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Photosynthesis in Higher Plants NCERT Textbook With Solutions PDF Free Download
Chapter 13: Photosynthesis in Higher Plants
All animals including human beings depend on plants for their food. Have you ever wondered where plants get their food?
Green plants, in fact, have to make or rather synthesize the food they need and all other organisms depend on them for their needs.
The green plants make or rather synthesize the food they need through photosynthesis and are therefore called autotrophs.
You have already learned that autotrophic nutrition is found only in plants and all other organisms that depend on the green plants for food are heterotrophs.
Green plants carry out ‘photosynthesis’, a Physico-chemical process by which they use light energy to drive the synthesis of organic compounds. Ultimately, all living forms on earth depend on sunlight for energy.
The use of energy from sunlight by plants doing photosynthesis is the basis of life on earth. Photosynthesis is important due to two reasons: it is the primary source of all food on earth.
It is also responsible for the release of oxygen into the atmosphere by green plants.
Have you ever thought about what would happen if there were no oxygen to breathe?
This chapter focuses on the structure of the photosynthetic machinery and the various reactions
that transform light energy into chemical energy.
13.1 What Do We Know?
Let us try to find out what we already know about photosynthesis. Some simple experiments you may have done in the earlier classes have shown that chlorophyll (green pigment of the leaf), light, and CO2 are required for photosynthesis to occur.
You may have carried out the experiment to look for starch formation in two leaves – a variegated leaf or a leaf that was partially covered with black paper, and exposed to light.
On testing these leaves for the presence of starch it was clear that photosynthesis occurred only in the green parts of the leaves in the presence of light.
Another experiment you may have carried out is where a part of a leaf is enclosed in a test tube
containing some KOH-soaked cotton (which absorbs CO2 ), while the other half is exposed to air. The setup is then placed in light for some time.
On testing for the presence of starch later in the two parts of the leaf, you must have found that the exposed part of the leaf tested positive for starch while the portion that was in the tube, tested negative.
This showed that CO2 was required for photosynthesis. Can you explain how this conclusion could be drawn?
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NCERT Solutions Class 11 Biology Chapter 13 Photosynthesis in Higher Plants
1. By looking at a plant externally can you tell whether a plant is C3 or C4? Why and how?
Though plants that are adapted to dry tropical climates have the C4 pathway, they do not show any external morphologic characteristics. Hence it is not possible to say whether the plant is C3 or C4 by looking at its external appearance.
2. By looking at which internal structure of a plant can you tell whether a plant is C3 or C4? Explain.
Leaves of C4 plants have kranz anatomy which makes them different than C3 plants. C4 plants also have specialized cells called bundle sheath cells which surround the vascular bundles. Every cell of the bundle sheath has chloroplasts.
The mesophyll cells of leaves do not differentiate into the spongy and palisade parenchyma possessing fewer intercellular spaces while there is a normal occurrence of chloroplasts in the mesophyll cells.
But in C3 plants, the bundled health does not exhibit chloroplast and the mesophyll cells of the leaves are differentiated into the spongy and palisade parenchyma. Hence by examining the internal structure we can tell whether the plant is C3 or C4.
3. Even though very few cells in a C4 plant carry out the biosynthetic – Calvin pathway, they are highly productive. Can you discuss why?
Plant productivity can be estimated by the rate at which photosynthesis takes place. The amount of carbon dioxide in a plant is directly proportional to the rate of photosynthesis.
In C4 plants, very few of the cells carry out the biosynthetic pathway yet they are highly productive for the following reasons.
- They have a mechanism that increases the CO2 concentration at the site of the enzyme.
- Mesophyll cells are broken down in the bundle sheath cells resulting in CO2 release which in turn increases the intracellular CO2 concentration.
- Rubisco functions as a carboxylase minimizing the oxygenase activity.
- An increase in photosynthesis makes C4 plants more productive.
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?
The affinity of RUBISCO is much higher than its affinity for Oxygen. It is the concentration of Oxygen and CO2
that determines the binding of the enzyme. Mesophyll cells of C4 plants lack this enzyme but are found in the bundle sheath cells that girdle the vascular bundles where the Calvin cycle occurs.
RuBisco functions as oxygenase when the concentration of Oxygen is higher and it acts as carboxylase when the concentration of CO2 is high.
In the mesophyll cells, the primary carbon dioxide acceptor is a three-carbon compound – phosphoenol pyruvate which is converted into a four-carbon compound, oxaloacetic acid, or OAA.
This is converted further into malic acid which is transported to the bundle sheath cells where it undergoes decarboxylation and carbon fixation takes place through the Calvin cycle which prevents RuBisCo to serve as an oxygenase.
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?
In the absence of chlorophyll-a photosynthesis will not take place because chlorophyll-a is a reaction center responsible for the conversion of solar energy into chemical energy.
Although chlorophyll is the primary pigment that traps sunlight but accessory pigments like chlorophyll-b, xanthophylls and carotenoids also absorb sunlight and transfer energy to chlorophyll-a.
NCERT Class 11 Biology Textbook Chapter 13 Photosynthesis in Higher Plants With Answer PDF Free Download