Biology Class 11 NCERT Solutions: Chapter 13 Photosynthesis in Higher Plants Part 1

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Q: 1. By looking at a plant externally can you tell whether a plant is ? Why and how?

Answer

One cannot distinguish whether a plant is or by observing its leaves and other, morphological features externally. Unlike plants, the leaves of plants have a special 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 plant, while maize is a plant.

Image shows the C3 and C4 plant

Image Shows the C3 and C4 Plant

Image shows the C3 and C4 plant

Q: 2. By looking at which internal structure of a plant can you tell whether a plant is or ? Explain.

Answer

The leaves of plants have a special anatomy called Kranz anatomy. This makes them different from plants. Special cells, known as bundle-sheath cells, surround the vascular 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 plants, thereby increasing their ability to photosynthesise.

Image of the internal structure of c3 and c4 plant

Image of the Internal Structure of c3 and c4 Plant

Image of the internal structure of c3 and c4 plant

Q: 3. Even though a very few cells in a plant carry out the biosynthetic - Calvin pathway, yet they 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. plants have a mechanism for increasing the concentration of carbon dioxide. In plants, the Calvin cycle occurs in the bundle-sheath cells. The compound (malic acid) from the mesophyll cells is broken down in the bundle-sheath cells. As a result, is released. The increase in ensures that the enzyme does not act as an oxygenase, but as a carboxylase. This prevents photorespiration and increases the rate of photosynthesis. Thus, plants are highly productive.

Q: 4. is an enzyme that acts both as a carboxylase and oxygenase. Why do you think carries out more carboxylation in plants?

Answer:

The enzyme is absent from the mesophyll cells of plants. It is present in the bundle-sheath cells surrounding the vascular bundles. In plants, the Calvin cycle occurs in the bundle-sheath cells. The primary acceptor in the mesophyll cells is phosphoenol pyruvate - a three-carbon compound. It is converted into the four-carbon compound oxaloacetic acid is further converted into malic acid. Malic acid is transported to the bundle-sheath cells, where it undergoes decarboxylation and fixation occurs by the Calvin cycle. This prevents the enzyme from acting as an oxygenase.

Image of the Calvin cycle

Image of the Calvin Cycle

Image of the Calvin cycle

Q: 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?

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.

Q: 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?

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.

Q: 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?

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.

Image of the process of photosynthesis

Image of the Process of Photosynthesis

Image of the process of photosynthesis