NCERT Solved Exercise Questions – Class 11 Biology Chapter 13 Photosynthesis in Higher Plants
13.1 By looking at a plant externally can you tell whether a plant is C3 or C4 ? Why and how?
Ans – The C4 pathway is present in plants that are suited to dry tropical environments. They exhibit a response to highlight intensities, have a unique sort of leaf morphology, and can withstand greater temperatures. Examine two vertical leaf slices, one from a C3 plant and the other from a C4 plant.
13.2 By looking at which internal structure of a plant can you tell whether a plant is C3 or C4 ? Explain.
Ans – In hot, dry, arid, nonsaline, or saline environments, C4 plants can be found. The leaves’ internal anatomy is kranz anatomy. The mesophyll is undifferentiated in kranz anatomy, and its cells are found in concentric layers around vascular bundles. Large-sized bundle sheath cells that are organised in a wreath-like pattern surround the vascular bundles (kranz – wreath). The plasmodesmata or cytoplasmic bridges connect the mesophyll and bundle sheath cells.
The mesophyll cells’ chloroplasts are smaller. They lack starch but do have well-developed grana and a peripheral reticulum. Mesophyll cells have been specifically designed to produce assimilatory power, develop 02, and perform light reactions (ATP and NADPH). The bundle sheath cells’ chloroplasts are agranal and they also contain the enzyme PEPcase for the first fixation of CO2.
13.3. Even though a very few cells in a C4 plant carry out the biosynthetic – Calvin pathway, yet they are highly productive. Can you discuss why?
Ans – Although the earliest CO2 fixation product in these plants is C4 oxalacetic acid, the predominant metabolic pathway is the C3 pathway, also known as the Calvin cycle.
Photorespiration doesn’t happen in C4 plants. They have a mechanism that raises the CO2 concentration at the enzyme site, which explains this.
This occurs when the mesophyll’s C4 acid is broken down in the bundle cells to release CO2, which raises the level of CO2 inside the cells. As a result, the oxygenase activity of the Rubisco is reduced, ensuring its ability to operate as a carboxylase.
You can certainly see why C4 plants have higher productivity and yields now that you are aware of the lack of photorespiration in these plants. These plants also exhibit endurance for greater temperatures.
13.4. RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in C4 plants?
Ans – The enzyme called RuBisCO, also known as ribulose bisphosphate carboxylase-oxygenase, may bind to both C02 and O2. This agreement is cutthroat. Which of the two will bind to the enzyme depends on the relative levels of C02 and 02 in the sample.
Photorespiration doesn’t happen in C4 plants. This is due to the fact that they have a mechanism that raises the level of CO2 at the enzyme site.
Oxaloacetic acid is broken down in the bundle sheath cells, releasing C02 as a result.
The amount of C02 inside cells rises as a result. By doing so, the RuBisCO is guaranteed to act as a carboxylase while reducing oxygenase activity.
13.5. 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?
Ans – Other thylakoid pigments, known as accessory pigments, such as chlorophyll b, xanthophylls, and carotenoids, also absorb light and send the energy to “chlorophyll a,” despite the fact that chlorophyll is the main pigment responsible for capturing light.
In fact, they prevent “chlorophyll a” from being photo-oxidized and allow a larger range of incoming light wavelengths to be used for photosynthesis. Although the absorption cross-section is tiny, reaction centre chlorophyll-protein complexes are capable of directly absorbing light and carrying out charge separation events without the assistance of other chlorophyll pigments.
13.6. Why is the colour of a leaf kept in the dark frequently yellow, or pale green? Which pigment do you think is more stable?
Ans – All cells involved in photosynthesis have carotenoids as pigments. They are auxiliary pigments that are also present in roots, petals, etc. These pigments don’t degrade easily, thus after the decomposition of chlorophylls, unmasking causes them to momentarily expose their colour. Thus, a leaf preserved in darkness is yellow or light green in colour.
13.7.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?
Ans – The leaves of the shaded side are darker green than those kept in sunlight due to two reasons:
A limiting element for photosynthesis is light. In the shade, leaves receive less light for photosynthesis. As a result, leaves or plants kept in the shadow don’t perform as much photosynthesis as those kept in sunlight. The leaves in the shade have more chlorophyll pigments, which speeds up photosynthesis.
The amount of light absorbed by the leaves rises due to this increase in chlorophyll concentration, which in turn accelerates the process of photosynthesis. As a result, leaves or plants kept in the shade are greener than those kept in the light.
13.8. Figure 13.10 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?
(a) Because the rate of photosynthesis rises with light intensity, tight is a limiting factor in region “A” and half of “BT.”
(b) Every other factor besides light.
(c) C stands for a region where a limiting factor other than light is present, such as CO2. D is the level of light necessary for photosynthesis to occur at its highest rate under the circumstances.
13.9. Give comparison between the following:
(a) C3 and C4 pathways
(b) Cyclic and non-cyclic photophosphorylation (c) Anatomy of leaf in C3 and C4 plants
Ans – (a) The differences between the C3 and C4 pathways
(b) Cyclic and non-cyclic photophosphorylation differ in that
(c) The anatomy of the leaf differs between C3 and C4 plants in the following ways: