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Question-1

What is absorption spectrum and action spectrum?

Solution:
Absorption spectrum: A curve obtained by plotting the amount of absorption of different wavelengths of light by a particular pigment is called absorption spectrum.

Action spectrum: The curve obtained by plotting the relative rates of photosynthesis at different wavelengths of light is called action spectrum.

Question-2

Name the first formed category of photosynthetic organisms.

Solution:
Cyanobacteria is the first formed category of photosynthetic organisms.

Question-3

What is the role of carotenoids in photosynthesis?

Solution:
Carotenoids have two major roles in photosynthesis:

(i) Carotenoids absorb radiant energy in the mid-region of the visible spectrum and hand over the same to the chlorophyll, thus, functioning as accessory pigments.

(ii) They protect chlorophyll molecules from photo-oxidation by picking up nascent oxygen and changing the same into a harmless molecular state.

Question-4

Name the various photosynthetic pigments.

Solution:
Chlorophyll a, chlorophyll b, Carotene and Xanthophyll are the various photosynthetic pigments.

Question-5

Differentiate between photorespiration and dark respiration.

Solution:

Photorespiration

Dark respiration

(i) It occurs only inside photosynthetic cells. (ii) It occurs in all living cells.
(ii) It takes place only in the presence of light. (ii) It occurs in both light and dark.
(iii) The substrate is RuBP. (iii) The substrate is commonly glucose though other food materials may also be used.
(iv) The substrate is always recently formed. (iv) The substrate may be recently formed or be a stored one.
(v) The end products are CO2 and PGA. (v) The end products are CO2 and water.
(vi) Both the end products are reutilized in photosynthesis. (vi) The end products are not recycled in respiration.
(vii) It occurs in chloroplasts, peroxisomes and mitochondria. (vii) It occurs in cytoplasm and mitochondria.
(viii) Toxic H2O2 is produced during the process. (viii) H2O2 is not produced.
(ix) It is a wasteful process and does not produce energy. (ix) It produces energy for cellular maintenance, synthesis and work.
(x) It increases with the increase in the concentration of oxygen. (x) It is not influenced by change in oxygen concentration.

 

Question-6

What is the main function of chlorophyll pigment ?

Solution:
The main function of chlorophyll pigment is to absorb light of specific wavelength in the visible region.

Question-7

Where would you find the pigment in the chloroplast ?

Solution:
The pigment is seen in the thylakoid membrane of the chloroplast.

Question-8

Write the significance of C4 pathway.

Solution:
The significance of C4 pathway is as follows:

(i) C4 plants have closely packed mesophyll cells, which provide a smaller area for better utilization of available water and reduction of the intensity of solar radiations. Thus, C4 plants are better adapted to tropical climates.

(ii) They are able to maintain high photosynthetic rates under low concentration of CO2 because the latter have greater affinity for phosphoenol pyruvic acid.

(iii) They do not show photorespiration. Hence, their photosynthetic rate is quite high.

(iv) They can tolerate saline soils because of the presence of C4 organic acids.

(v) They are adapted to the high temperatures and intense radiation of tropical

Question-9

What is the importance of photosynthesis?

Solution:
Photosynthesis is important in the following aspects:

(i) Photosynthesis is the only known process on Earth by which autotrophic organisms trap solar energy and convert it into food for rest of the organisms.

(ii) All useful plant products such as fodder, timber, firewood, fibres, resin, rubber, drugs, etc., are produced by the process of photosynthesis.

(iii) Coal, petroleum and natural gas are fossil fuels formed by photosynthetic organisms, which lived millions of years ago and got buried under the rocks. These form an important source of energy that is used to run many of our machines.

(iv) Photosynthesis is the only known process by which oxygen is added to the atmosphere, to compensate for oxygen being used by organisms and burning of organic fuels.

(v) Photosynthesis keeps the concentration of CO2 and O2 constant in the atmosphere by absorbing CO2 and releasing O2.

Question-10

Where are photosynthetic pigments located in the cyanobacteria ?

Solution:
The photosynthetic pigments are located on the membrane of cyanobacteria.

Question-11

Distinguish between photosynthesis and chemosynthesis.

Solution:

Photosynthesis

Chemosynthesis

(i) Photosynthesis occurs in all the chlorophyll-containing organisms such as algae, green bacteria and higher plants. (i) Chemosynthesis occurs only in some colourless, aerobic bacteria.
(ii) This process involves the participation of chlorophyll and sunlight. (ii) This process occurs without the participation of chlorophyll and sunlight.
(iii) In photosynthesis, light energy is converted into chemical energy stored in the form of carbohydrates. (iii) In this process, the chemical energy released during oxidation of inorganic substances is used to synthesize carbohydrates.
(iv) This process involves pigment systems to absorb light energy. (iv) This process does not require pigments.
(v) Molecular oxygen is evolved during this process. (v) Molecular oxygen is not evolved during this process

 

Question-12

Who proposed C4 cycle ?

Solution:
M. D.Hatch and C.R. Slack proposed C4 cycle.

Question-13

Define translocation of photosynthates.

Solution:
The synthesis of carbohydrates, through the process of photosynthesis, occurs in green cells of plants. The non-green cells are, therefore, dependent on photosynthetic cells for their carbohydrate supply. The carbohydrates synthesized in the leaves are distributed to roots and storage organs. The regions, such as green tissues, which synthesize carbohydrates are called source. A source has higher concentration of diffusible carbohydrates. While the regions, such as non-green tissues, where carbohydrates are utilized or stored are known as sink. A sink has a lower concentration of diffusible carbohydrates. Therefore, there is a movement of diffusible carbohydrates from the source to the sink. This long distance movement of carbohydrates synthesized through photosynthesis is called translocation of photosynthates.

Question-14

Give two examples of C4 plants.

Solution:
The two examples of C4 plants are, Maize and Amaranthus.

Question-15

How do internal factors affect photosynthesis?

Solution:
Internal factors affect photosynthesis in the following ways:

(i) Age of the leaf: As the leaf develops, the rate of photosynthesis increases gradually reaching a maximum at its fully expanded stage. Later photosynthesis gradually decreases with the age of the leaf.

(ii) Leaf anatomy: The rate of photosynthesis is partly influenced by the anatomical features of a leaf. These features include the number and degree of opening of stomata, extent of venation and the volume of intercellular spaces.

(iii) Chlorophyll content: Under normal conditions, chlorophyll content of leaves is not generally a limiting factor in photosynthesis. For example, the sun plants contain less chlorophyll in their leaves than those of shade plants but still exhibit higher rate of photosynthesis over shade plants.

(iv) Protoplasmic factor: Protoplasmic factor, probably enzymatic in nature, seems to be essential for photosynthesis.

Question-16

Name the cell organelles involved in photorespiration.

Solution:
Chloroplast, mitochondria and peroxisomes are the cell organelles involved in photorespiration.

Question-17

Define ‘quantum yield’.

Solution:
Quantum yield is defined as the number of oxygen molecules released per photon or quantum of light. Its value is 1/8 to 1/10 i.e. evolution of one molecule of O2 requires 8-10 quanta.

Question-18

Name the CO2 acceptor in C4 cycle.

Solution:
PEP is the CO2 acceptor in C4 cycle.

Question-19

Name a plant that can carry out photosynthesis at night.

Solution:
Opuntia is a plant that can carry out photosynthesis at night.

Question-20

Explain Crassulacean Acid Metabolism (CAM).

Solution:
Crassulacean Acid Metabolism refers to a mechanism of photosynthesis that is different from the already discussed C3 and C4 pathways. This occurs only in succulents and other plants that normally grow in dry conditions. In CAM plants, CO2 is taken up by the leaves on green stems through stomata, which remain open at night. However, during the day, the stomata remain closed in these plants to conserve moisture. The CO2 taken up at night is fixed in the same way as it happens in C4 plants to form malic acid, which is stored in the vacuole. The malic acid thus formed at night, is used during the day as a source of CO2 for photosynthesis to proceed via the C3 pathway. Thus, CAM is a kind of adaptation that allows certain plants to carry out photosynthesis without much loss of water.

Question-21

Name the process by which solar energy is trapped by autotrophic organisms and converted into food for the rest of the organisms.

Solution:
Photosynthesis is the process by which solar energy is trapped by autotrophic organisms and converted into food for the rest of the organisms.

Question-22

What are the disadvantages of photorespiration? Explain how the photorespiratory losses are overcome in plants like sugarcane?

Solution:
The disadvantages of photorespiration are,

(i) No energy rich compound is produced during this process.

(ii) As much as half of the photosynthetically fixed carbon dioxide may be lost by photorespiration.

(iii) It is a loss of the net productivity of green plants.

To overcome the photorespiratory losses plants like sugarcane and tropical plants have developed an additional cycle called as C-4 pathway. C-4 plants consume more than the usual 18 ATP molecules of energy to produce 1 molecule of glucose. C4 pathway is more energy expensive than C3 pathway.

Question-23

Give one point of difference between chlorophyll a and chlorophyll b.

Solution:
The chlorophyll a has methyl (CH3) group at the third carbon position of II pyrrolering of porphyrin head while chlorophyll b has an aldehyde group (CHO) at the same position.

Question-24

Why does chlorophyll appear green in reflected light and red in transmitted light?

Solution:
It is the property of the chlorophyll to absorb certain rays of light. The light is split up into seven colours and these colours form a series called spectrum. The absorption spectrum of chlorophyll is maximum for the red light, 700nm and it appears red in the transmitted light. This absorption spectrum for red light facilitates the process of photosynthesis to the maximum. The chlorophyll appears green in reflected light because green light of sunlight is not absorbed by the chlorophyll but is reflected back.

Question-25

What would be the effect of removing all green plants from the face of the earth?

Solution:
By removing all green plants, there would be no photosynthesis. In the absence of photosynthesis solar energy cannot be converted into food. All other living beings will not get food. Ultimately they will die and this earth will become lifeless.

Question-26

What takes over the function of photosynthesis in Opuntia ?

Solution:
Photosynthesis function is taken over by the stems in Opuntia.

Question-27

Name two photosynthetic pigments belonging to chlorophyll.

Solution:
Chlorophyll a and chlorophyll b are the two photosynthetic pigments belonging to chlorophyll.

Question-28

Photorespiration poses threat to plants, yet it occurs in angiosperms. Why?

Solution:
Photorespiration is a threat to plants because no energy rich compound is produced in this process. Half of the photosynthetically fixed carbon dioxide may be lost by photorespiration. Moreover it is a loss to the net productivity of green and C3 plants. To overcome the photorespiratory loses in angiosperms like sugarcane, they have developed an additional cycle called as C4 pathway that requires 30 ATP molecules.

Question-29

Are the enzymes that catalyse the dark reactions of carbon fixation located inside the thylakoids or outside the thylakoids?

Solution:
The stroma contains enzymes, which are capable of utilizing ATP and NADPH2 to produce carbohydrate, during dark reaction. The carbon fixation occurs in the stroma by a series of enzymes catalysed steps which are located outside the thylakoids and not inside the thylakoids.

Question-30

Specify two conditions in which photorespiration may take place in green plants.

Solution:
Presence of oxygen and presence of RuBP carboxylase enzyme.

Question-31

Where does carbon fixation occur in chloroplast?

Solution:
Carbon fixation takes place in the stroma of chloroplast by a series of enzyme catalysed reactions.

Question-32

Cite two examples of photosynthetic microorganisms which also fix atomospheric nitrogen.

Solution:
Photosynthetic microorganisms capable of fixing atmospheric nitrogen are,

(i) Anabaena and

(ii) Nostoc.

Question-33

From where do chemoautotrophs derive energy for synthesis of their food?

Solution:
Chemoautotrophs derive energy for synthesis of their food by oxidizing same inorganic substances and not from sunlight.

Question-34

How many ATP molecules are utilized for the formation of one molecule of glucose through C4 pathway.

Solution:
30 ATP molecules are utilized for the formation of one molecule of glucose through C4 pathway.

Question-35

Which wavelengths of visible spectrum is maximally absorbed by chlorophyll and which wavelength of light is least absorbed?

Solution:
Blue and red wavelengths of light are maximally absorbed by chlorophyll pigment and green wavelength is least absorbed.

Question-36

What is the end product of light reaction.

Solution:
ATP, NADPH2 and O2 are the end product of light reaction.

Question-37

Where do light and dark reactions occur in the chloroplast ?

Solution:
Light reaction occurs in the grana and dark reaction occurs in stroma.

Question-38

What is quantasomes?

Solution:
It is a distinct morphological structural unit in the thylakoids which embodies a photosynthetic unit.

Question-39

Specify two conditions in which photorespiration may take place in green plants.

Solution:
The two conditions in which photorespiration may take place in green plants are,

(i) In the presence of oxygen and

(ii) In the presence of RuBP carboxylase enzyme.

Question-40

What is photophosphorylation ?

Solution:
Photophosphorylation is the production of ATP during light reaction.

Question-41

In which reaction, the fixation of carbon dioxide occurs?

Solution:
The fixation of carbon dioxide occurs in the dark reaction.

Question-42

Which compound accepts CO2 in C4 pathway and in which cells of leaf is CO2 accepted in this pathway ?

Solution:
Phosphoenol pyruvic acid, a 3-carbon compound, accepts CO2 in mesophyll cells of the leaf in C4 pathway.

Question-43

Name the technique which was used to study the pathway of carbon in dark reaction.

Solution:
Autoradiography is the technique, which was used to study the pathway of carbon in dark reaction.

Question-44

From where do chemoautotrophs derive energy for synthesis of their food ?

Solution:
Chemoautotrophs derive energy for synthesis of their food by oxidizing same inorganic substance and not from sunlight.

Question-45

Which is the most abundant protein on earth ?

Solution:
Rubisco enzyme is the most abundant protein on earth.

Question-46

How many ATP molecules are utilized for the formation of one molecule of glucose through C2 pathway ?

Solution:
18 ATP molecules are utilized for the formation of one molecule of glucose through C2 pathway.

Question-47

Define photorespiration.

Solution:
Light dependent uptake of oxygen and release of carbon dioxide by the photosynthetic system is known as photorespiration.

Question-48

Explain the photochemical phase of photosynthesis. Where odes it takes place?

Solution:
The process of photosynthesis is completed into two phases:

(i) Light reaction or photochemical phase.

(ii) Dark reaction or biosynthetic phase.

The photochemical phase is called Hill reaction because it was discovered by Hill. It is associated with the grana of chloroplast. It occurs in the presence of light only, therefore it is known as photochemical phase of photosynthesis. The light reaction may include the following steps:

(a) Excitation of chlorophyll – On illumination chlorophyll molecule absorbs energy which cause in the shifting of an electron from the ground state of chlorophyll molecule to the excited state.

(b) Photolysis of water molecules – The excited chlorophyll transfers an electron from chlorophyll through various steps. The chlorophyll loses its electron from the water molecule in an electron transport system. The water molecule splits into H+ and OH_ions. During splitting of water molecule, the oxygen is releades.

(c) Photophosphorylation – Formation of ATP by linking ADP and Pi during photosynthetic electron transport system is called photophosphorylation.
These processes involve two sets of photochemical reaction which are catalysed by pigment system known as photosystem I and photosystem II.

Question-49

Explain why photosynthesis is considered the most important process in the biosphere?

Solution:
Photosynthesis is considered as the most important process in the biosphere because, photosynthesis depends on the entire living world as would be clear from the following account. All living things require energy to carry out various life processes. The energy is derived directly or indirectly from the sun. Green plants, which contain chlorophyll, trap the solar energy. This energy, which is fixed during photosynthesis, flows from the producer to the primary consumer and from herbivores to carnivores. Every food chain ends at decomposers. Some of the energy also directly passes from plants to decomposers. A considerable amount of energy is lost at heat or as non-consumable part. The ultimate amount available energy is so small that only a few organisms can be supported at apex of the food pyramid. If the sun’s energy is not available the life would not be possible on this earth. Also during photosynthesis green plants take in carbon dioxide and give out oxygen and hence they purify our surroundings or atmosphere. In the absence of photosynthesis solar energy cannot be converted into food. All other living beings will not get food. Hence, photosynthesis is the most important process in the biosphere.

Question-50

Where does non-cyclic photophosphorylation takes place? Describe the process. Why is the process referred to as non-cyclic?

Solution:
Non-cyclic photophosphorylation takes place in the grana of chloroplasts. The process initiates with excitation of special types of chlorophyll molecules P680 and P700 (letter P stands for pigment and the figures for the wavelength of light at which these molecules absorb). P680 and P700 molecules of chlorophyll form the reaction centers or photocenters. The accessory pigments and other chlorophyll molecules harvest the solar energy and pass it on to the reaction centers. Thus, a photon absorbed anywhere in the harvesting zone of P680 centre can pass its energy to the P680 molecules. The luster of pigment molecules, which transfer their energy to P680, absorb at or below the wavelength of 680nm. Together with P680 they form the photosystem I or photosytem II. Similarly, P700 forms photosystem I or PSII along with pigment molecules which absorb at or below 700nm. When P680 is ultimately compensated by P700 which transfers it to ferredoxin. In turn, ferredoxin transfers the electron to NADP to generated NADPH. As synthesis of ATP occurs in light and the process is not cyclic. I.e., the same elelctron are not coming back to the oxidized photosystems or in other words, it needs a constant supply of water molecules to be oxidized and NADP to be reduced, the process is called non-cyclic photophosphorylation.

Question-51

Explain the biosynthetic phase in brief.

Solution:
The photosynthetic reactions that lead to the reduction of carbon dioxide to glucose using ATP and reduced NADP are called dark reactions. These reactions takes place very fast and can occur in complete darkness, if the plant has been previously exposed to sunlight.In 1945, Melvin Calvin and his colleagues started a series of investigations that resulted in the overall understanding of the dark reactions of photosynthesis. They used the unicellular green alga Chlorella in their work. Their aim was to determine the pathway by which carbon dioxide became fixed into carbohydrates. They used radioactive 14C to trace the fate of carbon dioxide. After injecting 14C into all illuminated suspension of algae that had been carrying out photosynthesis with normal CO2  the algae were killed after a preselected time. The radioactive compounds were separated and identified by the technique of two dimensional paper chromatography. The findings of Calvin and his colleagues proved to be true for a wide variety of photosynthetic organisms ranging from photosynthetic bacteria to higher plants. The pathway of CO2 fixation that Calvin and his colleagues discovered is now known as Calvin Benson cycle or C3 cycle.




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