NCERT Solutions Chapter 11 of Class 11 Biology – Photosynthesis in Higher Plants

NCERT Solutions for class 11 Chapter-11 Photosynthesis in Higher Plants: The chapter on Photosynthesis in Higher Plants is important for students approaching the board exams. This article introduces NCERT solutions designed to help students explain the concepts of further learning and how to write to get good grades on exams. The solutions are presented in very simple language for ease of understanding.

NCERT CBSE Chapter 11 Photosynthesis in Higher Plants Solutions of Class 11 explains the basic principles of photosynthesis in plants. They will be introduced to the different components involved in the process, including chloroplasts, pigments, and enzymes. They will also learn about the light-dependent and light-independent reactions that occur during photosynthesis. Revise the basic concepts of Photosynthesis in Higher Plants for quick revision and class notes.

Q1: By looking at a Plant externally can you tell whether a Plant is C3 or C4? Why and how?

Answer: 

It is not possible to tell if a plant is C3 or C4 from its external characteristics. However, a C4 plant can survive in drier climates and has certain cellular-level differences in comparison to the C3 plants. These differences cannot be seen with a cursory glance at the leaves alone.

Q2: By looking at which internal structure of a plant can you tell whether a plant is C₃ or C₄? Explain.

Answer: 

We can tell whether a plant is C₃ or C₄ by looking at its leaf anatomy, which is an internal structure of the plant. The leaf anatomy of C₃ and C₄ plants is different, and this difference can be used to distinguish between the two types of plants:

• C₃ plants have simple leaf anatomy with a single layer of mesophyll cells between the upper and lower epidermis. The mesophyll cells contain chloroplasts and are responsible for photosynthesis. The vascular bundles are scattered throughout the leaf and are not arranged in any specific pattern. C₃ plants do not have any specialized structures for photosynthesis.
• On the other hand, C₄ plants have a more complex leaf anatomy with two layers of mesophyll cells and a layer of bundle sheath cells surrounding the vascular bundles. The mesophyll cells are responsible for the initial fixation of carbon dioxide, while the bundle sheath cells are responsible for the final fixation of carbon dioxide. The vascular bundles are arranged in a ring-like pattern around the bundle sheath cells. This specialized structure is called the Kranz anatomy.

Q3: Even though very few cells in a C4 Plant carry out the Biosynthetic – Calvin pathway, they are highly Productive. Can you discuss why? 

Answer: 

Yes, C₄ plants are highly productive even though very few cells in the plant carry out the biosynthetic: Calvin pathway. This is because C4 plants have a specialized carbon dioxide concentrating mechanism that allows them to increase the efficiency of photosynthesis and reduce photorespiration. The rate of photosynthesis is dependent on the amount of carbon dioxide available.

In C₄ plants, the initial fixation of carbon dioxide occurs in mesophyll cells, which are located near the surface of the leaf. These cells contain a high concentration of the enzyme phosphoenolpyruvate carboxylase (PEPC), which has a high affinity for carbon dioxide. PEPC combines with carbon dioxide to form a four-carbon compound called oxaloacetate, which is then transported to bundle sheath cells that surround the leaf veins.

In the bundle sheath cells, oxaloacetate is decarboxylated to release carbon dioxide, which is then used in the Calvin cycle to produce sugars. The bundle sheath cells contain a high concentration of the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), which has a lower affinity for oxygen than PEPC. Due to an increase in CO₂ levels, RuBisCo works as a carboxylase rather than an oxygenase. This reduces photorespiration and increases the speed of photosynthesis for better yields.

Q4: RuBisCO is an Enzyme acting as a Carboxylase and Oxygenase. Why do you think RuBisCO carries out more carboxylation in C4 plants?

Answer: 

The reason why RuBisCO carries out more carboxylation in C₄ plants is that these plants have evolved a specialized carbon-fixation pathway that allows them to concentrate carbon dioxide (CO₂) in the vicinity of RuBisCO. This pathway involves the initial fixation of CO₂ by phosphoenolpyruvate carboxylase (PEPCase) in mesophyll cells, followed by the transfer of the resulting four-carbon compound to bundle-sheath cells, where it is decarboxylated to release CO₂ that can be fixed by RuBisCO. This spatial separation of CO₂ fixation and release reduces the likelihood of oxygenation by RuBisCO and thus enhances its carboxylase activity. Additionally, C₄ plants typically exhibit higher rates of photosynthesis and water-use efficiency than C₃ plants, which may also contribute to their greater reliance on RuBisCO for carboxylation.

Q5: Suppose there were Plants that had a high concentration of Chlorophyll b but lacked Chlorophyll a, would they carry out photosynthesis? Then why do plants have chlorophyll b and other accessory pigments?

Answer: 

Chlorophyll-a molecules work much like ‘antennas’, absorbing light and sending out electrons during both cyclic & non-cyclic photophosphorylation. This excites them, allowing them to perform their important functions. Chl-a plays an essential role in the process of photosynthesis, serving as the reaction centres for both Photosystem I and II. In photosynthesis, chlorophyll-b and other pigments like carotenoids and xanthophylls act as supporting absorbents. Carotenoids & xanthophylls absorb energy and then transfer it to the chlorophyll-a molecule. This helps protect the chlorophyll from photo-oxidation, which can be damaging if not prevented. Consequently, chlorophyll-a is necessary for the process of photosynthesis.

Q6: 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 colour of a leaf kept in the dark frequently becomes yellow or pale green because of the degradation of chlorophyll. Chlorophyll is not stable in the absence of light, and its breakdown products include yellow and colourless compounds such as pheophytin and pheophorbide. These pigments are less efficient at capturing light energy than chlorophyll and therefore do not support photosynthesis as effectively.

Out of the different pigments present in leaves, carotenoids are more stable than chlorophyll. Carotenoids are yellow, orange, and red pigments that help plants to capture light energy in regions of the spectrum where chlorophyll is less efficient. They also play a protective role by absorbing excess light energy and dissipating it as heat, thereby preventing damage to the photosynthetic apparatus. Carotenoids are more stable than chlorophyll because they do not have a central magnesium atom that is vulnerable to oxidative damage in the absence of light. Additionally, carotenoids are lipid-soluble, which makes them less susceptible to degradation by water and other environmental factors.

Q7: Look at the 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 of the same plant on the shady side are typically light green compared to leaves on the sunny side. This is because the chlorophyll content in leaves is directly related to the amount of sunlight they receive. Chlorophyll is the pigment responsible for the green colour of leaves, and it plays a crucial role in photosynthesis. When leaves receive less sunlight, they produce less chlorophyll, which makes them appear lighter in colour.

On the other hand, leaves on the sunny side are darker green because they receive more direct sunlight and are able to produce more chlorophyll. Chlorophyll is essential for capturing light energy and initiating the process of photosynthesis. When leaves receive more sunlight, they are able to absorb more light energy, which allows them to produce more chlorophyll and carry out photosynthesis more efficiently.

Similarly, potted plants kept in the sunlight tend to have darker green leaves compared to those in the shade. This is because plants that receive more sunlight are able to produce more chlorophyll, which makes their leaves appear darker green. In contrast, plants kept in the shade receive less sunlight and produce less chlorophyll, which makes their leaves appear lighter in colour.

Q8: Figure 11.10 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?

Answer: 

• (a) Light acts as the limiting factor in regions A and B.
• (b) Light acts as the limiting factor in region A.
• (c) By stage C, light ceases to be a factor in affecting photosynthesis rates. Point D is the peak of photosynthesis and any increase in light intensity no longer has an effect.

Q9: Give a 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

C3 Pathway	C4 pathway
RuBP, a five-carbon molecule acts as the primary acceptor of CO 2 .	Phosphoenol pyruvate, a 3 Crabon compound acts as the primary acceptor of CO 2 .
The first stable product is 3-phosphoglycerate.	The first stable product is oxaloacetic acid
The leaves only consist of mesophyll cells.	The leaves consist of both mesophyll and bundle sheath cells.
Carbon fixation occurs very slowly in this.	Carbon fixation occurs very fast in this pathway

• (b) Cyclic and non-cyclic photophosphorylation

Cyclic photophosphorylation	Non-cyclic photophosphorylation
It occurs only in Photosystem I.	This involves both Photosystem I and Photosystem ii.
ATP is synthesized during this process	ATP and NADPH2 are both formed during this process.
Oxygen is liberated during this process.	Oxygen is not liberated during this process.
The electrons circulate in a closed manner during this process.	Since this process is non-cyclic, the electrons do not move in a cyclic process.

•  (c) Anatomy of leaf in C₃ and C₄ plants 

C3 leaf Anatomy	C4 leaf Anatomy
The leaves of these plants do not have bundle sheath cells.	Bundle sheath cells are present in the leaves of this anatomy.
RuBISCo is present in the mesophyll cells.	RuBISCo is present only in the bundle sheath cells.
A three-carbon compound is the first stable product.	A four-carbon compound, oxaloacetic acid is the first stable product.
Photorespiration occurs in such plants.	Photorespiration does not occur in these plants.