Directions: In the following questions, a statement of assertion is followed by a statement of reason. Mark the correct choice as:
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of assertion.
(c) If the assertion is true but the reason is false.
(d) If the assertion is false but the reason is true.
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Assertion: Chloroplasts occur inside the leaves mostly in mesophyll cells along their walls.
Reason: The membrane system of chloroplast is responsible for trapping the light energy and also for the synthesis of ATP and NADPH.
b
Assertion: The colour of the leaf is due to the presence of four pigments-chlorophyll \(a\), chlorophyll \(b\), xanthophylls and carotenoids.
Reason: Chlorophyll \(b\) is the chief pigment associated with photosynthesis.
(c) Chlorophyll a is found in all photosynthetic plants except bacteria. Hence, it is termed as universal photosynthetic pigment. It is also called primary photosynthetic pigment because it performs primary reaction of photosynthesis which involves conversion of light into chemical energy.
Assertion: The splitting of water is associated with PS II.
Reason: Water is split into \(\mathrm{H}^{+}, \mathrm{O}_2\) and electrons.
b
Assertion: The stroma lamellae have both PS I and PS II.
Reason: Thylakoid membranes possess photosynthetic pigments and coupling factors.
(d) Thylakoid membranes possess photosynthetic pigments and coupling factors. They occur in specific groups called photosystems (previously called quantasomes). There are two photosystems. The grana lamellae have both PS I and PS II and the stroma lamellae lack PS II as well as NADP reductase enzyme.
Assertion: The proton gradient is broken down due to the movement of protons across the membrane to stroma through the transmembrane channel of the \(F_0\) of the ATPase.
Reason: The breakdown of the proton gradient leads to the release of energy.
b
Assertion: Dark reactions are called biosynthetic phase of photosynthesis.
Reason: Dark reactions do not directly depend on the presence of light but are dependent on the products of the light reaction, i.e., ATP and NADPH.
b
Assertion: The first product of \(\mathrm{CO}_2\) fixation in \(\mathrm{C}_3\) pathway is OAA.
Reason: The first product of \(\mathrm{CO}_2\) fixation in \(\mathrm{C}_4\) pathway is 4 carbon compound.
(d) The first product of \(\mathrm{CO}_2\) fixation in \(\mathrm{C}_3\) pathway is 3-phosphoglyceric acid (PGA) which is a 3-carbon compound while the first product of \(\mathrm{CO}_2\) fixation in \(\mathrm{C}_4\) pathway is oxaloacetic acid (OAA) which is a 4-carbon compound.
Assertion: The \(C_4\) plants have a special type of leaf anatomy called kranz anatomy.
Reason: Chloroplasts of bundle sheath cells have well-developed grana and starch grains.
(c) \(\mathrm{C}_4\) plants show kranz type of anatomy. In kranz anatomy, the mesophyll is undifferentiated and its cells occur in concentric layers around vascular bundles. The vascular bundles are surrounded by large sized bundle sheath cells which are arranged in wreath-like manner in one to several layers. Chloroplasts of mesophyll cells are smaller, have well developed grana and they do not produce starch. Chloroplasts of bundle sheath cells are larger and agranal. Starch is often present.
Assertion: The primary \(\mathrm{CO}_2\) acceptor in \(\mathrm{C}_4\) pathway is 3-carbon molecule phosphoenol pyruvate (PEP).
Reason: The enzyme responsible for this fixation is PEP carboxylase or PEPcase.
b
Assertion: In \(C_4\) plants, the bundle sheath cells are rich in an enzyme phosphoenol pyruvate carboxylase (PEPCase).
Reason: In \(C_4\) plants, the mesophyll cells lack enzyme Ribulose bisphosphate carboxylase-oxygenase (RuBisCO).
(d) The primary \(\mathrm{CO}_2\) acceptor in \(\mathrm{C}_4\) cycle is a 3 -carbon molecule phosphoenol pyruvate (PEP) present in the mesophyll cells. The enzyme responsible for this fixation is PEP carboxylase PEPCase. The mesophyll cells lack RuBis \(\mathrm{CO}\) enzyme. The \(\mathrm{C}_4\) acid OAA is formed in the mesophyll cells. The \(\mathrm{CO}_2\) released in the bundle sheath cells enters the \(\mathrm{C}_3\) or the Calvin pathway, a pathway common to all plants. The bundle sheath cells are rich in an enzyme Ribulose bisphosphate carboxylase-oxygenase (RuBisCO ), but lack PEPCase.
Assertion: In \(C_4\) plants, photorespiration does not occur.
Reason: \(C_4\) plants have a mechanism that increases the concentration of \(\mathrm{CO}_2\) at the enzyme site.
(a) \(\ln C_4\) plants, photorespiration does not occur because they have a mechanism that increases the concentration of \(\mathrm{CO}_2\) at the enzyme site. This takes place when the \(\mathrm{C}_4\) acid from the mesophyll is broken down in the bundle sheath cells to release \(\mathrm{CO}_2\). This results in increasing the intracellular concentration of \(\mathrm{CO}_2\). This ensures that the RuBisCO functions as a carboxylase, thus, minimising the oxygenase activity.
Assertion: Photorespiration is a wasteful process.
Reason: In the photorespiratory pathway, there is no synthesis of sugars or ATP.
(a) Photorespiration is the light dependent process of oxygenation of ribulose bisphosphate (RuBP) and release of carbon dioxide by the photosynthetic organs of a plant. In the photorespiratory pathway, there is neither synthesis of sugars nor of ATP. Rather, it results in the release of \(\mathrm{CO}_2\) with the utilisation of ATP. Therefore, photorespiration is a wasteful process.
Assertion: The external factors that affect photosynthesis are number, size, age and orientation of leaves, mesophyll cells and chloroplasts and the amount of chlorophyll.
Reason: The plant or internal factors are dependent on the genetic predisposition and the growth of the plant.
(d) Photosynthesis is under the influence of several factors, both internal (plant) and external. The plant factors include the number, size, age and orientation of leaves, mesophyll cells and chloroplasts, internal \(\mathrm{CO}_2\) concentration and the amount of chlorophyll. The plant or internal factors are dependent on the genetic predisposition and the growth of the plant. The external factors include the availability of sunlight, temperature, \(\mathrm{CO}_2\) concentration and water. During photosynthesis, all these factors simultaneously affect its rate.
Assertion: \(\mathrm{C}_3\) plants respond to increased \(\mathrm{CO}_2\) concentration by increasing rate of photosynthesis.
Reason: The higher productivity of some greenhouse crops such as tomatoes and bell pepper is due to increased \(\mathrm{CO}_2\) concentration.
b
Assertion: Tropical plants have a higher optimum temperature for photosynthesis than temperate plants.
Reason: The temperature optimum for photosynthesis of different plants depends on their habitat.
(a) The optimum temperature is \(10^{\circ}-25^{\circ} \mathrm{C}\) for \(\mathrm{C}_3\) plants and \(30-45^{\circ} \mathrm{C}\) for \(\mathrm{C}_4\) plants. The temperature optimum for photosynthesis of different plants also depends on habitat that they are adapted to. Tropical plants have a higher temperature optimum than the plants adapted to temperate climates.
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