Q9: Give comparison between the following: (a) C_3 and C_4 pathways (b) Cyclic and non-cyclic photophosphorylation (c) Anatomy of leaf in C_3 and C_4…

(a) Comparison between C_3 and C_4 pathways Feature C_3 Pathway C_4 Pathway:---:---:--- Primary CO_2 Acceptor RuBP (Ribulose-1,5-bisphosphate), a 5-carbon compound. PEP (Phosphoenolpyruvate), a 3-carbon compound. First Stable Product 3-PGA (3-Phosphoglyceric acid), a 3-carbon compound. OAA…

Photosynthesis in Higher Plants Class 11 Biology NCERT Solutions

Q1Exercises

By looking at a plant externally, can you tell whether a plant is $\mathrm{C}_{3}$ or $\mathrm{C}_{4}$ ? Why and how?

Solution

No, it is generally not possible to definitively tell whether a plant is

\mathrm{C}_{3}

or

\mathrm{C}_{4}

just by looking at its external features. The key differences between these plants are anatomical and physiological, not morphological.

Reasoning:

Anatomical Differences: The primary distinguishing feature is the internal leaf anatomy. $\mathrm{C}_{4}$ plants possess 'Kranz' anatomy, which is an internal characteristic and cannot be seen from the outside.
Physiological Differences: The type of photosynthetic pathway, the primary $\mathrm{CO}_{2}$ acceptor, and the response to temperature and light intensity are physiological traits.

While

\mathrm{C}_{4}

plants are often adapted to hot, dry climates (e.g., maize, sugarcane), many

\mathrm{C}_{3}

plants also grow in these regions. Therefore, external appearance is not a reliable indicator for classifying a plant as

\mathrm{C}_{3}

or

\mathrm{C}_{4}

.

Q2Exercises

By looking at which internal structure of a plant you can tell whether a plant is $\mathrm{C}_{3}$ or $\mathrm{C}_{4}$ ? Explain.

Solution

One can tell whether a plant is

\mathrm{C}_{3}

or

\mathrm{C}_{4}

by observing the internal structure of its leaf, specifically the arrangement of cells around the vascular bundles. This is known as leaf anatomy.

The key internal structure is the 'Kranz' anatomy, which is present in $\mathrm{C}_{4}$ plants but absent in $\mathrm{C}_{3}$ plants.

Explanation:

$\mathrm{C}_{4}$ Plants (Kranz Anatomy): The vascular bundles in the leaves of $\mathrm{C}_{4}$ plants are surrounded by large cells called bundle sheath cells. These cells form several layers in a wreath-like ('Kranz') arrangement. They are characterized by:
- Having a large number of chloroplasts.
- Thick walls that are impervious to gaseous exchange.
- No intercellular spaces.
$\mathrm{C}_{3}$ Plants (No Kranz Anatomy): The vascular bundles in $\mathrm{C}_{3}$ leaves are not surrounded by such a distinct layer of large bundle sheath cells. The mesophyll tissue is typically differentiated into two types: the upper palisade parenchyma and the lower spongy parenchyma, with air spaces.

By examining a vertical section of a leaf under a microscope, the presence or absence of this specialized Kranz anatomy allows for the identification of a plant as

\mathrm{C}_{4}

or

\mathrm{C}_{3}

.

Q3Exercises

Even though a very few cells in a $\mathrm{C}_{4}$ plant carry out the biosynthetic - Calvin pathway, yet they are highly productive. Can you discuss why?

Solution

Yes,

\mathrm{C}_{4}

plants are highly productive despite the Calvin pathway being restricted to the bundle sheath cells. The high productivity is due to their highly efficient mechanism for

\mathrm{CO}_{2}

fixation which minimizes a wasteful process called photorespiration.

Reasons for High Productivity:

$\mathrm{CO}_{2}$ Concentrating Mechanism: $\mathrm{C}_{4}$ plants have a special mechanism (the Hatch and Slack pathway) that acts as a $\mathrm{CO}_{2}$ pump. The mesophyll cells capture atmospheric $\mathrm{CO}_{2}$ and convert it into a 4-carbon acid (like malic acid). This acid is then transported to the bundle sheath cells.
Increased $\mathrm{CO}_{2}$ at RuBisCO Site: Inside the bundle sheath cells, the 4-carbon acid is broken down, releasing a high concentration of $\mathrm{CO}_{2}$ . This ensures that the enzyme RuBisCO is constantly supplied with a high concentration of its substrate, $\mathrm{CO}_{2}$ .
Absence of Photorespiration: Photorespiration is a wasteful process that occurs in $\mathrm{C}_{3}$ plants when RuBisCO binds with $\mathrm{O}_{2}$ instead of $\mathrm{CO}_{2}$ . In $\mathrm{C}_{4}$ plants, the high concentration of $\mathrm{CO}_{2}$ in the bundle sheath cells minimizes the oxygenase activity of RuBisCO. By avoiding photorespiration, $\mathrm{C}_{4}$ plants conserve energy and carbon, leading to greater biomass production.

This efficient system makes

\mathrm{C}_{4}

plants more productive than

\mathrm{C}_{3}

plants, especially under conditions of high light intensity, high temperatures, and limited water supply.

Q4Exercises

RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in $\mathrm{C}_{4}$ plants?

Solution

RuBisCO (Ribulose-1,5-bisphosphate carboxylase-oxygenase) carries out more carboxylation in

\mathrm{C}_{4}

plants because they have a mechanism that increases the concentration of carbon dioxide at the enzyme's active site.

Explanation:

Competitive Binding: The active site of RuBisCO can bind to both $\mathrm{CO}_{2}$ (carboxylation) and $\mathrm{O}_{2}$ (oxygenation). The binding is competitive, and the outcome depends on the relative concentrations of $\mathrm{CO}_{2}$ and $\mathrm{O}_{2}$ .
$\mathrm{CO}_{2}$ Pumping Mechanism in $\mathrm{C}_{4}$ Plants: $\mathrm{C}_{4}$ plants have a unique leaf anatomy ('Kranz' anatomy) and a biochemical pathway (Hatch and Slack pathway) that functions as a $\mathrm{CO}_{2}$ pump. Atmospheric $\mathrm{CO}_{2}$ is first fixed into a 4-carbon acid in the mesophyll cells. This acid is then transported to the bundle sheath cells, where RuBisCO is located.
High Intracellular $\mathrm{CO}_{2}$ Concentration: In the bundle sheath cells, the 4-carbon acid is decarboxylated (broken down) to release $\mathrm{CO}_{2}$ . This process significantly increases the intracellular concentration of $\mathrm{CO}_{2}$ around the RuBisCO enzyme, far above the atmospheric concentration.
Favoring Carboxylation: This high $\mathrm{CO}_{2} : \mathrm{O}_{2}$ ratio ensures that RuBisCO predominantly binds with $\mathrm{CO}_{2}$ , thus maximizing its carboxylase activity and minimizing its oxygenase activity. This effectively prevents the wasteful photorespiration process and makes the overall photosynthetic process highly efficient.

Q5Exercises

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?

Solution

Part 1: Photosynthesis without Chlorophyll a

No, a plant that lacked chlorophyll a would not be able to carry out photosynthesis. Chlorophyll a is the essential, primary photosynthetic pigment. It functions as the reaction centre in both Photosystem I (P700) and Photosystem II (P680). It is the only pigment that can directly convert light energy into chemical energy by transferring excited electrons to an electron acceptor.

Even if chlorophyll b and other pigments absorb light energy, they cannot initiate the electron transport chain themselves. They must transfer the absorbed energy to the chlorophyll a reaction centre.

Part 2: Role of Chlorophyll b and other Accessory Pigments

Plants have chlorophyll b and other accessory pigments (like carotenoids and xanthophylls) for two main reasons:

Broadening the Absorption Spectrum: Chlorophyll a absorbs light most effectively in the blue and red regions of the spectrum. Accessory pigments absorb light at wavelengths where chlorophyll a does not absorb efficiently (e.g., chlorophyll b absorbs well in the blue-green region). This allows the plant to utilize a wider range of incoming sunlight for photosynthesis, thereby increasing the overall efficiency of light capture.
Protection from Photo-oxidation: Accessory pigments, particularly carotenoids, play a crucial role in protecting the primary pigment, chlorophyll a, from damage caused by excess light energy (photo-oxidation). They dissipate excess energy, preventing the formation of harmful reactive oxygen species.

Q6Exercises

Why is the colour of a leaf kept in the dark frequently becomes yellow, or pale green? Which pigment do you think is more stable?

Solution

When a leaf is kept in the dark, it becomes yellow or pale green because the chlorophyll pigments degrade, and the more stable yellow and orange accessory pigments become visible.

Explanation:

Chlorophyll Synthesis and Degradation: Chlorophyll molecules are not permanent. They are continuously being synthesized and broken down in the leaf. The synthesis of chlorophyll is a light-dependent process.
Effect of Darkness: In the absence of light, the synthesis of new chlorophyll molecules stops. However, the process of degradation continues. As the green chlorophyll pigments break down, their color fades.
Unmasking of Accessory Pigments: Leaves also contain other pigments called carotenoids (yellow-orange) and xanthophylls (yellow). These pigments are present all the time but are usually masked by the abundance of green chlorophyll. When chlorophyll degrades, these yellow pigments are unmasked, causing the leaf to appear yellow.

More Stable Pigment:

The carotenoids and xanthophylls are more stable than chlorophylls. This is evident because they remain in the leaf after the less stable chlorophyll pigments have been degraded in the dark.

Q7Exercises

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?

Solution

Observation: The leaves on the shady side (or plants kept in the shade) are darker green compared to the leaves on the sunny side.

Reason:

Leaves adapt to the amount of light they receive to maximize photosynthesis. The darker green color is due to a higher concentration of chlorophyll.

Low Light Conditions (Shade): In the shade, light is a limiting factor for photosynthesis. To compensate for the low light intensity, the plant produces a higher concentration of chlorophyll molecules per unit area of the leaf. This increases the leaf's ability to capture as much of the scarce light as possible, making the leaf appear darker green.
High Light Conditions (Sun): In direct sunlight, light is abundant and not a limiting factor. The plant does not need to produce excess chlorophyll to capture light. In fact, very high light intensity can damage chlorophyll molecules (photo-oxidation). Therefore, leaves in the sun have a relatively lower concentration of chlorophyll, making them appear lighter green. This also helps protect the photosynthetic machinery from photodamage.

Q8Exercises

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 light is a limiting factor?

(b)

What could be the limiting factor/s in region A ?

(c)

What do C and D represent on the curve?

Solution

(a) Light is a limiting factor in the region of the curve where the rate of photosynthesis increases with an increase in light intensity. This is represented by region A. At point A, the rate is directly proportional to the light intensity.

(b) In region A, the rate of photosynthesis is directly limited by the availability of light. Therefore, the limiting factor is light intensity. Other factors like

\mathrm{CO}_{2}

concentration and temperature are assumed to be available at optimal levels and are not limiting the rate in this region.

(c)

Point C: This point represents the light saturation point. At this point, the rate of photosynthesis has reached its maximum and does not increase further with an increase in light intensity. This indicates that light is no longer the limiting factor. The rate is now limited by other factors, such as the concentration of $\mathrm{CO}_{2}$ or the temperature.
Region D: This region represents the light-saturated plateau. Here, the rate of photosynthesis is constant despite the increase in light intensity, because it is being limited by other factors (e.g., $\mathrm{CO}_{2}$ availability). If light intensity increases to very high levels beyond this point, it can cause photo-oxidation of chlorophyll, leading to a decrease in the rate of photosynthesis.

Q9Exercises

Give comparison between the following:

(a)

\mathrm{C}_{3}

and

\mathrm{C}_{4}

pathways

(b)

Cyclic and non-cyclic photophosphorylation

(c)

Anatomy of leaf in

\mathrm{C}_{3}

and

\mathrm{C}_{4}

plants

Solution

(a) Comparison between $\mathrm{C}_{3}$ and $\mathrm{C}_{4}$ pathways

Feature	$\mathrm{C}_{3}$ Pathway	$\mathrm{C}_{4}$ Pathway
Primary $\mathrm{CO}_{2}$ Acceptor	RuBP (Ribulose-1,5-bisphosphate), a 5-carbon compound.	PEP (Phosphoenolpyruvate), a 3-carbon compound.
First Stable Product	3-PGA (3-Phosphoglyceric acid), a 3-carbon compound.	OAA (Oxaloacetic acid), a 4-carbon compound.
Location	Occurs entirely in the mesophyll cells.	Initial fixation in mesophyll cells; Calvin cycle in bundle sheath cells.
Enzymes	RuBisCO is the primary carboxylating enzyme.	PEPcase in mesophyll cells and RuBisCO in bundle sheath cells.
Photorespiration	Occurs at high light and temperature, reducing efficiency.	Negligible or absent due to high $\mathrm{CO}_{2}$ concentration at the RuBisCO site.
Optimal Temperature	Lower (20-25°C)	Higher (30-40°C)
Productivity	Less productive, especially in tropical climates.	More productive and efficient in terms of carbon fixation.
Examples	Rice, Wheat, Soybean	Maize, Sugarcane, Sorghum

(b) Comparison between Cyclic and Non-cyclic Photophosphorylation

Feature	Cyclic Photophosphorylation	Non-cyclic Photophosphorylation
Photosystems Involved	Only Photosystem I (PS I) is functional.	Both Photosystem I (PS I) and Photosystem II (PS II) are involved.
Electron Source	The electron is cycled back to the PS I reaction centre.	Electrons come from the splitting of water ( $\ ext{H}_2\ ext{O}$ ).
Electron Flow	Electron flows in a cyclic manner.	Electron flows in a non-cyclic manner (Z-scheme).
Products	Only ATP is synthesized.	Both ATP and NADPH are synthesized.
Oxygen Evolution	No oxygen is released as water is not split.	Oxygen is released as a by-product of water splitting.
Location	Occurs mainly in the stroma lamellae.	Occurs in the grana lamellae (thylakoids).

(c) Comparison between Anatomy of Leaf in $\mathrm{C}_{3}$ and $\mathrm{C}_{4}$ Plants

Feature	$\mathrm{C}_{3}$ Leaf Anatomy	$\mathrm{C}_{4}$ Leaf Anatomy
Specialized Anatomy	No specialized 'Kranz' anatomy.	Presence of 'Kranz' anatomy ('Kranz' means wreath).
Bundle Sheath Cells	Bundle sheath cells are present but are small and lack a high number of chloroplasts.	Large bundle sheath cells are arranged in a tight ring around the vascular bundles.
Characteristics of Bundle Sheath Cells	Thin walls, few chloroplasts.	Thick walls impervious to gases, contain a large number of chloroplasts.
Mesophyll	Differentiated into an upper palisade layer and a lower spongy layer with intercellular spaces.	Not differentiated into palisade and spongy layers. Cells are compactly arranged.
Location of RuBisCO	Present in all mesophyll cells.	Absent in mesophyll cells, present only in bundle sheath cells.
Location of PEPcase	Absent.	Present in the cytoplasm of mesophyll cells.

Photosynthesis in Higher PlantsClass 11 Biology NCERT Solutions

By looking at a plant externally, can you tell whether a plant is C3\mathrm{C}_{3}C3​ or C4\mathrm{C}_{4}C4​? Why and how?

Solution

By looking at which internal structure of a plant you can tell whether a plant is C3\mathrm{C}_{3}C3​ or C4\mathrm{C}_{4}C4​? Explain.

Solution

Even though a very few cells in a C4\mathrm{C}_{4}C4​ plant carry out the biosynthetic - Calvin pathway, yet they are highly productive. Can you discuss why?

Solution

RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in C4\mathrm{C}_{4}C4​ plants?

Solution

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?

Solution

Why is the colour of a leaf kept in the dark frequently becomes yellow, or pale green? Which pigment do you think is more stable?

Solution

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?

Solution

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

Solution

Give comparison between the following:

Solution

By looking at a plant externally, can you tell whether a plant is $\mathrm{C}_{3}$ or $\mathrm{C}_{4}$ ? Why and how?

By looking at which internal structure of a plant you can tell whether a plant is $\mathrm{C}_{3}$ or $\mathrm{C}_{4}$ ? Explain.

Even though a very few cells in a $\mathrm{C}_{4}$ plant carry out the biosynthetic - Calvin pathway, yet they are highly productive. Can you discuss why?

RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in $\mathrm{C}_{4}$ plants?