Question

With reference to factors affecting the rate of photosynthesis, which of the following statements is not correct? [NEET 2017]

(a) Light saturation for $\mathrm{CO}_2$-fixation occurs at $10 \%$ of full sunlight

(b) Increasing atmospheric $\mathrm{CO}_2$ concentration upto $0.05 \%$ can enhance $\mathrm{CO}_2$-fixation rate

(c) $\mathrm{C}_3$-plants respond to higher temperature with enhanced photosynthesis, while $\mathrm{C}_4$-plants have much lower temperature optimum

(d) Tomato is a greenhouse crop, which can be grown in $\mathrm{CO}_2$ enriched atmosphere for higher yield

Answer/Explanation

Ans. (c)

In $\mathrm{C}_4$-plants, the initial fixation of $\mathrm{CO}_2$ occurs in mesophyll cells. The primary acceptor of $\mathrm{CO}_2$ is Phosphoenol Pyruvate (PEP). It combines with $\mathrm{CO}_2$ in the presence of enzyme PEP carboxylase to form the first stable product, i.e. Oxalo Acetic Acid (OAA). Where as $\mathrm{C}_3$-plants lack PEPcarboxylase enzyme. They possess RuBisC0 enzyme. This enzyme can work as both carboxylase (fixation of $\mathrm{CO}_2$ ) and oxygenase (fixation of $\mathrm{O}_2$ ). RuBiscO has a much greater affinity for $\mathrm{CO}_2$ than for $\mathrm{O}_2$ and the binding is competitive. At higher temperature, its affinity for $\mathrm{CO}_2$ decrease and it works as oxygenase.
Therefore, at higher temperature photosynthesis decrease in $\mathrm{C}_3$-plants, while in $\mathrm{C}_4$-plants it increases.

Question

 Emerson’s enhancement effect and red drop have been instrumental in the discovery of [NEET 2016, Phase I]

(a) two photosystems operating simultaneously

(b) photophosphorylation and cyclic electron transport

(c) oxidative phosphorylation

(d) photophosphorylation and non-cyclic electron transport

Answer/Explanation

Ans. (a)

Emerson performed photosynthetic experiment on Chlorella. He provided monochromatic light of more than 680 $\mathrm{nm}$ and observed decrease in rate of photosynthesis known as red drop. Later, he provided synchronised light of $680 \mathrm{~nm}$ and $700 \mathrm{~nm}$ and observed increase in rate of photosynthesis, known as enhancement effect. This experiment led to discovery of two photosystems -PS-I and PS-II operating in photosynthesis.

Question

 The oxygen evolved during photosynthesis comes from water molecules. Which one of the following pairs of elements involved in this reaction? [NEET 2016, Phase I]

(a) Manganese and chlorine

(b) Manganese and potassium

(c) Magnesium and molybdenum

(d) Magnesium and chlorine

Answer/Explanation

Ans. (a)

Photolysis of water during photosynthesis evolve nascent oxygen in the presence of manganese, calcium and chloride ions.

Question

 Plants adapted to low light intensity have [CBSE AIPMT 2004]

(a) larger photosynthetic unit size than the sun plants

(b) higher rate of $\mathrm{CO}_2$ fixation than the sun plants

(c) more extended root system

(d) leaves modified to spines

Answer/Explanation

Ans. (a)

Shade tolerant plants have lower photosynthetic rates and hence, lower growth rates. On the other hand, these plants have larger photosynthetic unit size than the sun plants.

Question

 The principle of limiting factors was proposed by [CBSE AIPMT 1996]

(a) Blackmann

(b) Hill

(c) Arnon

(d) Liebig

Answer/Explanation

Ans. (a)

The principle of limiting factors was given by Blackmann, a British plant physiologist in 1905, according to him, light intensity, carbon dioxide concentration and temperature are the limiting factors in photosynthesis. 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 factor.

Question

Photosynthetically active radiation is represented by the range of wavelength [CBSE AIPMT 1996, 2004, 05]

(a) $340-450 \mathrm{~nm}$

(b) $400-700 \mathrm{~nm}$

(c) $500-600 \mathrm{~nm}$

(d) $400-950 \mathrm{~nm}$

Answer/Explanation

Ans. (b)
Photosynthetically Active Region (PAR) of solar radiation is visible region. It consists of radiations having wavelength betwen 400 to $700 \mathrm{~nm}$.
Green plants use this wavelength in the process of manufacture of food, i.e. photosynthesis.

Question

 A photosynthesising plant is releasing ${ }^{18} 0$ more than the normal. The plant must have been supplied with [CBSE AIPMT 1993]

(a) $\mathrm{O}_3$

(b) $\mathrm{H}_2 \mathrm{O}$ with ${ }^{19} \mathrm{O}$

(c) $\mathrm{CO}_2$ with ${ }^{18} \mathrm{O}$

(d) $\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6$ with ${ }^{18} \mathrm{O}$

Answer/Explanation

Ans. (b)

Ruben, Hassid and Kamen (1941) using heavy isotope of oxygen, $0^{18}$ in water, found that oxygen evolved in photosynthesis comes from water. Evolution of oxygen does not require carbon dioxide.

Question

 At a temperature above $35^{\circ} \mathrm{C}$ [CBSE AIPMT 1992]

(a) rate of photosynthesis will decline earlier than that of respiration

(b) rate of respiration will decline earlier than that of photosynthesis

(c) there is no fixed pattern

(d) both decline simultaneously

Answer/Explanation

Ans. (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.

Question

 During monsoon, the rice crop of Eastern states of India shows lesser yield due to limiting factor of [CBSE AIPMT 1991]

(a) $\mathrm{CO}_2 $

(b) light

(c) temperature

(d) water

Answer/Explanation

Ans. (b)
According to the principle of limiting factor, the rate of the process is limited by the pace of the slowest factor. Light intensity varies with latitude, altitude, season, topography, presence or absence of interceptors like cloud, dust, fog, humidity, etc. In Eastern states, low light intensity during monsoon results in low photosynthesis and hence, lesser yield.

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