11.12 NCERT Exercise problems

Q1. By looking at a plant externally can you tell whether a plant is \(\mathrm{C}_3\) or \(\mathrm{C}_4\)? Why and how?

Answer: One cannot distinguish whether a plant is \(\mathrm{C}_3\) or \(\mathrm{C}_4\) by observing its leaves and other morphological features externally. Unlike \(\mathrm{C}_3\) plants, the leaves of \(\mathrm{C}_4\) plants have a special anatomy called Kranz anatomy and this difference can only be observed at the cellular level. For example, although wheat and maize are grasses, wheat is \(\mathrm{C}_3\) plant, while maize is a \(\mathrm{C}_4\) plant.

Q2. By looking at which internal structure of a plant can you tell whether a plant is \(\mathrm{C}_3\) or \(\mathrm{C}_4\)? Explain.

Answer: The leaves of \(\mathrm{C}_4\) plants have a special anatomy called Kranz anatomy. This makes them different from \(\mathrm{C}_3\) plants. Special cells, known as bundle-sheath cells, surround the vascular bundles. These cells have a large number of chloroplasts. They are thick-walled and have no intercellular spaces. They are also impervious to gaseous exchange. All these anatomical features help prevent photorespiration in \(\mathrm{C}_4\) plants, thereby increasing their ability to photosynthesise.

Q3. Even though a very few cells in a \(\mathrm{C}_4\) plant carry out the biosynthetic – Calvin pathway, yet they are highly productive. Can you discuss why?

Answer: The productivity of a plant is measured by the rate at which it photosynthesises. The amount of carbon dioxide present in a plant is directly proportional to the rate of photosynthesis. \(\mathrm{C}_4\) plants have a mechanism for increasing the concentration of carbon dioxide. In \(\mathrm{C}_4\) plants, the Calvin cycle occurs in the bundle sheath cells. The \(\mathrm{C}_4\) compound (malic acid) from the mesophyll cells is broken down in the bundle sheath cells. As a result, \(\mathrm{CO}_2\) is released. The increase in \(\mathrm{CO}_2\) ensures that the enzyme RuBisCo does not act as an oxygenase but as a carboxylase. This prevents photorespiration and increases the rate of photosynthesis. Thus, \(\mathrm{C}_4\) plants are highly productive.

Q4.RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in \(\mathrm{C}_4\) plants?

Answer: The enzyme RuBisCo is absent from the mesophyll cells of \(\mathrm{C}_4\) plants. It is present in the bundle-sheath cells surrounding the vascular bundles. In \(\mathrm{C}_4\) plants, the Calvin cycle occurs in the bundle-sheath cells. The primary \(\mathrm{CO}_2\) acceptor in the mesophyll cells is phosphoenol pyruvate – a three-carbon compound. It is converted into the four-carbon compound oxaloacetic acid (OAA). OAA is further converted into malic acid. Malic acid is transported to the bundle-sheath cells, where it undergoes decarboxylation and \(\mathrm{CO}_2\) fixation occurs by the Calvin cycle. This prevents the enzyme RuBisCo from acting as an oxygenase.

Q5. Suppose there were plants that had a high concentration of Chlorophyll \(b\), but lacked chlorophyll \(a\), would it carry out photosynthesis? Then why do plants have chlorophyll \(b\) and other accessory pigments?

Answer: Chlorophyll-a molecules act as antenna molecules. They get excited by absorbing light and emit electrons during cyclic and non-cyclic photophosphorylations. They form the reaction centres for both photosystems I and II. Chlorophyll-b and other photosynthetic pigments such as carotenoids and xanthophylls act as accessory pigments. Their role is to absorb energy and transfer it to chlorophyll-a. Carotenoids and xanthophylls also protect the chlorophyll molecule from photo-oxidation.
Therefore, chlorophyll-a is essential for photosynthesis. If any plant were to lack chlorophyll-a and contain a high concentration of chlorophyll-b, then this plant would not undergo photosynthesis.

Q6. Why is the colour of a leaf kept in the dark frequently yellow, or pale green? Which pigment do you think is more stable?

Answer: Since leaves require light to perform photosynthesis, the colour of a leaf kept in the dark changes from a darker to a lighter shade of green. Sometimes, it also turns yellow. The production of the chlorophyll pigment essential for photosynthesis is directly proportional to the amount of light available. In the absence of light, the production of chlorophyll-a molecules stops and they get broken slowly. This changes the colour of the leaf gradually to light green. During this process, the xanthophyll and carotenoid pigments become predominant, causing the leaf to become yellow. These pigments are more stable as light is not essential for their production. They are always present in plants.

Q7. Look at leaves of the same plant on the shady side and compare it with the leaves on the sunny side. Or, compare the potted plants kept in the sunlight with those in the shade. Which of them has leaves that are darker green? Why?

Answer: Light is a limiting factor for photosynthesis. Leaves get lesser light for photosynthesis when they are in shade. Therefore, the leaves or plants in the shade perform lesser photosynthesis as compared to the leaves or plants kept in sunlight. In order to increase the rate of photosynthesis, the leaves present in the shade have more chlorophyll pigments. This increase in chlorophyll content increases the amount of light absorbed by the leaves, which in turn increases the rate of photosynthesis. Therefore, the leaves or plants in the shade are greener than the leaves or plants kept in the sun.

Q8. Figure 11.10 shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions:
(a) At which point/s (A, B or C) in the curve is light a limiting factor?
(b) What could be the limiting factor/s in region A?
(c) What do C and D represent on the curve?

Answer: (a) Generally, light is not a limiting factor. It becomes a limiting factor for plants growing in the shade or under tree canopies. In the given graph, light is a limiting factor at the point where photosynthesis is the minimum. The least value for photosynthesis is in region \(\mathbf{A}\). Hence, light is a limiting factor in this region.
(b) Light is a limiting factor in region A. Water, temperature, and the concentration of carbon dioxide could also be limiting factors in this region.
(c) Point \(\mathbf{D}\) represents the optimum point and gives the light intensity at which the maximum photosynthesis is recorded. The rate of photosynthesis remains constant after this point, even though the intensity of light.

Q9. Give a comparison between the following:
(a) \(\mathrm{C}_3\) and \(\mathrm{C}_4\) pathways
(b) Anatomy of leaf in \(\mathrm{C}_3\) and \(\mathrm{C}_4\) plants
(c) Cyclic and non-cyclic photophosphorylation

Answer: (a) \(\mathrm{C}_3\) and \(\mathrm{C}_4\) pathways

\(C _3\) Pathways	\(C _4\) Pathways
The primary acceptor of \(CO_2\) is RUBP-a six-carbon compound.	The primary acceptor of \(CO _2\) is phosphoenol pyruvate – a three carbon compound.
The first stable product is 3-phosphoglycerate.	The first stable product is oxaloacetic acid.
It occurs only in the mesophyll cells of the leaves.	It occurs in the mesophyll and bundle-sheath cells of the leaves.
It is a slower process of carbon fixation and photo-respiratory losses are high.	It is a faster process of carbon fixation and photo-respiratory losses are low.

(b)

\(C_3 \text { leaves }\)	\(C_4 \text { leaves }\)
Bundle-sheath cells are absent	Bundle-sheath cells are present
RuBisCo is present in the mesophyll cells.	RuBisCo is present in the bundle-sheath cells.
The first stable compound produced is 3-phosphoglycerate –a three-carbon compound.	The first stable compound produced is oxaloacetic acid – a four-carbon compound.
Photorespiration occurs	Photorespiration does not occur

(c)

Cyclic photophosphorylation	Non-cyclic photophosphorylation
It occurs only in photosystem I.	It occurs in photosystems I and II.
It involves only the synthesis of ATP.	It involves the synthesis of ATP and \(NADPH _2\).
In this process, photolysis of water does not occur. Therefore, oxygen is not produced.	In this process, photolysis of water takes place and oxygen is liberated.
In this process, electrons move in a closed circle.	In this process, electrons do not move in a closed circle.

Exemplar Section

VERY SHORT ANSWER TYPE QUESTIONS

Q1. Examine the figure

a. Is this structure present in animal cell or plant cell?
b. Can these be passed on to the progeny? How?
c. Name the metabolic processes taking place in the places marked
(1) and (2).

Answer: a. Plant cell.
b. Yes, through female gametes.
c. In part (1)-Photophosphorylation. In part (2)-Calvin cycle.

Q2. \(2 H_2 O \longrightarrow 4 H^{+}+O_2+4 e^{-}\)
Based on the above equation, answer the following questions:
a. Where does this reaction take place in plants?
b. What is the significance of this reaction?

Answer: a. Lumen of the thylakoids.
b. \(O _2\) is evolved during this reaction; moreover, electrons are made available to PS -II continuously.

Q3. Cyanobacteria and some other photosynthetic bacteria don’t have chloroplasts. How do they conduct photosynthesis? 

Answer: Cyanobacteria and other photosynthetic bacteria have thylakoids suspended freely in the cytoplasm (i.e., they are not enclosed in membrane), and they have bacteriochlorophyll.

Q4. a. NADP reductase enzyme is located on _________________.
b. Breakdown of proton gradient leads to release of _________________.

Answer: a. Grana-lamellae.
b. Energy.

Q5. Can girdling experiments be done in monocots? If yes, How? If no, why not?

Answer: No, because vascular bundles are scattered in monocot.

Q6.

\(
\begin{aligned}
& 3 CO_2+9 ATP+6 NADPH+\text { Water } \longrightarrow \text { glyceraldehyde } 3-\text { phosphate }+ \\
& 9 ADP+6 NADP^{+}+8 Pi
\end{aligned}
\)
Analyze the above reaction and answer the following questions:
a. How many molecules of ATP \& NADPH are required to fix one molecule of \(CO _2\) ?
b. Where in the chloroplast does this process occur?

Answer: a. Three molecules of ATP and two molecules of NADPH are required to fix one molecule of \(CO_2\)
b. Stroma of chloroplast

Q7. Does moonlight support photosynthesis?

Answer: As the intensity of moonlight is much less than the sunlight, so it does not support photosynthesis.

Q8. Some of these terms/chemicals are associated with the \(C _4\) cycle. Explain.
a. Hatch slack pathway
b. Calvin cycle
c. PEP carboxylase
d. Bundle sheath cells

Answer: Though \(C _4\) plants have \(C _4\) oxaloacetic acid as is the first \(CO _2\) fixation product they use the \(C _3\) pathway or Calvin cycle as the main biosynthetic pathway. \(C _4\) pathway is also called Hatch and Slack Pathway

• \(1^{\circ} CO _2\) acceptor in \(C _4\) plants is a 3-C molecule PEP (phosphoenol pyruvate) and is present in the mesophyll cells. The enzyme responsible for the fixation is PEPcase (PEP carboxylase) is found only in mesophyll cells. Bundle sheath cells lack PEPcase enzyme.
• \(C _4\) acid (OAA) is formed by carboxylation is mesophyll cells; therefore, initial carboxylation reaction occurs in mesophyll cells (also in \(C _3\) pathway). OAA forms other 4-C compounds like malic acid or aspartic acid in the mesophyll cells itself, which are transported to the bundle sheath cells.
• \(CO _2\) released in the bundle sheath cells enters the \(C _3\) or the Calvin pathway, a pathway common to all plants. The bundle sheath cells are rich in RuBisCO enzyme (necessary for the \(C _3\) or the Calvin cycle), but lack PEPcase.
• Calvin pathway in \(C _4\) plants takes place only in bundle sheath cells (because RuBisCO is present) but does not take place in the mesophyll cells because lack of RuBisCO enzyme in mesophyll cells of \(C _4\) plants like maize, sorghum, sugarcane, Jowar, Euphorbia, Atriplex.

Q9. Where is NADP reductase enzyme located in the chloroplast? What is the role of this enzyme in proton gradient development?

Answer: The NADP reductase enzyme is located on the stroma side of the membrane. Along with electrons that come from the acceptor of electrons of PSI protons are necessary for the reduction of \(NADP ^{+}\)to \(NADPH + H ^{+}\). These protons are also removed from the stroma.

Q10. ATPase enzyme consists of two parts. What are those parts? How are they arranged in the thylakoid membrane? Conformational change occur in which part of the enzyme?

Answer: ATPase enzyme consists of two parts:
i. One portion called \(F_0\) is imbedded in the membrane and forms a transmembrane channel that carries out facilitated diffusion of protons across the membrane.
ii. The other portion is called ‘F। and protrudes on the outer surface-of the thylakoid membrane on the side that faces stroma.
The breakdown of the gradient provides enough energy to cause a conformational change in the \(F\), particle of the ATPase, which makes the enzyme synthesise several molecules of energy-packed ATP.

Q11. Which products formed during the light reaction of photosynthesis are used to drive the dark reaction?

Answer: ATP and NADPH

Q12. What is the basis for designating C₃ and C₄ pathways of photosynthesis?

Answer: The number of carbon atoms in first stable product of carbon dioxide fixation is the basis for designating C₃ and C₄ pathways of photosynthesis.

SHORT ANSWER TYPE QUESTIONS

Q1. Succulents are known to keep their stomata closed during the day to check transpiration. How do they meet their photosynthetic \(CO _2\) requirements?

Answer: Succulent (water storing) plants such as cacti, euphorbias fix \(CO _2\) into organic compound using PEP carboxylase at night, when the stomata are open.
\(
\begin{aligned}
& PEP+CO_2 \longrightarrow OAA \\
& OAA \xrightarrow{\text { Conversion }} \text { Malic acid }
\end{aligned}
\)
The organic compound (malic acid) accumulates throughout the night and is decarboxylated during the day to produce \(CO _2\).

Q2. Chlorophyll ‘a’ is the primary pigment for light reaction. What are accessory pigments? What is their role in photosynthesis?

Answer: Accessory pigments are those pigments, which assist in photosynthesis by capturing energy from light of different wavelengths, e.g., chlorophyll b, Xanthophylls and carotenoids.

Role in Photosynthesis:

1. They absorb wavelength of light not absorbed by chlorophyll a and transfer the energy to chlorophyll.
2. They also protect chlorophyll a from photo-oxidation.

Q3. Do reactions of photosynthesis called, as ‘Dark Reaction’ need light? Explain.

Answer: ATP and NADPH are used to drive the processes leading to the synthesis of food, more accurately, sugars. This is the biosynthetic phase or dark reaction of photosynthesis. This process does not directly depend on the presence of light but is dependent on the products of the light reaction, i.e., ATP and NADPH, besides \(CO _2\) and \(H _2 O\).

Q4. How are photosynthesis and respiration related to each other?

Answer:Photosynthesis and respiration are related to each other as

1. Both processes take place in double membrane bound organelles.
2. In both processes ATP synthesis takes place.
3. In both processes electron transport system requires.

Q5. If a green plant is kept in dark with proper ventilation, can this plant carry out photosynthesis? Can anything be given as supplement to maintain its growth or survival?

Answer: No, this plant cannot photosynthesise in the absence of light. Only sunlight can be given as supplement to maintain its growth or survival.

Q6. 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?

Answer: 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?

Q7. In tropical rain forests, the canopy is thick and plants growing below receive filtered light. How are they able to carry out photosynthesis ?

Answer: In tropical rain forests, the canopy is thick and shorter plants growing below it called sciophytes (shade loving plants). They can photosynthesise in very low light conditions. They have larger photosynthetic units and hence they are able to carry out photosynthesis in filtered light.

Q8. What conditions enable Rubirco to function as an oxygenase? Explain the ensuing process.

Answer: In the first step of the Calvin pathway RuBP combines with \(CO _2\) to form 2 molecules of \(3 P G A\), that is catalysed by RuBisCO.
\(
RuPB+CO_2 \xrightarrow{\text { RuBisCo }} 2 \times 3 PGA
\)
RuBisCO that is the most abundant enzyme in the world is characterised by the fact that its active site can bind to both \(CO _2\) and \(O _2-\) hence the name. RuBisCO has a much greater affinity for \(CO _2\) than for \(O _2\). This binding is competitive. It is the relative concentration of \(O _2\) and \(CO _2\) that determines which of the two will bind to the enzyme.

Q9. Why does the rate of photosynthesis decrease at higher temperatures?

Answer: The rate of photosynthesis decreases at higher temperatures because at high temperatures the enzymes become denatured (destroy).

Q10. Explain how during light reaction of photosynthesis, ATP synthesis is a chemiosmotic phenomenon.

Answer: In the light reaction within the chloroplast, protons in the stroma decrease in number, while in the lumen there is accumulation of protons. This creates a proton gradient across the thylakoid membrane as well as a measurable decrease in pH in the lumen. This gradient is important because it is the breakdown of this gradient that leads to release of energy. The gradient is broken down due to the movement of protons across the membrane to the stroma through the transmembrane channel of the \(F_0\) of the ATPase. The ATPase enzyme consists of two parts: one part called the \(F_0\) is embedded in the membrane and forms a transmembrane channel that carries out facilitated diffusion of protons across the membrane. The other portion is called F1 and protrudes on the outer surface of the thylakoid membrane on the side that faces the stroma. The breakdown of the gradient provides enough energy to cause a conformational change in the \(F_1\) particle of the ATPase, which makes the enzyme synthesise several molecules of energy-packed ATP.

Q11. Find out how Melvin Calvin worked out the complete biosynthetic pathway for synthesis of sugar.

Answer: Just after World War II, among the several efforts to put radioisotopes to beneficial use, the work of Melvin Calvin is exemplary. The use of radioactive \(C ^{14}\) by him in algal photosynthesis studies led to the discovery that the first \(CO _2\) fixation product was a 3-carbon organic acid. He also contributed to working out the complete biosynthetic pathway; hence, it was called Calvin cycle after him. The first product identified was 3-phosphoglyceric acid or in short PGA.

Q12. Six turns of Calvin cycle are required to generate one mole of glucose. Explain.

Answer: The fixation of 6 molecules of \(CO _2\) and 6 turns of the cycle are required for the removal of one molecule of glucose from the pathway. Hence for every \(CO _2\) molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required. To make one molecule of glucose 6 turns of the cycle are required.
\(
\begin{array}{|l|l|}
\hline \text { In } & \text { Out } \\
\hline \text { Six } CO_2 & \text { One glucose } \\
\hline 18 \text { ATP } & \text { 18 ADP } \\
\hline 12 NADPH & 12 NADP \\
\hline
\end{array}
\)

Q13. Complete the flow chart for cyclic photophosphorylation of the photosystem-I

Answer: 

Q14. 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?

Answer: On studying vertical sections of leaves, one of a \(C _3\) plant and the other of a \(C _4\) plant. The particularly large cells around the vascular bundles of the \(C _4\) pathway plants are called bundle sheath cells, and the leaves which have such anatomy are said to have ‘Kranz’ anatomy. ‘Kranz’ means ‘wreath’ and is a reflection of the arrangement of cells. The bundle sheath cells may form several layers around the vascular bundles; they are characterised by having a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces. Leaves of \(C _4\) plants – maize or sorghum lack photorespiration. In addition these plants show tolerance to higher temperatures. Plants that are adapted to dry tropical regions have the \(C _4\) pathway.

Q15. In a way green plants and cyanobacteria have synthesized all the food on the earth. Comment.

Answer:  a. Respiration
b. Catabolic process (actually amphibolic pathway)
c. Glucose

Q16. Tomatoes, carrots and chillies are red in colour due to the presence of one pigment. Name the pigment. Is it a photosynthetic pigment? 

Answer: Tomatoes, carrots and chillies are red in colour due to the presence of carotene pigment. It is an accessory photosynthetic pigment.

Q17. Why do we believe chloroplast and mitochondria to be semi-autonomous organelle?

Answer: Mitochondria and Chloroplast are semi-autonomous organelles or endosymbionts of cells because they
i. Possess their own nucleic acid (DNA molecule).
ii. Can form some of the required protein but for most of the proteins these are dependent on nuclear DNA and cytoplasmic ribosome.
iii. Do not arise de novo.
iv. Have membrane similar to those of bacteria.

Q18. Observe the diagram and answer the following.

a. Which group of plants exibits these two types of cells?
b. What is the first product of \(C _4\) cycle?
c. Which enzyme is there in bundle sheath cells and mesophyll cells?

Answer: a. \(C_4\) plants
b. OAA (Oxaloacetic acid)
c. Phosphoenol pyruvate (PEP) is present in the mesophyll cells. Enzyme Ribulose bisphosphate carboxylase-oxygenase (RuBisCO) is present in bundle sheath cells.

Q19. A cyclic process is occurring in \(C_3\) plant, which is light dependent, and needs \(O _2\). This process doesn’t produce energy rather it consumes energy.
a. Can you name the given process?
b. Is it essential for survival?
c. What are the end products of this process?
d. Where does it occur?

Answer: a. Photorespiration
b. No
c. \(CO_2\) and \(NH_3\)
d. Photorespiration involves a complex network of enzyme reactions that exchange metabolites between chloroplasts, leaf peroxisomes and mitochondria.

LONG ANSWER TYPE QUESTIONS

Q1. Is it correct to say that photosynthesis occurs only in leaves of a plant? Besides leaves, what are the other parts that may be capable of carrying out photosynthesis? Justify.

Answer: Photosynthesis does take place in the green leaves of plants but it does so also in other green parts of the plants. The mesophyll cells in the leaves, have a large number of chloroplasts. Usually the chloroplasts align themselves along the walls of the mesophyll cells, such that they get the optimum quantity of the incident light.

• Photosynthetic or Assimilatory roots: They are green roots which are capable of PHS, e.g., Trapa bispinosa (water chestnut = Singhara), Tmospora (Gillow or Gurcha), Podostemum.
• Some plants of arid regions modify their stems into flattened (Opuntia), or fleshy cylindrical (Euphorbia) structures. These modified stems of indefinite growth are called phylloclades. They contain chlorophyll and carry out photosynthesis.
• One intemode long phylloclade or stem which is leaf like is called cladode. Cladode is capable of photosynthesis. Cladode is found in certain xerophytes, e.g., Ruscus and Asparagus.

Q2. The entire process of photosynthesis consists of a number of reactions. Where in the cell do each of these take place?
a. Synthesis of ATP & NADPH ____________
b. Photolysis of water ______________
c. Fixation of \(CO _2\) _______________
d. Synthesis of sugar molecule _____________
e. Synthesis of starch ____________________

Answer: a. Synthesis of ATP and NADPH: Membrane system (Grana)
b. Photolysis of water: Inner side of the membrane of thylakoid
c. Fixation of \(CO2\) : Stroma of chloroplast
d. Synthesis of sugar molecule: Stroma of chloroplast
e. Synthesis of starch: Stroma of chloroplast

Q3. 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?

Answer: Pigments are substances that have an ability to absorb light, at specific wavelengths. This property of the pigment is responsible for its ability to initiate the process of photosynthesis.
The wavelengths at which there is maximum absorption by chlorophyll a, i. e. in the blue and the red regions, also shows higher rate of photosynthesis.

Q4. What can we conclude from the statement that the action and absorption spectrum of photosynthesis overlap? At which wavelength do they show peaks?

Answer: The wavelengths at which there is maximum absorption by chlorophyll a, i.e. in the blue and the red regions, also shows higher rate of photosynthesis. Hence, we can conclude that chlorophyll a is the chief pigment associated with photosynthesis. The action spectrum of photosynthesis superimposed on absorption spectrum of chlorophyll a.

Q5. Under what conditions are C4 plants superior to C3?

Answer: C4 plants are special:
i. They have a special type of leaf anatomy. 
ii: They tolerate higher temperatures. 
iii. They show a response to high light intensities.
iv. They lack a process called photorespiration.
v. They have greater productivity of biomass.

Q6. In the figure given below, the black line (upper) indicates 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?
b. How can we derive an absorption spectrum for any substance?

Answer: a. Action spectrum of photosynthesis superimposed on absorption spectrum of chlorophyll a. The wavelengths at which there is maximum . absorption by chlorophyll a, i.e. in the blue and the red regions, also shows higher rate of photosynthesis. Hence, one can conclude that chlorophyll a is the chief pigment associated with photosynthesis.
b. Absorption spectrum for any substance can be derived by plotting the different wavelengths of light.

Q7. List the important events and end products of the light reaction?

Answer:  Light reactions or the ‘Photochemical’ phase include light absorption, water splitting, oxygen release, and the formation of high-energy chemical intermediates, ATP and NADPH. Several complexes are involved in the process. The pigments are organised into two discrete photochemical light harvesting complexes (LHC) within the Photosystem I (PS I) and Photosystem II (PS II). These are named in the sequence of their discovery, and not in the sequence in which they function during the light reaction. The LHC are made up of hundreds of pigment molecules bound to proteins. Each photosystem has all the pigments (except one molecule of chlorophyll a) forming a light harvesting system also called antennae. These pigments help to make photosynthesis more efficient by absorbing different wavelengths of light. The single chlorophyll a molecule forms the reaction centre. The reaction centre is different in both the photosystems. In PS I the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called P700,while in PS II it has absorption maxima at 680 nm, and is called P680.

Q8. In the diagram given below what is label A, B and C. What type of phosphorylation is possible in this?

Answer: It’s feasible to do cyclic photophosphorylation here. The creation of ATP from ADP and inorganic phosphate in the presence of light is known as photophosphorylation.

Q9. Why is the RuBisCo enzyme more appropriately called RUBP Carboxylase-Oxygenase and what important role does it play in photosynthesis?

Answer: For ease of understanding, the Calvin cycle can be described under three stages: carboxylation, reduction and regeneration. Carboxylation is the fixation of \(CO_2\) into a stable organic intermediate. Carboxylation is the most crucial step of the Calvin cycle where \(CO_2\) is utilised for the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBP carboxylase which results in the formation of two molecules of 3-PGA. Since this enzyme also has an oxygenation activity it would be more correct to call it RuBP carboxylase-oxygenase or RuBisCO.

Q10. What special anatomical features are displayed by leaves of \(C_4\) plants? How do they provide advantage over the structure of \(C_3\) plants?

Answer: Study vertical sections of leaves, one of a \(C_3\) plant and the other of a \(C_4\) plant. The particularly large cells around the vascular bundles of the \(C_4\) pathway plants are called bundle sheath cells, and the leaves which have such anatomy are said to have ‘Kranz’ anatomy. ‘Kranz’ means ‘wreath’ and is a reflection of the arrangement of cells. The bundle sheath cells may form several layers around the vascular bundles; they are characterised by having a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces. \(C_4\) plants lack photorespiration. In addition these plants show tolerance to higher temperatures. Plants that are adapted to dry tropical regions have the \(C_4\) pathway.
\(C_4\) plants are special:
i. They have a special type of leaf anatomy.
ii. They tolerate higher temperatures.
iii. They show a response to high light intensities.
iv. They lack a process called photorespiration.
v. They have greater productivity of biomass.

Q11. Name the two important enzymes of \(C _4\) pathway, and explain their role in fixing \(CO _2\) ?

Answer: The important enzyme of \(C_3\) pathway is RuBisCO and that of \(C_4\) pathway is PEPcase. Carboxylation in the \(C_3\) pathway is the fixation of \(CO_2\) into a stable organic intermediate. Carboxylation is the most crucial step of the Calvin cycle, where \(CO_2\) is utilised for the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBP carboxylase which results in the formation of two molecules of 3-PGA. The primary \(CO_2\) acceptor in the \(C_4\) pathway is a 3-carbon molecule phosphoenol pyruvate (PEP) and is present in the mesophyll cells. The enzyme responsible for this fixation is PEP carboxylase or PEPcase.

Q12. Why is RuBisCo enzyme the most abundant enzyme in the world?

Answer: RuBisCo enzyme is the most abundant enzyme in the world because this enzyme is responsible for photosynthesis and present in all green parts of the plants including leaves.

Q13. Why photorespiration does not take place in \(C_4\) plants?

Answer: In \(C_4\) plants photorespiration does not occur. This is because they have a mechanism that increases the concentration of \(CO_2\) at the enzyme site. This takes place when the \(C_4\) acid from the mesophyll is broken down in the bundle sheath cells to release \(CO_2\) – this results in increasing the intracellular concentration of \(CO_2\). In turn, this ensures that the RuBisCO functions as a carboxylase minimising the oxygenase activity.