Practice Corner MCQs

Hint

(a) Photosynthesis is an enzyme regulated anabolic process which involves the synthesis of organic compounds, inside the chlorophyll containing cells, from \(\mathrm{CO}_2\) and \(\mathrm{H}_2 \mathrm{O}\) with the help of sunlight as a source of energy. Oxygen evolved during photosynthesis comes from water. The equation for photosynthesis can be summarised as follows:

\(
6 \mathrm{CO}_2+12 \mathrm{H}_2 \mathrm{O} \underset{\text { Radiant energy }}{\stackrel{\text { Chlorophyll }}{\longrightarrow}}
\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{H}_2 \mathrm{O}+6 \mathrm{O}_2 \uparrow
\)

Hint

(d) Photosynthesis is an enzyme regulated anabolic process which involves the synthesis of organic compounds, inside the chlorophyll containing cells, from \(\mathrm{CO}_2\) and \(\mathrm{H}_2 \mathrm{O}\) with the help of sunlight as a source of energy. Oxygen evolved during photosynthesis comes from water. The equation for photosynthesis can be summarised as follows:

\(
6 \mathrm{CO}_2+12 \mathrm{H}_2 \mathrm{O} \underset{\text { Radiant energy }}{\stackrel{\text { Chlorophyll }}{\longrightarrow}}
\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{H}_2 \mathrm{O}+6 \mathrm{O}_2 \uparrow
\)

Hint

b

Hint

(a) Joseph Priestley (1770) observed that a burning candle and mouse were kept together in a closed space – a bell jar, the candle soon gets extinguished. Similarly, the mouse soon get suffocated and die. However, when he placed a mint plant in the same bell jar, he found that the mouse stayed alive and candle continued to burn. Priestley hypothesised that foul air or phlogiston produced during burning of candles or animal (mice) respiration could be converted into pure air or dephlogiston by plants (mint). In 1774, Priestley discovered oxygen.

Hint

(b) Jan Ingenhousz, in his experiment on an aquatic plant, showed that in bright sunlight, small bubbles were formed around the green parts while in the dark, no such bubbles formed. He found out these bubbles to be of oxygen. He thus, confirmed that the purification of air or formation of dephlogiston is carried out by green plants only in the presence of sunlight.

Hint

(c) Jan Ingenhousz, in his experiment on an aquatic plant, showed that in bright sunlight, small bubbles were formed around the green parts while in the dark, no such bubbles formed. He found out these bubbles to be of oxygen. He thus, confirmed that purification of air or formation of dephlogiston is carried out by green plants only in presence of sunlight.

Hint

(a) Sachs provided the evidence that green plant parts produce glucose which is stored in the form of starch. Starch is the first visible product of photosynthesis.

Hint

(d) Engelmann split light into its spectral components by the prism and then illuminated Cladophora (a green alga) was placed in a suspension of aerobic bacteria. The bacteria were used to detect the sites of \(\mathrm{O}_2\) evolution. He found that bacteria accumulated in the region of blue and red light of the split spectrum. He thus, discovered the effect of different wavelengths of light on photosynthesis and plotted the first action spectrum of photosynthesis.

Hint

(c) van Niel, on the basis of his studies with purple and green sulphur bacteria, demonstrated that photosynthesis is a light dependent reaction in which hydrogen from an oxidisable compound reduces \(\mathrm{CO}_2\) to form sugar.
\(
2 \mathrm{H}_2 \mathrm{~A}+\mathrm{CO}_2 \stackrel{\text { Light }}{\longrightarrow} 2 \mathrm{~A}+\mathrm{CH}_2 \mathrm{O}+\mathrm{H}_2 \mathrm{O}
\)
In green sulphur bacteria, when \(\mathrm{H}_2 \mathrm{~S}\) instead of \(\mathrm{H}_2 \mathrm{O}\) was used as hydrogen donor, \(\mathrm{no} \mathrm{O}_2\) was evolved.
\(
6 \mathrm{CO}_2+12 \mathrm{H}_2 \mathrm{~S} \longrightarrow \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{H}_2 \mathrm{O}+12 \mathrm{~S}
\)
From the above observation, van Niel proposed that oxygenic photosynthesis of all organisms is actually an oxidation-reduction reaction involving the action of hydrogen of water over carbon dioxide to form organic compounds. He also propounded that oxygen evolved from water.

Hint

(d) Ruben and Kamen, while working on Chlorella (unicellular green alga) found that oxygen liberated during photosynthesis comes from water.

(i) When normal \(\mathrm{H}_2 \mathrm{O}\) and radioactive \(\mathrm{CO}_2\) (i.e., \(\mathrm{CO}_2^{18}\) ) were used, normal \(\mathrm{O}_2\) is evolved.
\(
n \mathrm{CO}_2^{18}+n \mathrm{H}_2 \mathrm{O} \frac{\text { Light }}{\text { Chlorophyll }}\left(\mathrm{CH}_2 \mathrm{O}^{18}\right)_n+n \mathrm{O}_2 \uparrow(\text { normal })+\mathrm{H}_2 \mathrm{O}
\)
(ii) When normal \(\mathrm{CO}_2\) and radioactive \(\mathrm{H}_2 \mathrm{O}\left(\right.\) i.e., \(\left.\mathrm{H}_2 \mathrm{O}^{18}\right)\) were used, radioactive \(\mathrm{O}_2\) (i.e., \(\mathrm{O}_2^{18}\) ) is evolved.
\(
\mathrm{nCO}_2+\mathrm{nH}_2 \mathrm{O}^{18} \longrightarrow\left(\mathrm{CH}_2 \mathrm{O}\right)_n+\mathrm{O}_2^{18} \uparrow \text { (radioactive oxygen) }+\mathrm{H}_2 \mathrm{O}
\)

Hint

(a) Ruben and Kamen, while working on Chlorella (unicellular green alga) found that oxygen liberated during photosynthesis comes from water.

(i) When normal \(\mathrm{H}_2 \mathrm{O}\) and radioactive \(\mathrm{CO}_2\) (i.e., \(\mathrm{CO}_2^{18}\) ) were used, normal \(\mathrm{O}_2\) is evolved.
\(
n \mathrm{CO}_2^{18}+n \mathrm{H}_2 \mathrm{O} \frac{\text { Light }}{\text { Chlorophyll }}\left(\mathrm{CH}_2 \mathrm{O}^{18}\right)_n+n \mathrm{O}_2 \uparrow(\text { normal })+\mathrm{H}_2 \mathrm{O}
\)
(ii) When normal \(\mathrm{CO}_2\) and radioactive \(\mathrm{H}_2 \mathrm{O}\left(\right.\) i.e., \(\left.\mathrm{H}_2 \mathrm{O}^{18}\right)\) were used, radioactive \(\mathrm{O}_2\) (i.e., \(\mathrm{O}_2^{18}\) ) is evolved.
\(
\mathrm{nCO}_2+\mathrm{nH}_2 \mathrm{O}^{18} \longrightarrow\left(\mathrm{CH}_2 \mathrm{O}\right)_n+\mathrm{O}_2^{18} \uparrow \text { (radioactive oxygen) }+\mathrm{H}_2 \mathrm{O}
\)

Hint

(a) During highlight intensity, the chloroplast align themselves in vertical position along the lateral walls. During moderate light intensity, they align themselves along tangential walls. They change their position in mesophyll cells to receive optimum light.

Hint

(b) Dark reaction or \(\mathrm{CO}_2\) fixation takes place in stroma, trapping of light and synthesis of ATP and NADPH takes place in thylakoids or membrane system.

Hint

(a) Paper chromatography is a technique used to separate pigments according to their relative solubility and molecular weight. A chromatographic separation of the leaf pigments shows that the colour that we see in leaves is not due to a single pigment but due to four pigments-Chlorophyll a (appears bright or blue green in the chromatogram), chlorophyll \(\boldsymbol{b}\) (yellow green), xanthophylls (yellow) and carotenoids (yellow to yellow-orange).

Hint

(a) Paper chromatography is a technique used to separate pigments according to their relative solubility and molecular weight. A chromatographic separation of the leaf pigments shows that the colour that we see in leaves is not due to a single pigment but due to four pigments-Chlorophyll a (appears bright or blue green in the chromatogram), chlorophyll \(\boldsymbol{b}\) (yellow green), xanthophylls (yellow) and carotenoids (yellow to yellow-orange).

Hint

(b) Chlorophyll a is the primary photosynthetic pigment, which is bluish green in the pure state. It has an empirical formula of \(\mathrm{C}_{55} \mathrm{H}_{72} \mathrm{O}_5 \mathrm{~N}_4 \mathrm{Mg}\) and molecular weight of 893 . It absorbs more red wavelength of light than violet-blue. It is soluble in a number of organic solvents such as petroleum ether.

Hint

(b) Carotenes are hydrocarbons with a general formula of \(\mathrm{C}_{40} \mathrm{H}_{56}\). Red colour of tomato, carrots and chillies is due to carotene called lycopene. The most common carotene is \(\beta\)-carotene. It is converted to vitamin \(\mathrm{A}\) by animals and human beings.
\(
\begin{aligned}
& \mathrm{C}_{40} \mathrm{H}_{56}+2 \mathrm{H}_2 \mathrm{O} \longrightarrow 2 \mathrm{C}_{19} \mathrm{H}_{27} \mathrm{CH}_2 \mathrm{OH} \\
& \beta \text {-carotene } \quad \quad \quad \quad \text { Vitamin A } \\
\end{aligned}
\)

Hint

(a) Xanthophylls (=Carotenols) are oxygen containing derivatives of carotenes, e.g., \(\mathrm{C}_{40} \mathrm{H}_{56} \mathrm{O}\) (cryptoxanthin), \(\mathrm{C}_{40} \mathrm{H}_{56} \mathrm{O}_2\) (lutein, zeaxanthin). Yellowish colour of autumnal foliage is due to lutein. The characteristic xanthophyll of brown algae is fucoxanthin \(\left(\mathrm{C}_{40} \mathrm{H}_{56} \mathrm{O}_6\right)\).

Hint

(d) Phycobilins are open tetrapyrroles which neither contain magnesium nor phytol. Phycobilins are water-soluble. The pigments are of two types-blue (phycocyanin, allophycocyanin) and red (phycoerythrin). The pigments are useful in chromatic adaptations. They are important accessory pigments of blue-green algae, cryptomonads and red algae.

Hint

(c) Light is the visible part of electromagnetic radiations. Visible light consists of radiations having a wavelength between \(390-760 \mathrm{~nm}\) (or \(3900-7600 \) )Å. It can be resolved into light of different colours – violet \((390-430 \mathrm{~nm})\), blue or indigo \((430-470 \mathrm{~nm})\), blue-green or blue \((470-500 \mathrm{~nm})\), green \((500-580 \mathrm{~nm})\), yellow \((580-600 \mathrm{~nm})\), orange \((600-650 \mathrm{~nm})\), orange-red \((650-660 \mathrm{~nm})\) and red \((660\) \(760 \mathrm{~nm}\) ).

Hint

(a) Light is the visible part of electromagnetic radiations. Visible light consists of radiations having a wavelength between \(390-760 \mathrm{~nm}\) (or \(3900-7600 \) )Å. It can be resolved into light of different colours – violet \((390-430 \mathrm{~nm})\), blue or indigo \((430-470 \mathrm{~nm})\), blue-green or blue \((470-500 \mathrm{~nm})\), green \((500-580 \mathrm{~nm})\), yellow \((580-600 \mathrm{~nm})\), orange \((600-650 \mathrm{~nm})\), orange-red \((650-660 \mathrm{~nm})\) and red \((660\) \(760 \mathrm{~nm}\) ).

Hint

(b) The absorption spectrum of chl a shows maximum absorption in blue and red regions of visible light spectrum.

Hint

(d) The graphical curve showing the amount of energy of different wavelengths of light absorbed by a pigment is called absorption spectrum. It is studied with the help of spectrophotometer. The absorption spectra of chlorophylls \(a\) and \(b\) show that they absorb maximum light in the blue-violet and red wavelengths. The pigments are often known after the wavelength which is absorbed to the maximum, e.g., \(\mathrm{Chl} a_{673}, \mathrm{Chl} a_{683}\left(\mathrm{~P}_{680}\right), \mathrm{Chl} a_{703}\left(\mathrm{~P}_{700}\right)\).

Hint

d

Hint

(d) Chlorophyll \(a\) is found in almost all photosynthetic plants except bacteria. It is called primary photosynthetic pigment because it performs primary reactions of photosynthesis which involve conversion of light energy into chemical energy. Other photosynthetic pigments like chlorophyll \(b\), xanthophylls and carotenes are called accessory pigments. 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 and also protect chlorophyll \(a\) from photo-oxidation.

Hint

c

Hint

(c) Photochemical phase, also called light or Hill reaction, occurs inside the thylakoids, especially those of grana region. Photochemical step is dependent upon light. The function of this phase is to produce assimilatory power consisting of reduced coenzyme NADPH and energy rich ATP molecules.

Hint

(d) The complete process of photosynthesis is studied under two phases – photochemical phase and biosynthetic phase. Photochemical phase involves absorption of light energy, photolysis of water with the release of \(\mathrm{O}_2\) and production of assimilatory power (i.e., ATP + NADPH). \(\mathrm{CO}_2\) fixation occurs during biosynthetic phase of photosynthesis.

Hint

b

Hint

(b) The reaction center of photosystem I (PS I) is referred to as \(P_{700}\), where “P” stands for “Pigment” and ” 700 ” stands for the wavelength of light that this particular chlorophyll molecule absorbs most strongly. The reaction center of photosystem II (PS II) is referred to as \(P_{680}\) for comparable reasons.

Hint

(a) The reaction center of photosystem I (PS I) is referred to as \(P_{700}\), where “P” stands for “Pigment” and ” 700 ” stands for the wavelength of light that this particular chlorophyll molecule absorbs most strongly. The reaction center of photosystem II (PS II) is referred to as \(P_{680}\) for comparable reasons.

Hint

d

Hint

(d) During Z-scheme, the electron extruded by PS I passes through special chlorophyll \(X\), Fe-S, ferredoxin, to finally reach NADP+. \(\mathrm{NADP}^{+}\)is ultimately reduced by combining with \(\mathrm{H}^{+}\)(released during photolysis) with the help of NADP+ reductase enzyme.

Hint

(d) In non-cyclic photophosphorylation, the whole scheme of transfer of electrons, starting from the PS II, uphill to the acceptor, down the electron transport chain to PS I, excitation of electrons, transfer to another acceptor, and finally downhill to NADP+ causing it to be reduced to \(\mathrm{NADPH}+\mathrm{H}^{+}\)is called as the \(\mathrm{Z}\) scheme, due to its characteristic shape. This shape is formed when all the carriers are placed in a sequence on the redox potential scale.

Hint

(c) NADPH is formed during non-cyclic photophosphorylation wherein both PS I and PS II are involved.

Hint

(d) In non-cyclic photophosphorylation, the whole scheme of transfer of electrons, starting from the PS II, uphill to the acceptor, down the electron transport chain to PS I, excitation of electrons, transfer to another acceptor, and finally downhill to NADP+ causing it to be reduced to \(\mathrm{NADPH}+\mathrm{H}^{+}\)is called as the \(\mathrm{Z}\) scheme, due to its characteristic shape. This shape is formed when all the carriers are placed in a sequence on the redox potential scale.

Hint

a

Hint

c

Hint

(b) The splitting of water is associated with the PS II. Water split into \(\mathrm{H}^{+},[\mathrm{O}]\) and electrons. The electrons needed to replace those removed from photosystem I are provided by photosystem II.
\(
2 \mathrm{H}_2 \mathrm{O} \longrightarrow 4 \mathrm{H}^{+}+\mathrm{O}_2+4 e^{-}
\)
Water splitting complex is associated with the PSII, which itself is physically located on the inner side of the membrane of the thylakoid. The liberation of \(\mathrm{O}_2\) during photolysis of water involves the role of \(\mathrm{Mn}^{2+}, \mathrm{Ca}^{2+}\) and \(\mathrm{Cl}^2\) ions.

Hint

(a) The splitting of water is associated with the PS II. Water split into \(\mathrm{H}^{+},[\mathrm{O}]\) and electrons. The electrons needed to replace those removed from photosystem I are provided by photosystem II.
\(
2 \mathrm{H}_2 \mathrm{O} \longrightarrow 4 \mathrm{H}^{+}+\mathrm{O}_2+4 e^{-}
\)
Water splitting complex is associated with the PSII, which itself is physically located on the inner side of the membrane of the thylakoid. The liberation of \(\mathrm{O}_2\) during photolysis of water involves the role of \(\mathrm{Mn}^{2+}, \mathrm{Ca}^{2+}\) and \(\mathrm{Cl}^2\) ions.

Hint

(d) The splitting of water is associated with the PS II. Water split into \(\mathrm{H}^{+},[\mathrm{O}]\) and electrons. The electrons needed to replace those removed from photosystem I are provided by photosystem II.
\(
2 \mathrm{H}_2 \mathrm{O} \longrightarrow 4 \mathrm{H}^{+}+\mathrm{O}_2+4 e^{-}
\)
Water splitting complex is associated with the PSII, which itself is physically located on the inner side of the membrane of the thylakoid. The liberation of \(\mathrm{O}_2\) during photolysis of water involves the role of \(\mathrm{Mn}^{2+}, \mathrm{Ca}^{2+}\) and \(\mathrm{Cl}^2\) ions.

Hint

(b) The process by which ATP is synthesised by cells (in mitochondria and chloroplasts) is named as phosphorylation. Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate \((P i)\) in the presence of light, inside chloroplast. Photophosphorylation is of two main types – Cyclic and non-cyclic photophosphorylation.

Hint

(b) The given diagram represents – cyclic photophosphorylation that involves only PS I.

Hint

(a) PS II is present on innerside (appressed part) of thylakoid membrane.

Hint

(c) PSI is involved in both cyclic and non-cyclic photophosphorylation. PS II is involved only in non-cyclic photophosphorylation. PS II is present in the appressed (inner) part of grana thylakoids. PSI is located in the non- appressed (outer) part of grana thylakoids as well as stroma thylakoids.

Hint

b

Hint

(c) In cyclic photophosphorylation, an electron expelled by the excited photocentre \(\left(P_{700}\right)\) is returned to it after passing through a series of electron carriers. Its photocentre \(P_{700}\) extrudes an electron with a gain of \(23 \mathrm{kcal} / \mathrm{mole}\) of energy after absorbing a photon of light (hv). After losing the electron the photocentre becomes oxidised. The expelled electron passes through a series of carriers including \(X\) or \(A_0\) (a special \(P_{700}\) chlorophyll molecule), \(A_1\) (a quinone), Fes complexes \(\left(\mathrm{FeS}_{\mathrm{X}}, \mathrm{FeS}_{\mathrm{A}}, \mathrm{FeS}_{\mathrm{B}}\right)\), ferredoxin \((\mathrm{Fd})\), plastoquinone \((\mathrm{PQ})\), cytochrome b-f complex and plastocyanin (PC) before returning to photo centre.

Hint

b

Hint

(a)  In photosynthesis and respiration, ATP synthesis is linked to development of proton gradient across a membrane. The only difference is that in photosynthesis proton accumulation is towards the lumen of thylakoid and in respiration, proton accumulate in the intermembrane space of the mitochondria.

Hint

b

Hint

(a) In the given figure, the ATP synthesis through chemiosmosis is shown. The electron transport chain forms a proton gradient across the inner mitochondrial membrane, which drives the synthesis of ATP through chemosmosis. Here A represents the thylakoid lumen. The ATP synthase consists of two functional domain: \(\mathrm{CF}_1\) situated in mitochondrial matrix and \(\mathrm{CF}_0\) located to the inner mitochondrial membrane. \(B\) represents \(C_0\) and \(C\) represents \(C F_1\).

Hint

c

Hint

c

Hint

(c) For fixation of one sugar molecule, \(6 \mathrm{CO}_2\) molecules are required and for each \(\mathrm{CO}_2\) molecule, one Calvin cycle is run. Therefore 6 Calvin cycles are required for fixation of one molecule of sugar and 12 Calvin cycles are required for fixation of two molecules of sugar.

Hint

(c) Biosynthetic phase (Dark or Blackman’s reaction) catalyses the assimilation of \(\mathrm{CO}_2\) to carbohydrates. These reactions occur in stroma or matrix of chloroplasts and all the enzymes required for the processes are present in the stroma/matrix of chloroplasts. These reactions do not require light, instead assimilatory power (ATP and NADPH) produced during photochemical (light) phase is used in fixation and reduction of \(\mathrm{CO}_2\). However, this should not be construed to mean that they occur in darkness or that they are not light dependent.

Hint

(d) Light reactions are also called as photochemical phase whereas dark reactions are called as biosynthetic phase.

Hint

c

Hint

c

Hint

(b) Light reactions (or photochemical phase) of photosynthesis mainly occur on the grana thylakoids. Dark reactions (or biosynthetic phase) which involve synthesis of carbohydrates by \(\mathrm{CO}_2\) fixation, occur in the stroma (or matrix) of chloroplasts. The chloroplast matrix of higher plants stores starch temporarily in the form of starch granules.

Hint

(b)

Hint

(a) Carboxylation is the fixation of \(\mathrm{CO}_2\) into a stable organic intermediate. It is the most crucial step of Calvin cycle where \(\mathrm{CO}_2\) is utilised for the carboxylation of RuBP through the use of ATP and NADPH generated by the light reactions. This reaction is catalysed by enzyme RuBis \(\mathrm{CO}\) which results in the formation of two molecules of 3-phosphoglyceric acid or PGA, which is the first stable product of photosynthesis.

Hint

c

Hint

(a) Carboxylation is the fixation of \(\mathrm{CO}_2\) into a stable organic intermediate. It is the most crucial step of Calvin cycle where \(\mathrm{CO}_2\) is utilised for the carboxylation of RuBP through the use of ATP and NADPH generated by the light reactions. This reaction is catalysed by enzyme RuBis \(\mathrm{CO}\) which results in the formation of two molecules of 3-phosphoglyceric acid or PGA, which is the first stable product of photosynthesis.

Hint

(c) The enzyme RuBisCO carries out photosynthetic carboxylation of RuBP. The enzyme was previously called as carboxydismutase. RuBisCO is the most abundant protein of the biological world. RuBisCO is located in the stroma on the outer surface of thylakoid membranes. As it is capable of carrying out both carboxylation and oxygenation reactions in \(C_3\) and \(C_4\) plants respectively, thus it is correctly named as RuBP carboxylaseoxygenase.

Hint

(c) During reduction stage of \(C_3\) cycle, a series of reactions lead to the formation of glucose. The steps involve utilisation of two molecules of ATP for phosphorylation and two of NADPH for reduction per \(\mathrm{CO}_2\) molecule fixed. The fixation of six molecules of \(\mathrm{CO}_2\) and 6 turns of the cycle are required for the removal of one molecule of glucose from the pathway.

Hint

(a) For every \(\mathrm{CO}_2\) molecule entering the Calvin cycle, 3 molecules of ATP and 2 molecules of NADPH are required. To make one molecule of glucose, 6 turns of the cycles are required \((6 \times 3\) ATP \(=18[latex] ATP and [latex]6 \times 2 \mathrm{NADPH}=12 \mathrm{NADPH})\).

Hint

(c) For every \(\mathrm{CO}_2\) molecule entering the Calvin cycle, 3 molecules of ATP and 2 molecules of NADPH are required. To make one molecule of glucose, 6 turns of the cycles are required \((6 \times 3\) ATP \(=18[latex] ATP and [latex]6 \times 2 \mathrm{NADPH}=12 \mathrm{NADPH})\).

Hint

(c) Six molecules of \(\mathrm{CO}_2\) are required to synthesise one molecule of glucoses, each of which undergoes a \(\mathrm{C}_3\) cycle. Hence, 6 molecules of \(\mathrm{CO}_2\) synthesise a glucose molecule after 6 turns of Calvin cycle.

Hint

(c) \(\ln C_3\) plants, bundle sheath cells usually do not contain chloroplasts and the whole \(C_3\) cycle operates in mesophyll cells of leaves.

Hint

(d) Kranz anatomy is shown by \(\mathrm{C}_4\) plants such as Sorghum, sugarcane, maize, Cyperus rotundus, etc.

Hint

(d) Kranz anatomy is a characteristic of \(\mathrm{C}_4\) plants. 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. The chloroplasts of mesophyll cells are smaller, they have well-developed grana and a peripheral reticulum but no starch.

Hint

b

Hint

(b) Mesophyll cells in \(C_4\) plants are specialised to perform light reaction, evolve \(\mathrm{O}_2\) and produce assimilatory power. They possess enzyme PEPCase for initial fixation of \(\mathrm{CO}_2\).

Hint

(b) In \(C_4\) plants, Calvin cycle enzymes are present in chloroplasts of bundle sheath cells.

Hint

(a) In \(C_4\) plants showing Hatch-Slack cycle, initial fixation of \(\mathrm{CO}_2\) occurs in mesophyll cells followed by final fixation in bundle sheath cells.

Hint

d

Hint

c

Hint

(a) In \(\mathrm{C}_4\) plants, the first stable photosynthetic product is a 4-carbon compound, i.e., oxaloacetic acid (OAA), which is formed by initial fixation of \(\mathrm{CO}_2\) by the carboxylation of phosphoenol pyruvate in the mesophyll cells. So, when radioactive \({ }^{14} \mathrm{CO}_2\) is introduced into the reaction, it is first incorporated into the OAA.
\(
\text { PEP }+{ }^{14} \mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \stackrel{\text { PEP carboxylase }}{\longrightarrow} \mathrm{OAA}+\mathrm{H}_3 \mathrm{PO}_4
\)
In a CAM plant, although the first product formed is OAA, but here the initial \(\mathrm{CO}_2\) fixation occurs at night.

Hint

(a) Both malic acid and aspartic acid are 4 -carbon compounds formed in the mesophyll cells during \(\mathrm{C}_4\) cycle. PGA (3-phosphoglyceric acid) is a 3-carbon compound, which is the first stable product of Calvin cycle \(\left(\mathrm{C}_3\right.\) cycle). RuBP (Ribulose-1,5-biphosphate) is a 5-carbon compound, which is the primary acceptor of \(\mathrm{CO}_2\) during \(\mathrm{C}_3\) cycle.

Hint

a

Hint

(a) Crassulacean acid metabolism is a mechanism of photosynthesis involving double fixation of \(\mathrm{CO}_2\) which occurs in succulents belonging to Crassulaceae, Cacti, Euphorbiaceae and some other plants of dry habitats where the stomata remain closed during the daytime and open only at night, e.g., Sedum, Kalanchoe, Opuntia. Sunken stomata are deep-seated stomata in which subsidiary cells lie above the guard cells.

Hint

c

Hint

a

Hint

(a) At mild temperature, RuBis \(\mathrm{CO}\) has high affinity for \(\mathrm{CO}_2\) that is about 80 times higher than its affinity for oxygen. In addition, RuBis \(\mathrm{CO}\) has higher affinity for \(\mathrm{O}_2\) when temperature increases.

Hint

(c) At high temperature, RuBisCO functions as oxygenase and instead of fixing \(\mathrm{CO}_2\), oxidises RuBP to produce a \(3-\mathrm{C}\) phosphoglyceric acid and a 2-C phosphoglycolate. This is the first reaction of photorespiration or \(C_2\) cycle.

Hint

(a) Photorespiration (Photosynthetic carbon oxidative or PCO cycle) is the light dependent process of oxygenation of ribulose-1, 5-biphosphate (RuBP) and release of \(\mathrm{CO}_2\) by the photosynthetic organs of a plant. Under normal conditions, in Calvin cycle, carboxylation of RuBP takes place whereas during photorespiration instead of carboxylation, oxygenation of RuBP takes place. This is due to abnormal behaviour of enzyme RuBisCO, which at high temperature (more than \(35^{\circ} \mathrm{C}\) ), functions as oxygenase (instead of carboxylase). Instead of fixing \(\mathrm{CO}_2\), it performs oxygenation of RuBP to produce a 3-carbon phosphoglyceric acid (PGA) and a 2-carbon phosphoglycolate. It is the first reaction of photorespiration and can be represented as :
\(
\text { RuBP }+\mathrm{O}_2 \underset{\text { oxygenase }}{\stackrel{\text { RuBP }}{\longrightarrow}} \mathrm{PGA}+\text { Phosphoglycolate }
\)
In this way, photorespiration interferes with the successful functioning of Calvin cycle by causing oxygenation of RuBP instead of carboxylation.

Hint

(d) Photorespiratory pathway (or \(C_2\) pathway) results in the release of \(\mathrm{CO}_2\) with the utilisation of ATP. In the photorespiratory pathway, there is no synthesis of ATP or NADPH.

Hint

(b) Photorespiratory pathway (or \(C_2\) pathway) results in the release of \(\mathrm{CO}_2\) with the utilisation of ATP. In the photorespiratory pathway, there is no synthesis of ATP or NADPH.

Hint

(d) The site for photorespiration is chloroplast, peroxisome and mitochondrion.

Hint

(a) Blackman’s (1905) law of limiting factors states that if a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value; it is the factor which directly affects the process if its quantity is changed.

Hint

(d) A limiting factor is defined as a factor which is deficient to such an extent that increase in its magnitude directly increases the rate of the process.
In the given graph, rate of photosynthesis initially increases with an increase in light intensity (region A) but soon it levelled off. Thus, initially light intensity was limiting the rate of photosynthesis. However, when light intensity was present in sufficient amounts (region C), rate of photosynthesis did not increase further. This is due to the fact that in region \(\mathrm{C}\), some other factor (e.g. \(\mathrm{CO}_2\) concentration) became the limiting factor. At this region, the rate of photosynthesis could be further enhanced only by the increase in availability of other limiting factor \(\left(e, g . \mathrm{CO}_2\right)\).

Hint

c

Hint

(b) \(\mathrm{CO}_2\) compensation point or threshold value is that concentration of \(\mathrm{CO}_2\) at which illuminated plant parts stop absorbing carbon dioxide from their environment. At this value, \(\mathrm{CO}_2\) fixed in photosynthesis is equal to \(\mathrm{CO}_2\) evolved in respiration and photorespiration. The value is \(25-100 \mathrm{ppm}\) in \(\mathrm{C}_3\) plants and \(0-10\) ppm in \(\mathrm{C}_4\) plants. The reason for low compensation value for \(\mathrm{C}_4\) plants is the greater efficiency of \(\mathrm{CO}_2\)-fixation through PEP-carboxylase. The optimum \(\mathrm{CO}_2\) concentration for \(\mathrm{C}_4\) plants is \(360 \mathrm{ppm}\) and for \(\mathrm{C}_3\) plants, it is \(450 \mathrm{ppm}\).

Hint

(a) \(\mathrm{CO}_2\) compensation point or threshold value is that concentration of \(\mathrm{CO}_2\) at which illuminated plant parts stop absorbing carbon dioxide from their environment. At this value, \(\mathrm{CO}_2\) fixed in photosynthesis is equal to \(\mathrm{CO}_2\) evolved in respiration and photorespiration. The value is \(25-100 \mathrm{ppm}\) in \(\mathrm{C}_3\) plants and \(0-10\) ppm in \(\mathrm{C}_4\) plants. The reason for low compensation value for \(\mathrm{C}_4\) plants is the greater efficiency of \(\mathrm{CO}_2\)-fixation through PEP-carboxylase. The optimum \(\mathrm{CO}_2\) concentration for \(\mathrm{C}_4\) plants is \(360 \mathrm{ppm}\) and for \(\mathrm{C}_3\) plants, it is \(450 \mathrm{ppm}\).

Hint

(b) Rate of photosynthesis usually increases with an increase in temperature upto \(40^{\circ} \mathrm{C}\). Above this temperature, rate of photosynthesis starts decreasing due to inactivation of enzymes. The minimum temperature at which most plants start photosynthesis is \(0^{\circ}\) \(-5^{\circ} \mathrm{C}\) but it can be as low as \(-20^{\circ} \mathrm{C}\) for lichens and \(-35^{\circ} \mathrm{C}\) for some gymnosperms. The maximum temperature at which photosynthesis can occur is \(55^{\circ} \mathrm{C}\) in some desert plants and \(75^{\circ} \mathrm{C}\) for hot spring algae. When temperature is increased from minimum to optimum, the rate of photosynthesis doubles for every \(10^{\circ} \mathrm{C}\) rise in temperature.

Hint

(c) The \(C_4\) plants respond to higher temperatures and show higher rate of photosynthesis while \(C_3\) plants have a much lower temperature optimum. The temperature optimum for photosynthesis of different plants depends on the habitat that they are adapted to. Tropical plants have a higher temperature optimum than the plants adapted to temperate climates as tropical areas have higher temperature as compared to temperate areas.

Hint

(a) Oxygen is a product of photosynthesis. A small quantity of \(\mathrm{O}_2\) is essential for photosynthesis to take place. But as \(\mathrm{O}_2\) concentration rises, rate of photosynthesis decreases, which may be because (i) oxygen takes part in oxidation of photosynthetic pigments, intermediates and enzymes in the presence of strong light (photo-oxidation), (ii) oxygen is a strong quencher of excited state of chlorophyll. Oxygen competes with \(\mathrm{CO}_2\) for reducing power. It converts RuBP-carboxylase to RuBP-oxygenase. At very high oxygen concentration, the rate of photosynthesis begins to decline in all plants. This phenomenon is referred to as Warburg effect.

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(c) Water indirectly affects the process of photosynthesis by causing water stress due to its unavailability. It causes closing of stomata which further results in \(\mathrm{CO}_2\) unavailability and hence affecting photosynthesis.

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(d) The process of photosynthesis is influenced by several factors, which include both internal (plant) and external (environmental) factors. The plant factors include the number, size, age and orientation of leaves, presence or absence of hormones, leaf anatomy, amount of chlorophyll, etc. The external factors would include the availability of sunlight, temperature, concentration of \(\mathrm{CO}_2\) and \(\mathrm{O}_2\) and water.

Hint

b

Hint

(b) Glyceraldehyde-3-phosphate (G3P) or 3-phosphoglyceraldehyde (PGAL), also known as triose phosphate, is a chemical compound that occurs as an intermediate in several central metabolic pathways of all organisms. It is an intermediate in photosynthesis. A part of triose phosphate is translocated to cytosol from chloroplast and is utilised for the synthesis of glucose, sucrose, starch, etc.

Hint

(c) The light intensity at which a plant can achieve maximum rate of photosynthesis is called as light saturation point. Its value is \(50-70 \%\) of full sunlight in \(C_3\) sun plants and upto \(200 \%\) of full sunlight in \(\mathrm{C}_4\) sun plants (e.g., sugarcane). Beyond saturation point (it is seldom realised in nature in \(C_4\) sun plants), the rate of photosynthesis begins to decline. Optimum temperature at which the rate of photosynthesis is maximum is \(10^{\circ} \mathrm{C}-25^{\circ} \mathrm{C}\) for \(\mathrm{C}_3\) plants and \(30-45^{\circ} \mathrm{C}\) for \(\mathrm{C}_4[latex] plants. Thus, it is clear that in the given graph, plant [latex]L\) is a \(C_4\) plant and plant \(M\) is a \(C_3\) plant.

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(d) The given cross section of the leaf represents kranz anatomy in a \(\mathrm{C}_4\) plant. \(P\) are the chloroplasts of mesophyll cells, which are granal (i.e, thylakoids are stacked to form grana) and are specialised to perform light reaction of photosynthesis, evolve \(\mathrm{O}_2\) and produce assimilatory power (ATP + NADPH). Q are the chloroplasts of bundle sheath cells which are agranal, i.e., grana are absent and the thylakoids occur as stroma lamellae. In these chloroplasts, final fixation of \(\mathrm{CO}_2\) takes place through Calvin cycle where \(\mathrm{CO}_2\) is fixed by RuBP in the presence of enzyme RuBisCO.

Hint

(a) Wheat is a \(C_3\) plant and sugarcane is a \(C_4\) plant. In a \(C_3\) plant, first product of photosynthesis is 3-phosphoglycerate whereas in a \(C_4\) plant, first product of photosynthesis is a \(4-C\) organic acid i.e., oxaloacetic acid which is reduced to malic acid (malate) or transaminated to form aspartate. In sugarcane, after exposure to \({ }^{14} \mathrm{CO}_2\) for approximately one second, more than \(94 \%\) of fixed radioactivity is found to be present in malate, aspartate and oxaloacetate.

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(a) Carboxylation is the most crucial step of Calvin cycle where \(\mathrm{CO}_2\) is utilised for carboxylation of RuBP. This reaction is catalysed by enzyme RuBP carboxylase which results in the formation of 2 molecules of \(3-P G A\).

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(d) The rate of photosynthesis is more in intense light than diffused light. The light intensity is more in heliophytes (sun loving plants) and less in sciophytes (shade loving plants). For a plant, rate of photosynthesis increases with increase in light intensity except very high light intensity, which causes solarisation.

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(b) At N, rate of photosynthesis in increasing with increasing light intensity so light is limiting the rate of photosynthesis. After \(\mathrm{N}\), i.e., at M rate of photosynthesis does not increase further even on increasing the light intensity. It implies that some other factor like \(\mathrm{CO}_2\) is now limiting the rate of photosynthesis.