RBSE Solutions for Class 11 Biology Chapter 13 Photosynthesis in Higher Plants

Rajasthan Board RBSE Solutions for Class 11 Biology Chapter 13 Photosynthesis in Higher Plants Textbook Exercise Questions and Answers.

RBSE Class 11 Biology Solutions Chapter 13 Photosynthesis in Higher Plants

RBSE Class 11 Biology Photosynthesis in Higher Plants Textbook Questions and Answers

Question 1. 
By looking at a plant externally can you tell whether a plant is C₃ or C₄? Why and how?
Answer:
C₃ plants generally grown in temperate region. They have low capacity to use CO₂ when its quantity is above 50 ppm in atmosphere. The optimum temperature for C₃ plants is 20 - 25°C. Due to having photorespiration, the possibility of energy loss sustain in them. These plants loose water in transpiration and their productivity is also low, mostly broad leaves plants having reticulate venation are C₃ plants.

C₄ plants are found in tropical - regions. The leaves of C₄ plants have kranz anatomy. They are narrow/ribbon like with parallel venation e.g. sugarcane leaves/maize leaves. They can tolerate high temperature. The optimum temperature for these plants is 30 - 350. They can do photosynthesis at low concentration of CO₂ (10 ppm) in atmosphere. They lack photorespiration therefore, they are good producers. Usually monocotyledons are C₄ plants.

Question 2. 
By looking at which internal structure of a plant can you tell whether a plant is C₃ or C₄? Explain.
Answer:
The internal structure of leaves of C₄ plant is differ from C₃ - plants. The C₄ plants have kranz anatomy in their leaves. In these plants, the mesophyll tissue of leaf is not divided into palisade tissue and spongy tissue as found in C₃ plants. In C₄ plants bundle sheath cells are found around vascular bundle. These cells are large in size having big chloroplasts and less grana. While mesophyll cells have small chloroplast having more number of grana. In C₃ plants mesophyll tissue is differentiated into palisade and spongy tissues. They lack bundle sheath cells. The cells have the same type of chloroplasts.

Question 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?
Answer:
Both C₃ and C₄ plants evenly have Calvin pathway. The C₃ plants can do photosynthesis of high concentration of CO₂ in atmosphere while C₄ plants can do photosynthesis at low concentration of CO₂. Therefore less CO₂ is available to C₃ plants than that of C₄ plants. CO₂ acts as limiting factor for C₃ plants. In C₄ plants, Calvin cycle (or C₃ cycle) takes place only in bundle sheath cells. There Calvin cycle is not found in their mesophyll cells. These cells possess only Hatch - Slack cycle.

In C₃ plant some oxygen binds with RUBISCO, thereby carbondioxide accumilation reduces. Here Ribulose - biphosphate (RuBP) combines with oxygen than that of turning into 3 - phosphoglyceric acid and forms phosphoglycolate. It is called photorespiration. In this process sugar and ATP are not formed. Therefore, it is a senseless process. In C₄ plants, the biomass produced in big amount due to lack of photorespiration. Hence, they are high producers.

Question 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?
Answer:
RuBisCO is a protein enzyme mostly found in world. It can bind with both CO₂ and O₂ but it has more affinity to CO₂ than O₂. This affinity depends on the relative concentration of O₂ and CO₂.
In Calvin cycle RuBisCO catalyses the reaction of CO₂ and RUBP, as a result 3 - phosphoglyceric acid is formed. In C₃ plants, some O₂ binds with RuBisCO. Thereby fixation of O₂ reduces, because RuBisCO combines with O₂ to forms phosphoglycolate molecule. This process is called photorespiration. In this, the sugar formation does not occur, and energy does not store in ATP also.

Contrary to C₃ plants, C₄ plants lack photorespiration. Malic acid present in mesophyll cells of leaves of C₄ plants breaks into pyruvic acid and CO₂ in bundle sheath cells. Thereby concentration of CO₂ in bundle sheath cells increases and RuBisCO functions as to carboxylase. As a result the productivity of sugar increases here. Here RuBisCO does not function as oxygenase.

Question 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:
If there were complete absence of chlorophyll a in a plant, it would not carry out photosynthetic activity at all because chlorophyll a is the chief pigment associated with photosynthesis as it traps sunlight. Other accessory pigments like chlorophyll - b, xanthophylls and carotenoids are equally essential as they also absorb light and transfer energy to chlorophyll - a. They also enable a wider range of wavelength of incoming light to be utilised for photosynthesis and protect chlorophyll from photo - oxidation.

Question 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:
The green coloifr of the plants is due to presence of chlorophyll which is found in chloroplast. The plants can perform photosynthesis in the presence of chlorophyll. There leucoplast is found in storage parts and chromoplast is found in colourful parts of the plant. These three plastids can change to each other.
Grana lamellae of chloroplast have chlorophyll, carotenoids. Carotenoids are of two types: Xanthophyll and carotene. They are yellow and orange pigments respectively. Light is essential for synthesis of chlorophyll. When a plant is kept in dark, the amount of chloroplast reduces and the process of photosynthesis also reduces. The stored substances in plant gets finished. Thereby chlorophyll of leaves is denatured. Now due to presence of carotenoids the leaves observe green - yellow. Thus, carotenoids and xanthophyll are more stable.

Question 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:
As leaves in shade get lesser light for photosynthesis so they perform lesser photosynthesis as compared to the leaves or plants kept in sunlight. 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 which makes the leaves or plants in shade greener than the leaves or plants kept in the sun.

Question 8. 
Given below figure 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?
Answer:
(a) In the given graph, light is the limiting factor where photosynthesis is minimum. Hence, at point A light is the limiting factor.

(b) Light is a limiting factor; water, temperature and the concentration of carbon dioxide could also be limiting factors in the region A.
(c) C represents the stage beyond which light is not a limiting factor. D represents the stage beyond which intensity of light has no effect on the rate of photosynthesis as the maximum rate of photosynthesis has been attained at this point.

Question 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.
Answer:
(a) C₃ and C₄ pathways:

 
(b) Cyclic and non - cyclic photophosphorylation:

(c) Difference between the anatomy of C₃ and C₄ plants