Chapter 11: Photosynthesis in Higher Plants

Class 11 Biology Chapter 11 Photosynthesis in Higher Plants Exercise Questions and Answers :

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

Answer : We cannot determine whether a plant is or solely by external appearance. The distinction is based on the internal photosynthetic mechanisms . plants initially fix into 3-carbon PGA, whileplants first fix into a 4-carbon compound in mesophyll cells. Bundle sheath cells then perform the Calvin cycle.

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

Answer : To determine whether a plant is or, we need to examine its leaf anatomy. In  plants, mesophyll cells are the primary site of fixation, and photorespiration occurs. Inplants, fixation initially happens in mesophyll cells, and the Calvin cycle takes place in bundle sheath cells. This separation of functions minimizes photorespiration. The presence of Kranz anatomy, with distinct mesophyll and bundle sheath cells, indicates aplant .

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

Answer : plants are highly productive despite a relatively small number of cells conducting the Calvin pathway (usually in bundle sheath cells) because they have evolved an efficient concentration mechanism. In plants, the initial carbon fixation occurs in mesophyll cells, where carbon is fixed into a 4-carbon compound, often malate or oxaloacetate. This 4-carbon compound is then transported to the bundle sheath cells, where it is decarboxylated to release . This spatial separation of fixation and Calvin cycle allows for a higher concentration of in the vicinity of RuBisCO in bundle sheath cells, reducing the likelihood of oxygenation and photorespiration.

This concentrated enhances the efficiency of RuBisCO as a carboxylase, resulting in more effective photosynthesis even in conditions of high temperature and aridity. This adaptation enablesplants to outperform plants in terms of productivity, especially in environments with high light and temperature levels, where water loss through transpiration is a concern.

4. RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation inplants?

Answer : RuBisCO, while capable of both carboxylation and oxygenation, carries out more carboxylation inplants due to their unique anatomical and biochemical adaptations. Inplants, is initially fixed into a 4-carbon compound in mesophyll cells and subsequently released as in bundle sheath cells. This spatial separation concentrates around RuBisCO in the bundle sheath cells. As a result, the intracellular concentration of is higher, favoring the carboxylation reaction over oxygenation. This minimizes photorespiration, allowing RuBisCO inplants to predominantly act as a carboxylase, enhancing overall photosynthetic efficiency in environments with high temperatures and intense light.

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 : Plants that solely possess chlorophyll b and lack chlorophyll a would not carry out photosynthesis effectively, as chlorophyll a is essential for primary energy conversion. Chlorophyll b and accessory pigments aid chlorophyll a in capturing a wider spectrum of light, increasing photosynthetic efficiency. These pigments enable plants to adapt to varying light conditions and optimize energy production. They serve to broaden the range of light wavelengths that can be utilized for photosynthesis, enhancing a plant's ability to thrive in diverse environments.

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 :  In the absence of light, chlorophyll, the green pigment responsible for photosynthesis, breaks down, revealing other pigments in the leaf. Carotenoids, responsible for yellow and orange hues, are more stable than chlorophyll and become more prominent, giving leaves a yellow or pale green color in the dark.

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 : Leaves on the sunny side of a plant or those exposed to direct sunlight are typically darker green than the leaves on the shady side or those in the shade. This is because sunlight is essential for photosynthesis, and leaves exposed to more sunlight have higher chlorophyll content, resulting in a darker green color. Chlorophyll is the primary pigment involved in photosynthesis and gives leaves their green color. In contrast, leaves in the shade receive less light and, therefore, have lower chlorophyll levels, appearing lighter in color.

8. Figure 11.10 shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions:

(a) At which points (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) Light is a limiting factor in regions A and B of the curve.

(b) In region A, limiting factors could include factors other than light, such as availability, temperature, or nutrient levels. In region A', light can still be a limiting factor.

(c) Region C represents the point where the rate of photosynthesis does not increase with higher light intensity. Region D indicates that factors other than light become limiting for photosynthesis at that point.

9. Give comparison between the following:

(a) andpathways

(b) Cyclic and non-cyclic photophosphorylation

(c) Anatomy of leaf in andplants

Answer : (a) and pathways

(b) Cyclic and non-cyclic photophosphorylation

(c) Anatomy of leaf in and plants