Photosynthesis In Higher Plants

It is essential for life on Earth, providing the primary source of energy for most ecosystems.

Key Components of Photosynthesis:

• Chloroplasts: The organelles responsible for photosynthesis, containing chlorophyll pigments.
• Chlorophyll: Green pigments that absorb sunlight.
• Light Reaction: The initial stage of photosynthesis, occurring in the thylakoid membranes of chloroplasts. It involves the conversion of light energy into chemical energy (ATP and NADPH).
• Dark Reaction (Calvin Cycle): The second stage of photosynthesis, occurring in the stroma of chloroplasts. It involves the use of ATP and NADPH to convert carbon dioxide into glucose.

Factors Affecting Photosynthesis:

• Light intensity: More light generally leads to higher rates of photosynthesis, up to a certain point.
• Light quality: Red and blue light are most effective for photosynthesis.
• Temperature: Photosynthesis increases with temperature up to an optimum point, beyond which it decreases due to enzyme denaturation.
• CO2 concentration: Increased CO2 concentration generally leads to higher rates of photosynthesis.
• Water availability: Water is essential for photosynthesis as it is a reactant in the light reaction.

Significance of Photosynthesis:

• Energy production: Photosynthesis provides the primary source of energy for most ecosystems.
• Oxygen production: Photosynthesis releases oxygen into the atmosphere, essential for aerobic respiration.
• Carbon fixation: Photosynthesis removes carbon dioxide from the atmosphere, helping to regulate the Earth’s climate.

Exercise

1. By looking at a plant externally can you tell whether a plant is C3 or C4 ? Why and how?

Ans :

Yes, in some cases, you can visually distinguish between C3 and C4 plants based on their external characteristics. However, it’s not always definitive, and other factors like leaf anatomy and physiology may be required for a more accurate determination.

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

Ans : 

The Kranz anatomy is the key internal structure that can help you distinguish between C3 and C4 plants.

• Bundle Sheath Cells: A ring of cells surrounding the vascular bundle (a cluster of xylem and phloem tissue).
• Mesophyll Cells: Cells located between the bundle sheath and the upper and lower epidermis.

In C3 plants, the mesophyll cells are not organized in a distinct Kranz arrangement, and the bundle sheath cells are relatively small.

In C4 plants, the Kranz anatomy is clearly visible, with a well-defined ring of bundle sheath cells and larger mesophyll cells. This anatomical structure is essential for the C4 photosynthetic pathway, which involves a spatial separation of the light-dependent and light-independent reactions.

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? 

Ans : The high productivity of C4 plants, despite the relatively few cells carrying out the Calvin cycle, is primarily due to their specialized anatomical structure and biochemical adaptations.

The high productivity of C4 plants is a result of their unique anatomical adaptations, biochemical pathways, and ability to minimize photorespiration. These factors allow them to efficiently capture and utilize CO2, leading to higher rates of photosynthesis and overall productivity.

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?

Ans : RuBisCO’s preference for carboxylation over oxygenation in C4 plants is primarily due to the specialized anatomical structure and biochemical adaptations of these plants.

By creating a favorable environment with a high concentration of CO2 and using a less oxygen-sensitive enzyme, C4 plants can effectively promote carboxylation by RuBisCO, leading to higher photosynthetic efficiency compared to C3 plants.

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?

Ans : No, plants that had a high concentration of Chlorophyll b but lacked Chlorophyll a would not be able to carry out photosynthesis.

It is essential for capturing sunlight and transferring the energy to the electron transport chain, which drives the production of ATP and NADPH. While Chlorophyll b can also absorb sunlight, it is less efficient than Chlorophyll a at transferring energy to the electron transport chain.

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?

Ans : 

The color of a leaf kept in the dark frequently appears yellow or pale green due to the breakdown of chlorophyll.

Chlorophyll, the primary pigment responsible for the green color of plants, is a relatively unstable molecule. It requires sunlight to maintain its structure and function. When a leaf is kept in the dark, chlorophyll production slows down or ceases, leading to its breakdown. As chlorophyll breaks down, other pigments, such as carotenoids and xanthophylls, become more visible, giving the leaf a yellow or pale green hue.

Carotenoids and xanthophylls are generally more stable than chlorophyll. These pigments are responsible for the orange, yellow, and brown colors observed in leaves during autumn, when chlorophyll production declines. They are more resistant to degradation and can persist in the leaf tissue even after chlorophyll has been broken down.

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?

Ans : 

Leaves on the sunny side of a plant are generally darker green than those on the shady side.

This is because plants exposed to more sunlight produce more chlorophyll, the pigment responsible for the green color of leaves. When plants are exposed to more sunlight, they need more chlorophyll to capture the energy efficiently.

Similarly, potted plants kept in sunlight will typically have darker green leaves than those kept in the shade. The increased light exposure stimulates chlorophyll production, resulting in a deeper green 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 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?

Ans : 

(a) In regions A and B, light is the limiting factor. 

(b) In region A’, light may act as a limiting factor. 

(c) In region C, the rate of photosynthesis does not increase with higher light intensity, and at point D, other factors become limiting.

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 

Ans : 

(a) C3 and C4 pathways 

(b) Cyclic and non-cyclic photophosphorylation 

(c) Anatomy of leaf in C3 and C4 plants