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Photosynthetic Active Radiation (PAR) has the following range of wavelengths.[AIPMT 2005, 2004, 1996]
(b) Wavelengths between 400 and \(700 \mathrm{~nm}\), which comprise the visible range of the electromagnetic spectrum are capable of causing photosynthesis. These are called photosynthetically active radiations. Chlorophyll a and b absorb too much light in the blue and red regions of the spectrum of light. Carotenoids mostly absorb is the blue region of the spectrum.
As compared to a \(\mathrm{C}_{3}\)-plant, how many additional molecules of ATP are needed for the net production of one molecule of hexose sugar by \(\mathrm{C}_{4}\)-plants? [AIPMT 2005]
(c) In \(\mathrm{C}_{4}\) plants the cost of concentrating \(\mathrm{CO}_{2}\) within the bundle sheath cell is \(2 \mathrm{ATP}\) per \(\mathrm{CO}_{2}\).
In \(\mathrm{C}_{4}\) plants \(12 \mathrm{ATP}\) molecules are required for producing one hexose sugar. In the bundle sheath cells \(\mathrm{C}_{3}\) cycle operates which requires \(18 \mathrm{ATP}\) and \(12 \mathrm{NADPH}_{2}\) molecules. So total 30 ATP and 12 \(\mathrm{NADPH}_{2}\) molecules are required in \(\mathrm{C}_{4}\) cycle. Whereas in \(\mathrm{C}_{3}\) cycle \(18 \mathrm{ATP}\) and \(12 \mathrm{NADPH}_{2}\) molecules are required.
Photosynthesis in \(\mathrm{C}_{4}\) plants is relatively less limited by atmospheric \(\mathrm{CO}_{2}\) levels because [AIPMT 2005]
(d) The fixation of \(\mathrm{CO}_{2}\) in \(\mathrm{C}_{4}\)-plants takes place in two places and by two different organic compounds. Phosphoenol Pyruvate(PEP) is found in mesophyll cells which primarily fixes atmospheric \(\mathrm{CO}_{2}\) into oxalo acetic acid (4C). RuBisCO is present in bundle sheath cells where final fixation of \(\mathrm{CO}_{2}\) in hexose sugars takes place. \(\mathrm{CO}_{2}\) is primarily fixed by PEP carboxylase because this enzyme has a greater affinity to \(\mathrm{CO}_{2}\) than RuBisCO.
In \(\mathrm{C}_{3}\) plants, the first stable product of photosynthesis during the dark reaction is [AIPMT 2004]
(c) The Calvin cycle is also known as \(\mathrm{C}_{3}\) cycle because \(\mathrm{CO}_{2}\) reduction is a cyclic process and first stable product in this cycle is a 3-C compound (i.e., 3-phosphoglyceric acid or 3-PGA).
In this cycle, \(\mathrm{CO}_{2}\) acceptor molecule is RuBP or RuDP (i.e., Ribulose 1, 5-biphosphate or Ribulose 1,5 -diphosphate). There occurs covalent bonding of \(\mathrm{CO}_{2}\) to \(\mathrm{RuBP}\) and the enzyme catalyzing this reaction is RuBP-carboxylase/oxygenase (RuBisCO).
Plants adapted to low light intensity have [AIPMT 2004]
(a) To absorb more sunlight (quantitatively), the plants growing in low light conditions have a larger photosynthetic unit size. It means that they have more number of chlorophyll molecules per reaction center. to trap more light energy available to them.
Chlorophyll in chloroplasts is located in [CBSE AIPMT 2004]
(c) The thylakoids of the chloroplast are flattened vesicles arranged as a membranous network within the stroma. \(50 \%\) of chloroplast proteins and various components involved ( namely chlorophyll, carotenoids, and plastoquinone) in photosynthesis are present in thylakoid membranes.
Which fractions of the visible spectrum of solar radiations are primarily absorbed by carotenoids of the higher plants? [AIPMT 2003]
(d) Carotenoids of higher plants are fat-soluble compound that includes carotenes and xanthophylls. Most of them absorb light of the violet and blue range. Green light is absorbed in less amount.
Which one of the following is wrong in relation to photorespiration? [AIPMT 2003]
(c) Photorespiration is absent in \(\mathrm{C}_{4}\) plants due to the presence of a special structure called as kranz anatomy in their leaves. It is a process which takes place in \(\mathrm{C}_{3}\) plants when carbon dioxide concentration in plants becomes low.
In sugarcane plant \({ }^{14} \mathrm{CO}_{2}\) is fixed to malic acid, in which the enzyme that fixes \(\mathrm{CO}_{2}\) is [AIPMT 2003]
(b) \(C_{4}\) pathway was first reported in members of the family Gramineae (grasses) like sugarcane, maize, sorghum, etc.
In \(\mathrm{C}_{4}\) plants PEPCo (PEP carboxylase) is the key enzyme used to fix \(\mathrm{CO}_{2}\) in \(\mathrm{C}_{4}\) plants.Oxaloacetic acid is a \(4-C\) compound and is the first stable product so this pathway is known as \(\mathrm{C}_{4}\) cycle.
Phosphoenol pyruvic acid + Carbon dioxide
\(\stackrel{P E P-c a r b o x y l a s e}{\longrightarrow}\) Oxaloacetic acid
Stomata of CAM plants [AIPMT 2003]
(c) Stomata of most plants open at sunrise and close in darkness to allow the entry of \(\mathrm{CO}_{2}\) needed for photosynthesis during the daytime.
Certain succulents that are native to hot, dry conditions (e.g., cacti, Kalanchoe, and Bryophyllum) act in an opposite manner. They open their stomata at night, fix carbon dioxide into organic acids in the dark, and close their stomata during the day. This is an appropriate way to absorb \(\mathrm{CO}_{2}\) through open stomata at night, when transpiration stress is low, and conserve water during the heat of the day. These plants show Crassulacean Acid Metabolism (CAM).
Which element is located at the centre of the porphyrin ring in chlorophyll? [AIPMT 2003]
(b) Chlorophyll is the green pigment present in plants and some photosynthetic bacteria.
The empirical formula of chlorophyll- \(a\) molecule is \(\mathrm{C}_{55} \mathrm{H}_{72} \mathrm{O}_{5} \mathrm{~N}_{4} \mathrm{Mg}\). It consists of a porphyrin head and a phytol tail. Porphyrin is a cyclic tetrapyrrole structure, having a magnesium atom in the centre. The tail consists of phytol alcohol and it is attached to one of the pyrrole rings.
Which one of the following concerns photophosphorylation? [AIPMT 2003]
(b) The light dependent production of ATP from \(\mathrm{ADP}+\mathrm{P} i\) in the chloroplasts is called photophosphorylation.
Photophosphorylation is of 2 types -Cyclic photophosphorylation – It involves only PS-I, water is not utilized and so no oxygen is evolved. Here two ATP molecules are produced.
Non-cyclic photophosphorylation – It involves both PS-I and PS-II, water is utilized and so oxygen is evolved. Here one ATP molecule and one \(\mathrm{NADPH}_{2}\) molecule are produced.
Which of the following absorb light energy for photosynthesis? [AIPMT 2002]
(a) Photosynthesis occurs in chloroplasts that contain photosynthetic pigments – chlorophylls, carotenoids, etc. The light energy required for photosynthesis comes from sunlight. The sunlight travels in the form of small particles called photons. Each photon has a quantum of energy. This quantum of energy is absorbed by a single antenna chlorophyll and then migrates from one molecule to the other till it reaches the reaction center of photosystems. The reaction center is also \(\mathrm{P}_{700}\) chlorophyll molecule that releases electrons as a result of transferred energy. This electron is transferred between various acceptors and generates ATP and \(\mathrm{NADPH}_{2}\) in the light reaction of photosynthesis.
In photosynthesis energy from light reaction to dark reaction is transferred in the form of [AIPMT 2002]
(b) Photosynthesis consists of light-dependent phase and light-independent phase or dark reaction. Light-dependent phase occurs in the grana fraction of chloroplast. It involves cyclic and non-cyclic photophosphorylation where assimilatory powers (ATP and \(\mathrm{NADPH}_{2}\) ) are produced. In dark reaction, which occurs in stroma fraction of chloroplast, the actual reduction of \(\mathrm{CO}_{2}\) to carbohydrates takes place using the assimilatory powers (ATP and \(\mathrm{NADPH}_{2}\) ) produced in the light-dependent phase.
It needs \(18 \mathrm{ATP}\) and \(12 \mathrm{NADPH}_{2}\) molecules to produce one molecule of glucose.
Which pigment absorbs the red and far-red light? [AIPMT 2002]
(b) : Phytochrome has a light absorbing or light detecting portion (the chromophore) attached to small protein of about \(1,24,000\) daltons. Phytochrome occurs in 2 forms, i.e., \(\mathrm{P}_{\mathrm{R}}\) and \(\mathrm{P}_{\mathrm{FR}}\) (i.e., red light and far red light absorbing forms) and these 2 forms are interconvertible.
Cytochromes are electron transferring proteins. They contain iron porphyrin or copper porphyrin as prosthetic groups. Chlorophyll is the fundamental green pigment of photosynthesis. It is localized in the chloroplasts. Carotenoids are lipid compounds and they are yellow, orange, purple, etc. in colour. These are found in higher plants red algae, green algae, fungi, and photosynthetic bacteria.
The enzyme that catalyses initial carbon dioxide fixation in \(\mathrm{C}_{4}\)-plants is [CBSE AIPMT 1992, 2002]
(b) In \(C_{4}\)-plants, mesophyll cells fix carbon dioxide with the help of
phosphoenol-pyruvate (the first acceptor) in the presence of PEP carboxylase to a compound oxaloacetic acid(first product).
What is true for photolithotrophs? [AIPMT 2001]
(b) Photolithotrophs are those plants that obtain energy from radiation and hydrogen from inorganic compounds.
Which pigment system is inactivated in red drop? [AIPMT 2001]
(c) Emerson and Lewis worked on Chlorella and calculated the quantum yield for different wavelengths. Emerson observed that rate of photosynthesis declines in the red region of the spectrum. This decline in photosynthesis is called “Red drop”. It was observed that the quantum yield falls when the light of wavelengths more than 680 or \(690 \mathrm{~nm}\) are supplied. As the PS-II \(\mathrm{P}_{680}\) is driven by red light, so it remains inactive during the red drop.
Which pair is wrong? [AIPMT 2001]
(a) Maize is a \(\mathrm{C}_{4}-\) plant. \(\mathrm{C}_{4}-\) plants have Kranz type anatomy of leaves.
PGA (3-Phosphoglyceric Acid) is formed during the Calvin cycle.
OAA(Oxalo Acetic Acid) a 4C compound is formed during Hatch and Slack cycle (C \(C_{4}\) cycle).
Glycolate induces opening of stomata in [AIPMT 2001]
(b) Light causes photosynthesis which leads to reduction in \(\mathrm{CO}_{2}\) concentration \(\rightarrow\) oxidation of glycolate \(\rightarrow\) ATP synthesis \(\rightarrow\) activation of \(\mathrm{K}^{+}\)pump \(\rightarrow\) movement of \(\mathrm{K}^{+}\)in guard cell \(\rightarrow\) movement of water into guard cell \(\rightarrow\) swelling of guard cell \(\rightarrow\) opening of stomata.
Which is the first \(\mathrm{CO}_{2}\) acceptor enzyme in \(\mathrm{C}_{4}\) plants? [AIPMT 2000]
(d) \(C_{4}\) pathway was first reported in members of family Gramineae (grasses) like sugarcane, maize, sorghum, etc. These \(C_{4}\) plants have a characteristic leaf anatomy called Kranz anatomy. So in chloroplasts of mesophyll cells -PEPCo (PEP carboylase) is the key enzyme used to fix \(\mathrm{CO}_{2}\) in \(\mathrm{C}_{4}\) plants Oxaloacetic acid is a 4-C compound and is the first stable product so this pathway is known as \(\mathrm{C}_{4}\) cycle.
For assimilation of one \(\mathrm{CO}_{2}\) molecule, the energy required in form of \(\mathrm{ATP}\) and \(\mathrm{NADPH}_{2}\) are [AIPMT 2000]
(c) Photosynthesis is actually oxidation-reduction process in which water is oxidised and \(\mathrm{CO}_{2}\) is reduced to carbohydrates. The reduction of \(\mathrm{CO}_{2}\) to carbohydrates needs assimilatory powers, i.e., ATP and \(\mathrm{NADPH}_{2}\).
The process of photosynthesis involves two steps-
(i) Light dependent phase or photochemical reaction.
(ii) Light independent phase or dark reaction.
In Calvin cycle, \(\mathrm{CO}_{2}\) acceptor molecule is RuBP or RuBP. The enzyme catalyzing this reaction is RuBPcarboxylase/oxygenase (RuBisCO). As Calvin cycle takes in only one carbon \(\left(\mathrm{as} \mathrm{CO}_{2}\right)\) at a time, so it takes six turns of the cycle to produce a net gain of six carbons (i.e., hexose or glucose). In this cycle, for the formation of one mole of hexose sugar (Glucose), 18 ATP and \(12 \mathrm{NADPH}_{2}\) are used.
For 6 molecules of \(\mathrm{CO}_{2}\) it needs 18 ATP and 12 \(\mathrm{NADPH}_{2}\) molecules so for assimilation of one molecule of \(\mathrm{CO}_{2}\) it needs 3 ATP and \(2 \mathrm{NADPH}_{2}\) molecules.
For the synthesis of one glucose molecule the Calvin cycle operates for [AIPMT 2000]
(c) Conversion of \(\mathrm{CO}_{2}\) to simple (reduced) organic compounds is called \(\mathrm{CO}_{2}\) assimilation or \(\mathrm{CO}_{2}\) fixation or carbon fixation. This fixation pathway was elucidated in the early 1950 s by Melvin Calvin and Coworkers and is often called as Calvin cycle.
Since one molecule of carbon is fixed in one turn of the Calvin cycle. So, six turns of the cycle are required to fix the glucose molecule containing 6 carbon atoms.
The first step for initiation of photosynthesis will be [AIPMT 2000]
(b) The process of photosynthesis involves two steps:
(i) Light-dependent phase or photochemical reaction.
(ii) Light independent phase or dark reactions.
The light reaction occurs in grana fraction of chloroplast and in this reaction are included those activities, which are dependent on light.
The grana of chloroplasts contain many collaborating molecules of pigment. A quantum of light is absorbed by a single antenna chlorophyll, then it migrates from one molecule to the other till it reaches the reaction center. This quantum of light is used for generating ATP and NADPH, which is later consumed in dark reactions produce sugars by fixing \(\mathrm{CO}_{2}\) molecules.
Photochemical reactions in the chloroplast are directly involved in [CBSE AIPMT 2000]
(d) \(\mathrm{CO}_{2}\) is fixed in the stroma of the chloroplast leading to the synthesis of PGA from which glyceraldehyde phosphate is formed. From
glyceraldehyde phosphate, sugar and starch are formed. All these do not require light. However, photolysis of water and phosphorylation of ADP to ATP requires light energy.
Fixation of one \(\mathrm{CO}_{2}\) molecule through Calvin cycle requires [CBSE AIPMT 2000]
(c) 2 ATP are required during the conversion of PGA to 1,3 diphosphoglyceric acid and 1
ATP during conversion of glyceraldehyde phosphate to ribulose biphosphate. 2 \(\mathrm{NADPH}_{2}\) molecules are utilised for converting 1,3 diphosphoglyceric acid to glyceraldehyde phosphate.
The enzyme which fixes \(\mathrm{CO}_{2}\) in \(\mathrm{C}_{4}\) plants is [AIPMT 2000]
(b) The three most important roles that PEP carboxylase plays in plants and bacteria metabolism are in the \(\mathrm{C}_{4}\) cycle, the CAM cycle and the Citric Acid Cycle biosynthesis flux. PEP carboxylase plays the key role of binding \(\mathrm{CO}_{2}\) to create oxaloacetate in the mesophyll tissue.
\(\mathrm{C}_{4}\) plants have carboxylating enzymes that have a higher affinity for \(\mathrm{CO}_{2}\), compared to rubisco, especially at low \(\mathrm{CO}_{2}\) concentrations & high temperatures. The \(\mathrm{C}_{4}\) pathway uses PEP carboxylase, which has a much higher affinity for \(\mathrm{CO}_{2}\) than rubisco; in addition, PEP carboxylase does NOT have oxygenase activity.
Cytochrome is [AIPMT 2001]
(b) Cytochrome is an iron-containing porphyrin pigment. These are electron transferring proteins often regarded as enzymes.
Carbon dioxide acceptor in \(\mathrm{C}_{3}\)-plants is [AIPMT 1999, 1995, 1993, 1996]
(c) An enzyme ribulose biphosphate carboxylase catalyses the dark reaction, pertaining to the addition of \(\mathrm{CO}_{2}\) to Ribulose-1-5-diphosphate. It is found in abundance in leaves and it is believed that it is the single most abundant protein on earth. It is clear that the first acceptor of \(\mathrm{CO}_{2}\) is Ribulose 1-5 diphosphate and the first product formed after fixation of \(\mathrm{CO}_{2}\) is 3 -phosphoglyceric acid.
The rate of photosynthesis is higher in [AIPMT 1999]
(c) Plants can use a small portion of the light which falls upon them. Chlorophyll- \(a\) and chlorophyll- \(b\) absorb too much light in the blue and red region. Carotenoids absorb light mostly in the blue region of the spectrum of light. In monochromatic lights, maximum photosynthesis occurs in red light, followed by blue light and poor photosynthesis in green light. Under very high light intensity solarization phenomenon occurs. It involves photooxidation of different cellular components including chlorophyll.
Which enzyme is most abundantly found on earth? [CBSE AIPMT 1999]
(b) RuBisCO (RuBP carboxylase) is the most abundant protein on this planet.
RuBisCO constitutes \(16 \%\) of chloroplast protein. It is required for \(\mathrm{CO}_{2}\) fixation with RuBP(Ribulose Biphosphate) in
Calvin cycle.
Protochlorophyll differs from chlorophyll in lacking [CBSE AIPMT 1998]
(a) Protochlorophyll differs from chlorophyll in lacking 2 hydrogen atoms in one of its pyrrole rings.
Which one of the following statements about cytochrome P-450 is wrong? [AIPMT 1998]
(c) Cytochrome P-450 is not a coloured cell, instead, this is a respiratory pigment-mixture of iron and protein which are electron acceptors. It contains haeme groups and are primarily responsible for the generation of ATP via electron transport.
Chlorophyll \(a\) molecule at its carbon atom 3 of the pyrrole ring II has one of the following [AIPMT 1997]
(d) The empirical formula of chlorophyll \(a\) molecule is \(\mathrm{C}_{55} \mathrm{H}_{72} \mathrm{O}_{5} \mathrm{~N}_{4} \mathrm{Mg}\). It has tadpole like configuration. It consists of a porphyrin head and a phytol tail. Porphyrin is a cyclic tetrapyrol structure, having a magnesium atom in the centre. In chlorophyll \(a\), a methyl group \(\left(\mathrm{CH}_{3}\right)\) is attached to the third carbon in the porphyrin head.
The core metal of chlorophyll is [AIPMT 1997]
(d) The empirical formula of chlorophyll molecule is \(\mathrm{C}_{55} \mathrm{H}_{72} \mathrm{O}_{5 \mathrm{~N}}^{4} \mathrm{Mg}\). It has tadpole-like configuration. It consists of a porphyrin head and a phytol tail. Porphyrin is a cyclic tetrapyrrole structure, having a magnesium atom in the centre. Chlorophyll a methyl group (CH 3 ) is attached to the third carbon in the porphyrin head.
\(\mathrm{NADPH}_{2}\) is generated through [AIPMT 1997]
(d) Non-cyclic photophosphorylation involves both PS-I and PS-II. Here electrons are not cycled back and are used in the reduction of NADP to \(\mathrm{NADPH}_{2}\). The electrons generated by PSII are passed over a series of electron carriers in a downhill journey and handed over to the reaction centre of PSI. PSI again passes the electrons to \(\mathrm{NADP}^{+}\)which combines with \(\mathrm{H}^{+}\)ions to form NADPH.
Suppose an aquatic plant is placed in a test tube containing distilled water and the tube is stoppered. The tube is left outdoors for 24 hours and the \(\mathrm{pH}\) value of the water is then measured at regular intervals. Which of the following is the most probable result? [AIPMT 1997]
(a) After sunrise, photosynthesis is carried out using \(\mathrm{CO}_{2}\). Hence concentration of \(\mathrm{CO}_{2}\) decreases increasing \(\mathrm{pH}\).
‘The law of limiting factors’ was proposed by [AIPMT 1996]
(c) Blackman (1905) gave the law of limiting factors which states that when a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest process. It is the factor which is present in the minimum amount.
What will be the number of Calvin cycles to generate one molecule of hexose? [AIPMT 1996]
(d) Carbon dioxide fixation occurs during the Calvin cycle, which occurs in three stages- carboxylation, reduction, and regeneration. One turn of the Calvin cycle fixes one molecule of carbon dioxide. Hence, to produce one molecule of hexose sugar, six turns of the Calvin cycle are required.
Photorespiration is favoured by [AIPMT 1996]
(c) Photorespiratory loss of \(\mathrm{CO}_{2}\) occurs when RuBisCo starts functioning as an oxygenase instead of carboxylase under conditions of high \(\mathrm{O}_{2}\) and low \(\mathrm{CO}_{2}\). It involves three organelles chloroplast, mitochondria, and peroxisomes. Half of the photosynthetically fixed carbon (in the form of RuBP) may be lost into the atmosphere through this process and no ATP formation occurs.
In \(\mathrm{C}_{4}\) plants, \(\mathrm{CO}_{2}\) combines with [AIPMT 1996]
(a) \(C_{4}\) pathway was first reported in members of family Gramineae (grasses) like sugarcane, maize, sorghum, etc. These \(C_{4}\) plants have a characteristic leaf anatomy called Kranz anatomy. So in chloroplasts of mesophyll cells – PEPCo (PEP carboylase) is the key enzyme used to fix \(\mathrm{CO}_{2}\) in \(\mathrm{C}_{4}\) plants Oxaloacetic acid is a \(4-\mathrm{C}\) compound and is the first stable product so this pathway is known as \(\mathrm{C}_{4}\) cycle.
In \(\mathrm{C}_{4}\) plants, \(\mathrm{CO}_{2}\) fixation is done by [AIPMT 1996]
(c) The \(\mathrm{C}_{4}\) plants have a characteristic leaf anatomy called kranz anatomy. Here two types of chloroplasts are present – bundle sheath chloroplasts and mesophyll chloroplasts. In \(\mathrm{C}_{4}\) plants, there are two carboxylation reactions which occur first in mesophyll chloroplasts and then in bundle sheath chloroplasts. \(\mathrm{CO}_{2}\) acceptor molecule in mesophyll chloroplasts is PEP (Phospho-enol pyruvate) and not Ribulose 1 , 5-biphosphate. Further it has enzyme PEPcarboxylase for initial \(\mathrm{CO}_{2}\) fixation. \(\mathrm{RuBP}-\) carboxylase is absent in mesophyll chloroplasts but is present in bundle sheath chloroplasts. The first product formed is oxaloacetic acid and hence it is known as \(\mathrm{C}_{4}\) cycle. Bundle sheath cells fix \(\mathrm{CO}_{2}\) through \(\mathrm{C}_{3}\) cycle.
Chlorophyll-a molecule at its carbon atom 3 of the pyrrole ring-ll has one of the following [CBSE AIPMT 1996]
(a) Chlorophyll has a tetrapyrrole porphyrin head and a long-chain alcohol called phytol tail. Each pyrrole is a 5-member ring with one nitrogen and four carbon. A non-ionic Mg atom lies in the centre of porphyrin, attached to nitrogen atoms of pyrrole rings. Chlorophyll-a has a methyl group at carbon 3 of the pyrrole ring and chlorophyll-b has a formyl (aldehyde) group attached to this atom.
Which one of the following is represented by Calvin cycle? [CBSE AIPMT 1996]
(a) In the dark phase or Calvin cycle, carbon dioxide is assimilated with the help of assimilatory power (ATP and \(\mathrm{NADPH}_{2}\) ) to produce organic acid. The cycle involves the reduction of carbon involving carboxylation, glycolytic reversal, and regeneration of \(\mathrm{RuBP}. \mathrm{C}_{3}\) cycle is also known as reductive pentose pathway or Photosynthetic Carbon Reduction (PCR).
The primary acceptor, during \(\mathrm{CO}_{2}\) fixation in \(\mathrm{C}_{3}\) plants, is [AIPMT 1995]
(b) The Calvin cycle is also known as \(\mathrm{C}_{3}\) cycle because \(\mathrm{CO}_{2}\), reduction is cyclic process and first stable product in this cycle is a \(3-\mathrm{C}\) compound (i.e., 3-phosphoglyceric acid or 3-PGA). In this cycle, \(\mathrm{CO}_{2}\) acceptor molecule is RuBP or RuDP (i.e., Ribulose 1,5 -biphosphate or Ribulose 1,5 -diphosphate). There occurs covalent bonding of \(\mathrm{CO}_{2}\), to RuBP and the enzyme catalyzing this reaction is RuBPcarboxylase/oxygenase (RuBisCO).
The \(\mathrm{CO}_{2}\) fixation during \(\mathrm{C}_{4}\) pathway occurs in the chloroplast of [AIPMT 1995]
(c) The Carbon fixation pathway in \(\mathrm{C}_{4}\) plants begins in mesophyll cells, where the \(\mathrm{CO}_{2}\) gets converted to bicarbonate, which is then added to the three-carbon acid phosphoenolpyruvate (PEP) by the action of enzyme phosphoenolpyruvate carboxylase. So, the correct answer is ‘Mesophyll cells’.
The specific characteristic of \(\mathrm{C}_{4}\)-plants is [AIPMT 1995]
(c) Anatomy of leaves of \(\mathrm{C}_{4}\) plant is called kranz anatomy. In \(\mathrm{C}_{4}\) plants, the mesophyll cells cluster around the bundle-sheath cell in a wreath formation (Kranz means ‘Wreath or ring). Also, the number of chloroplasts observed in bundle sheath cells is more than that in the mesophyll cell. This entire structure is densely packed and plays a major role in \(\mathrm{C}_{4}\) photosynthesis.
Which of the following pigments acts as a reaction-centre during photosynthesis? [AIPMT 1994]
(c) During photosynthesis a portion of light energy absorbed by chlorophyll and carotenoids is eventually stored as chemical energy via the formation of chemical bonds. This conversion of energy from one form to another is a complex process that depends on cooperation between many pigment molecules and a group of electron transfer proteins. The majority of pigments serve as an antenna complex, collecting light and transfusing energy to the reaction center complex. There are two photochemical complexes, known as photosystem I and II. PSII absorbs far-red light of wavelengths greater than \(680 \mathrm{~nm}\) and PSI absorbs red light of wavelengths greater than \(700 \mathrm{~nm}\). Both these complexes are involved in light reactions of photosynthesis.
During the light reaction of photosynthesis, which of the following phenomenon is observed during cyclic phosphorylation as well as non-cyclic phosphorylation? [AIPMT 1994]
(b) Photolysis of water, the release of oxygen, and the formation of NADPH take place during non-cyclic photophosphorylation, while ATP is synthesized in both the cyclic as well as non-cyclic photophosphorylation. In cyclic photophosphorylation, while passing between ferredoxin and plastoquinone and or over the cytochrome complex, the electron losses sufficient energy to form ATP from ADP and inorganic phosphate. In noncyclic photophosphorylation, while passing over the cytochrome complex, the electrons lose sufficient energy for the synthesis of ATP.
Nine-tenth of all photosynthesis of world ( \(85-90 \%\) ) is carried out by [CBSE AIPMT 1994]
(b) \(90 \%\) of total photosynthesis is carried out by aquatic plants, chiefly algae ( \(80 \%\) in oceans and \(10 \%\) in freshwater). \(10 \%\) of total photosynthesis is performed by land plants.
\(\mathrm{C}_{4}\)-cycle was discovered by [CBSE AIPMT 1994]
(a) \(C_{4}\) pathway or dicarboxylic acid pathway is an alternative path of \(\mathrm{CO}_{2}\)-fixation in photosynthesis. It was discovered by MD Hatch and CR Slack in 1967, also known as Hatch- Slack cycle.
Series of reactions which can convert fatty acids to sugars in plants but not in animals is [AIPMT 1994]
(b) Several plant structures e.g. pollen, and seeds contain fats. The fats are hydrolysed and consumed as a source of energy. This is accomplished through oxidative degradation called the glyoxylate cycle.
A photosynthesising plant is releasing \({ }^{18} \mathrm{O}\) more than the normal. The plant must have been supplied with [AIPMT 1993]
(b) Water molecule breaks up into hydrogen and oxygen in the illuminated chloroplasts. This is called photolysis of water. If a photosynthesising plant is releasing \({ }^{18} \mathrm{O}\) more than the normal, the plant must have been supplied with \(\mathrm{H}_{2} \mathrm{O}\) with \({ }^{18} \mathrm{O}\).
Maximum solar energy is trapped by [AIPMT 1993]
(c) Maximum solar energy is trapped by growing algae in tanks. The light spectrum of red and blue light are most effective in performing photosynthesis for growing algae.
The carbon dioxide acceptor in Calvin cycle/ \(\mathrm{C}_{3}\)-plants is [AIPMT 1993, 1995, 1996, 1999]
(b) In Calvin cycle, \(\mathrm{CO}_{2}\) is accepted by Ribulose – 1, 5 – diphosphate (RuDP) already present in the cells and a 6-carbon addition compound is formed which is unstable. It soon gets converted into 2 molecules of 3 -phosphoglyceric acid due to hydrolysis and dismutation. Phosphoenol pyruvate (PEP) and Phosphoglyceric acid (PGA) are formed in glycolysis.
Which one is a \(\mathrm{C}_{4}\)-plant? [AIPMT 1993]
(d) For a long time, Calvin cycle \(\left(\mathrm{C}_{3}\right.\) cycle) was considered to be the only photosynthetic pathway for reduction of \(\mathrm{CO}_{2}\), into carbohydrates. M.D. Hatch and Slack reported that a 4-C compound oxaloacetic acid \((O A A)\) is the first stable product in \(\mathrm{CO}_{2}\), reduction process. This led to an alternative pathway of \(\mathrm{CO}_{2}\), fixation which is known as Hatch and Slack’s cycle or \(\mathrm{C}_{4}\) cycle. It occurs in sugarcane, maize, etc. These \(\mathrm{C}_{4}\) plants have a characteristic leaf anatomy called Kranz anatomy.
Maximum solar energy is trapped by [CBSE AIPMT 1993]
(d) Maximum solar energy is trapped by growing grasses, as they have the largest surface area for absorption. Limited number of algal individuals are growing in the tank so, they absorb limited amount of light.
Chlorophyll \(a\) occurs in [AIPMT 1992]
(c) Chlorophyll-a occurs in all photosynthesizing plants except bacteria. Chlorophyll- \(a\) is the only one common to all organisms that possess chlorophyll (the only one in blue-green algae) and is believed to be specifically required. In a few photosynthetic bacteria other kinds of chlorophyll, bacteriochlorophylls, occur.
Photosystem II occurs in [AIPMT 1992]
(c) Photosystem II has almost equal number of chlorophyll \(a\) and chlorophyll \(b\) molecules. It is dark green in colour and located mostly in the appressed parts of grana thylakoids towards the inner surface of membranes.
The enzyme that catalyses carbon dioxide fixation in \(\mathrm{C}_{4}\) plants is [AIPMT 1992]
(b) \(C_{4}\) pathway was first reported in members of family Gramineae (grasses) like sugarcane, maize, sorghum, etc. These \(C_{4}\) plants have a characteristic leaf anatomy called Kranz anatomy. So in chloroplasts of mesophyll cells – PEPCo (PEP carboylase) is the key enzyme used to fix \(\mathrm{CO}_{2}\) in \(\mathrm{C}_{4}\) plants Oxaloacetic acid is a 4-C compound and is the first stable product so this pathway is known as \(\mathrm{C}_{4}\) cycle.
The first carbon dioxide acceptor in \(\mathrm{C}_{4}\)-plants is [AIPMT 1992, 1990]
(a) The primary acceptor of \(\mathrm{CO}_{2}\) is a 3-carbon compound phosphoenol pyruvic acid. Addition of \(\mathrm{CO}_{2}\) to any compound is called carboxylation. In \(\mathrm{C}_{4}\) cycle, \(\mathrm{CO}_{2}\) combines with phosphoenol pyruvic acid to form oxaloacetic acid, with the help of enzyme phosphoenol pyruvate carboxylase (PEPCo). The oxaloacetic acid breaks up into pyruvic acid and \(\mathrm{CO}_{2}\) which combines with RuBP to form PGA as in the Calvin cycle.
Formation of ATP in photosynthesis and respiration is an oxidation process which utilises the energy from [CBSE AIPMT 1992]
(c) Cytochromes (Keilin; 1925) are the electron transport intermediates containing heme (or related prosthetic groups) in which the iron undergoes valency changes during electron transfer and produces energy (ATP) in both photosynthesis and respiration.
At a temperature above \(35^{\circ} \mathrm{C}\) [CBSE AIPMT 1992]
(a) Optimum temperature for photosynthesis is \(10^{\circ}-25^{\circ} \mathrm{C}\) for \(\mathrm{C}_{3}\)-plants and \(30^{\circ}-45^{\circ} \mathrm{C}\) for \(\mathrm{C}_{4}\)-plants. Optimum temperature for respiration is
\(20^{\circ} \mathrm{C}-30^{\circ} \mathrm{C}\), i.e. respiration has a higher temperature optimum than photosynthesis and thus declines later.
All types of plastids possess essentially the same structure because they [AIPMT 1992]
(d) Depending upon their colour, plastids are of three main types – leucoplasts, chromoplasts, and chloroplasts.
Ferredoxin is a constituent of [AIPMT 1991]
(a) In photosystem-I, the ejected electron is trapped by FRS (ferrodoxin reducing substance) which is an unknown oxidation-reduction system. The electron is now transferred to a non-heme iron protein called ferrodoxin (Fd) from which the electron is transferred to \(\mathrm{NADP}^{+}\)intermediate protein electron carrier ferrodoxin-NADP reductase. So that \(\mathrm{NADP}^{+}\)is reduced to \(\mathrm{NADPH}+\mathrm{H}^{+}\).
During monsoon, the rice crop of eastern states of India shows lesser yield due to limiting factor of [AIPMT 1991]
(b) Rate of yield is dependent on light as photosynthesis is dependent on light. The maximum rate of photosynthesis occurs when light is brightest (high light intensity). But during monsoon, the light is dim (low light intensity) and so this reduces the rate of photosynthesis and hence yield.
Which technique has helped in investigation of Calvin cycle? [AIPMT 1991]
(c) By employing \({ }^{14} \mathrm{C}\) labelled carbon dioxide \({ }^{14} \mathrm{CO}_{2}\) in photosynthesis and observing the appearance of characteristic radiations in different reaction intermediates and product in different experiments. Calvin and his co-workers were able to formulate the complete metabolic path of carbon assimilation in the form of the cycle which is called as Calvin cycle.
Dark reactions of photosynthesis occur in [AIPMT 1991]
(c) The dark reactions of photosynthesis is purely enzymatic and slower than the primary photochemical reaction. It takes place in the 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.
Photosynthetic pigments found in the chloroplasts occur in [AIPMT 1991]
(a) Photosynthetic pigments are found in the thylakoid membrane of chloroplasts. The grana lamellae are paired to form sac-like structures and form thylakoids. Chlorophylls and other photosynthetic pigments are confined to grana.
Kranz anatomy is typical of [AIPMT 1990, 1995]
(a) Kranz anatomy is typical of \(C_{4}\) plants. It is found in leaves of \(C_{4}\) plants. The \(C_{4}\) pathway requires the presence of two types of cells i.e., mesophyll cells and bundle sheath cells. The particularly large cells around the vascular bundles of \(\mathrm{C}_{4}\) plants are called bundle sheath cells, these cells may form several layers around the vascular bundles, they are characterized by having a large number of chloroplasts, grana are absent, thick walls impervious to gaseous exchange and no extracellular spaces. This special anatomy of leaves of the \(\mathrm{C}_{4}\) plants is called Kranz anatomy. ‘Kranz’ means wreath and is a reflection of the arrangement of cells.
A very efficient converter of solar energy with net productivity of \(204 \mathrm{~kg} / \mathrm{m}^{2}\) or more is the crop [AIPMT 1989]
(b) A very efficient converter of solar energy with net productivity of \(2-4 \mathrm{~kg} / \mathrm{m}^{2}\) or more is sugarcane, maize which are \(\mathrm{C}_{4}\) plants. Others are \(\mathrm{C}_{3}\) plants.
In \(\mathrm{C}_{4}\) plants, Calvin cycle operates in [AIPMT 1989]
(a) In mesophyll cells, carbon dioxide is accepted by PEP and form oxaloacetic acid and further forms malic acid which enters the chloroplasts of bundle sheath cells and undergoes decarboxylation yielding pyruvic acid and \(\mathrm{CO}_{2}\). It is released in the stroma of bundle sheath cells to bind with RuBisCO and undergoes the Calvin cycle to form glucose.
The substrate for photorespiration is [AIPMT 1989]
(b) Photorespiration leads to loss of fixed carbon dioxide in \(\mathrm{C}_{3}\) plants by the enzyme RuBisCo. During photorespiration, these enzymes react with oxygen instead of carbon dioxide to ribulose 1,5 bisphosphate to produce phosphoglycolate as the first product. The phosphoglycolate is dephosphorylated to generate glycolate, which is the actual substrate of photorespiration.
The size of chlorophyll molecule is [AIPMT 1989, 1988]
(c) Chlorophyll are the magnesium porphyrin (head, \(15 \times 15 \AA\) ) compounds. The porphyrin ring consists of four pyrrole rings joined together by \(\mathrm{CH}\) bridges. A long chain of \(\mathrm{C}\) atoms called as phytol (tail; \(20 \AA\) ) chain is attached to porphyrin ring.
\(\mathrm{NADP}^{+}\)is reduced to NADPH in [AIPMT 1988]
(d) In photosynthesis during non-cyclic photophosphorylation involving both PS-I and PS-II, electrons released during photolysis of water are transfered to PS-II and then PS-I via a series of electron carriers. \(P_{700}\) of PS-I releases electrons after absorbing light energy. This electron passes through chlorophyll X, Fe-S, ferredoxin and finally to \(\mathrm{NADP}^{+}\). \(\mathrm{NADP}^{+}\)combines with \(\mathrm{H}^{+}\)(released during photolysis) with the help of NADP reductase to form NADPH.
\(
\mathrm{NADP}^{+}+2 \mathrm{e}^{-}+\mathrm{H}^{+} \stackrel{\begin{array}{l}
\text { NADP } \\
\text { reductase }
\end{array}}{\longrightarrow} \mathrm{NADPH}
\)
Carbon dioxide joins the photosynthetic pathway in [AIPMT 1988]
(d) The dark reactions or Blackman’s reactions or biosynthetic phase involves three steps: (i) Fixation of \(\mathrm{CO}_{2}\), (ii) Reduction of \(\mathrm{CO}_{2}\) and (iii) Synthesis of other compounds from glucose. It is independent of light. Pigment System I is relatively very weakly fluorescent while pigment system II is strongly fluorescent. Light energy absorbed by the pigment in the two systems is ultimately trapped by the \(P_{700}\) and \(P_{680^{\circ}}\) Light reaction is associated with the grana of the chloroplasts. It takes place in the presence of light only.
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