NCERT Solutions for Class 11-science Biology Chapter 13 - Photosynthesis In Higher Plants
Chapter 13 - Photosynthesis In Higher Plants 224
The plants which are adapted to dry tropical regions have the C 4 pathway, but they do not show any characteristic in external morphology. Therefore, we cannot say whether a plant is C3 or C4 by looking at its external appearance.
C4 plants differ from C3 plants in having 'Kranz' type of anatomy. They have a large parenchymatous sheath around the vascular bundles. Each cell of the bundle sheath possesses chloroplasts. The mesophyll cells of leaves are not differentiated into palisade and spongy parenchyma. They have less intercellular spaces, and the mesophyll cells possess normal chloroplasts.
In C3 plants, bundle sheath cells do not possess chloroplasts. The mesophyll cells of leaves are differentiated into palisade and spongy parenchyma.
In C4 plants, the biosynthetic Calvin cycle occurs in bundle sheaths. Despite a few number of cells being involved in the Calvin cycle, C4 plants have a high rate of photosynthesis due to the following factors:
- Rapid supply of CO2 from mesophyll cells taking part in initial carbon dioxide fixation.
- Absence of photorespiration due to the absence of photolysis in bundle sheath cells.
- Rapid withdrawal of photosynthates from the bundle sheath cells as they lie over the vascular bundles.
- Photosynthesis continues even when stomata are closed due to fixation of CO2 released through respiration.
Chapter 13 - Photosynthesis In Higher Plants 225
(a) C3 and C4 pathways:
i. In the C3 pathway, the first product of carbon fixation is a 3 carbon atom compound 3-phosphoglyceric acid.
ii. Photorespiration occurs in C3 plants.
iii. Plants are adapted to all climates except saline conditions.
iv. This pathway is less expensive and requires only 18 molecules of ATP for the synthesis of one molecule of glucose.
i. In the C4 pathway, the first product of carbon fixation is oxaloacetic acid, a 4 carbon atom compound.
ii. Photorespiration rarely occurs.
iii. Plants are adapted to tropical climate and can tolerate halophytic conditions.
iv. This pathway is more energy expensive and requires 30 molecules of ATP for the synthesis of one molecule of glucose.
(b) Cyclic and non-cyclic photophosphorylation:
i. It involves the participation of only PS I.
ii. This system is not concerned with photo-oxidation of water.
iii. It is only concerned with the production of ATP.
iv. It occurs under special conditions when NADPH starts accumulating.
i. It involves the participation of both PS I and PS II.
ii. The first step of this system is photo-oxidation of water resulting into H+, e- and release of O2.
iii. It is concerned with the reduction of NADP+ and production of ATP.
iv. It occurs under normal conditions.
(c) Anatomy of leaf in C3 and C4 plants:
Anatomy of leaf in C3 plants
Anatomy of leaf in C4 plants
i. Plants do not posses 'Kranz' anatomy of leaves.
ii. Chloroplasts are not dimorphic.
iii. Mesophyll cells have intercellular spaces.
a) At A and 50% of B, light is the limiting factor because the rate of photosynthesis is increasing with the intensity of light.
b) Apart from light, other limiting factors will be CO2 and H2O.
c) C represents a stage beyond which light is not a limiting factor and D is the line beyond which the intensity of light does not affect the rate of photosynthesis.
Photosynthesis will not occur in plants which do not possess chlorophyll a because the chlorophyll a molecule acts as a reaction centre. Reaction centres convert light energy into chemical or electron energy.
Although chlorophyll is the major pigment responsible for trapping light, other thylakoid pigments such as chlorophyll b, xanthophylls and carotenoids, which are called accessory pigments, also absorb light and transfer the energy to chlorophyll a.
RuBisCO has a much greater affinity for CO2 than for O2. It is the relative concentration of O2 and CO2 which determines which of the two will bind to the enzyme.
RuBisCO functions as oxygenase only when there is a higher concentration of oxygen and lower concentration of carbon dioxide. In C4 plants, the enzyme RuBisCO is absent in mesophyll chloroplasts. It is present only in bundle sheath chloroplasts which get continuous supply of CO2 even when the stomata are closed through conversion of malic acid to pyruvic acid. Therefore, RuBisCO usually acts as carboxylase rather than oxygenase.
Chlorophyll is unable to absorb energy in the absence of light and loses its stability, giving the leaf a yellowish colour. This shows that carotenoids which provide yellow colour to the leaf are more stable.
The shady side of the plant/potted plants kept in the shade will have more concentration of chlorophyll and hence a darker green colour of leaves. This is because of the following two reasons:
- The chloroplasts of the mesophyll cells are irregularly arranged, and their orientation is in vertical files along the walls in strong light.
- Photooxidation of chloroplasts occurs in bright light and its non-oxidation occurs in shaded areas.
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