence of sunlight and chlorophyll pigment. In the dark or during the absence of light, photosynthesis does not take place, therefore there is no synthesis of starch and chlorophyll pigment. The most stable pigment is chlorophyll a.

Q.12. List out the differences in anatomy of leaf in C 3  and C 4  plants?

Q.1. Does photosynthesis occur in leaves only? If no, what are the other parts that are capable of carrying out photosynthesis? Justify.

A.1.  The process of  photosynthesis  primarily occurs in leaves of all green plants. The leaves are designed in such a way that they capture sunlight and convert solar energy into chemical energy effectively. However, in some plants, modified parts other than leaves carry out photosynthesis. In some plants, roots develop chlorophyll and initiate photosynthesis, they are referred to as assimilatory roots. Example – Tinospora, Trapa. The stem in some plants such as the Opuntia gets modified and adapts to perform the functions of leaves, be turning thick, succulent and flattened, performing photosynthesis, these structures are referred to as phylloclade. In Australian Acacia, the petiole takes the shape and function of photosynthesis when stamina of leaf fails.

Q.2. List the location in the cell where the following reactions take place during the process of photosynthesis.

a) Outer side of the thylakoid membrane

b) The inner side of the thylakoid membrane

Q.3. Name the pigment that is responsible for its ability to initiate the process of photosynthesis. The rate of photosynthesis is higher in the red and blue regions of the spectrum of light, why?

A.3.  The thylakoid membranes contain the chlorophyll pigments which have the property of excitability and emits  e –  in the excited stage, though it is replaced and transferred by the  e –  produced from splitting of water molecules. Chlorophyll pigments absorb the maximum energy possessed by the red and blue light, thereby getting excited and initiating photosynthesis. Its wavelength lies between the PAR (Photosynthetic Active Radiation). Hence the rate is higher in the red and blue regions.

Q.4. What are the important events and end products of light reaction?

A.4.  The following are the events: Emission of a pair of electrons due to excitation of chlorophyll molecules, using this energy to form ATP from ADP + Pi, the process is referred to as photophosphorylation, a water molecule is split

End products are ATP and NADPH. The light reaction produces the reducing power i.e., the ATP and NADPH 2  molecules which are utilized in the dark reaction. A by-product of splitting water is O 2 .

Q.5. Why photorespiration does not take place in C4 plants?

A.5.  Photorespiration is related to C 3  cycle, where plants lose carbon dioxide fixation because of the increase in the concentration ion of oxygen and modification in the nature of the activity of RuBP carboxylase oxygenase. C 4  plants have developed a mechanism to avoid the loss of carbon dioxide. There is not a direct contact of RuBP carboxylase oxygenase as C 3  cycle functions in bundle sheath cells. The enzyme PEP carboxylase in mesophyll cells carries out carbon dioxide fixation, wherein oxaloacetate is formed which is converted to malic acid which is passed to bundle sheath cells where carbon dioxide is utilized in the Calvin cycle that functions in bundle sheath cells of C 3 plants.

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In C3 plants, chloroplasts are present only in mesophyll cells.

In C4 plants, chloroplasts are present both in mesophyll and bundle sheath cells.

Only one type of chloroplasts is present.

Di-morphic types of chloroplasts are present.

Kranz anatomy is absent in the leaves of C3 plants.

Kranz anatomy is present in the leaves of C4 plants.