NCERT Exemplar for Class 11 Biology Chapter 13

NCERT Exemplar for Class 11 Biology will help students to deepen their knowledge related to the topic of Photosynthesis in Higher Plants. This provided exemplar by SimplyAcad allows students to cover all the areas and sections comprising the chapter 13 of the biology textbook. The NCERT exemplar for Class 11 Biology is an effective study material to boost the knowledge of students through different types of questions. The subject experts have prepared these questions including MCQs, very short, short and long answer type questions to sharpen the understanding of learners. It will be an advantage for students as they become more familiar with these different patterns. Students can easily access this NCERT exemplar for class 11 biology in this article below to perform incredibly well in their upcoming examinations. Along with this, there are several other NCERT exemplar for class 11 science of all the chapters provided in a detailed manner.

Multiple Choice Questions

1. Which metal ion is a constituent of chlorophyll?

a. Iron

b. Copper

c. Magnesium

d. Zinc

Ans: (c) Magnesium

Explanation: Chlorophyll is a chlorine pigment. At the center of chlorine, the ring is a magnesium ion.

2. Which pigment acts directly to convert light energy to chemical energy?

a. Chlorophyll a

b. Chlorophyll b

c. Xanthophyll

d. Carotenoid

Ans: (a) Chlorophyll a

Explanation: Absorption of light is maximum by chlorophyll a.

3. The range of wavelength (in nm) is called photosynthetically active radiation (PAR10).

a.100 – 390

b. 390 – 430

c. 400 – 700

d.760 – 1000

Ans: (c) 400 – 700

4. Which light range is least effective in photosynthesis

a. Blue

b. Green

c. Red

d. Violet

Ans: (b) Green

Explanation: Because chlorophyll, the pigment involved in photosynthesis, is green (for the most part, there are other forms that are not green) and because it is green it reflects green wavelengths of light and does not absorb them.

5. Chemosynthetic bacteria obtain energy from

a. Sun

b. Light

c. Organic chemicals

d. Inorganic chemicals

Ans: (d) Inorganic chemicals

Explanation: Chemosynthetic bacteria utilize inorganic chemicals to make organic chemicals.

6. Energy required for ATP synthesis in PSII comes from

a. Proton gradient

b. Electron gradient

c. Reduction of glucose

d. Oxidation of glucose

Ans: (a) Proton gradient

7. During light reaction in photosynthesis the following are formed:

a. ATP and sugar

b. Hydrogen, O_2, and sugar

c. ATP, hydrogen donor and O₂

d. ATP, hydrogen and O₂ donor

Ans: (c) ATP,hydrogen and O₂ donor

Explanation: Water molecules are split during the light reaction; releasing hydrogen and O₂.

8. Dark reaction in photosynthesis is called so because

a. It can occur in dark also

b. It does not depend on light energy

c. It cannot occur during daylight

d. It occurs more rapidly at night

e. It cannot occur during daylight

Ans: (b) It does not depend on light energy.

Explanation: Splitting of the water molecule and formation of ATP are directly driven by light and hence are called light reactions. The rest of the steps are not directly light-driven and hence are called dark reactions (as per convention). This does not mean that they happen in the dark and not during the day.

9. PEP is the primary CO₂ acceptor is

a. C4 plants

b. C3 plants

c. C2 plants

d. ATP synthase

Ans: (a) C4 plants

Explanation: PEP is absent in C3 plants and PEP carboxylase is also absent.

10. Splitting of water is associated with

a. Photosystem-I

b. Lumen of thylakoid

c. Both Photosystem I and Photosystem II

d. Inner surface of thylakoid membrane

Ans: (d) Inner surface of thylakoid membrane

11. The correct sequence of flow of electrons in the light reaction is

a. PSII, plastoquinone, cytochromes, PSI, Ferredoxin

b. PSI, plastoquinone, cytochromes,PSII, Ferredoxin

c. PSI, ferredoxin, PSII

d. PSI, plastoquinone, cytochromes, PSI, Ferredoxin

Ans: (a) PSII, plastoquinone, cytochromes, PSI, Ferredoxin

12. The enzyme that is not found in a C3 plant is

a. RuBP carboxylase

b. PEP Carboxylase

c. NADP reductase

d. ATP synthase

Ans: (b) PEP carboxylase

13. The reaction that is responsible for the primary fixation of CO₂ is catalyzed by

a. RuBP carboxylase

b. PEP carboxylase

c. RuBP carboxylase and PEP carboxylase

d. PGA synthase

Ans: (c) RuBp carboxylase and PEP carboxylase.

Explanation: For C3 plants it is RuBP carboxylase and for C4 plants it is PEP carboxylase.

14. When C2 is added to PEP, the first stable product synthesis is:

a. Pyruvate

b. Glyceraldehyde-3-phosphate

c. Phosphoglycerate

d. Oxaloacetate

Ans: (d) Oxaloacetate

Very Short Answer Questions

1. Examine the figure:

a. Is this structure present in an animal cell or plant cells?

Ans: This structure is present in a plant cell.

b. Can these be passed on to the progeny? How?

Ans: Chloroplast has the ability of self-replication. It can also be passed onto the progeny.

c. Name the metabolic processes taking place in the places marked (1) and (2)

Ans: (1) shows dark reaction and

(2) shows the site of replication of chloroplast.

2. 2H₂O 4H⁺ + O₂ + 4e^–

Based on the above equation, answer the following questions:

a. Where does this reaction take place in plants?

Ans: It shows the decomposition of water molecules. It takes place on PS II and is located on the inner surface of the thylakoid membrane.

b. What is the significance of this reaction?

Ans: Splitting of water continuously furnishes electrons to the electron transport chain; for further stages of photosynthesis.

3. Cyanobacteria and some other photosynthetic bacteria don’t have chloroplasts. How do they conduct photosynthesis?

Ans: Cyanobacteria and some other photosynthetic bacteria do not have chloroplasts. But they have folds in their inner membrane where photosynthesis occurs. They have bluish pigment phycocyanin which captures solar energy to carry out photosynthesis.

4. a. NADP reductase enzyme is located on _________________.

Ans: the outer side of the thylakoid membrane.

b. Breakdown of proton gradient leads to release of _________________.

Ans: ATP molecules.

5. Can girdling experiments be done in monocots? If yes, how? If no, why not?

Ans: Girdling experiments cannot be done in monocots. The monocot has vascular bundles scattered all over the width of the stem. Hence, we cannot reach a specific band of phloem for girdling.

6. 3CO₂ + 9ATP + 6NADPH + Water glyceraldehyde 3 – phosphate + 9 ADP + 6 NADP+ + 8 Pi

Analyze the above reaction and answer the following questions:

a. How many molecules of ATP & NADPH are required to fix one molecule of CO₂?

Ans: 3 molecules of ATP are required for phosphorylation and 2 molecules of NADPH are required for the reduction of carbon dioxide.

b. Where in the chloroplast does this process occur?

Ans: This reaction occurs in the stroma of the chloroplast.

7. Does moonlight support photosynthesis? Find out.

Ans: Moonlight does not have sufficient energy to excite chlorophyll molecules. Hence, moonlight cannot support photosynthesis.

8. Some of these terms/chemicals are associated with the C4 cycle. Explain.

a. Hatch slack pathway

Ans: The process of synthesis of glucose in C4 plants is called Hatch and Slack Pathway. It was observed by MD Hatch and CR Slack in 1977.

b. Calvin cycle

Ans: The steps after splitting of water molecules lead to the formation of carbohydrates. This happens in a cyclical manner and is called the Calvin Cycle. This was first observed by Melvin Calvin and his co-workers.

c. PEP carboxylase

Ans: PEP carboxylase is an enzyme. It is present in mesophyll cells of C4 plants. It fixes carbon to form oxaloacetate.

d. Bundle sheath cells

Ans: The specialized sclerenchymatous cells present around vascular bundles in the veins of C4 plants are called bundle sheath cells.

9. Where is the NADP reductase enzyme located in the chloroplast? What is the Role of this enzyme in proton gradient development?

Ans: NADP reductase enzyme is located on the outer side of the thylakoid membrane. It facilitates the breakdown of the proton gradient to release energy, i.e. NADPH.

10. ATPase enzyme consists of two parts. What are those parts? How are they arranged in the thylakoid membrane? Conformational change occurs in which part of the enzyme?

Ans: ATPase enzyme consists of two parts; called F0 head and F1 head. The F0 head is towards the inner side of the thylakoid, while F1 is towards the outer side of the thylakoid. The conformational change occurs in the F1 part of the enzymes.

11. Which products formed during the light reaction of photosynthesis are used to drive the dark reaction?

Ans: ATP and NADPH

12. What is the basis for designating C3 and C4 pathways of photosynthesis?

Ans: In the case of C3 pathways, carbon is fixed into a 3-carbon compound, i.e. 3-PGA. On the other hand, in the case of C4 pathways, carbon is fixed into a 4-carbon compound, i.e. oxaloacetic acid. Thus, the amount of carbon atoms in the end product is the purpose for establishing C3 and C4 pathways of photosynthesis.

Short Answer Questions

1. Succulents are known to keep their stomata closed during the day to check transpiration. How do they meet their photosynthetic CO₂ requirements?

Ans: Succulent plants keep their stomata close during the day to check transpiration. This also prevents the entry of carbon dioxide during the day. These plants have arranged a unique method to secure the supply of carbon dioxide during the daytime. These plants fix carbon dioxide in the form of malic acid during the night.

2. Chlorophyll ‘a’ is the primary pigment for a light reaction. What are accessory pigments? What is their role in photosynthesis?

Ans: Pigments that aid chlorophyll in trapping solar radiation are called accessory pigments. Chlorophyll b, xanthophylls, and carotenoids are the accessory pigments. They aid in harvesting solar radiation and pass it on to chlorophyll a. Thus, accessory pigments play a supportive role in light-harvesting.

3. Do reactions of photosynthesis called ‘Dark Reaction’ need light? Explain.

Ans: Dark Reaction comprises a biosynthetic phase of photosynthesis during which synthesis of carbohydrates takes place. This part of the photosynthesis is indirectly dependent on light and hence is called Dark Reaction. However, this does not mean that a dark reaction takes place in complete darkness rather it goes on even during the daytime. So, a dark reaction indirectly requires light but does not depend on light to proceed with.

4. How are photosynthesis and respiration related to each other?

Ans: Photosynthesis and respiration are associated with each other in specific ways. We know that respiration comprises the oxidation of carbohydrates to develop energy. Since carbohydrates for respiration are prepared during photosynthesis, respiration cannot occur without photosynthesis. Moreover, respiration also requires oxygen which is a by-product of photosynthesis. Also, carbon dioxide is a significant raw material for photosynthesis and much of the carbon dioxide enters as a by-product of respiration.

5. If a green plant is kept in dark with proper ventilation, can this plant carry out photosynthesis? Can anything be given as a supplement to maintain its growth or survival?

Ans: If a plant is kept in dark with adequate ventilation, it will proceed to get carbon dioxide. But it is bereft of sunlight. Hence, this plant cannot carry out photosynthesis. For sustaining its survival, the plant can be provided with an adequate proportion of water. But the plant would ultimately die because of the absence of nutrients.

6. Photosynthetic organisms occur at different depths in the ocean. Do they receive qualitatively and quantitatively the same light? How do they adapt to carry out photosynthesis under these conditions?

Ans: Light is rarely a limiting factor for photosynthesis because light saturation occurs at 10% of the full sunlight. So, except for plants in shade or dense forest; light is rarely a limiting factor. Photosynthetic organisms in oceans are found at various depths and the amount of light available to them is enough for carrying out photosynthesis. Moreover, these organisms show great variations in photosynthetic pigments. These pigments help these organisms to carry out photosynthesis even in low light conditions.

7. In tropical rainforests, the canopy is thick and plants growing below receive filtered light. How are they able to carry out photosynthesis?

Ans: Light is rarely a limiting factor for photosynthesis because just 10% of the full sunlight is sufficient to ensure light saturation. In plants that thrive under a large canopy in forests; a less percentage of light may slow down the rate of photosynthesis but it cannot halt photosynthesis during daytime. Hence, these plants are able to carry on photosynthesis.

8. What conditions enable Rubisco to function as an oxygenase? Explain the ensuing process.

Ans: RuBisCO has an affinity with carbon dioxide as well as with oxygen. But RuBisCO’s binding with either of them is strong. This means that in case of a higher concentration of CO₂, the enzyme would act as a carboxylase. But in the case of a higher concentration of O₂, the enzyme would act as an oxygenase.

9. Why does the rate of photosynthesis decrease at higher temperatures?

Ans: Photosynthesis is an enzyme-mediated process. Enzymes work in optimum range temperature. When the temperature goes past this range, an enzyme cannot work. Due to this, the proportion of photosynthesis declines at higher temperatures.

10. Explain how during light reaction of photosynthesis, ATP synthesis is a chemiosmotic phenomenon.

Ans: The activity of ions across a semipermeable membrane and under an electronic gradient is called chemiosmotic movement. In ATP synthesis during light reaction of photosynthesis, movement of ions takes place across the thylakoid membrane which is a semi-permeable membrane. Moreover, ions are pumped by a proton pump utilizing a proton gradient; and eventually, ATP is synthesized. Due to these characteristics, ATP synthesis during the light reaction is called a chemiosmotic phenomenon.

11. Find out how Melvin Calvin worked out the complete biosynthetic pathway for the synthesis of sugar.

Ans: Melvin Calvin and his coworkers discovered the complete pathway for the synthesis of sugar by performing experiments at the University of California. For this, they followed the following steps:

For this experiment, carbon dioxide was tagged with C14 (a radioactive isotope of carbon) and was provided to the plants. After that, the pathway of C14 was identified and analyzed under different circumstances, e.g. in dark and light. Once the experiment with the live plant was over, the plant was killed and the tagged compound was taken out from the dead plant for further examination. Based on the estimation in live and dead plants, Calvin and his coworkers ultimately found out the pathway during light dominant reactions.

12. Six turns of the Calvin cycle are required to generate one mole of glucose. Explain.

Ans: During the Calvin cycle, 3 molecules of ATP and 2 molecules of NADPH are required for the reduction of one molecule of CO₂. Since glucose is a 6-carbon compound, 6 molecules of carbon dioxide are required to prepare one molecule of glucose. Hence, six turns of the Calvin Cycle are needed to generate one mole of glucose. Hence a total of 18 ATP and 12 NADPH is required for the synthesis of one molecule of glucose.

13. Complete the flowchart for cyclic photophosphorylation of the photosystem-I

Ans:

14. In what kind of plants do you come across ‘Kranz’ anatomy? To which conditions are those plants better adapted? How are these plants better adapted than the plants which lack this anatomy?

Ans: Kranz’s anatomy is found in C4 plants; such as maize, sugarcane, etc. These plants can resist high temperatures and high intensities of light. These plants are also modified to live in an insufficient supply of nitrogen and carbon dioxide. These plants do not carry photorespiration; unlike C3 plants. This aids in making optimum amounts of glucose. Hence, C4 plants produce more biomass compared to C3 plants.

15. A process is occurring throughout the day, in ‘X’ organisms. Cells participating in this process ATP, CO₂, and water are evolved. It is not a light-dependent process.

a. Name the process.

Ans: Respiration

b. Is it a catabolic or an anabolic process?

Ans: This is a catholic process

c. What could be the raw material of this process?

Ans: Carbohydrate

16. Tomatoes, carrots, and chilies are red due to the presence of one pigment. Name the pigment. Is it a photosynthetic pigment?

Ans: The color of tomatoes, carrots, and chilies is red because of the pigment Lycopene. This is not a photosynthetic pigment. This pigment is which exists in the plastic is called

a. Oxaloacetic acid

b. PEPcarboxylase or PEPcase.

17. Why do we believe chloroplast and mitochondria to be semi-autonomous organelle?

Ans: Chloroplast and mitochondria have extra genomic DNA. Due to this, these organelles can replicate themselves. Hence, these are called semi-autonomous organelles.

18. Observe the diagram and answer the following.

a. Which group of plants exhibits these two types of cells?

Ans: Monocot plants display bundle sheath cells and mesophyll cells.

b. What is the first product of the C4 cycle?

Ans: Oxaloacetic acid

c. Which enzyme is there in bundle sheath cells and mesophyll cells?

Ans: PEPcarboxylase or PEPcase.

19. A cyclic process is occurring in a C3 plant, which is light-dependent and needs O2. This process doesn’t produce energy rather it consumes energy.

a. Can you name the given process?

Ans: Photorespiration

b. Is it essential for survival?

Ans: It is not essential for survival.

c. What are the end products of this process?

Ans: Hydrogen peroxide

d. Where does it occur?

Ans: It occurs in chloroplast, mitochondria, and peroxisome.

20. Suppose Euphorbia and Maize are grown in the tropical area.

a. Which one of them do you think will be able to survive under such conditions?

Ans: Euphorbia, because it is a CAM plant.

b. Which one of them is more efficient in terms of photosynthetic activity?

Ans: Maize is a C4 plant and hence it is more fertile in terms of photosynthetic activity.

c. What differences do you think are there in their leaf anatomy?

Ans: Kranz Anatomy is seen in maize plants, while it is absent in Euphoria plants.

Long Answer Questions

1. Is it correct to say that photosynthesis occurs only in the leaves of a plant? Besides leaves, what are the other parts that may be capable of carrying out photosynthesis? Justify.

Ans: If the part of the plant is green in color then photosynthesis can occur in that part also. So, it is false to say that photosynthesis happens only in the leaves of a plant. Most of the other plant parts are capable of carrying out photosynthesis. Some examples are following:

a. Roots, Trapa, and Tinospora have chlorophyll and they carry out photosynthesis.

b. In Opuntia, the stem is altered into a fleshy green pattern, and leaves are altered into thorns to curtail transpiration. In these plants, photosynthesis takes place in the stem. The modified stem of Opuntia is called phylloclade.

c. In many plants, sepals are green in color and carry out photosynthesis.

2. The entire process of photosynthesis consists of a number of reactions. Where in the cell does each of these take place?

a. Synthesis of ATP & NADPH __________________________

Ans: Outer side of thylakoid membrane

b. Photolysis of water __________________________

Ans: The inner side of thylakoid membrane

c. Fixation of CO₂ __________________________

Ans: The stroma of chloroplast

d.Synthesis of sugar molecule __________________________

Ans: Chloroplast

e. Synthesis of starch __________________________

Ans: Cytoplasm

3. Which property of the pigment is responsible for its ability to initiate the process of photosynthesis? Why is the rate of photosynthesis higher in the red and blue regions of the spectrum of light?

Ans: Pigments are substances that can absorb light at specific wavelengths. It means that different pigments can absorb light of different colors. So, if pigment A can get in an excited state by color X, pigment B may get excited by color Y. Once a pigment gets active after absorbing light, it gives energy so that light energy can be utilized for further stages of photosynthesis. Chlorophyll a is the most substantial pigment in plants. This pigment shows excellent efficiency between the wavelengths of blue and red light. As shown by these graphs, other pigments also show their optimum efficiency between blue and red light.

When this graph is correlated with the graph (showing rate of photosynthesis), it is clear that photosynthesis is at an optimum level between blue and red wavelengths. This primarily happens because of chlorophyll a. But this also happens because of the activity of accessory pigments. Due to this, the rate of photosynthesis is higher in the range of red and blue lights.

4. What can we conclude from the statement that the action and absorption spectrum of photosynthesis overlap? At which wavelength do they show peaks?

Ans: To understand this, we need to analyze the following graph.

The first graph shows the rate of photosynthesis; as measured by the release of oxygen. The second graph displays absorption of various wavelengths of light by chlorophyll a; superimposed with the rate of photosynthesis. Both the graphs appear to be imitating each other; the graphs appear to be imitating each in their peaks and troughs. The graphs are peaking at more than 400 nm wavelength which is similar to blue color. After that, both the graphs show trough. Then they peak again between 600 and 700 nm which is equivalent to red color. So, this shows that action and absorption spectra overlap. The black line displays the action spectrum of photosynthesis, while the blue line displays the absorption spectrum. 430 – 470 nm and 660 – 670 nm are the maximum wavelengths.

5. Under what conditions are C4 plants superior to C3?

Ans: C4 plants are superior to C3 plants under the following conditions:

C4 plants can perform photosynthesis at low CO₂ concentrations. C4 plants show saturation at about 360 L-1, while C3 plants show saturation only beyond 450 L-1. This means that the availability of CO₂ is a limiting factor for C3 plants; which is not the case with C4 plants. C4 plants show higher temperature optimum but C3 plants show a lower temperature optimum. So, C4 plants can carry on photosynthesis even at high temperatures; which is not the case with C3 plants. C4 plants are not affected by high oxygen levels in the atmosphere because the plants are not affected by Kranz’s anatomy. Kranz’s anatomy ensures that photorespiration does not take place in C4 plants. But this is not the case with C3 plants.

6. In the figure given below, the black line (upper) indicates the action spectrum for photosynthesis and the lighter line (lower) indicates the absorption spectrum of chlorophyll a. Answer the following

a. What does the action spectrum indicate? How can we plot an action spectrum?

Ans: Action spectrum indicates the rate of photosynthesis. This is measured by release of oxygen. We can plan an action spectrum by plotting the amount of oxygen released on the y-axis and wavelength on the x-axis. This can be shown by plotting absorption of different wavelengths by any pigment, e.g. chlorophyll a as shown in this graph.

b. How can we derive an absorption spectrum for any substance?

Ans: This can be done by plotting the absorption of different wavelengths for a particular pigment; like chlorophyll a; chlorophyll b or any other pigment.

c. If chlorophyll a is responsible for light reaction of photosynthesis, why do the action spectrum and absorption do not overlap?

Ans: This happens because of the presence of other pigments as well. Other pigments; known as the accessory pigments; help in optimum utilization of solar energy. Thus, the action spectrum is invariably at a higher grade as compared to the absorption spectrum. Thus, the action spectrum does not overlap the absorption spectrum.

7. List the important events and end products of the light reaction?

Ans: Important events and end products of the light reaction are as follows. It is important to note that these events are not in the order as they happen but in the order of their discovery by the scientists.

Light Absorption: Light absorption is carried out by Light-Harvesting Complex (LHC). Each photosystem has all the pigments (except one molecule of chlorophyll a) setting up a light-harvesting system. These pigments are also called the antennae. These pigments absorb light of different wavelengths and thus make the system more efficient. The single chlorophyll molecule functions as the reaction center.

Water Splitting: Energy from sunlight is utilized to split.the water molecule into hydrogen ions and oxygen. Extra electrons released after the split are utilized NADP+ to NADPH.

Oxygen Release: Oxygen which is released after the splitting of water molecules goes out through stomata. Formation of High Energy Intermediates (ATP and NADPH). At the end of the light reaction, two energy-rich compounds ATP and NADPH are formed.

8. In the diagram given below what are labels A, B, and C. What type of phosphorylation is possible in this?

Ans: This figure shows cyclic phosphorylation.

9. Why is the Rubisco enzyme more appropriately called RUBP Carboxylase-Oxygenase and what important role does it play in photosynthesis?

Ans: RuBisCO enzyme has an affinity towards carbon dioxide as well as towards oxygen. But this binding is competitive. It has a considerable affinity towards carbon dioxide than towards oxygenation. It is the relative concentration of CO₂ and O₂ which affects the binding of RuBisCO to a particular molecule. Since it acts like carboxylase as well as like oxygenase; it is more appropriately called RUBP Carboxylase–Oxygenase.

Importance of RuBisCO:

In C3 plants, RuBisCO bonds with CO₂ to generate 3PGA. In C3 plants, some of the RuBisCO binds with O₂ to produce phosphoglycerate and phosphoglycolate through a process called photorespiration. Photorespiration reduces carbon fixation in C3 plants.

10. What special anatomical features are displayed by the leaves of C4 plants? How do they provide an advantage over the structure of C3 plants?

Ans: The C4 plants have special anatomy in leaves. There is a sheath of enormous cells throughout the vascular bundles in leaves. This is called Kranz Anatomy. Several membranes of cells around the bundle cell are particularly impregnated with chloroplast. The thick walls of these cells are impenetrable to gaseous exchange. Intercellular space is absent in the sheath. The bundle sheath provides an advantage to C4 plants over C3 plants. This sheath helps in increasing carbon dioxide concentration in the leaves. The relative concentration of carbon dioxide and oxygen influences the binding of this enzyme to a particular molecule. The high concentration of carbon dioxide in the leaves of these plants ensures that all the RuBisCO binds with carbon dioxide and there is no binding with oxygen. Thus, photorespiration does not take place in these plants. This helps in preventing wastage of aids and carbon fixation is highly productive in these plants. Eventually, these plants establish a higher percentage of biomass compared to the C3 plants.

11. Name the two important enzymes of the C4 pathway, and explain their role in fixing CO₂?

Ans: The two important enzymes of C3 and C4 pathways are RuBisCO and PEPcase respectively. RuBisCO is the primary CO₂ acceptor in C3 plants, while PEPcase is the primary CO₂ acceptor in C4 plants.

Role of RuBisCO: RuBisCO is also called RUBP Carboxylase Oxygenase. As the name indicates; it has both carboxylase and oxygenase actions. This enzyme has more affinity towards carbon dioxide than oxygen. However, some of this enzyme fixes to oxygen in C3 plants through a process called photorespiration. Photorespiration is an inefficient process because it does not create any beneficial product for the plants. Moreover, it also reduces carbon fixation in C3 plants.

Role of PEPcase: This is the major carbon dioxide acceptor in C4 plants. However, RuBisCO is also present in these plants. PEPcase binds with carbon dioxide to produce a 4 carbon compound oxaloacetic acid (OAA). OAA is eventually converted into carbon dioxide. Carbon dioxide eventually enters the Calvin cycle and carbohydrates are generated. It is crucial to memorize that the Calvin Cycle is mutual in both C3 and C4 plants.

12. Why is the Rubisco enzyme the most abundant enzyme in the world?

Ans: RuBisCo catalyzes the first step in the conversion of carbon dioxide into sugar during the Calvin Cycle. This enzyme is present in all the photosynthetic organisms of the world. From the cyanobacteria to the leaves of huge trees; RuBisCO is precisely present everywhere. It can be said that all the carbon in the biosphere originated from carbon fixation held up by RuBisCO. Hence, this is the most substantial enzyme in the world.

13. Why does photorespiration not take place in C4 plants?

Ans: The bundle sheath provides an advantage to C4 plants over C3 plants. This sheath helps in increasing carbon dioxide concentration in the leaves. The relative concentration of carbon dioxide and oxygen influences the binding of this enzyme to a particular molecule. The high concentration of carbon dioxide in the leaves of these plants ensures that all the RuBisCO binds with carbon dioxide and there is no binding with oxygen. Thus, photorespiration does not take place in these plants.

NCERT Exemplar For Class 11 Science

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