GSEB Class 11 Biology Solutions Chapter 13 Photosynthesis in Higher Plants

Question 1. By looking at a plant externally can you tell whether a plant is C3 or C4? Why and how?
Answer: Often, plants thriving in arid environments utilize C4 pathways. It is not possible to state definitively if a plant is C3 or C4 from its outer look; however, an assumption can be formed by examining the thick leaf structure.
In simple words: Plants in dry places usually use C4 paths. You can't be sure if a plant is C3 or C4 just by looking at its outside, but you might guess by checking if its leaves are thick.

Exam Tip: Remember that external features provide clues, but internal anatomy is needed for a conclusive identification of C3 or 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 Calvin cycle happens in all mesophyll cells of a C3 plant. For C4 plants, it does not occur in mesophyll cells, but instead only in the bundle sheath cells. A further process causing a key distinction between C3 and C4 plants is photorespiration.
In simple words: The Calvin cycle's location tells you if a plant is C3 or C4. In C3 plants, it's in mesophyll cells. In C4 plants, it's only in bundle sheath cells. Photorespiration is also a key difference.

Exam Tip: Focus on the location of the Calvin cycle (mesophyll vs. bundle sheath cells) and the presence or absence of photorespiration as key distinguishing internal features.

Question 3. Even though very few cells in a C4 plant carry out the biosynthetic-Calvin pathway, yet they are highly productive. Can you discuss why?
Answer: C4 plants chemically attach carbon dioxide within mesophyll cells by joining it to the three-carbon molecule phosphoenolpyruvate. This reaction is aided by an enzyme named PEP carboxylase, which then forms the four-carbon organic acid, oxaloacetic acid. The oxaloacetic acid or malate created by this method then moves to specialized bundle sheath cells. In these cells, the enzyme rubisco and other Calvin cycle enzymes are found. There, CO2, freed by breaking down the four-carbon acids, is fixed by rubisco to form the three-carbon sugar 3-phosphoglyceric acids.
In simple words: C4 plants fix carbon dioxide in mesophyll cells using PEP carboxylase to make a four-carbon acid. This acid then moves to bundle sheath cells, where CO2 is released and fixed by rubisco. This two-step process allows C4 plants to be very efficient at photosynthesis, even though few cells do the Calvin cycle.

Exam Tip: When discussing C4 plant productivity, emphasize the two-step carbon fixation process involving PEP carboxylase in mesophyll cells and the concentration of CO2 in bundle sheath cells, which minimizes photorespiration.

Question 4. Rubisco is an enzyme that acts both as a carboxylase and oxygenase. Why do you think the RuBisco carries out more carboxylation in C4 plants?
Answer: The Rubisco enzyme, Earth's most common enzyme, can bind both CO2 and O2 at its active site. RuBisco shows a much stronger attraction for CO2 compared to O2. Consider what would occur if this were not true. This attachment is competitive. The varying amounts of O2 and CO2 decide which one will connect with the enzyme. Photorespiration does not happen in C4 plants. This is because they possess a system that boosts the CO2 level at the enzyme's location. This occurs when C4 acid from the mesophyll breaks down in bundle sheath cells, releasing CO2, which raises the internal CO2 concentration. As a result, this guarantees that RuBisco acts primarily as a carboxylase, thus reducing oxygenase activity.
In simple words: RuBisco prefers CO2 over O2. C4 plants have a special way to increase CO2 concentration where RuBisco works. They move a C4 acid to bundle sheath cells, where it releases CO2. This high CO2 level makes RuBisco mostly bind to CO2, doing more carboxylation and less oxygenation.

Exam Tip: Explain the competitive binding of Rubisco and then highlight the C4 mechanism that concentrates CO2 around Rubisco in bundle sheath cells, suppressing photorespiration and promoting carboxylation.

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: No, a plant lacking chlorophyll (a) would likely not carry out photosynthesis effectively. Chlorophyll (a) is known as the primary photosynthetic pigment, directly involved in converting light energy into chemical energy. Pigments are substances able to absorb light at particular wavelengths. We know about the visible light spectrum and its wavelengths, like VIBGYOR. Figure 13.1a shows chlorophyll (a) absorbs light most in blue and red regions. Figure 13.1b indicates that these same wavelengths lead to the highest rates of photosynthesis. So, chlorophyll (a) is the main pigment for photosynthesis. Other pigments, such as chlorophyll (b), xanthophylls, and carotenoids, are known as accessory pigments. These also take in light and pass the energy to chlorophyll (a). They assist by allowing a broader range of light wavelengths to be used for photosynthesis and also help protect chlorophyll (a) from damage due to too much light.
In simple words: A plant without chlorophyll (a) probably couldn't photosynthesize well because chlorophyll (a) is the main pigment that changes light into energy. Plants have chlorophyll (b) and other accessory pigments because they absorb different colors of light and pass that energy to chlorophyll (a). This helps the plant use more light for photosynthesis and protects chlorophyll (a) from harm.

Exam Tip: Clearly state that chlorophyll (a) is essential for primary photosynthesis, and then explain the dual role of accessory pigments: broadening the absorption spectrum and providing photoprotection for chlorophyll (a).

Question 6. Why is the color of a leaf kept in the dark frequently yellow, or pale green? Which pigment do you think is more stable?
Answer: Because leaves need light for photosynthesis, a leaf kept in the dark shifts color from darker to paler green. Occasionally, it also becomes yellow. The creation of chlorophyll, a pigment vital for photosynthesis, directly depends on the light quantity present. Without light, 'chlorophyll-a' molecule creation halts, and existing ones gradually break down. This causes the leaf's color to slowly fade to light green. In this period, xanthophyll and carotenoid pigments become dominant, making the leaf turn yellow. These pigments are more resilient because light is not necessary for them to be produced. They exist in plants constantly.
In simple words: Leaves turn pale green or yellow in the dark because chlorophyll, which needs light, stops being made and breaks down. Other pigments like xanthophylls and carotenoids, which are more stable and don't need light, then become more visible, giving the leaf a yellow color.

Exam Tip: Explain the degradation of chlorophyll in the absence of light and the unmasking of more stable accessory pigments (carotenoids and xanthophylls) to answer this question fully.

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: Leaves from plants kept in direct sunlight show a darker green color compared to those kept in shade. This occurs because of the interaction between the chlorophyll within the leaf and sunlight.
In simple words: Leaves in the sun are darker green than those in the shade. This is because sunlight helps make more chlorophyll in the leaves.

Exam Tip: Connect the intensity of sunlight directly to chlorophyll production; more sunlight generally leads to more chlorophyll and thus a darker green color.

Question 8. The figure shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions: 1. At which point/s (A, B, or C) in the curve is light a limiting factor? 2. What could be the limiting factor/s in region A? 3. What do C and D represent on the curve?
Answer:
1. Usually, light is not a restricting factor. It becomes a limiting factor for plants growing in shade or beneath tree cover. In the given graph, light is a limiting factor at the point where photosynthesis is at its lowest. The lowest value for photosynthesis is in region A. Therefore, light is a limiting factor here.
2. Light acts as a limiting factor in region A. Water, temperature, and the amount of carbon dioxide can also serve as restricting factors in this area.
3. D indicates the best point, showing the light intensity at which photosynthesis reaches its peak. The rate of photosynthesis stays constant beyond this point, even as light intensity goes up in region C.
In simple words: 1. Light is a limiting factor in region A because photosynthesis is lowest there. 2. In region A, besides light, water, temperature, and carbon dioxide levels could also be limiting photosynthesis. 3. Point D shows the perfect light level for maximum photosynthesis, after which (in region C), more light doesn't make photosynthesis faster.

Exam Tip: For graphs showing limiting factors, identify regions where the rate of the process is directly proportional to the factor (limiting) and where it plateaus (another factor is limiting or optimum has been reached).

Question 9. Give a comparison between the following:
(a) C2 and C4 pathways
Answer:

Exam Tip: When comparing C3 and C4 pathways, highlight the primary CO2 acceptor, the first stable product, the location of carbon fixation, and the efficiency regarding photorespiration.

Question 9. Give a comparison between the following:
(b) Cyclic and non-cyclic photophosphorylation
Answer:

Exam Tip: Distinguish between cyclic and non-cyclic photophosphorylation based on the photosystems involved, the products formed (ATP, NADPH), whether water is split, and the electron flow (cyclic or non-cyclic).

Question 9. Give a comparison between the following:
(c) Anatomy of leaf in C3 and C4 plants
Answer:

Exam Tip: For leaf anatomy comparison, focus on the presence/absence of bundle sheath cells (Kranz anatomy), the location of RuBisCo, and the primary carbon fixation product to differentiate between C3 and C4 plants effectively.