NCERT Solutions for Class 11 Biology Chapter 11 Photosynthesis in Higher Plants

NCERT
SOLUTIONS
CLASS - 11th

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Class : 11th
Subject : Biology
Chapter : 13
Chapter Name : Photosynthesis In Higher Plants

Q1 By looking at a plant externally can you tell whether a plant is C or C ? Why and how?
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Answer. One cannot distinguish whether a plant is C or C by observing its leaves and other
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morphological features externally. Unlike C plants, the leaves of C plants have a special
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anatomy called Kranz anatomy and this difference can only be observed at the cellular level. For
example, although wheat and maize are grasses, wheat is a C plant, while maize is a C plant.
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Page : 224 , Block Name : Exercise

Q2 By looking at which internal structure of a plant can you tell whether a plant is C or C ?
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Explain.

Answer. The leaves of C plants have a special anatomy caned Kranz anatomy. This makes them
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different from C plants. Special cells, known as bundle-sheath cells, surround the vascular
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bundles. These cells have a large number of chloroplasts. They are thick-walled and have no
intercellular spaces. They are also impervious to gaseous exchange. All these anatomical features
help prevent photorespiration in C Plants, thereby increasing their ability to photosynthesise.
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Page : 224 , Block Name : Exercise

Q3 Even though a very few cells in a C plant carry out the biosynthetic – Calvin pathway, yet they
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are highly productive. Can you discuss why?

Answer. The productivity of a plant is measured by the rate at which it photosynthesises. The
amount of carbon dioxide present in a plant is directly proportional to the rate of photosynthesis.
C plants have a mechanism for increasing the concentration of carbon dioxide. In C plants, the
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Calvin cycle occurs in the bundle-sheath cells. The C compound (malic acid) from the mesophyll
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cells is broken down in the bundle- sheath cells. As a result, CO is released. The increase in CO
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ensures that the enzyme RuBisCo does not act as an oxygenase, but as a carboxylase. This prevents
photorespiration and increases the rate of photosynthesis. Thus, C plants are highly productive.
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Page : 224 , Block Name : Exercise

Q4 RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think
RuBisCO carries out more carboxylation in C plants?
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Answer. The enzyme RuBisCo is absent from the mesophyll cells of C plants. It is present in the
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bundle-sheath cells surrounding the vascular bundles. In C plants, the Calvin cycle occurs in the
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bundle-sheath cells. The primary CO acceptor in the mesophyll cells is phosphoenolpyruvate —
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a three-carbon compound. It is converted into the four-carbon compound oxaloacetic acid (OAA).
OAA is further converted into malic acid. Malic acid is transported to the bundle-sheath cells,
where it undergoes decarboxylation and CO xation occurs by the Calvin cycle. This prevents the
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enzyme RuBisCo from acting as an oxygenase.

Q5 Suppose there were plants that had a high concentration of Chlorophyll b, but lacked
chlorophyll a, would it carry out photosynthesis? Then why do plants have chlorophyll b and other
accessory pigments?

Answer. Chlorophyll-a molecules act as antenna molecules. They get excited by absorbing light
and emit electrons during cyclic and non-cyclic photophosphorylations. They form the reaction
centres for both photosystems I and II. Chlorophyll-b and other photosynthetic pigments such as
carotenoids and xanthophylls act as accessory pigments. Their role is to absorb energy and
transfer it to chlorophyll-a. Carotenoids and xanthophylls also protect the chlorophyll molecule
from photo-oxidation. Therefore, chlorophyll-a is essential for photosynthesis. If any plant were
to lack chlorophyll-a and contain a high concentration of chlorophyll-b, then this plant would not
undergo photosynthesis.

Q6 Why is the colour of a leaf kept in the dark frequently yellow, or pale green? Which pigment do
you think is more stable?

Answer. Since leaves require light to perform photosynthesis, the colour of a leaf kept in the dark
changes from a darker to a lighter shade of green. Sometimes, it also turns yellow. The production
of the chlorophyll pigment essential for photosynthesis is directly proportional to the amount of
light available. In the absence of light, the production of chlorophyll-a molecules stops and they
get broken slowly. This changes the colour of the leaf gradually to light green. During this process,

the xanthophyll and carotenoid pigments become predominant, causing the leaf to become yellow.
These pigments are more stable as light is not essential for their production. They are always
present in plants.

Q7 Look at leaves of the same plant on the shady side and compare it with the leaves on the sunny
side. Or, compare the potted plants kept in the sunlight with those in the shade. Which of them
has leaves that are darker green ? Why?

Answer. Light is a limiting factor for photosynthesis. Leaves get lesser light for photosynthesis

when they are in shade. Therefore, the leaves or plants in shade perform lesser photosynthesis as
compared to the leaves or plants kept in sunlight.
In order to increase the rate of photosynthesis, the leaves present in shade have more chlorophyll
pigments. This increase in chlorophyll content increases the amount of light absorbed by the
leaves, which in turn increases the rate of photosynthesis. Therefore, the leaves or plants in shade
are greener than the leaves or plants kept in the sun.

Q8 Figure shows the effect of light on the rate of photosynthesis. Based on the graph, answer the
following questions:
(a) At which point/s (A, B or C) in the curve is light a limiting factor?
(b) What could be the limiting factor/s in region A?
(c) What do C and D represent on the curve?

Answer.

(a) Generally, light is not a limiting factor. It becomes a limiting factor for plants growing in shade
or under tree canopies. In the given graph, light is a limiting factor at the point where
photosynthesis is the minimum. The least value for photosynthesis is in region A. Hence, light is a
limiting factor in this region.
(b) Light is a limiting factor in region A. Water, temperature, and the concentration of carbon
dioxide could also be limiting factors in this region.
(c) Point D represents the optimum point and gives the light intensity at which the maximum

photosynthesis is recorded. The rate of photosynthesis remains constant after this point, even
though the intensity of light.

Q9 Give comparison between the following:
(a) C and C pathways
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(b) Cyclic and non-cyclic photophosphorylation
(c) Anatomy of leaf in C and C plants
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Answer. (a) C and C pathways
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(b) Cyclic and non-cyclic photophosphorylation

(c) Anatomy of leaf in C and C plants
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