Gujarat Board GSEB Textbook Solutions Class 11 Biology Chapter 13 Photosynthesis in Higher Plants Textbook Questions and Answers.
Gujarat Board Textbook Solutions Class 11 Biology Chapter 13 Photosynthesis in Higher Plants
GSEB Class 11 Biology Photosynthesis in Higher Plants Text Book Questions and Answers
By looking at a plant externally can you tell whether a plant is C3 or C4? Why and how?
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, some guess can be made by looking at the fleshy leaf structure.
By looking at which internal structure of a plant can you tell whether a plant is C3 or C4? Explain.
The Calvin pathway occurs in all the mesophyll cells of the C3 plant. In the C4 plants, it does not take place in the mesophyll cells but does so only in the bundle sheath cells. Understand one more process that creates an ” important difference between C3 and C4 plants-Photorespiration.
Even though very few cells in a C4 plant carry out the biosynthetic-Calvin pathway, yet they are highly productive. Can you discuss why?
C4 plants chemically fix carbon dioxide in the cells of the mesophyll by adding it to the three-carbon molecule phosphoenolpyruvate, a reaction catalyzed by an enzyme called PEP carboxylase and which creates the four-carbon organic acid, oxaloacetic acid. Oxaloacetic acid or malate synthesized by this process is then translocated to specialized bundle sheath cells where the enzyme, rubisco, and other Calvin cycle enzymes are located and where CO2 released by decarboxylation of the four-carbon acids is fixed by rubisco activity to the three-carbon sugar 3 – phosphoglyceric acids.
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?
Rubisco enzyme is the most abundant enzyme in the world is characterized by the fact that its active site can bind to both CO2 and O2. The RuBisco has a much greater affinity for CO2 than for O2. Imagine what would happen if this were not so This binding is competitive. It is the relative concentration of O2 and CO2 that determines which of the two will bind to the enzyme.
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.
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?
Pigments are substances that have the ability to absorb light, at specific wavelengths. With the ability of chlorophyll a pigment to absorb light of different wavelengths of course we are familiar with the wavelength of the visible spectrum of light as well as the VIBGYOR.
From Figure 13.1a can we determine the wavelength (color of light) at which chlorophyll (a) shows maximum absorption? Now, look at figure 13.1b showing the wavelengths at which maximum photosynthesis occurs in a plant. We can see that the wavelengths at which there is maximum absorption by chlorophyll (a) i.e. in the blue and the red regions, also show a higher rate of photosynthesis. Hence, we can conclude that chlorophyll (a) is the chief pigment associated with photosynthesis. But by looking at figure 13.1c can we say that there is a complete one-to-one overlap between the absorption spectrum of chlorophyll (a) and the action spectrum of photosynthesis?
These graphs, together, show that most of the photosynthesis takes place in the blue and red regions of the spectrum; some photosynthesis does take place at the other wavelengths of the visible spectrum. Let us see how this happens. 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 wavelength of incoming light to be utilized for photosynthesis but also protect chlorophyll (a) from photooxidation.
Why is the color of a leaf kept in the dark frequently yellow, or pale green? Which pigment do you think is more stable?
Since leaves require light to perform photosynthesis, the colour of a leaf kept in the dark changes from a darker to a lighter shade of green. Sometimes, it also turns yellow. The production of the chlorophyll pigment essential for photosynthesis is directly proportional to the amount of light available. In the absence of light, the production of ‘chlorophyll – a‘ molecules stops and they get broken slowly.
This changes the colour of the leaf gradually to light green. During this process, the xanthophyll and carotenoid pigments become predominant, causing the leaf to become yellow. These pigments are more stable as light is not essential for their production. They are always present in plants.
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?
The leaves of the pot kept in sunlight are darker in color than the leaves of the pot kept in the shade. This is due to the reaction between the chlorophyll present in the leaf and the sunlight.
The figure shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions:
- At which point/s (A, B, or C) in the curve is light a limiting factor?
- What could be the limiting factor/s in region A?
- What do C and D represent on the curve?
1. Generally, light is not a limiting factor. It becomes a limiting factor for plants growing in shade or under tree canopies. In the given graph, light is a limiting factor at the point where photosynthesis is the minimum. The least value for photosynthesis is in region 4. Hence, light is a limiting factor in this region.
2. Ught is a limiting factor in region A water, temperature, and the concentration of carbon dioxide can also be limiting factors in this region.
3. D represents the optimum point and gives the light intensity at which the maximum photosynthesis is recorded. The rate of photosynthesis remains constant after this point, even though the intensity of light increases in region C.
Give a comparison between the following:
(a) C2 and C4 pathways
|C3 pathway||C4 pathway|
|(1) The primary acceptor of CO2 is RUBP – a five-carbon compound||The primary acceptor of CO2 is phosphoenolpyruvate a three-carbon compound.|
|(2) The first stable product is
3 – phosphoglycerate
|The first- stable product is oxaloacetic acid|
|(3) It occurs only in the mesophyll cells of the leaves||It occurs in the mesophyll and bundle sheath cells of leaves|
|(4) It is a slow process of carbon fixation and photo-respiratory losses are high||It is a faster process of carbon fixation and photo-respiratory losses are low|
(b) Cyclic and non-cyclic photophosphorylation
|Cyclic photophosphorylation||Non-Cyclic photophosphorylation|
|(1) It occurs only in photosystem I||It occurs in photosystem I and II|
|(2) It involves only the synthesis of ATP||It involves the synthesis of ATP and NADPH2|
|(3) In this process, photolysis of water does not occur. Therefore, oxygen is not produced||In this process, photolysis of water takes place and oxygen is liberated|
|(4) In this process, electrons move in a closed circle||In this process, electrons do not move in a closed circle.|
(c) Anatomy of leaf in C3 and C4 plants
|C3 leaf||C4 leaf|
|(1) Bundle-sheath cells are absent||Bundle-sheath cells are present|
|(2) RuBisCo is Present in the mesophyll cells||RubisCo is present in the bundle sheath cells|
|(3) The first stable compound produced is 3- phosphoglycerate a three-carbon compound||The first stable compound produced is oxaloacetic acid – a four-carbon compound|
|(4) photorespiration occurs||photorespiration does not occur|