Set-I: Past Entrance papers

Hint

(d) Only photosystem I is involved in the cyclic photophosphorylation process. Cyclic photophosphorylation is a process in which an electron expelled by the excited photo centre is returned to it after passing through a series of electron carriers. The excited electron does not pass on to NADP \({ }^{+}\)but is cycled back to the PS I complex through the electron transport chain. Non-cyclic photophosphorylation involves both photosystems I and II. The electron follows a non-cyclic pathway in it. The representation of it is also called the Z scheme.

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(b) Carbon fixation or carbon assimilation is the process by which inorganic carbon (particularly in the form of carbon dioxide) is converted to organic compounds by living organisms. The compounds are then used to store energy and as structure for other
biomolecules. Most of the plants that are adapted to dry tropical regions form C-4 acid i.e. oxalic acid as their first stable product. These plants are called \(C_{4}\) plants. Sugarcane, maize, Sorghum, etc. are examples of these plants.

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(d) In light reaction, plastoquinone facilitates the transfer of electrons from PS II to cytochrome \(b_{6}\) f complex (non-cyclic photophosphorylation)
process of light reaction starts with PS II \((680 \mathrm{~nm})\). When sunlight falls on the reaction center (chlorophyll, a) it abosrbs \(680 \mathrm{~nm}\) wavelength of red light causing electrons to become excited
and jump into an orbit farther from the atomic nucleus. These electrons are picked by the electron acceptor which passes them to an electron transport system consisting of cytochrome \(b_{6}\) complex.

Hint

(b) Statement(b) is incorrect and can be corrected as \(\ln \mathrm{C}_{4}\) plants, Kranz anatomy in leaf is found due to the presence of two types of cells viz., mesophyll cells and bundle sheath cells. The mesophyll cells are specialised to perform the light reactions, evolve \(\mathrm{O}_{2}\) and produce assimilatory power. The bundle sheath cells possess RuBisCO and thus, perform RuBisCO activity at this site.

Hint

(a) In photorespiration, \(\mathrm{O}_{2}\) binds to RubisCo. As a result RuBP instead of being converted to 2 molecules of PGA bind with \(\mathrm{O}_{2}\) to form one molecule each of phosphoglycerate (3 carbon compound) and phosphoglycolate ( 2 carbon compound).

Photorespiration is the oxygenation of RuBP by RUBISCO followed by photorespiratory glycolate metabolism. Competition between \(\mathrm{O}_{2}\) and \(\mathrm{CO}_{2}\) reduces the rate of carbon assimilation, energetic efficiency of photosynthesis, and may reduce the photosynthetic quotient \(\left(\mathrm{PQ}=\mathrm{O}_{2}\right.\) evolved \(/ \mathrm{CO}_{2}\) assimilated).

Hint

(b) During non-cyclic photophosphorylation, electrons expelled by the excited PS-II
photo centre does not return to it. Therefore, it requires an external electron donor and that purpose is served by water.
\(\mathrm{H}_{2}\) Oundergo photolysis and the electrons thus released are picked up by PS-II ( \(\left.\mathrm{P}_{680}\right)\) and handed over to PS-I or \(P_{700}\).

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(d) Engelmann used a prism to split light into its spectral components and then illuminated a green alga, Cladophora placed in a suspension of aerobic bacteria (Azotobacter). The bacteria were used to detect the sites of oxygen evolution.
He observed that bacteria mainly accumulated in the region of blue and red light of the split spectrum, thus giving the first action spectrum of photosynthesis.

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(c) In Hatch and Slack pathway, the primary \(\mathrm{CO}_{2}\) acceptor is phosphoenol pyruvate. This occurs in \(\mathrm{C}_{4}\)-plants. Phosphoenol pyruvate, a 3-carbon compound, accepts \(\mathrm{CO}_{2}\) and forms oxaloacetic acid which is a 4-carbon compound.

Hint

(b) During the light reaction of photosynthesis NADPH, ATP and oxygen are formed.
Oxygen is liberated by the photolysis of water. The electrons released during the photolysis of water are picked up by \(\)P_{680}\(\) photo centre of PS-II. On receiving light energy photo centre expels an electron which passes over a series of carriers. As a result assimilatory power, i.e. ATP and NADPH is produced. NADH is formed during respiration.

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(a) Green sulphur bacteria do not use \(\mathrm{H}_{2} \mathrm{O}\) as source of proton, therefore they do not evolve \(\mathrm{O}_{2}\)

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(b) \(\mathrm{C}_{4}\) plants respond to higher temperature with enhanced photosynthesis while \(\mathrm{C}_{3}\) plants have lower temperature optimum.

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(a) Phosphoenol Pyruvate (PEP) is found in the mesophyll cell, which accepts the atmospheric \(\mathrm{CO}_{2}\) in \(\mathrm{C}_{4}-\) plants and  \(\begin{aligned}
&\text { converts it to oxalo acetate }–\mathrm{a} \mathrm{C}_{4} \\
&\text { compound. It is the first stable } \\
&\text { compound of } \mathrm{C}_{4}-\text { plants. } \\
&\text { Concept Enhancer } \mathrm{C}_{4} \text {-plants possess } \\
&\text { special adaptation anatomy in their } \\
&\text { leaves to cope up the photorespiratory } \\
&\text { losses. There are dimorphic chloroplast } \\
&\text { present in them-agranal in bundle } \\
&\text { sheath cells and granal in mesophyll } \\
&\text { cells. }
\end{aligned}\)

Hint

(c) Photorespiration is the light-dependent process of oxygenation of ribulose biphosphate \((\mathrm{RuBP})\) and release of carbon dioxide by the photosynthetic organs of a plant. It leads to oxidation of considerable amount of photosynthetic products to \(\mathrm{CO}_{2}\) and \(\mathrm{H}_{2} \mathrm{O}\) without the production of useful energy. Photorespiration occurs only in \(\mathrm{C}_{3}\) plants because at high temperature and high oxygen concentration RuBP carboxylase changes to RuBP oxygenase. Photorespiration is absent in \(\mathrm{C}_{4}\) plants. Peroxisome and mitochondria are required for completing the process.

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(d) Transpirational loss of water occurs through stomata & gaseous exchange also occurs through stomata in terrestrial plants. Simultaneously as both are the process of simple diffusion occurs in order of diffusion pressure gradient or diffusion coefficient.

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(d) Highest number of protons found in lumen of thylakoid of chloroplast during day time due to photolysis of water and active transport of protons by plastoquinone from stroma to lumen of thylakoid.

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(d) Emerson et al. (1957) found that if the light of shorter wavelengths was provided at the same time as the longer red wavelengths, photosynthesis was even faster than the sum of the two rates with either colour alone. This synergism or enhancement became known as the Emerson enhancement effect. The two separate groups of pigments or photosystems cooperate in photosynthesis and such long red wavelengths are absorbed only by one photosystem, called photosystem I (PS I). The second photosystem, photosystem II (PS II), absorbs wavelengths shorter than \(690 \mathrm{~nm}\), and for maximum photosynthesis wavelengths absorbed by both systems must function together. The two photosystems normally cooperate to cause photosynthesis at all wavelengths shorter than \(690 \mathrm{~nm}\), because both photosystems absorb those wavelengths. The importance of Emerson’s work is that it suggested the presence of two distinct photosystems.

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(d) This plant is a \(\mathrm{C}_{4}\) plant. \(\mathrm{C}_{4}\) plants are special as they have a special type of leaf anatomy called Kranz Anatomy. They can tolerate higher temperatures, they show a response to high intensities of light, they lack a wasteful process called photorespiration and they have improved efficiency of nitrogen utilization. Most of the plants that are adapted to dry tropical regions have the \(\mathrm{C}_{4}\) pathway and are known as \(\mathrm{C}_{4}\) plants. In these plants, double fixation of carbon dioxide occurs.

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(b) Anthocyanins are water-soluble pigments, which commonly occur in membrane-enclosed vacuoles. They are responsible for colour of fruits and flower petals.

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(a) Photolysis of water during photosynthesis evolves nascent oxygen in the presence of manganese, calcium, and chloride ions.

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(b) The light-independent reactions (dark or Blackman’s reactions) of photosynthesis take place in the stroma or matrix of chloroplasts. These reactions are enzymatic reactions which catalyse assimilation of \(\mathrm{CO}_{2}\) into carbohydrates.

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(c) Chromatophores are the internal membrane systems of photosynthetic forms which possess photosynthetic pigments. They occur in photoautotrophic bacteria, e.g., purple bacteria and green bacteria.

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(a) In chloroplast which are green-coloured plastids, thylakoids are arranged in stacks like the pile of coins called grana.

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(b) Cytochromes are found in mitochondria. These are located on the inner membrane of mitochondria and are related to phosphorylation.

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(a) In Rhodospirillum, an electron donor is an organic compound during photosynthesis. It helps an organism trap light energy and stores it as chemical energy.

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(c) In some photoautotrophic bacteria, bacteriochlorophyll and bacteriophaeophytin or bacterioviridin pigments containing membraneous extensions called chromatophores are present. These are sites of bacterial photosynthesis.

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(a) In \(\mathrm{C}_{4}\) plants, bundle sheath cells are rich in RuBisCO, but lack PEPcase. The \(\mathrm{CO}_{2}\) released in the bundle sheath cells enters the \(\mathrm{C}_{3}\) or the calvin pathway, common to all plants. Thus, the basic pathway that results in the formation of sugars, the calvin pathway is common to the \(\mathrm{C}_{3}\) and \(\mathrm{C}_{4}\) plants.

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(d) Photorespiration is a light-dependent process which occurs in \(C_{3}\)-plants. It is opposite to photosynthesis because, during this process, uptake of \(\mathrm{O}_{2}\) and release of \(\mathrm{CO}_{2}\) take place. Due to the presence of Kranz anatomy, \(\mathrm{C}_{4}\)-plants do not show photorespiration.

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(c) Photorespiration required three cell organelles in a sequence of chloroplast, peroxisome, and mitochondria.

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(a) Polyandrous condition (having a large and indefinite number of stamens) is present in Gloriosa (family Liliaceae) and Petunia (family Solanaceae). Nitrogen fixation is the conversion of inert atmospheric nitrogen into utilisable compounds of nitrogen like nitrate, ammonia, amino acids, etc. Biological nitrogen fixation is performed by free living and symbiotic bacteria and cyanobacteria. Symbiotic nitrogen fixers occur in association with roots of higher plants. For e.g., Rhizobium is nitrogen fixing bacterial symbiont of papilionaceous roots and Frankia is a symbiont in root nodules of several nonleguminous plants like Casuarina. Both Rhizobium and Frankia live free as aerobes in the soil and develop the ability to fix nitrogen only as symbionts when they become anaerobic.

Hint

(a) Crassulacean acid metabolism (CAM) is photosynthesis by the \(\mathrm{C}_{4}\) pathway in which carbon dioxide is taken up during the night when the plant’s stomata are open and fixed into malic acid. During the day, when the stomata are closed, carbon dioxide is released from malic acid for use in the Calvin cycle. This is important for plants that live in arid conditions as it enables them to keep their stomata closed during the day to reduce water loss from evaporation. Crassulacean acid metabolism is common in succulent plants of desert regions, including cacti and spurges, and in certain ferns.

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(b) The \(\mathrm{C}_{4}\) plants are adapted to dry tropical regions and have greater productivity of biomass. They have special type of leaf anatomy known as Kranz anatomy. In this type of anatomy, the bundle sheath cells 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.

Hint

(a) Manganese \(\left(\mathrm{Mn}^{2+}\right)\) is used for photolysis of water to produce oxygen and electrons during the light reaction of photosynthesis. It is the phenomenon of breaking up water into hydrogen and oxygen in the illuminated chloroplast. It acts as an essential cofactor.

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(b) PAR(Photosynthetically Active Radiation) designates the spectral range of solar radiation from 400-700 \(\mathrm{nm}\) that photosynthetic organisms are
able to use in the process of photosynthesis. Of the total incident solar radiation, the proportion of PAR is less than \(50 \%\).

Hint

(d) Calvin, Benson and their colleagues in California, U.S.A. fed Chlorella and Scenedesmus with radioactive \({ }^{14} \mathrm{C}\) in carbon dioxide. Radioactive carbon, \({ }^{14} \mathrm{C}\) has a half-life of 5568 years. Therefore, the path of \(\mathrm{CO}_{2}\) fixation can be easily traced with its help. Algal suspension, illuminated and carrying out photosynthesis with normal carbon dioxide, was supplied \({ }^{14} \mathrm{CO}_{2}\). The alga was killed at intervals in near-boiling methanol. It immediately stopped photosynthesis activity due to the denaturation of enzymes. Alcohol was evaporated and after crushing the alga, the product was made into a paste. The paste was placed on a paper chromatogram and the different compounds were separated by two-dimensional chromatography. The radioactive compounds were identified by comparing their position on the chromatogram with standard chemicals. Calvin and co-workers found that after three seconds, radioactivity appeared in phosphoglyceric acid or PGA. Phosphoglyceric acid is, therefore, the first stable product of photosynthesis.

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(d) Rate of net photosynthesis in \(\mathrm{C}_{3}\) plants is 15-35 mg CO\(/ 2 / \mathrm{dm}^{2} /\) hr while in \(\mathrm{C}_{4}\) plants it \(40-80 \mathrm{mg}\) \(\mathrm{CO}_{2} / \mathrm{dm}^{2} / \mathrm{hr}\). This variation in rate is due to photorespiration. Photorespiration is an inhibitory process which decreases the rate of photosynthesis. In excess of oxygen RuBP carboxylase converts to RuBP oxygenase. As a result glycolate synthesis is enhanced and leads to begin photorespiration. Photorespiration is negligible or absent in \(\mathrm{C}_{4}\) plants and present only in \(\mathrm{C}_{3}\) plants. So \(\mathrm{C}_{4}\) plants are photosynthetically more efficient.

Hint

(a) \(\mathrm{Z}\) scheme involves both PSI and PSII to transfer electron excited by light starting from PSII uphill to the acceptor, down to the electron transport chain to PSI, which further comprises of excitation of electrons, transfer to another acceptor and finally downhill to \(\mathrm{NADP}^{+}\)causing reduction of it to \(\mathrm{NADPH}+\mathrm{H}^{+}\). Stroma lamella contains PSI only.

Hint

(c)

A – Fixation of \(\mathrm{CO}_{2}\) by PEP carboxylase
\(\mathrm{B}\) – Decarboxylation
\(\mathrm{C}\) – Regeneration

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(c) In Kranz anatomy, the mesophyll is undifferentiated and its cells occur in concentric layers around vascular bundles. Vascular bundles are surrounded by large-sized bundle sheath cells which are arranged in a wreath-like manner in one to several layers. \(\mathrm{C}_{4}\) plants, both monocots and dicots, such as sugarcane, maize, and sorghum have Kranz anatomy in leaf.

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(c) In cyclic photophosphorylation, 2 molecules of ATP are synthesised which are used in dark reactions. Cyclic photophosphorylation is not concerned with the photolysis of water. So \(\mathrm{O}_{2}\) is not evolved and NADPH is also not produced.

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(d) The dark reactions of photosynthesis is purely enzymatic and slower than the primary photochemical reaction. It takes place in stroma portion of the chloroplast and is independent of light i.e., it can occur either in the presence or in absence of light provided that assimilatory power is available.

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(b) Oscillatoria is a photosynthetic cyanobacterium. In this cyanobacteria during photosynthesis water is an electron donor and oxygen is a byproduct, i.e., oxygenic photosynthesis occurs. Rhodospirillum and Chlorobium are non-oxygenic photosynthetic, purple non-sulphur and green-sulphur bacteria. Chromatium is a purple sulphur bacterium, also a non-oxygenic photosynthetic.

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(a) The electrons released during photolysis of water are picked up by \(\mathrm{P}_{680}[latex] photo centre of photosystem II. The electron extruded by the photo centre of photosystem II picked up by the quencher phaeophytin. From here the electron passes over a series of carriers in a downhill journey losing its energy at every step. The major carriers are plastoquinone (PQ), cytochrome [latex]b-f\) complex and plastocyanin \((P C)\). While passing over the cytochrome complex, the electron loses sufficient energy for the creation of proton gradient and synthesis of ATP from ADP and inorganic phosphate. The process is called photophosphorylation (noncyclic).

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(b) \(\mathrm{C}_{4}\) plants are photosynthetically more efficient than \(\mathrm{C}_{3}[latex] plants because [latex]\mathrm{C}_{4}\) plant contain two types of chloroplast i.e., bundle sheath chloroplast and mesophyll chloroplast. So such plants operate a dicarboxylic acid cycle in addition to Calvin cycle. \(\mathrm{CO}_{2}\) acceptor molecule (PEP) is present in large bundle sheath cell which has higher efficiency in picking up \(\mathrm{CO}_{2}\). Thus, photosynthesis continues even at low \(\mathrm{CO}_{2}\) concentration and the rate of photorespiration is also negligible.

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(d) \(\mathrm{C}_{4}\) plants show kranz anatomy i.e. the mesophyll is undifferentiated and its cells occur in concentric layers around the vascular bundle, which is surrounded by large-sized bundle sheath cells, in a wreath like manner. In this type of plant, the initial fixation of \(\mathrm{CO}_{2}\) occur in mesophyll cell. The primary acceptor (phosphoenol pyruvate) combines with \(\mathrm{CO}_{2}\) to form oxaloacetic acid which later reduces to malic acid. Malic acid is then translocated to bundle sheath cell for further decarboxylation.

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(c) As starch is a high molecular weight polymer of D-glucose in \(\alpha \mapsto 4\) linkage. It is synthesised in chloroplasts as one of the stable end products of photosynthesis. It is the most abundant and common storage polysaccharide in plants hence, the most staple food for man and herbivores. It is a mixture of two types of glucose homopolysaccharide viz, amylose and amylopectin. During daytime the starch synthesis in the chloroplast is coordinated with sucrose synthesis in the cytosol. Typically about \(90 \%\) of total solute carried in phloem is the carbohydrate sucrose, a disaccharide. This is relatively inactive and highly soluble sugar playing a little direct role in metabolism and so, making it an ideal transport sugar.

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(d) Mesophyll is the site in plant leaves, where chlorophyll pigment is present. Thus carbon dioxide fixation occurs there, which produces malic acid. It is then transferred to agranal chloroplast in bundle sheath.

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(c) Type I photosystems use ferredoxin-like iron-sulphur cluster proteins as terminal electron acceptors, while type II photosystems ultimately shuttle electrons to a quinone terminal electron acceptor. One has to note that both reaction centres types are present in chloroplasts of plants and cyanobacteria, working together to form a unique photosynthetic chain able to extract electrons from water, evolving oxygen as a byproduct.

Hint

(a) The Pr form absorbs light between 660 to \(680 \mathrm{~nm}\) and absorbs at a peak of \(666 \mathrm{~nm}\). It is the form synthesized in dark-grown seedlings. When Pr absorbs red light, it is converted to the Pfr form.

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(c) The first reaction of photorespiration occurs in the stroma of the chloroplast. In this reaction, the RuBP (Ribulose 1-5 biphosphate) consumes one oxygen molecule in presence of the enzyme RuBisCO. In peroxisome the glycolate transferred from chloroplast takes up \(\mathrm{O}_{2}\) and formed the glyoxylate whereas, the \(\mathrm{H}_{2} \mathrm{O}_{2}\) released as by-product.

Hint

(a) In light reaction of photosynthesis two types of photosystems are involved. PS-I consists of plenty of chlorophyll- \(a\) and very less quantity of chlorophyll- \(b\). These pigments absorb light energy and transfer it to the reaction centre \(-\mathrm{P}_{700}\). After absorbing an adequate amount of light energy electron gets excited from \(\mathrm{P}_{700}\) molecule and moves to iron-sulphur protein complex, designated as A (Fe-S). It gets reduced after accepting electrons. It later gives these electrons to ferredoxin and gets oxidized again.