PSEB 11th Class Biology Solutions Chapter 13 Photosynthesis in Higher Plants

Punjab State Board PSEB 11th Class Biology Book Solutions Chapter 13 Photosynthesis in Higher Plants Textbook Exercise Questions and Answers.

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

PSEB 11th Class Biology Guide 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:
Usually plants growing in dry conditions use C₄-pathways. It cannot be said conclusively, if the plant is a C₃ or C₄ by looking at external appearance.

Question 2.
By looking at which internal structure of a plant can you tell whether a plant is C₃ or C₄? Explain.
Answer:
The particularly large cells around the vascular bundles of the C₄– pathway plants are called bundle sheath cells and the leaves, which have such anatomy are said to have ‘Kranz’ anatomy.
‘Kranz’ means ‘wreath’ and is a reflection of the arrangement of cells.
The bundle sheath cells may form several layers around the vascular bundles they are characterised by having a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces.

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:
The productivity of a plant is measured by the rate at which it photosynthesizes. The amount of carbon dioxide present in a plant is directly proportional to the rate of photosynthesis. C₄ plants have a mechanism for increasing the concentration of carbon dioxide. In C₄ plants, the Calvin cycle occurs in the bundle-sheath cells.

The C₄ compound (malic acid) from the mesophyll cells is broken down in the bundle sheath cells. As a result, CO₂ is released. The increase in CO₂ ensures that the enzyme RuBisCo does not act as an oxygenase, but as a carboxylase.
This prevents photorespiration and increases the rate of photosynthesis. Thus, C₄ plants are highly productive.

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 has a much greater affinity for CO₂ than for O₂. It is the relative concentration of O₂ and CO₂ that determines which of the two will bind to the enzyme. In C₄ -plants some O₂ does bind to RuBisCO and hence, CO₂ fixation is decreased. Here the RuBP instead of being converted to two molecules of PGA binds with O₂ to form one molecule and phosphoglycolate in a pathway called photorespiration.

In the photorespiratory pathway, there is neither synthesis of sugars, nor of ATP. Rather it results in the release of CO₂ with the utilization of ATP. in the photorespiratory pathway, there is no synthesis of ATP or NADPH. Therefore, photorespiration is a wasteful process.

In C₄-plants, photorespiration does not occur. This is because they have a mechanism that increases the concentration of CO₂ at the enzyme site. This takes place when the C₄ acid from the mesophyll is broken down in the bundle cells to release CO₂, this results in increasing the intracellular concentration of CO₂. In turn, this ensures that the RuBisCO functions as a carboxylase minimizing the oxygenase activity.

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:
‘Though chlorophyll is the major pigment responsible for trapping light, other thylakoid pigments like chlorophyll-b, xanthophylls and carotenoids, which are called accessory pigments, also absorb light and transfer the energy to chlorophyll-a. Indeed, they not only enable a wider range of wavelengths of incoming light to be utilized for photosynthesis but also protect chlorophyll-a from photo-oxidation.

Question 6.
Why is the colour ola leaf kept n the dark frequently yellow or pale green? Which pigment do you think is more stable?
Answer:
This is due to the interconversion of pigments, i.e., change of green chlorophyll pigment into yellow-colored carotenoids. The carotene pigment is 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:
The leaves of the same plant on the sunny side are dark green as compare it with the leaves on the sunny side due to more chlorophyll pigment.

Question 8.
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) Points K-C of the curve, the rate did not increase with an increase in its concentration because under these conditions, light becomes limiting factor.

(b) The rate of photosynthesis shows proportionate increase upto a certain CO₂ concentration (In region A of the curve), beyond which the rate again hcomes constant, not showing any increase by increasing CO₂ concentration.

(c) lithe light inrensiry is doubled, i.e., the plants are exposed to 2 units of light, CO₂ concentration again becomes limiting factor beyond this concentration (Points C and D represent on the curve.)

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) Comparison between C₃ and C₄ pathways

C3 Pathway	C4 Pathway
The primary acceptor of CO2 is RuBP, a 5 carbon compound.	The primary acceptor of CO2 is PEP, a 3 carbon compound.
It operated under low concentration of CO2 in mesophyll cells.	It can operate under very low CO2 concentration in mesophyll cells.
CO2 once fixed is not released back.	CO2 once fixed is released back in bundle sheath cells.
Fixation of one molecule of CO2 needs 3ATP and 2 NADPH2 molecules. It requires 18ATP for the synthesis of one molecule of glucose.	C4 pathway requires 30 ATP for the synthesis of one molecule of glucose.
C3 -cycle operates in all categories of plants.	It operates in only C4-plants.

(b) Comparison between cyclic and non-cyclic photophosphorylation

Cyclic Photophosphorylation	Non-cyclic Photophosphorylation
It occurs in photosystem-I in stromal or intergranal thylakoids.	It is carried out by both PS-I and PS-II in the granal thylakoids.
It is not connected to photolysis of water so no oxygen is evolved.	It is connected with photolysis of water, so oxygen is evolved in it.
It is activated by light of 700 nm wavelength.	It occurs in 680 nm as well as 700 nm wavelength.
It generates ATP only there is no formation of NADPH2.	It produces both ATP as well as NADPH2.
Chlorophyll does not receive any electrons from donor.	The source of electrons is photolysis of water.
This system does not take part in photosynthesis except in bacteria.	This system is connected with CO2 fixation and is dominant in green plants.

(C) Comparison between C₃ and C₄ leaves.

C3 Leaves	C4 Leaves
Bundle-sheath cells are absent.	Bundle-sheath cells are present.
RuBisCo is present in the mesophyll cells.	RuBisCo is present in the bundle sheath cells.
The first stable compound produced is 3-phosphoglycerate, a three-carbon compound.	The first stable compound produced is oxaloacetic acid; a four-carbon compound.
‘ Photorespiration occurs.	‘ Photorespirarion does not occur.