NCERT Exemplar Class 11 Biology Solutions Photosynthesis in Higher Plants
Multiple Choke Questions
1.Which metal ion is a constituent of chlorophyll?
(a) Iron (b) Copper
(c) Magnesium(d) Zinc
Soln. (c) : Magnesium is a constituent of chlorophyll. Chlorophylls are the green photosynthetic pigments. Five types of chlorophylls n, h, c, d and e occur in plants. Out of these only chlorophylls a and b occur in the chloroplasts of higher plants. Molecular formula of chlorophyll a, b, c and d are C55H72O5N4Mg, C55H70O6N4Mg and C54H70O6N4Mg respectively. Chlorophyll e is a rare type found in some golden algae.
2.Which pigment acts directly to convert light energy to chemical energy?
(a) Chlorophyll a (b) Chlorophyll b
(c) Xanthophyll (d) Carotenoid
Soln.(a) : Chlorophyll a is termed as universal photosynthetic pigment as it is found in all photosynthetic plants except bacteria. It is also called primary photosynthetic pigment because it performs primary reaction of photosynthesis which involves conversion of light into chemical energy. Other photosynthetic pigments chlorophyll b and carotenoids (carotenes and xanthophylls) are called accessory pigments, as they absorb light energy of different wavelengths, broaden the spectrum of light absorption and hand over the energy to chlorophyll a through electron spin resonance.
3.Which range of wavelength (in nm) is called photosynthetically active radiation (PAR)?
(a) 100-390 (b) 390-430
(c) 400-700 (d) 760-100,00
Soln.(c): Sunlight or solar radiations reaching the earth have wavelength between 300 nm (in the ultraviolet range) to 2600 nm (in the infra-red range). Part of the spectrum used in photosynthesis has a wavelength between 400-700 nm, it is called as photosynthetically active radiation (PAR). The energy reaching
the earth’s surface consists largely of visible light and infra-red components. On a clear day, radiant energy reaching the earth’s surface is about 5% UV, 45% visible and 50% infrared. Of this energy reaching the earth’s surface (incident solar radiation), less than 50% is photosynthetically active radiation (PAR). Only 2-10% of the PAR is used by green plants for photosynthesis and the rest is absorbed as heat by ground vegetation or water. Infact, only about 0.02% of the sunlight reaching the atmosphere is used in photosynthesis.
4.Which light range is least effective in t photosynthesis?
(a) Blue (b) Green
(c) Red (d) Violet
Soln.(b): Blue and red regions of the light spectrum are most effective in photosynthesis. Blue wavelengths of light carry more energy while red wavelengths have lesser energy. Therefore, the most efficient wavelengths of light effective in photosynthesis are those of red light. Green light is least effective in photosynthesis.
5.Chemosynthetic bacteria obtain energy from
(b) infra red rays
(c) organic substances
(d) inorganic chemicals.
Soln.(d) : Chemosynthetic autotrophic bacteria (chemoautotrophic bacteria) are bacteria which are able to manufacture their organic food from inorganic raw materials with the help of energy derived from exergonic chemical reactions involving oxidation of an inorganic substance present in the external medium. The chemical energy obtained from oxidation reaction is trapped in ATP molecules. They play a great role in recycling nutrients like nitrogen, phosphorous, iron and sulphur. There are several types of chemoautotrophic bacteria but the well known examples are nitrifying bacteria, sulphur oxidising bacteria, and iron bacteria.
6.Energy required for ATP synthesis in PSII comes from
(a) proton gradient
(b) electron gradient
(c) reduction of glucose
(d) oxidation of glucose.
Soln.(a):PhotosystemII (PSII) is a photosynthetic pigment system along with some electron carriers that is located in the appressed part of the grana thylakoids. PSII has chlorophyll a, chlorophyll b and carotenoids. It picks up electron released during photolysis of water. The same is extruded on absorption of light energy. As the extruded electron passes over cytochrome b6-f complex, it energises passage of protons picked up by PQ to create proton gradient for synthesis of ATP from ADP and inorganic phosphate. This photophosphorylation is noncyclic.
7.During light reaction in photosynthesis the following are formed.
(a) ATP and sugar
(b) hydrogen, O2 and sugar
(c) ATP, hydrogen donor and O2
(d) ATP, hydrogen and 02 donor.
Soln.(c) : Light reaction occurs inside thylakoids. It is dependent upon light. It involves photolysis of water and production of assimilatory power (NADPH and ATP). Electrons released during photolysis of water are picked up by P680 photocentre ofphotosystem II. From here electrons passes over series of carrier which include PQ, cytochrome b-f complex and PC. While passing over cytochrome complex, the electron losses sufficient energy for creation of proton gradient and ATP from ADP andinorganic phosphate. From PC electron is picked by the trap centre P700 of photosystem I which pushes out electron after absorbing light energy. Electron passes over carriers FeS, ferredoxin and NADP-reductase which gives electron to NADP+ for combining with
H+ to produce NADPH.
8.Dark reaction in photosynthesis is called so because
(a) it can occur in dark also
(b) it does not directly depend on light energy
(c) it dannot occur during day light
(d) it occurs more rapidly at night.
Soln.(b) :Biosynthetic phase (dark or Blackman’s reaction) catalyses assimilation of CO2 to carbohydrates. The reactions are called carbon reactions. They occur in stroma of chloroplasts. The reactions do not require light. Instead assimilatory power (ATP and NADPH) produced during photochemical phase is used in fixation and reduction of carbon dioxide. All the enzymes required for the process are present in the matrix or stroma of the chloroplast.
9.PEP is primary CO2 acceptor in
(b) C3 plants
(c) C2 plants
(d) both C3 and C4 plants.
Soln.(a) : Mechanism of C4 pathway (or Dicarboxylic acid pathway or Hatch and Slack pathway) can be illustrated as follows:
Initial fixation: In C4 plants, initial fixation of CO2 or carboxylation occurs in mesophyll cells. The chloroplasts of mesophyll cells possess enzyme PEP carboxylase (or PEP case) for initial fixation of CO2 The primary acceptor of CO2 is phosphoenol pyruvate or PEP It combines with CO2 in the presence of PEP carboxylase (or PEPcase) to form oxaloacetic acid or oxaloacetate (OAA).
10. Splitting of water is associated with
(a) photosystem I
(b) lumen of thylakoid
(c) both photosystem I and II
(d) inner surface of thylakoid membrane.
Soln.(d) : Light reactions occur inside the thylakoids, especially those of grana region. It involves two types of reactions photolysis of water and production of assimilatory power. The phenomenon of breaking up of water into hydrogen and oxygen in the illuminated chloroplasts is called photolysis or photocatalytic splitting of water. Water splitting complex is associated with the PSII, which itself is physically located on the 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, PSII, 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.
13. The reaction that is responsible for the primary fixation of CO2 is catalysed by
(a) RuBP carboxylase
(b) PEP carboxylase
(c) RuBP carboxylase and PEP carboxylase
(d) BGA synthase.
Soln. (c) : Carboxylation is the addition of carbon dioxide to an acceptor. Photosynthetic carboxylation requires ribulose-1, 5-biphos ¬phate or RuBP as acceptor of carbon dioxide and RuBP- carboxylase oxygenase as enzyme in C3 plants. In C4 plants, initial fixation of CO2 occurs in mesophyll cells. The primary acceptor of CO2 is phosphoenol pyruvate which combines with CO2 in presence of PEP carboxylase to form oxaloacetic acid or oxalo ¬acetate.
14. When CO2 is added to PEP, the first stable product synthesised is
Soln.(d): Refer answer 9.
Short Answer Type Questions
1. 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).
Soln.(a) The given figure represents structure of chloroplast. They are photosynthetic organelles in plants. This structure is present in plant cell.
(b) Yes, these can be passed on to the progeny. Chloroplasts are semi-autonomous cell organelles as they possess DNA. During cytokinesis chloroplast as well as other cell organelles are distributed to the daughter cells.
(c) In the given figure, part marked as (1) is stroma of chloroplast where dark reaction of photosynthesis takes place. Marked part (2) is extranuclear DNA or chloroplast DNA. Chloroplast DNA is responsible for replication of chloroplast.
2. 2H2O –>2H+ + O2 + 4e– 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?
Soln. (a) The given equation shows splitting or photolysis of water which takes place on the inner surface of thylakoid membrane.
(b) Splitting of water is an important event in photosynthesis as
(i) It evolves oxygen as byproduct of photosynthesis which is essential for living beings.
(ii)Hydrogen ions produced during pho-tolysis are utilised in reducing NADP to NADPH.
(iii)The electrons released are transferred from PSII to PSI through a series of electron carriers thus, creating a gradient are for the synthesis of ATP.
3. Cyanobacteria and some other photosynthetic bacteria do not have chloroplasts. How do they conduct photosynthesis?
Soln. In cyanobacteria, complex lamellar systems (thylakoids) are present instead of chloroplast. These thylakoids are functionally analogous to the plastids of eukaryotic cells. Pigments like chlorophyll a, C-phycocyanin, C-phycoerythrin, embedded in these lamellar system and they trap solar energy and perform photosynthesis. They perform oxygenic photosynthesis. Photosynthetic bacteria possess related pigments called bacteriochlorophylls, which are of different types (a, b, c, d, e, f and g). Groups that contain bacteriochlorophyll, perform photosynthesis, but do not evolve oxygen. Bacteriochlorophylls are photoreceptors similar to chlorophylls, except for the reduction of an additional pyrrole ring and other minor differences that shift their absorption maxima to near infrared, to wavelengths as long as 1000 nm. Thus, they utilize light wavelengths not used by green plants pr cyanobacteria. Bacteriopheophytin is a variant of bateriochlorophyll that has two protons instead of a magnesium ion at its centre.
4. (a)NADP reductase enzyme is located on__________
(b) Breakdown of proton gradient leads to release of__________
Soln. (a) NADP reductase enzyme is located on the outer side of thylakoid membrane.
(b) Breakdown of proton gradient leads to release of ATP.
5.Can girdling experiments bedone in monocots? If yes, How? If no, why not?
Soln. The girdling experiment cannot be done in monocots. The monocot stem has vascular bundles scattered all over the width of stem, so we cannot get the specific band of the phloem tissue which we get in dicots.
6. 3CO2 + 9ATP + 6NADPH + Water â€”â€”â€”>glyceraldehyde 3 phosphate + 9 ADP + 6 NADP+ + 8 Pi Analyse the above reaction and answer the following questions:
(a) How many molecules of ATP & NADPH are required to fix one molecule of CO2?
(b) Where in the chloroplast does this process occur?
Soln.(a) To fix one molecule of CO2, 3 molecules of ATP and 2 molecules of NADPH are required.
(b) This process occurs in the stroma of the chloroplast.
7. Does moonlight support photosynthesis? Find out.
Soln. Moonlight does not carry enough energy to excite chlorophyll molecules, i.c., reaction centres PSI and PSII, so light dependent reactions are not initiated. Thus, photosynthesis cannot occur in presence of moonlight.
8.Some of these terms/chemicals are associated with the C4 cycle. Explain.
(a) Hatch Slack pathway
(b) Calvin cycle
(c) PEP carboxylase
(d) Bundle sheath cells
Soln.(a) In 1966, Hatch and Slack reported that C4 acid (oxaloacetic acid) is the first stable product in carbon dioxide reduction process by conducting experiments on leaves of maize and grasses therefore, C4 pathway is called Hatch and Slack pathway.
(b) In C4plants, Calvin cycle occurs in the chloroplasts of bundle sheath cells.
(c) In C4 plants, primary acceptor of C02 is phosphoenol pyruvic acid and carboxylation is catalysed by PEP carboxylase or PEPcase.
(d) Bundle sheath cells are specialised sclerenchymatous cells present around the vascular bundle, in the veins of monocot leaves. These have agranal chloroplasts and contain starch grains. They possess RuBisCO enzyme and are well protected from oxygen released from mesophyll cells.C3 cycle occurs in these cells to manufacture glucose in C4 plants.
9.Where is NADP reductase enzyme located in the chloroplast? What is the role of this enzyme in proton gradient development?
Soln. NADP reductase is situated on the outerside of thylakoid membrane. It obtains electron from PS I and protons from matrix to reduce NADP+ to NADP + H+ state.
10. 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?
Soln. ATP synthetase is a group of polypeptide that converts ATP and inorganic phosphate (Pi) to ATP and water. It is called coupling factor because it couples ATP formation to transport of electrons and H+ across the thylakoid membrane. It exists along with photosystem I, only in stroma thylakoids and the nonappressed regions of grana thylakoids. It contains two major parts: a stalk called CF0 that extends from the lumen across the thylakoid membrane to the stroma and a spherical (headpiece) part called CF1 that lies in the stroma. A total of nine polypeptides exist in CF1 and 3 polypeptides in CF0 of ATP synthase.
11.Which products formed during the light reactidn of photosynthesis are used to drive the dark reaction?
Soln. Assimilatory power containing ATP (energy rich molecules) and N ADPH (reduced coenzyme) formed during light reaction of photosynthesis are used in dark reaction for fixing CO2 and to form glucose molecule.
12.What is the basis for designating C3 and C4 pathways of photosynthesis?
Soln. In C4 cycle, 3-phosphoglyceric acid is first stable product which is 3-carbon compound. In C4 cycle, oxaloacetic acid is the first stable product which is 4-carbon compound. Accord ing to the fi rst stable productsC3 and C4 pathways of photosynthesis are designated.
Short Answer Type Questions
1. Succulents are known to keep their stomata closed during the day to check transpiration. How do they meet their photosynthetic CO2 requirements?
Soln. Crassulacean acid metabolism (CAM) is a mechanism of photosynthesis involving double fixation of CO2 which occurs in succulents and some other plants of dry habitats, in which the stomata remain closed during the daytime and open only at night. The process of photosynthesis is similar to that of C4 plants but instead of spatial separation of initial PEPcase fixation and final RuBisCO fixation of CO,, the two steps occur in the same cells but at different times, night and day, e.g., Sedum, Kalanchoe, Opuntia, pineapple, etc. The initial fixation of CO2 occurs at night and final fixation occurs during daytime. This results in conserving water.
2.Chlorophyll ‘a’ is the primary pigment for light reaction. What are accessory pigments? What is their role in photosynthesis?
Soln. Chlorophylls are green photosynthetic pigments. Five types of chlorophylls occur in plants other than bacteria – a, b, c, d and c. Out of these, only chlorophyll a and b occur in the chloroplasts of higher plants. Chlorophyll a is called primary photosynthetic pigment because it performs primary reaction of photosynthesis which involves conversion of light into chemical or electrical energy. Other phytosynthetic pigments are called accessory pigments. They absorb light energy of different wavelengths, broaden the specturm of light absorption and hand over the energy to chlorophyll a through electron spin resonance. Chlorophyll b, c, d, e, carotenoids and phycobilins are accessory pigments.
Role of accessory pigments in the photosynthesis are as follows:
(i) They absorb light energy in different regions of spectrum and pass it over to the main photocentre (chlorophyll ‘a’ molecule): Thus they increase the efficiency of photosynthesis.
(ii)They prevent solarisation (disintegration) of chlorophyll molecules under high light
intensities upto an extent, by absorbing a major fraction of incident light.
3.Do reactions of photosynthesis called, as’Dark Reaction’need light? Explain.
Soln. Dark reactions are actually light independent reactions. CO2 is reduced through various biochemical reactions to produce C6H12O6 (glucose) which does not need light. But they depend on the products formed during light reaction i.e., NADPH and ATP in fixation and reduction of carbon dioxide.
4.How are photosynthesis and respiration related to each other?
Soln. Photosynthesis and respiration are complementary process. Photosynthesis is photochemical process where plants can capture and fix the energy of the sun. The chemical reaction of photosynthesis is as follows:
Respiration is the typical process where mitochondria of cells release chemical energy from sugar and other organic molecules through chemical oxidation. This process occurs in both plants and animals. In most organisms, respiration releases the energy required for all metabolic process
C6H12O6 + 6O2â€”â€”â€”> 6CO2 + 6H2O + 686kcal.
Through the process of photosynthesis, green plants absorb solar energy and take carbon dioxide from the atmosphere to produce carbohydrates (sugars). Plants use these carbohydrates through the process of respiration, which releases the energy contained in carbohydrates for various body functions.
5.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?
Soln. Plant which is kept in dark with proper ventilation cannot carry out photosynthesis. In this case, the plant will survive till its reserve food material is not exhausted completely, afterwards, it will die. Such a plant can survive in only one condition, i.e., when provided with artificial light source like a lightened bulb or tube light along with water. So it can carry out photosynthesis and grow.
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?
Soln.Deeper layers of water body receive less light with short wavelengths whereas surface of water receives more amount of light with high wavelengths. So, photosynthetic organisms occurring at different depths in the ocean receive qualitatively and quantitatively different light. These plants contain accessory pigments which helps to carryout photosynthesis under different light conditions.
7.In tropical rainforests, the canopy is thick and shorter plants growing below it, receive filtered light. How are they able to carry out photosynthesis ?
Soln. Shorter plants growing under canopy of rainforest trees are shade plants. The shade plants are adapted to carry out photosynthesis in low light intensity which come down in filtered light. For this the shade plants possess, good quantity of carotenoids and xanthophy 11s that can absorb green wavelength and hand over the energy to chlorophyll a molecules for photoconversion.
8.What conditions enable RuBisCO to function as an oxygenase? Explain the ensuing process.
Soln. Ribulose biphosphate carboxylase (RuBisCO), the main enzyme of Calvin cycle fixes CO2 , acts as ribulose biphosphate oxy-genase under low atmospheric concentration of CO2 and increased concentration of 02. In presence of high concentration of 02 the en-zyme RuBP oxygenase splits a molecule of Ribulose-1, 5 biphosphate into one molecule each of 3-Phosphoglyceric acid and 2-phos- phoglycolic acid.
9.Why does the rate of photosynthesis decrease at higher temperatures?
Soln. Temperature does not influence light reactions of photosynthesis but affects the enzyme controlled dark reactions. The optimum temperature for photosynthesis is 18 to 35 °C. When temperature is increased from minimum to optimum, the rate of photosynthesis doubles for every 10 °C rise in temperature. Above the optimum temperature, the rate of photosynthesis shows an initial increase for short duration but later declines.
10. Explain how during light reaction of photosynthesis, ATP synthesis is a chemiosmotic phenomenon.
Soln. Electron transport during light reaction increase in proton concentration in the lumen of thylakoids. Lumen of thylakoid becomes enriched with H+ ions due to photolytic splitting of water.
Primary acceptor of electron is located on the duterside of thylakoid membrane. It transfers its electrons to an H-carrier. The carrier removes a proton from stroma while transporting electron to the innerside of membrane. The proton is released’into the lumen while the electron passes to the next carrier. NADP reductase is situated on the outerside of thylakoid membrane. It obtains electron from PS I and protons from matrix to reduce NADP+ to NADPH + H+ state. The consequences of the three events is that concentration of protons decreases in matrix or stroma region while their concentration in thylakoid lumen rises resulting in decrease in pH. .
A high proton gradient brings about chemiosmosis. It activates coupling factors. Protons pass through their CF0 channels and bring about conformational change in CF, components to bring about synthesis of ATP from ADP and inorganic phosphate.
11.Find out how Melvin Calvin worked out the complete biosynthetic pathway for synthesis of sugar.
Soln. In 1954, Calvin, Benson and their coworkers worked out the path of carbon. This path of carbon was studied with the help of radioactive tracer technique using Chlorella, a unicellular green alga and CO2 with C atomic weight (C14O2.). By providing
radioactive carbon one is able to trace the sequence of steps through which this carbon passes i.e., what are the intermediate steps to dark fixation of C14O2.
12. Six turns of Calvin cycle are required to generate one mole of glucose. Explain.
Soln. Calvin cycle has to take place six turns for the formation of one molecule of glucose because glucose is a six carbon compound. Precursor of carbohydrate (CH20) is formed when 1 molecule of CO2 participates in the carboxylation process.
13. Complete the flow chart for cyclic photophos-phorylation of the photosystem-l.
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?
Soln. Mostly in plants like Atriplex, sugarcane, maize, Cyperus, Amaranthus etc. kranz anatomy occurs. These plants are also called C4 plants.
The vascular bundles are surrounded by sheath of large parenchymatous cells called bundle sheath cells, which are in turn
surrounded by mesophyll cells. These cells are connectedly plasmodemata. Two types of chloroplasts are characteristic, i.e.,
– Bundle sheath chloroplasts: Larger in size, lack grana (agranal chloroplasts) and contain starch grains. They possess RuBisCO enzyme and are well protected from oxygen being released from mesophyll cells.
– Mesophyll chloroplasts: Smaller in size, contain grana and lack starch grains. Mesophyll cells are specialised to perform light reaction, evolve oxygen and produce assimilatory power – ATP and NADPH.
C4 plants are well adapted for drought and dessication. These plants show C4cycle and there occur two carboxylation reactions, first in mesophyll chloroplast and second in bundle sheath chloroplast and phosphoenol pyruvic acid is first CO2 acceptor.
They conserve water into mesophylls while fixing CO2 at a rate much higher to that of C3 plants. Further, higher concentration of CO2 in bundle sheath suppresses photorespiration which help the C4 plants to photosynthesise more efficiently even at higher temperature than theC3 plants.
15. A process is occurring throughout the day, in ‘X’ organism. Cells are participating in this process. During this process ATP, CO2 and water are evolved. It is not a light dependent process.
(a) Name the process.
(b) Is it a catabolic or an anabolic process?
(c) What could be the raw material of this process?
Soln.(a) The name of this process is cellular respiration.
(b) It is a catabolic process which involves breakdown of glucose molecule.
(c) Raw material for this process is glucose molecule and oxygen, which are products of photosynthesis.
16. Tomatoes, carrots and chillies are red in colour due to the presence of one pigment. Name the pigment. Is it a photosynthetic pigment?
Soln. Those pigments which occur on photosynthetic thylakoids of chloroplasts and take part in absorption of light energy for the purpose of photosynthesis is known as photosynthetic pigments. Photosynthetic pigments are of two types – chlorophylls and carotenoids.
(i) Carotenoids: Carotenoids are a group of yellow, brown to reddish pigments which are associated with the chlorophylls inside the chloroplasts, but also occur inside the chromoplasts.
They are also called accessory pigments, because they hand over the energy absorbed by them to chlorophyll a.
They are of two types – carotenes and xanthophylls. ‘
Red colour of tomato and chillies is due to carotene called lycopene. The most common carotene called (3 carotene is present in carrot.
Lycopene and (3 carotene cannot be called as photosynthetic pigments in this case as chlorophyll (primary pigment) is not present in tomato, chilly and carrot and these pigments supplement absorption of photosyntetically useful light wavelength only in the presence of light. ^ ,
17. Why do we believe chloroplast and mitochon ¬dria to be semi-autonomous organelle?
Soln. Chloroplasts and mitochondria are semi-autonomous organelles. They show a degree of autonomy or independence in their functioning. However, they are not
fully autonomous because their structures and functioning are partiajjy controlled by nucleus of the cell and availability pf materials from cytoplasm. So, it is more appropriate to call them semi-autonomous organelles. Semi-Autonomy is because of following reasons:
(i) They have their own DNA which can replicate independently.
(ii)All three types of RNA are present e.g. wRNA, fRNA and rRNA.
(iii)They possess their own ribosomes (70S).
(iv)They synthesise some of their own structural proteins.
18.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 C4 cycle?
(c) Which enzyme is there in bundle sheath cells and mesophyll cells?
Soln.(a) C4 plants such as monocots e.g., sugarcane, maize etc. and dicots e.g. Amaranthus possess these two types of cells. i.e., bundle sheath cells and mesophyll cells (in kranz anatomy).
(b) A four-carbon compound, oxaloacetic acid is the first product of C4 cycle.
(c) Mesophyll cells have PEP carboxylase to fix atmospheric CO2 to form a 4-carbon compound oxaloacetic acid, whereas bundle sheath cells have RuBP carboxylase which fixes CO2 to form three carbon compound 3 PGA (3 phosphoglyceric acid).
19. A cyclic process is occurring in 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?
(b) Is it essential for survival?
(c) What are the end products of this process?
(d) Where does it occur?
Soln.(a) Photorespiration is the light dependent process of oxygenation of ribulose biphosphate (RuBP) and release of carbon dioxide by the photosynthetic organs of plant.
(b) Photorespiration does not produce energy or reducing power, rather it consumes energy. Further, it undoes the work of photosynthesis. Therefore, photorespiration is a highly wasteful process. It is not essential for survival.
(c) The end product of this process is phosphoglycerate.
(d) The site of photorespiration is chloroplast whereas, peroxisome and mitochondria are required for completing the process.
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?
(b) Which one of them is more efficient in terms of photosynthetic activity?
(c) What difference do you think are there in their leaf anatomy?
Soln.(a) Euphorbia is a CAM plant. It fixes CO2 during night and uses it in daytime. It will be able to survive in hot tropical climate. Maize is a tropical plant. It undergoesC4 cycle (Hatch Slack Pathway) and can easily survive in tropical climate (hot).
(b) Maize being a C4 plant is more efficient in terms of photosynthetic activity C4 plants
have lowCO2 compensation point (0-10 ppm) and rate of carbon assimilation is rapid. Rate of photorespiration is negligible. C4 plants can perform photosynthesis even when the stomata are closed (from CO2 produced in respiration).
(c) Euphorbia has large sized succulent cells and stomata having reniform guard cells closed during the day. Maize plant leaves have kranz anatomy. Stomata possess dumb-bell shaped guard cells. The stomata remain open during the day.
Long Answer Type Questions
1.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.
Soln. Photosynthesis mainly occurs in leaves of a plant. Sometime some other parts of plant’ get modified to perform photosynthesis.
Other parts of plants modified to perform photosynthesis are as follows:
(i) Modified adventitious roots such as assimilatory roots perform photosynthesis. They are green roots which are capable of photosynthesis. In Trapa the green assimilatory roots are submerged like other roots. They develop from the stem nodes and are highly branched to increase photosynthetic area. Photosynthetic roots are also found in Tinospora. They are like green hanging threads which arise from the stem nodes during the rainy seasons and shrivel during drought.
(ii)Modified aerial roots such as phylloclades and cladodes perform photosynthesis. Phylloclades are flattened (e.g., Opuntia or cylindrical (e.g., Causuarina, Euphorbia royleana, E. tirucalli) green stems of unlimited growth which have taken over the function of photosynthesis. Cladodes (Cladophylls) are green stems of limited growth (usually one internode long) which have taken over the function of photosynthesis from the leaves, e.g., Ruscus.
Modified petiole such as Australian Acacia and Parkinsonia perform photosynthesis. . Lamina in some compound leaves falls
off soon and petiole gets modified into leaf-like structure and synthesise food, e.g., Australian Acacia and Parkinsonia.
2.The entire process of photosynthesis consists of a number of reactions. Where in the ceil do each of these take place?
(a) Synthesis of ATP & NADPH__________
(b) Photolysis of water__________
(c) Fixation of CO2__________
(d) Synthesis of sugar molecule __________
(e) Synthesis of starch __________
Soln.(a) Synthesis of ATP and NADPH takes place on outerside of thylakoid membrane. Assimilatory power ATP and NADPH are produced during photochemical process and utilised during biochemical process.
(b) Photolysis of water takes place on innerside of thylakoid membrane. Photolysis of water is breaking of water into hydrogen and oxygen in the illuminated chloroplast.
complex Mn2 , Ca , Cl
(c) Fixation of CO2 takes place on stroma of chloroplast. It occurs in biosynthetic phase. During this phase NADPH and ATP are consumed.
(d) Synthesis of sugar molecule (glucose) takes place in chloroplast. It is transported in the form of sucrose (formed in cytoplasm).
(e) Synthesis of starch takes place in chloroplast.
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?
Soln. Pigments absorbs the radiant energy of light. Light initiates the mechanism of photosynthesis by transferring its electron and getting excited. Red and blue light have maximum energy which is absorbed by chlorophyll pigment which then gets excited and initiates the process of photosynthesis. Also, its wavelength are (400 – 700 nm) i.e., between the photosynthetic active region. Thus, the rate of photosynthesis is higher in the red and blue regions of the spectrum of light.
4. What’can we conclude from the statement that the action and absorption spectrum of photosynthesis overlap? At which wavelength do they show peaks?
Soln.The action spectrum of photosynthesis and absorption spectrum of pigment overlap. It shows that maximum absorption and thus maximum photosynthesis occurs in the same range of wavelengths viz in blue and red regions of light.
5.Under what conditions are C4 plants superior to C3?
Soln. C4 plants are superior to C3 in the following conditions:
(i) They are specialised to live in area with high temperature as found in tropics. It is because their enzymes, PEP carboxylase and phosphopyruvic dikinase are functional only at higher temperature.
(ii)They are the only plants which grow under full sunlight, the optimum for C3 sun plants is 70% of full sunlight.
abundant occurrence of organic acids in them.
(iv) They are more efficient in picking up CO2 even in low concentration. Therefore, the rate of carbon assimilation is almost double the rate of most efficient C3 plants. Most of the highly productive crop plants belong to C4 group e.g., sugarcane, maize, millet, sorghum etc.
(v) Transpiration ratio for C4 plant is 200-350 and for C3 plants the ratio is 500-800. It shows that C4 plants transpire less and are more efficient in conserving water than most of C3 plants.
(vi) Normal atmospheric oxygen
concentration is inhibitory to C3 plants while C4 plants grow optimally at this concentration.
(vii) There is little loss of photosynthetic activity on account of photorespiration which is absent in C4 plants.
(viii)They perform photosynthesis even when their stomata are closed.
6.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? Explain with an example.
(b) How can we derive an absorption spectrum for any substance?
(c) If chlorophyll a is responsible for light reaction of photosynthesis, why do the action spectrum and absorption spectrum not overlap?
Soln.(a) Action spectrum depicts the relative rates of photosynthesis at different wavelengths of light. Action spectrum for photosynthesis can be plotted by measurements of oxygen evolution at different wavelengths. Englemann (1882) by using a green algae plotted action spectrum.
(b) Absorption spectrum of a substance can be derived by calculating amount of energy of different wavelength of light absorbed.
(c) Chlorophyll a is responsible for light reaction of photosynthesis, but the action spectrum and absorption spectrum do not overlap because, though chlorophyll a is the main pigment responsible for absorption of light, other thylakoid pigments like chlorophyll b, xanthophylls, carotenoid, which are accessory pigments, also absorb and transfer the energy to chlorophyll a. Indeed they not only enable • a wider range of wavelength of incoming light to be utilised for photosynthesis but also protect chlorophyll a from photooxidation.
7. What are the important events and end products of the light reaction?
Soln. The important events of the light reaction are photolysis of water and production of assimilatory power (NADPH and ATP). Photolysis of water: The phenomenon of breaking up of water into hydrogen and oxygen in the illuminated chloroplasts is called photolysis or photocatalytic splitting of water. Light energy, an oxygen evolving complex (OEC) and an electron carrier are required. Oxygen evolving complex was formerly called Z-enzyme. The enzyme has four Mn ions. Light energised changes in Mn (Mn2+, Mn3+, Mn4+) remove electrons from OH- component of water forming oxygen. Liberation of 02 also requires two other ions, Ca2+ and Cl–. Electron carrier transfer the released electrons to P680
Production of assimilatory power (NADPH and ATP): Formation of NADPH takes place during nori-cyclic photophosphorylation while ATP is synthesised in both the cyclic as well as non cyclic photophosphorylation.
8. In the diagram shown below label A, B, C. What type of phosphorylation is possible in this?
The cyclic photophosphorylation is shown in the above figure. ,
9. Why is the RuBisCO enzyme more appropriately called RUBP Carboxylase-Oxygenase and what important role does it play in photosynthesis?
Soln. RuBisCO or RuBP carboxylase – oxygenease has dual nature. It has affinity for both CO2 and 02 but has more affinity for CO2 than 02 Thus, the concentrations of two determines which of the two will bind to the enzyme.
(i) In a normal condition of C3 plants when CO2 and 02 concentrations are normal,
it acts as carboxylase and fix CO2 by combining with ribulose biphosphate and C3 cycle operates normally, producing glucose molecule as by product of photosynthesis.
6RuBP + 6 CO2 + 18 ATP + 12 NADPH -> 6 RuBP + glucose + 18 ADP + 18 P + 12 NADP
(ii) In C3 plants when O2 concentration goes up and CO2 goes down,, it starts acting as an oxygenase enzyme and C2 cycle (photorespiration) starts where RuBP binds with CO2 to from phosphoglycolate and phosphoglyceric
(iii) In C4 plants RuBP acts as carboxylase and accept liberated CO2 to form phosphoglyceric acid.
10. What special anatomical features are displayed by leaves of C4 plants? How do they provide advantage over the structure of C3 plants?
Soln. C4 plant leaves show kranz type of anatomy. In kranz anatomy, the mesophyll is undifferentiated and its cells occur in concentric layers around vascular bundles. The vascular bundles are surrounded by large sized bundle sheath cells which are arranged in wreath like manner in one to several layers.
Chloroplasts of mesophyll cells are smaller, grana rich and they do not produce starch. Chloroplasts of bundle sheath cells are larger and lack grana. A joint peripheral reticulum occur inside the chloroplasts. Starch is often present. The kranz anatomy of C4 plants gives them the following advantages over the C3 plants.
(i) The localisation of PEPcase and RuBisCO in different cells suppresses photorespiration as the concentration of CO2 is always more than that of 02, in cells where RuBisCO is present. It ensures that RuBisCO functions as carboxylase only.
(ii)The mesophyll cells have PEPcase as the carbon dioxide acceptor molecule. The substrate for PEPcase is HCO3 and PEPcase shows a high affinity for HCO3 and the enzyme is saturated by HCO3 in equilibrium with air leaves of CO2. Furthermore, HCO3, HCO3, oxygen is not a competitor, thus PEPcase is highly efficient. It enables C4 plants to reduce the stomatal aperture and thereby conserve water while fixing C02 at rates equal to or greater than those of C3 plants.
(iii)Initially, organic acids are produced in the mesophyll cells. This abundance of organic acids makes them tolerant to saline conditions.
(iv)They are specialised to live in area with high temperature as found in tropics. It is because their enzymes, PEP carboxylase and phosphopyruvic dikinase are functional only at higher temperature.
11. Name the two important enzymes of C3and C4 pathway, respectively? What important role do they play in fixing CO2 ?
Soln. The important enzyme of C3 cycle is RuBP carboxylase oxygenase which catalyses carboxylation of ribulose bisphosphate, which is 5-carbon compound to form PGA, the first stable product in C3 cycle.
In C4 cycle, the important enzyme is phosphoenol pyruvate carboxylase (PEP carboxylase) which helps in fixing CO2 to form oxaloacetate (4-carbon compound) the
first stable product of dark reaction is C4 cycle.
Phosphoenol pyruvic acid + CO2 + H20
12. Why is RuBisCO enzyme the most abundant enzyme in the world?
Soln. RuBisCO (Ribulose 1, 5-biphosphate carboxylase/oxygenase) is used in the Calvin cycle to catalyse the first major step of carbon fixation. It is thought to be the most abundant enzyme in the world because it is present in every plant that undergoes photosynthesis and molecular synthesis through the Calvin cycle. It is also present in anoxygenic photosynthetic bacteria, oxygen producing cyanobacteria, and other bacterial species that convert CO2to organic matter. It is present even in dinoflagellates and unicellular photosynthetic algae. The number of members of these groups is very high specially that of photosynthetic microorganisms. It supports RuBisCO’s status is the most abundant enzyme in world.
13. Why does not photorespiration take place in C4 plants?
Soln. Ribulose biphosphate carboxylase oxygenase (RuBisCO), the main enzyme of Calvin cycle which fixes CO2, acts as both oxygenase and carboxylase. In presence of high concentration of 02, the enzyme RuBisCO acts as oxygenase and splits a molecule of ribulose- 1, 5 biphosphate into one molecule each of 3-phosphoglyceric acid and 2-phosphoglycolic acid. This causes the loss of fixed CO2 and wastes the work already done. This process is called photorespiration. Photorespiration occurs when intracellular concentration of carbon dioxide is inC4 plants, RuBisCO is located only in bundle sheath cells where photosynthetic release of oxygen does not occur. Bundle sheath cells have a high intracellular concentration of CO2 due to flow of C4 acids and their decarboxylation to releaseC O2 Therefore, RuBisCO functions purely as carboxylase in C4 plants and no photorgspiration occurs.
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