Question

The first stable product of $\mathrm{CO}_2$ fixation in Sorghum is [NEET 2021]

(a) pyruvic acid

(b) oxaloacetic acid

(c) succinic acid

(d) phosphoglyceric acid

Answer/Explanation

Ans. (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 $\mathrm{C}_4$ plants. Sugarcane, maize, Sorghum, etc. are the examples of these plants.

Question

 Which of the following statements is incorrect? [NEET (Oct.) 2020]

(a) RuBisCO is a bifunctional enzyme

(b) In $\mathrm{C}_4$ plants the site of RuBisCO activity is mesophyll cell

(c) The substrate molecule for RuBisCO activity is a 5 -carbon compound

(d) RuBisCO action requires ATP and NADPH

Answer/Explanation

Ans. (b)

Statement (b) is incorrect and can be corrected as in C, plants, Kranz anatomy in leat is found due to the presence of two type of cells viz., mesophyll cells and bundle sheath cells.
The mesophyll cells are specialised to perform light reaction, evolveO ${ }_2$ and produce assimilatory power. The bundle sheath cells possess RuBisCO and thus, perform RuBisC0 activity at this site.

Question

 The oxygenation activity of RuBisCO enzyme in photorespiration leads to the formation of [NEET (Sep.) 2020]

(a) 1 molecule of 3-C compound

(b) 1 molecule of 6-C compound

(c) 1 molecule of 4-C compound and 1 . molecule of $2-\mathrm{C}$ compound

(d) 2 molecules of $3-C$ compound

Answer/Explanation

Ans. (a)

The coxygenation activity of RuBisCO enzyme in photorespiration leads to the formation of 1 molecule of $3 C$ compound (phosphoglycerate). In $\mathrm{C}_s$ plants during oxygen fixation, one molecule ot PGA3Cland one molecule of 2-phosphoglycolate(2C) are formed. The latter is then converted back to PGA in the photorespiratory cycle. Photorespiration occurs at high concentration of oxygen and temperature in the environment.

Question

 In Hatch and Slack pathway, the primary $\mathrm{CO}_2$ acceptor is [NEET (Odisha) 2019]
(a)oxaloacetic acid

(b) phosphoglyceric acid

(c) phosphoenol pyruvate

(d) RuBisCo

Answer/Explanation

Ans. (c)

In Hatch and Slack pathway, the primary COn acceptor is phosphoenol pyruvate. This occurs in C–plants. Phosphoenol pyruvate, a 3-carbon compound, accepts $\mathrm{CO}_2$ and forms oxaloacetic acid which is a 4-carbon compound.

Question

 Phosphoenol Pyruvate $(\mathrm{PEP})$ is the primary $\mathrm{CO}_2$ acceptor in [NEET 2017]
$(a) C_3$-plants $

(b) C_4$-plants

(c) $C_2$-plants $

(d) C_2$ and $C_4$-plants

Answer/Explanation

Ans. (b) 
Phosphoenol Pyruvate (PEP) is found in the mesophyll cell, which accepts the atmospheric $\mathrm{CO}_2$ in $\mathrm{C}_4$-plants and converts it to oxalo acetate $-\mathrm{aC}_i$ compound. It is the first stable compound of $\mathrm{C}_{+}$-plants. Concept Enhancer $\mathrm{C}_4$-plants possess special adaptation anatomy in their leaves to cope up the photorespiratory losses. There are dimorphic chloroplast present in them-agranal in bundle sheath cells and granal in mesophyll cells.

Question

 A plant in your garden avoids photorespiratory losses, has improved water use efficiency. shows high rates of photosynthesis at high temperatures and has improved efficiency of nitrogen 9utilisation. In which of the following physiological groups would you assign this plant? [NEET 2016, Phase 1]
(a) $\mathrm{C}_4 $

(b) CAM

(c) Nitrogen fixer

(d) $\mathrm{C}_1$

Answer/Explanation

Ans. (a)

This plant is a $\mathrm{C}_4$-plant as these group ot plants shows little photorespiration, efficient in binding to $\mathrm{CO}_2$ even at low concentrations, better utilisation of water as well as high rates of photosynthesis even at high temperatures, i.e. tropical region. Besides, they can also tolerate excess of salts due to presence of organic acids.

Question

 PGA as the first $\mathrm{CO}_2$-fixation product was discovered in photosynthesis of [CBSE AIPMT 2010]
(a)bryophyte

(b) gymnosperm

(c) angiosperm

(d) alga

Answer/Explanation

Ans. (d)

The use of radioactive “Cby Melvin Calvin in algal (Chlorella) photosynthesis studies led to the discovery that the first $\mathrm{CO}_2$ fixation product was a 3-carbon organic acid. The first product identified was 3-phosphoglyceric acid (PGA).

Question

$ C_4$-plants are more efficient in photosynthesis than $C_3$-plants due to [CBSE AIPMT 2010, 08]

(a) higher leaf area

(b) presence of larger number of chloroplasts in the leaf cells

(c) presence of thin cuticle

(d) lower rate of photorespiration

Answer/Explanation

Ans. (b)

$C_4$-plants are more efficient in photosynthesis than $\mathrm{C}_3$-plants but use more energy. They possess the larger number of chloroplasts in the leat cells. In the leaves of $\mathrm{C}_4$-plants, the vascular bundles are surrounded by bundle sheath cells which in turn are surrounded by mesophyll cells. Chloroplast in bundle sheath cells are larger and always contain grana, whereas chioroplasts in mesophyll cells are smaller.

Question

In the leaves of $C_4$-plants, malic acid formation during $\mathrm{CO}_2$-fixation occurs in the cells of [CBSE AIPMT 2007, 08]

(a) mesophyll

(b) bundle sheath

(c) phloem

(d) epidermis

Answer/Explanation

Ans. (a)

The oxalic acid is reduced to malic acid in mesophylicells, from chloroplast of mesophyll cells the malic acid is transterred to the chloroplast of bundle sheath cels where, it is decarboxylated to form $\mathrm{CO}_2$ and proruvic acid.

Question

 As compared to a $C_3$-plant, how many additional molecules of ATP are needed for net production of one molecule of hexose sugar by $\mathrm{C}_4$-plants [CBSE AIPMT 2005]
(a) 2

(b) 6

(c) 12

(d) zero

Answer/Explanation

Ans. (c)
In $\mathrm{C}_4$-plants every $\mathrm{CO}_2$ molecule has to be fixed twice, so these plants are needed more energy for the synthesis of hexose sugar molecules than $\mathrm{C}_3$-plants in which $\mathrm{CO}_2$ has to be fixed only ance. 18 ATP molecules are required by $\mathrm{C}_5$-plants for the synthesis of one molecule of hexose sugar while 30 ATP molecules are needed by the $\mathrm{C}_4$-plants for the same. Thus, $\mathrm{C}_4$-plants have a need of 12 ATP molecules extra than $\mathrm{C}_3$-plants for the synthesis of one molecule of hexose sugar.

Question

Photosynthesis in $\mathrm{C}_4$-plants is relatively less limited by atmospheric $\mathrm{CO}_2$ levels because [CBSE AIPMT 2005]

(a) effective pumping of $\mathrm{CO}_2$ into bundle sheath cells

(b) RuBisC0 in $\mathrm{C}_4$-plants has higher affinity for $\mathrm{CO}_2$

(c) four carbon acids are the primary initial $\mathrm{CD}_2$-fixation products

(d) the primary fixation of $\mathrm{CD}_2$ is mediated vio PEP carboxylase

Answer/Explanation

Ans. (d)

The fixation of $\mathrm{CO}_2$ in $\mathrm{C}_4$-plants takes place in two places and by two ditferent arganic compounds. Phosphoenol Pyruvate [PEP) is tound in mesophyl cells which primarily tixes atmospheric $\mathrm{CD}_2$ into pxalo acetic acid (4EL
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 encyme has greater atfinity to $\mathrm{CO}_2$ than RuBisCO.

Question

 In $C_2-p l a n t s_1$ the first stable product of photosynthesis during the dark reaction is [CBSE AIPMT 2004]
(a) malic acid

(b) oxaloacetic acid

(c)3-phosphoglyceric acid

(d) phosphoglyceraldehyde

Answer/Explanation

Ans. (c)

In $C_3$-plants the first stable product formed during dark reaction is 5-phosphoglyceric acid. Since, it is a 3 carbon compound hence, the pathway Is reterred as $\mathrm{C}_3$-pathway. Oxalo Acetic Acid (OAA) is the first stable compound in $\mathrm{C}_4$-plants. It is a $4 \mathrm{C}$ compound.

Question

 In sugarcane plant ${ }^* \mathrm{CO}_2$ is fixed in malic acid, in which the enzyme that fixes $\mathrm{CO}_2$ is [CBSE AIPMT 2003]
(a) fructose phosphatase

(b) ribulose bisphosphate carboxylase

(c)phosphoenol pyruvic acid carboxylase

(d) ribulose phosphate kinase

Answer/Explanation

Ans. (c)

In $\mathrm{C}_4$-plants, $\mathrm{CO}_2$ is taken up by. Phosphoenol- Pyruvate (PEP) and the reaction being catalysed by PEP carboxylase.

Question

 In photosynthesis energy from light reaction to dark reaction is transferred in the form of [CBSE AIPMT 2002]

(a) ADP

(b) ATP

(c) RuDP

(d) chiorophyll

Answer/Explanation

Ans. (b)

As a result of light reaction, oxygen, NADPH and ATP are formed. Oxygen is released into the atmosphere while NADPH and ATP are utilised for reduction of $\mathrm{CO}_2$ to carbohydrate in dark reaction.

Question

 Which pair is wrong? [CBSE AIPMT 2001]

(a) $\mathrm{C}_3-$ Maize

(b) $\mathrm{C}_4-\mathrm{Kranz}$ anatomy

(c) Calvin cycle-PGA

(d) Hatch and Slack Pathway-0xalo acetic acid

Answer/Explanation

Ans. (a)

Maize is a $\mathrm{C}_4$-plant. $\mathrm{C}_4$-plants have Kranz type anatomy of leaves. PGA(3-Phosphoglyceric Acid) is farmed during Calvin cycle. OAA (Oxalo Acetic Acid) a $4 \mathrm{C}$ compound is formed during Hatch and Slack cycle (C cycle).

Question

How many turns of Calvin cycle yield one molecule of glucose? [CBSE AIPMT 2000]

(a) 8

(b) 2

(c) 6

(d) 4

Answer/Explanation

Ans. (c)

Conversion of $\mathrm{CO}_2$ to simple (reduced) organic compounds is called $\mathrm{CD}_2$ 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 cantaining 6 carbon atoms.

Question

 Fixation of one $\mathrm{CO}_2$ molecule through Calvin cycle requires [CBSE AIPMT 2000]

(a) 1 ATP and 2NADPH

(b) 2 ATP and 2NADPH 2

(c) 3 ATP and 2NADPH ${ }_2$

(d) 2ATP and 1NADPH ${ }_2$

Answer/Explanation

Ans. (c)
2 ATP are required during conversion of PGA to 1, 3 diphosphoglyceric acid and 1 ATP during conversion of glyceraldehyde phosphate to ribulase biphosphate. 2
$N A D P H_2$ molecules are utilised for converting 1,3 diphosphoglyceric acid to glyceraldehyde phosphate.

Question

 Which one of the following is represented by Calvin cycle? [CBSE AIPMT 1996]

(a) Reductive carboxylation

(b) Oxidative carboxylation

(c) Photophosphorylation

(d) Oxidative phosphorylation

Answer/Explanation

Ans. (a)

In dark phase or Calvin cycle, carbon dioxide is assimilated with the help of assimilatory power (ATP and NADPH, ) to produce organic acid. The cycle involves reduction of carban involving carboxylation, glycolytic reversal and regeneration of RuBP. $\mathrm{C}_3$ cycle is also known as reductive pentose pathway or Photosynthetic Carbon Reduction (PCR).

Question

$ \mathrm{C}_4$-cycle was discovered by [CBSE AIPMT 1994]

(a) Hatch and Slack

(b) Calvin

(c) Hill

(d) Arnon

Answer/Explanation

Ans. (a)

$\mathrm{C}_4$ 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, so also known as Hatch- Slack cycle.

Question

The carbon dioxide acceptor in Calvin cycle/ $\mathrm{C}_3$-plants is [CBSE AIPMT 1993, 95, 96, 99]

(a) Phosphoenol Pyruvate (PEP)

(b) Ribulose 1,5-Diphosphate (RuDP)

(c) Phosphoglyceric Acid (PGA)

(d) Ribulose Monophosphate (RMP)

Answer/Explanation

Ans. (b)

In $\mathrm{C}_3$-plants, $\mathrm{CD}_2$ combines with ribulose biphosphate (acceptor molecule) in the presence of RuBisCO (RuBP carboxylase) and form two molecules of 3-Phosphoglyceric acid or PGA (first stable product).

Question

 Which one is a $\mathrm{C}_4$-plant? [CBSE AIPMT 1992]

(a) Papaya

(b) Pea

(c) Potato

(d) Maize/Corn

Answer/Explanation

Ans. (d)
The plants in which the first stable product af dark reaction of photosynthesis is a 4-carbon

Question

compound are called $\mathrm{C}_4$-plants, e.g. sugarcane, maize, sorghum, etc. These plants show characteristic Kranz anatomy. The first $\mathrm{CD}_2$ acceptor in these plants is Phosphoenol Pyruvate (PEP). The enzyme that catalyses initial carbon dioxide fixation in $\mathrm{C}_4$-plants is [CBSE AIPMT 1992, 2002]

(a) RuBP carboxylase

(b) PEP carboxylase

(c) carbonic anhydrase

(d) carboxydismutase

Answer/Explanation

Ans. (b)

In $\mathrm{C}_4$-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).

Question

Dark reactions of photosynthesis occur in [CBSE AIPMT 1991]

(a) granal thylakoid membranes

(b) stromal lamella membranes

(c) stroma outside photosynthetic lamellae

(d) periplastidial space

Answer/Explanation

Ans. (c)

Light reaction of photosynthesis occurs in granal thylakoid membranes of chloraplast while dark reaction occurs in the stroma or matrix, i.e. outside the photosynthetic lamellae of chloroplast.

Question

Which technique has helped in investigation of Calvin cycle? [CBSE AIPMT 1991]

(a) X-ray crystallography

(b) X-ray technique

(c) Radioactive isotope technique

(d) Intermittent light

Answer/Explanation

Ans. (c)

Calvin, Benson and Basshan utilised $\mathrm{C}^{14}$ (with long life) to trace the path of carbon in photosynthesis. Calvin was awarded Nobel Prize in 1961 in recognition to his work with $C^{14}$ isatope. He discovered the cycle involved in carbon assimilation, known as Calvin cycle or $\mathrm{C}_3$-cycle.

Question

Kranz anatomy is typical of [CBSE AIPMT 1990, 95]

(a)C $\mathrm{C}_4$-plants

(b) $\mathrm{C}_3$-plants

(c) $\mathrm{C}_2$-plants

(d) CAM plants

Answer/Explanation

Ans. (a)

Leaves of $\mathrm{C}_4$-plants (e.g. sugarcane, maize) are characterised by Kranz anatomy in which 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 (Kranz- wreath).

Question

 The first carbon dioxide acceptor in $\mathrm{C}_4$-plants is [CBSE AIPMT 1990, 92]

(a) phosphoenol-pyruvate

(b) ribulose 1,5-diphosphate

(c) oxalo acetic acid

(d) phosphoglyceric acid

Answer/Explanation

Ans. (a)

In $\mathrm{C}_4$-plants, phosphoenol-pyruvate is the first acceptor of $\mathrm{CO}_2$ while ribulose bi-phosphate is the second acceptor. Oxalo Acetic Acid (OAA) is the first product of $\mathrm{C}_4$-cycle.

Question

$ \operatorname{InC}_4$-plants, Calvin cycle operates in [CBSE AIPMT 1989]

(a) stroma of bundle sheath chloroplasts

(b) grana of bundle sheath chloroplasts

(c) grana of mesophyll chloroplasts

(d) stroma of mesophyll chloroplasts

Answer/Explanation

Ans. (a)

$\mathrm{C}_4$-plants possess two types of chloroplasts granal in mesophyll cells and agranal in bundle sheath cells.
Mesophyll cells are specialised to perform light reaction and bundle sheath cells possess RuBisCO, here $\mathrm{CO}_2$ is fixed through Calvin cycle.

Question

 A very efficient converter of solar energy with net productivity of 2-4 $\mathrm{kg} / \mathrm{m}^2$ or more is the crop of [CBSE AIPMT 1989]

(a) wheat

(b) sugarcane

(c) rice

(d) bajra

Answer/Explanation

Ans. (b)

In $\mathrm{C}_4$-plants, (e.g. maize, sugarcane, sorghum)optimum temperature of photosynthesis is $30-45^{\circ} \mathrm{C}$. In $\mathrm{C}_4$-plants, rate of net photosynthesis in full sunlight is $\left(40-80 \mathrm{mg} \mathrm{CO} / \mathrm{dm}^2 / \mathrm{hr}\right)$ which is more than the rate of net photosynthesis $\left(15-35 \mathrm{mg} \mathrm{CO} / 2 \mathrm{dm}^2 / \mathrm{hr}\right)$ at optimum sunlight in $\mathrm{C}_3$-plants.

Question

Carbon dioxide joins the photosynthetic pathway in [CBSE AIPMT 1988]

(a) PS-I

(b) PS-II

(c) light reaction

(d) dark reaction

Answer/Explanation

Ans. (d)

In dark reaction of photosynthesis, reducing agent (NADPH) and source of energy (ATP) formed during light reaction, are utilised in the conversion of $\mathrm{CO}_2$ to carbohydrates.

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