Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Balbharti Maharashtra State Board 11th Biology Important Questions Chapter 12 Photosynthesis Important Questions and Answers.

Maharashtra State Board 11th Biology Important Questions Chapter 12 Photosynthesis

Question 1.
What is the percentage of C02 in atmosphere?
Answer:
Atmosphere contains only about 0.03 percent C02 by volume.

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 2.
Define photosynthesis and give its reaction with the help of solar energy trapped by pigments like chlorophyll.
Answer:
Photosynthesis is defined as synthesis of carbohydrates (glucose) from inorganic materials like CO2 and H20 with the help of solar energy trapped by pigments like chlorophyll.

Question 3.
Explain diagrammatically the ultrastructure of chloroplast.
Answer:

  1. The chloroplasts are discoid and lens shaped in higher plants. Chloroplast is bounded by a double membrane.
  2. System of chlorophyll bearing a double-membrane sac is present inside the stroma. These are stacked one above the other to form grana.
  3. Individual sacs in each granum is are known as thylakoid.
  4. All the pigments chlorophylls, carotenes and xanthophylls are located in thylakoid membranes.
  5. These pigments are fat soluble and are present in lipid part of membrane also they absorb light of specific spectrum in the visible regions.

Question 4.
Distinguish between grana and stroma.
Answer:

Grana Stroma
1. These are formed of stacks of thylakoids. It is the matrix of chloroplast.
2. Light reaction occurs in grana. Dark reaction occurs in stroma.

Question 5.
Describe a structure of a chlorophyll molecule.
Answer:
1. Chemically chlorophyll molecule consists of two parts head of tetrapyrrole the porphyrin ring and a long hydrocarbon tail called phytol attached to the porphyrin group.
2. Both chlorophyll-a and chlorophyll-b are similar in their molecular structure, except that the methyl group (-CH3) in chlorophyll-a is replaced with an aldehyde group (-CHO) in chlorophyll-b.

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 6.
What are Carotenoids?
Answer:

  1. Carotenoids are lipid compound present universally in almost all the higher plants and several microorganisms.
  2. They are usually red, orange, yellow, brown, and are associated with chlorophyll. They are of two types – the carotenes and xanthophylls.
  3. The carotenes (C40H56) are orange red and xanthophylls contain oxygen.
  4. The light energy absorbed by the carotenoids is transferred to chlorophyll-a to be utilized in photosynthesis.
  5. All photosynthetic plants have these pigments that absorb light between the red and blue region of the
    spectrum.
  6. Carotenoids found mainly in higher plants, absorb primarily in the violet to blue regions of the spectrum.
  7. They not only absorb light energy and transfer it to chlorophyll but also protect the chlorophyll molecule from photo-oxidation.

Question 7.
Write a short note on nature of light.
Answer:

  1. Light is a form of energy.
  2. It travels as stream of tiny particles called photons.
  3. A photon contains a quantum of light.
  4. Light has different wavelengths having different colors.
  5. One can see electromagnetic radiation with wavelengths ranging from 390nm to 730nm. This part of the spectrum is called the Visible light.
  6. It lies between wavelengths of ultraviolet and infra-red.

Question 8.
Draw neat and labelled diagram of absorption and action spectrum.
Answer:
Absorption and action spectra

Question 9.
Write the chemical reaction given by Van Niel in case of bacteria that use H2S and C02 to synthesize carbohydrates. Write his postulate.
Answer:
Chemical reaction given by Van Niel:
6C02 + 12H2S → C6H1206 + 6H20 + 12S ↓
Van Niel’s postulate:
Green plants use water in place of H2S and 02 is evolved in place of sulphur.

Question 10.
What was used by Ruben to confirm that the source of oxygen evolved in photosynthesis is water?
Answer:
Ruben used heavy isotope of oxygen (1802) to confirm that the source of oxygen evolved in photosynthesis is water.

Question 11.
Explain Hill reaction.
Answer:
In this experiment, Hill cultured isolated chloroplasts in a medium containing C02 free water, haemoglobin and ferric compound.

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 12.
Give reason. Why photosynthesis is a redox reaction?
Answer:
Photosynthesis is considered as a redox reaction as it involves both reduction and oxidation reactions.
Water is oxidized by the removal of H+ while C02 is reduced by the addition of H+.
The redox reactions of photosynthesis are necessary for the conversion of light energy into chemical energy.

Question 13.
Which are the two reactions involved in the process of photosynthesis.
Answer:

  1. The process of photosynthesis is an oxidation and reduction process in which water is oxidized (to release 02) and C02 is reduced to form sugar. It consists of two successive series of reactions.
  2. The first reaction requires light and is called Light reaction.
  3. Second reaction does not require light and is called Dark or Blackman reaction.
  4. Of the two reactions, the former is a photochemical reaction, while the latter is a biochemical reaction.

Question 14.
What is light reaction?
Answer:
1. The light reaction is a reaction in which solar energy is trapped by chlorophyll and stored in the form of chemical energy as ATP and in the form of reducing power as NADPH2.
2. Oxygen is evolved in the light reaction by splitting of water.

Question 15.
Define: Quantum energy
Answer:
A certain minimum quantity of energy a photon must have to boost an electron is called quantum energy.

Question 16.
What is excited state?
Answer:
A molecule that has absorbed a photon is in energy rich excited state. An excited state of an atom means that the valence electron has moved from its ground state orbital to high energy orbital.

Question 17.
What is ground state?
Answer:
When the light source is turned off, the high energy electrons return rapidly to their normal low energy orbitals as the excited molecule reverts to its original stable condition, called the ground state.

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 18.
Explain the structure of reaction centre.
Answer:
Solar energy is trapped by chlorophyll and stored in the form of chemical energy as ATP and in the form of reducing power as NADPH2. Oxygen is evolved in the light reaction by splitting of water. The components of light reaction are as follows:

Reaction centre:

  1. The light absorbing pigments present in thylakoid membranes are arranged in clusters of chlorophyll and accessory pigments.
  2. P-680 and P-700 are the special type of chlorophyll molecules which form the reaction centres or photocentres.
  3. Solar energy is harvested by accessory pigments and other chlorophyll molecule and is passed on to the reaction centre.
  4. These (accessory pigments) are known as light harvesting or antenna molecule. Their function is to absorb light energy and transmit at a very high rate to the reaction centre where the photochemical reactions occur.

Question 19.
What are the two types of photosystems?
Answer:
1. Photosystems:
a. Two kinds of photosystems are present in thylakoid membranes of chloroplasts.
b. Each has its own set of light harvesting chlorophyll and carotenoid molecules.
c. Chlorophyll and accessory pigments help to capture light energy over large area and pass it on to the photocenters.
d. Thus, a photon absorbed anywhere in the harvesting zone of P-680 center can pass its energy to the P680 molecule.
e. The cluster of pigments which transfer their energy to P-680 absorb at or below 680nm.
f. Together with P-680 they form Photosystem – II.
g. Likewise, P-700 forms Photosystem – I along with pigment molecule which absorbs light at or below 700 nm.

2. Photosystem II:
a. Photolysis of water and release of oxygen takes place in this system.
b. In this process, when PS-II absorbs light, electrons are released and chlorophyll molecule is oxidized.
c. Electrons emitted by P680 (PS-II) are ultimately trapped by P700 (PS -1).
d. Oxygen is the byproduct by the photosynthesizing plants.
e. Protons accumulate inside the thylakoid resulting in a Proton gradient.
f. When the protons diffuse across the thylakoid membrane into stroma against the H+ gradient, energy, is released.
g. This is used to produce ATP.

3. Photosystem 1:
a. Upon absorption of light quanta by PS-I (P700) reaction center emits energy rich electrons.
b. These flow down a chain of electron carriers to NADP along with the proton generated by splitting of water.
c. This result in the formation of NADPH.
d. Hydrogen attached to NADPH is used for reduction of C02 in dark reaction also called as reducing power of the cell.

Question 20.
The oxidized P-680 regains its electrons by the photolysis of water. Mention the reactions related to the same.
Answer:
The oxidized P-680 regains its electrons by the photolysis of water as follows:

  1. 4H2O → 4H+ + 40H
  2. 4OH → 4(OH) + 4e
  3. 4OH → 2H20 + 02
  4. 4H2O → 2H2O + 02 ↑ + 4H+ 4e Overall reaction

Question 21.
Distinguish between Photosystem I and Photosystem II
Answer:

  1. The movement of ions across a selectively permeable membrane, down their electrochemical gradient is called chemiosmosis.
  2. The movement of hydrogen ions across a membrane during cellular respiration in mitochondria and during photosynthesis in chloroplasts, leads to the generation of ATP.
  3. These membranes are thylakoids and the protons accumulate in the lumen during photosynthesis.
  4. An electrochemical concentration gradient forms when hydrogen ions (protons) diffuse from an area of higher proton concentration to an area of lower proton concentration.
  5. This electrochemical concentration gradient of protons across a membrane can be utilized to make
    ATP.
  6. ATP synthase is the enzyme that makes ATP by chemiosmosis.
  7. It allows protons to pass through the membrane using the kinetic energy to phosphorylate ADP making ATP.
  8. Splitting of water molecule on the inner side of the membrane results in the accumulation of hydrogen ions within the lumen of thylakoids.
  9. The enzyme, NADP reductase, is located in the stroma side of the membrane.
  10. For reduction of NADP to NADPH2, protons are required along with electrons that come from ferredoxin.
  11. Thus, within the chloroplast, the protons in the stroma decrease in number, while in the lumen, the number of protons increases.
  12. This creates a proton gradient across the thylakoid membrane.
  13. Energy,generated by the subsequent spontaneous movement of protons is used for the synthesis of ATP.

Question 22.
Draw neat and labelled diagram representing ATP synthesis through chemiosmosis.
Answer:
Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis 1

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 23.
What is photophosphorylation?
Answer:
Formation of ATP in the chloroplasts in presence of light is called photophosphorylation, light
Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis 2

Question 24.
Mention the light reactions occurring in in granum.
Answer:

  1. 24H2O → 24OH + 24H+
  2. 24OH → 240H + 24e
  3. 24e + 24H+ + 12NADP+ → 12NADPH2
  4. 18ADP + 18Pi → 18 ATP
  5. 240H → 12H2O + 6O2

Question 25.
Write the dark reaction occurring in stroma.
Answer:
602+ 18ATP + 12NADPH2 → C6H12O2 + 6H2O + 18ADP +18Pi+ 12NADP+

Question 26.
Name the following:
1. Name the first compound formed during C3 pathway.
2. Carbon fixation occurs in?
Answer:
1. 3-phosphoglyceric acid.
2. Stroma

Question 27.
Where does this reaction take place?
6C02 + 18ATP + 12NADPH2 → C6H12O6 + 6H20 + 18ADP + 18Pi + 12NADP+
Answer:
In stroma of a chloroplast.

Question 28.
Who discovered the Carbon fixation pathway? How was the pathway studied?
Answer:

  1. The path of carbon fixation in dark reaction through intermediate compounds leading to the formation of sugar and starch was discovered by Calvin, Benson and their co-workers discovered the carbon fixation pathway.
  2. Path of carbon was studied with the help of radioactive tracer technique using Chlorella, a unicellular green alga and radioactive 14C02.
  3. With the help of radioactive carbon, it becomes possible to trace the intermediate steps of fixation of 14C02.

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 29.
When does photorespiration occur?
Answer:
1. Photorespiration occurs under the conditions like high temperature, bright light, high oxygen and low C02 concentration. It is a wasteful process linked with C3-Cycle, where instead of fixation of C02, it is given out.
2. Photorespiration is a respiratory process in many higher plants by which they take up oxygen in the light and give outiearbon dioxide.

Question 30.
Diagrammatically explain the process of photorespiration.
Answer:
Mechanism:

  1. Photorespiration involves three organelles chloroplast, peroxisomes and mitochondria and occurs in a series of cyclic reactions which is also called PCO cycle. (Photosynthetic Carbon Cycle)
  2. Enzyme Rubisco acts as oxygenase at higher concentration of O2 and photorespiration begins.
  3. When RuBP reacts with 02 rather than C02 to form a 3-carbon compound (PGA) and 2-carbon compound phosphoglycolate.
  4. Phosphoglycolate is then converted to glycolate which is shuttled out of the chloroplast into the peroxisomes.
  5. In Peroxisomes, glycolate is converted into glyoxylate by enzyme glycolate oxidase.
  6. Glyoxylate is further converted into amino acid glycine by transamination.
  7. In mitochondria, two molecules of glycine are converted into serine (amino acid) and C02 is given out.
  8. Thus, it loses 25% of photosynthetically fixed carbon.
  9. Serine is transported back to peroxisomes and converted into glycerate.
  10. It is shuttled back to chloroplast to undergo phosphorylation and utilized in formation of 3-PGA, which get utilized in C3 pathway.

Question 31.
Explain Hatch – Slack pathway.
Answer:
1. M. D. Hatch and C. R. Slack while working on sugarcane found four carbon compounds (dicarboxylic acid) as the first stable product of photosynthesis.
2. It occurs in tropical and sub-tropical grasses and some dicotyledons.
3. The first product of this cycle is a 4-carbon compound oxaloacetic acid. Hence it is also called as C4 pathway and plants are called C4 plants.
Mechanism:

  1. C02 taken from atmosphere is accepted by a 3-carbon compound, phosphoenolpyruvic acid in the chloroplasts of mesophyll cells, leading to the formation of 4-C compound, oxaloacetic acid with the help of enzyme phosphoenolpyruvate carboxylase.
  2. It is converted to another 4-C compound, malic acid.
  3. It is transported to the chloroplasts of bundle sheath cells.
  4. Malic acid (4-C) is converted to pyruvic acid (3-C) with the release of C02 in the cytoplasm.
  5. Thus, concentration of C02 increases in the bundle sheath cells.
  6. Chloroplasts of these cells contain enzymes of Calvin cycle.
  7. Because of high concentration of C02, RuBP carboxylase participates in Calvin cycle and not photorespiration.
  8. Sugar formed in Calvin cycle is transported into the phloem.
  9. Pyruvic acid generated in the bundle sheath cells re-enter mesophyll cells and regenerates
    phosphoenolpyruvic acid by consuming one ATP.
  10. Since this conversion results in the formation of AMP (not ADP), two ATP are required to regenerate ATP from AMP. Thus, C4 pathway needs 12 additional ATP.
  11. The C3 pathway requires 18 ATP for the synthesis of one glucose molecule, whereas C4 pathway requires 30 ATP.
  12. Thus, C4 plants are better photosynthesizers as compared to C3 plants as there is no photorespiration in these plants.

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis 3

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 32.
Draw a neat labelled diagram of Kranz anatomy of C4 plant.
Answer:
Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis 4

Question 33.
Name the following:
1. Name the first product of C02 fixation in Hatch – Slack Pathway.
2. C4 plant leaf shows which type of anatomy?
3. Example of a C4 Plant
Answer:
1. Oxaloacetic acid.
2. Kranz Anatomy
3. Maize, Sugarcane, Sorghum, etc.

Question 34.
Short note on Kranz Anatomy of a C4 plant.
Answer:

  1. Anatomy of leaves of C4 plants is different from leaves of C3 plants.
  2. C4 plants show Kranz anatomy.
  3. In the leaves of such plants, there is a bundle sheath around the vascular bundles.
  4. The chloroplasts in the bundle – sheath cells are large and without or less developed grana, where as in the mesophyll cells the chloroplasts are small but with well-developed grana.

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 35.
Short note on: Crassulacean Acid Metabolism.
Answer:
Crassulacean Acid Metabolism (CAM).

  1. It is one more alternative pathway of carbon fixation found in desert plants.
  2. It was first reported in the family Crassulaceae, therefore called as CAM (Crassulacean Acid Metabolism).
  3. In CAM plants, stomata are scotoactive i.e. active during night, hence initial C02 fixation occurs in night.
  4. Thus, C4 pathway fix C02 at night and reduce C02 in day time via the C3 pathway by using NADPH formed during the day.
  5. PEP caboxylase and Rubisco are present in the mesophyll cell (no Kranz anatomy).
  6. Formation of malic acid during dark is called acidification (phase I).
  7. Malate is stored in vacuoles during the night.
  8. Malate releases C02 during the day for C3 pathway within the same cell is called deacidification (phase II).
  9. Examples of CAM plants: Kalanchoe, Opuntia, Aloe etc.
  10. The Chemical reactions of the carbon dioxide fixation and its assimilation are similar to that of C4 plants.

Question 36.
1. Draw a neat labelled diagram of CAM pathway.
2. In CAM plants, why does acid concentration increase during night?
Answer:
Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis 5
2. In CAM plants, malic acid accumulates during night, which is formed from Oxaloacetic acid in presence of the enzyme malate dehydrogenase.

Question 37.
What are the external factors which affects photosynthesis?
Answer:
External factors which affect photosynthesis are as follows:
1. Light:
a. It is an essential factor as it supplies the energy necessary for photosynthesis.
b. Quality and intensity of light affects the photosynthesis.
c. Highest rate of photosynthesis takes place in red light followed by blue light.
d. The rate of photosynthesis considerably decreases in plants which are growing under a forest canopy.
e. In most of the plants, photosynthesis is maximum in bright diffused sunlight.
f. Uninterrupted and continuous photosynthesis for a very long period of time may be sustained without any visible damage to the plant.

2. Carbon dioxide:
The main source of C02 in land plants is the atmosphere, which contains only 0.3% of the gas.
b. Under normal conditions of temperature and light, carbon dioxide acts as a limiting factor in photosynthesis.
c. Increase in concentration of CO2 increases the photosynthesis.
d. Increase in C02 to about 1% is advantageous to most of the plants.
e. Higher concentration of the gas has an inhibitory effect on photosynthesis.

3. Temperature:
a. Like all other physiological processes, photosynthesis also needs a suitable temperature.
b. The optimum temperature at which the photosynthesis is maximum is 25-30 °C. Except in plants like Opuntia, photosynthesis takes place at as high as 55 °C.
c. This is the maximum temperature. Minimum temperature is temperature at which photosynthesis process just starts.
d. In the presence of sufficient light and CO2, photosynthesis increases with the rise of temperature till it becomes maximum. After that there is a decrease or fall in the rate of the process.

4. Water:
a. Water is necessary for photosynthetic process.
b. An increase in water content of the leaf results in the corresponding increase in the rate of photosynthesis.
c. Thus, the limiting effect of water is not direct but indirect.
d. It is mainly due to the fact that it helps in maintaining the turgidity of the assimilatory cells and the proper hydration of their protoplasm.

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 38.
What are the internal factors which affects photosynthesis?
Answer:
Internal factors which affects photosynthesis are as follows:
1. Chlorophyll:
a. Though presence of chlorophyll is essential for photosynthesis but rate of photosynthesis is proportional to the quantity of chlorophyll present.
b. It is because of the fact that chlorophyll merely acts as a biocatalyst and hence a small quantity is quite enough to maintain the large bulk of the reacting substances.
2. Sugar:
The final product in the photosynthesis reaction is sugar and its accumulation in the cells slow down the process of photosynthesis.
3. Internal structures:
The thickness of cuticle and epidermis of the leaf, the size and distribution of intercellular spaces and the distribution of the stomata and the development of chlorenchyma and other tissues also affects the rate of photosynthesis.

Question 39.
State and explain the Blackman’s law of limiting factor.
Answer:

  1. The Blackman’s law of limiting factors states that when a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is controlled by the pace of the “slowest factor”.
  2. The slowest factor is that factor which is present in the lowest or minimum concentration in relation to others.
  3. The law of limiting factor can be explained by taking two external factors such as carbon dioxide and light.
  4. For example, a plant photosynthesizing at a fixed light intensity sufficient to utilize 10mg of C2 per hour only.
  5. Photosynthetic rate goes on increasing when concentration of CO2 increases.
  6. Further increase in CO2 concentration will not increase the rate of photosynthesis. In this case light becomes the limiting factor. Therefore, under such circumstances rate of photosynthesis can be increased only by increasing the light intensity.
  7. This proves that the rate of photosynthesis responds to one factor alone at a time and there would be a sharp break in the curve and a plateau formed exactly at the point where another factor becomes limiting.
  8. If any one of the other factors which is kept constant (e.g. Light) is increased, the photosynthetic rate increases again reaching the optimum where again another factor becomes limiting.

Question 40.
Give the significance of Photosynthesis.
Answer:

  1. Photosynthesis is anabolic process which uses inorganic substances and produces food for all life directly or indirectly.
  2. This process transforms solar energy into chemical energy.
  3. The released by product 02 is necessary not only for aerobic respiration in living organisms but also used in forming protective ozone layer around earth.
  4. It also helps us in providing fossil fuels, coals, petroleum and natural gas.

Question 41.
Apply Your Knowledge:

Question 1.
1. Identify ‘x’, ‘y’ and ‘z’ in the given diagrammatic representation of cyclic photophosphorylation.
2. During which steps ATP molecules are formed?
Answer:
Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis 6
1. ‘x’ is ferredoxin, ‘y’ is cytochrome-b6 and ‘z’ is plastocyanin.
2. During conversion of ferredoxin to cytochrome-b6 and from cytochrome-b6 to cytochrome-f ATP molecules are formed.

Question 42.
Quick Review

Cyclic and Non-cyclic photophosphorylation:

Cyclic Non-cyclic
Used Photons
1 ADP
1 Phosphate group
Photons
1 ADP1 Phosphate group
1 H20
1 NADP+
Produced 2 ATP 1 ATP
2 NADPH + H+
\(\frac { 1 }{ 2 }\)O2
Accomplished Captured energy in the form of ATP Captured energy in the form of ATP and NADPH2
Transfers hydrogen (as NADPH) to the dark reactions

Calvin’s Cycle:

Used (Reactants) Produced (Products)
6CO2 12 PGAL (2 become 1 glucose)
6 RUBP 18 Phosphates (return to light reactions)
18 ATP (from light reactions) 18 ADP (returns to light reactions)
12 NADPH (from light reactions) 12 NADP+ (return to light reactions)

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis 7

Abbrevations

ATP Adenosine triphosphate RuBisCO Ribulose bisphosphate carboxylase
ADP Adenosine diphosphate PGA Phosphoglyceric acid
Co-Q Co-enzyme quinone PGAL Phosphoglyceraldehyde
FRS Ferredoxin Reducing Substance DHAP Dihydroxyacetone phosphate
PC Plastocyanin NADP Nicotinamide adenine dinucleotide phosphate
PQ Plastoquinone NADPH2 Nicotinamide adenine dinucleotide hydrogen phosphate
RUBP Ribulose-1, 5-bisphosphate PEPA Phosphoenol pyruvic acid
RUMP Ribulose monophosphate OAA Oxaloacetic acid

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 43.
Exercise:

Question 1.
Why photosynthesis is known as redox reaction?
Answer:

  1. Photosynthesis is considered as a redox reaction as it involves both reduction and oxidation reactions.
  2. Water is oxidized by the removal of H+ while C02 is reduced by the addition of H.
  3. The redox reactions of photosynthesis are necessary for the conversion of light energy into chemical energy.

Question 2.
Sketch and label ‘Ultrastructure of Chloroplast’.
Answer:

  1. The chloroplasts are discoid and lens shaped in higher plants. Chloroplast is bounded by a double membrane.
  2. System of chlorophyll bearing a double-membrane sac is present inside the stroma.
  3. These are stacked one above the other to form grana.
  4. Individual sacs in each granum is are known as thylakoid.
  5. All the pigments chlorophylls, carotenes and xanthophylls are located in thylakoid membranes.
  6. These pigments are fat soluble and are present in lipid part of membrane also they absorb light of specific spectrum in the visible regions.

Question 3.
Name the various photosynthetic pigments.
Answer:

  1. Chemically chlorophyll molecule consists of two parts head of tetrapyrrole the porphyrin ring and a long hydrocarbon tail called phytol attached to the porphyrin group.
  2. Both chlorophyll-a and chlorophyll-b are similar in their molecular structure, except that the methyl group (-CH) in chlorophyll-a is replaced with an aldehyde group (-CHO) in chlorophyll-b.
  3. Carotenoids are lipid compound present universally in almost all the higher plants and several microorganisms.
  4. They are usually red, orange, yellow, brown, and are associated with chlorophyll. They are of two types – the carotenes and xanthophylls.
  5. The carotenes (C40H5) are orange red and xanthophylls contain oxygen.
  6. The light energy absorbed by the carotenoids is transferred to chlorophyll-a to be utilized in photosynthesis.
  7. All photosynthetic plants have these pigments that absorb light between the red and blue region of the
    spectrum.
  8. Carotenoids found mainly in higher plants, absorb primarily in the violet to blue regions of the spectrum.
  9. They not only absorb light energy and transfer it to chlorophyll but also protect the chlorophyll molecule from photo-oxidation. Xanthophylls (C40H56O2) are yellow pigments found in fruits and vegetables.

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 4.
What is the main function of accessory pigment?
Answer:
1. Accessory pigments are light absorbing molecules which are found in photosynthetic organisms.
2. They transfer the absorbed light to chlorophyll-a and thus increasing the photosynthetic rate.
3. In absence of accessory pigments less amount of light will be absorbed and also there would be no protection provided to chlorophyll molecule from photo-oxidation.

Question 5.
Explain the nature of light.
Answer:

  1. Light is a form of energy.
  2. It travels as stream of tiny particles called photons.
  3. A photon contains a quantum of light.
  4. Light has different wavelengths having different colors.
  5. One can see electromagnetic radiation with wavelengths ranging from 390nm to 730nm. This part of the spectrum is called the Visible light.
  6. It lies between wavelengths of ultraviolet and infra-red.

Question 6.
Which instrument is used for studying the absorption spectrum of photosynthetic pigments?
Answer:
In absorption spectrum, absorption of different wavelengths of light pigments can be measured by spectrophotometer.

Question 7.
Explain Hill’s reaction.
Answer:
Robert Hill proved that the source of oxygen evolved during photosynthesis is water and not carbon dioxide. Hence, it is called Hill’s Reaction.

  1. In this experiment, Hill cultured isolated chloroplasts in a medium containing C02 free water, haemoglobin and ferric compound.
  2. Ferric salts and haemoglobin were added in the medium as hydrogen and oxygen acceptors respectively.
  3. When the suspension was illuminated, he observed that haemoglobin turned into oxyhaemoglobin (red colour).
  4. This confirmed that water must have oxidized releasing 02, that reacted with haemoglobin. Reduction of ferric compound was also indicated by change in colour.
  5. The H2O molecule oxidized to evolve 02 as a by-product. Thus, Hill proved that the source of evolving 02 is H20 and not C02.
  6. This process of splitting up of water molecules under the influence of light in the presence of chlorophyll is called Photolysis of water or Hill Reaction.

Question 8.
Describe photoexcitation of chlorophyll-a.
Answer:

  1. Chlorophyll-a is an essential photosynthetic pigment as it converts light energy into chemical energy and acts as a reaction centre.
  2. Initially, it lies at ground state or singlet state but when it absorbs or receives photons (solar energy), it gets activated and goes in excited state or excited second singlet state.
  3. In the excited state, chlorophyll-a emits an electron. The emitted electron is energy rich, i.e. has extra amount of energy.
  4. Due to the loss of electron (e), chlorophyll-a becomes positively charged. This is the ionized state.
  5. Chlorophyll-a molecule cannot remain in the ionized state for more than 10‘9 seconds. Hence the photo-chemical reaction or electron transfer occurs very fast.
  6. The energy rich electron is then transferred through various electron acceptors and donors (carriers).
  7. During the transfer, the electron emits energy which is utilized for the synthesis of ATP. This shows that light energy is converted into chemical energy in the form of ATP.

Question 9.
Write the wavelengths required for proper functioning of PS-I and PS-II respectively?
Answer:
1. Together with P-680 they form Photosystem – II.
2. Likewise, P-700 forms Photosystem – I along with pigment molecule which absorbs light at or below 700 nm.

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 10.
Differentiate between PS-IandPS-II.
Answer:

  1. The movement of ions across a selectively permeable membrane, down their electrochemical gradient is called chemiosmosis.
  2. The movement of hydrogen ions across a membrane during cellular respiration in mitochondria and during photosynthesis in chloroplasts, leads to the generation of ATP.
  3. These membranes are thylakoids and the protons accumulate in the lumen during photosynthesis.
  4. An electrochemical concentration gradient forms when hydrogen ions (protons) diffuse from an area of higher proton concentration to an area of lower proton concentration.
  5. This electrochemical concentration gradient of protons across a membrane can be utilized to make
    ATP.
  6. ATP synthase is the enzyme that makes ATP by chemiosmosis. .
  7. It allows protons to pass through the membrane using the kinetic energy to phosphorylate ADP making ATP.
  8. Splitting of water molecule on the inner side of the membrane results in the accumulation of hydrogen ions within the lumen of thylakoids.
  9. The enzyme, NADP reductase, is located in the stroma side of the membrane.
  10. For reduction of NADP to NADPH2, protons are required along with electrons that come from ferredoxin.
  11. Thus, within the chloroplast, the protons in the stroma decrease in number, while in the lumen, the number of protons increases.
  12. This creates a proton gradient across the thylakoid membrane.
  13. Energy,generated by the subsequent spontaneous movement of protons is used for the synthesis of ATP.

Question 11.
Define photophosphorylation.
Answer:
Formation of ATP in the chloroplasts in presence of light is called photophosphorylation, light

Question 12.
Give graphic representation of cyclic photophosphorylation.
Answer:
Cyclic photophosphorylation:
a. Illumination of photosystem-I causes electrons to move continuously out of the reaction center of photosystem-I and back to it.
b. The cyclic electron-flow is accompanied by the photophosphorylation of ADP to yield ATP. This is termed as Cyclic photophosphorylation.
c. Since this process involves only pigment system I, photolysis of water and consequent evolution of oxygen does not take place.

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 13.
Give graphic representation of noncyclic photophosphorylation.
Answer:
Non-cyclic photophosphorylation:
a. It involves both photosystems- PS-I and PS-II.
b. In this case, electron transport chain starts with the release of electrons from PS-II.
c. In this chain high energy electrons released from PS-II do not return to PS-II but, after passing through an electron transport chain, reach PS-I, which in turn donates it to reduce NADP to NADPH.
d. The reduced NADP+ (NADPH) is utilized for the reduction of CO2 in the dark reaction.
e. Electron-deficient PS-II brings about oxidation of water-molecule. Due to this, protons, electrons and oxygen atom are released.
f. Electrons are taken up by PS-II itself to return to reduced state, protons are accepted by NADP+ whereas oxygen is released.
g. As in this process, high energy electrons released from PS-II do not return to PS-II and it is accompanied with ATP formation, this is called Non-cyclic photophosphorylation.

Question 14.
Write the link between light dependent and dark reactions.
Answer:
Link between light-dependent and dark reactions:

  1. The light reaction gives rise to two important products, a reducing agent NADPH2 and an energy-rich compound ATP. Both these are utilized in the dark phase of photosynthesis.
  2. ATP and NADPH2 molecules function as vehicles for transfer of energy of sunlight into dark reaction leaving to carbon fixation. In this reaction C02 is reduced to carbohydrate.
  3. During dark reaction, ATP and NADPH2 are transformed into ADP, iP and NADP which are transferred to the grana in which light reaction takes place.

Question 15.
Write the dark reaction occurring in stroma.
Answer:
602+ 18ATP + 12NADPH2 → C6H12O2 + 6H2O + 18ADP +18Pi+ 12NADP+

Question 16.
Explain how light and dark reactions of photosynthesis are interdependent.
Answer:
Link between light-dependent and dark reactions:

  1. The light reaction gives rise to two important products, a reducing agent NADPH2 and an energy-rich compound ATP. Both these are utilized in the dark phase of photosynthesis.
  2. ATP and NADPH2 molecules function as vehicles for transfer of energy of sunlight into dark reaction leaving to carbon fixation. In this reaction C02 is reduced to carbohydrate.
  3. During dark reaction, ATP and NADPH2 are transformed into ADP, iP and NADP which are transferred to the grana in which light reaction takes place.

Question 17.
Give schematic representation of Calvin Cycle.
Answer:
1. The entire process of dark reaction was traced by Dr. Melvin Calvin along with his co-worker, Dr. Benson. Hence, the process is called as Calvin cycle or Calvin- Benson cycle. Since the first stable product formed is a 3-carbon compound, it is also called as C3 pathway and the plants are called C14 plants.
2. Calvin carried out experiments on unicellular green algae (Chlorella), using radioactive isotope of carbon, C14 as a tracer. It is also called synthesis phase or second phase of photosynthesis.

The cycle is divided into the following phases:
1. Carboxylation phase:
a. Carbon dioxide reduction starts with a five-carbon sugar ribulose-l,5-bisphosphate (RuBP). It is a 5- carbon sugar with two phosphate groups attached to it.
b. RuBP reacts with CO2 to produce an unstable 6 carbon intermediate in the presence of Rubisco.
c. It immediately splits into 3 carbon compounds called 3-phosphoglyceric acid.
d. RuBisCO is a large protein molecule and comprises 16% of the chloroplast proteins.

2. Glycolytic reversal:
a. 3-phosphoglyceric acid form 1,3-diphosphoglyceric acid by utilizing ATP molecule.
b. These are then reduced to glyceraldehyde-3-phosphate (3-PGA) by NADPH supplied by the light reactions of photosynthesis.
c. In order to keep Calvin cycle continuously running there must be sufficient number of RuBP and regular supply of ATP and NADPH.
d. Out of 12 molecules of 3-phosphoglyceraldehyde, two molecules are used for synthesis of one glucose molecule.

3. Regeneration of RuBP:
a. 10 molecules of 3-phosphoglyceraldehyde are used for the regeneration of 6 molecules of RuBP at the cost of 6 ATP.
b. Therefore, six turns of Calvin cycle are needed to get one molecule of glucose.

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 18.
Describe Calvin cycle and its significance.
Answer:
Significance:
1. Carboxylation: RuBisCO is the most abundant enzyme in the world. It is responsible for fixing carbon in the form of C02 into sugar. As a result of Carboxylation, the first stable product of carbon fixation i.e. 3- PGA is synthesized.
2. Reduction/Glycolytic reversal: NADPH2 donates electrons to 1, 3-Bisphoshoglycerate to form 3- phosphoglyceraldehyde molecules. During this process ADP and NADP are generated which are used in light reaction.
3. Regeneration of RuBP: Some 3-phosphoglyceraldehyde molecules are involved in production of glucose while others are recycled to regenerate the 5-carbon compound RuBP which used to accept new carbon molecules. Thus, regeneration of RuBP is required for Calvin cycle to run continuously.

Question 19.
How was the carbon fixation pathway studied?
Answer:
1. Path of carbon was studied with the help of radioactive tracer technique using Chlorella, a unicellular green alga and radioactive 14C02.
2. With the help of radioactive carbon, it becomes possible to trace the intermediate steps of fixation of 14C02.

Question 20.
Describe photorespiration with the help of diagrammatic representation.
Answer:
Mechanism:

  1. Photorespiration involves three organelles chloroplast, peroxisomes and mitochondria and occurs in a series of cyclic reactions which is also called PCO cycle. (Photosynthetic Carbon Cycle)
  2. Enzyme Rubisco acts as oxygenase at higher concentration of O2 and photorespiration begins.
  3. When RuBP reacts with 02 rather than C02 to form a 3-carbon compound (PGA) and 2-carbon compound phosphoglycolate.
  4. Phosphoglycolate is then converted to glycolate which is shuttled out of the chloroplast into the peroxisomes.
  5. In Peroxisomes, glycolate is converted into glyoxylate by enzyme glycolate oxidase.
  6. Glyoxylate is further converted into amino acid glycine by transamination.
  7. In mitochondria, two molecules of glycine are converted into serine (amino acid) and C02 is given out.
  8. Thus, it loses 25% of photosynthetically fixed carbon.
  9. Serine is transported back to peroxisomes and converted into glycerate.
  10. It is shuttled back to chloroplast to undergo phosphorylation and utilized information of 3-PGA, which get utilized in C3 pathway.

Question 21.
Give significance of C4 pathway.
Answer:

  1. C4 plants have special type of leaf anatomy called Kranz anatomy.
  2. In C4 plants, C02 fixation occurs twice.
  3. In these plants, chloroplasts of mesophyll cells contain enzyme PEP carboxylase which fixes atmospheric C02. Thus, first C02 fixation occurs in mesophyll cells.
  4. Decarboxylation of malic acid in bundle sheath cells results in increase in C02 concentration.
  5. Thus, RuBisCO acts as carboxylase and brings about carboxylation of RuBP.
  6. Due to this oxygenation of RuBP and photorespiration is prevented.
  7. Thus, despite of having less number of bundle sheath cells carrying out Calvin cycle, C4 plants are highly productive.

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 22.
Who proposed C4 pathway?
Answer:
1. M. D. Hatch and C. R. Slack while working on sugarcane found four carbon compounds (dicarboxylic acid) as the first stable product of photosynthesis.
2. It occurs in tropical and sub-tropical grasses and some dicotyledons.
3. The first product of this cycle is a 4-carbon compound oxaloacetic acid. Hence it is also called as C4 pathway and plants are called C4 plants.
Mechanism:

  1. C02 taken from atmosphere is accepted by a 3-carbon compound, phosphoenolpyruvic acid in the chloroplasts of mesophyll cells, leading to the formation of 4-C compound, oxaloacetic acid with the help of enzyme phosphoenolpyruvate carboxylase.
  2. It is converted to another 4-C compound, malic acid. It is transported to the chloroplasts of bundle sheath cells.
  3. Malic acid (4-C) is converted to pyruvic acid (3-C) with the release of C02 in the cytoplasm.
  4. Thus, concentration of C02 increases in the bundle sheath cells.
  5. Chloroplasts of these cells contain enzymes of Calvin cycle.
  6. Because of high concentration of C02, RuBP carboxylase participates in Calvin cycle and not photorespiration.
  7. Sugar formed in Calvin cycle is transported into the phloem.
  8. Pyruvic acid generated in the bundle sheath cells re-enter mesophyll cells and regenerates
    phosphoenolpyruvic acid by consuming one ATP.
  9. Since this conversion results in the formation of AMP (not ADP), two ATP are required to regenerate ATP from AMP. Thus, C4 pathway needs 12 additional ATP.
  10. The C3 pathway requires 18 ATP for the synthesis of one glucose molecule, whereas C4 pathway requires 30 ATP.
  11. Thus, C4 plants are better photosynthesizers as compared to C3 plants as there is no photorespiration in these plants.

Question 23.
Give schematic representation of HSK pathway.
Answer:
1. M. D. Hatch and C. R. Slack while working on sugarcane found four-carbon compounds (dicarboxylic acid) as the first stable product of photosynthesis.
2. It occurs in tropical and sub-tropical grasses and some dicotyledons.
3. The first product of this cycle is a 4-carbon compound oxaloacetic acid. Hence it is also called as C4 pathway and plants are called C4 plants.

Mechanism:

  1. C02 taken from atmosphere is accepted by a 3-carbon compound, phosphoenolpyruvic acid in the chloroplasts of mesophyll cells, leading to the formation of 4-C compound, oxaloacetic acid with the help of enzyme phosphoenolpyruvate carboxylase.
  2. It is converted to another 4-C compound, malic acid. It is transported to the chloroplasts of bundle sheath cells.
  3. Malic acid (4-C) is converted to pyruvic acid (3-C) with the release of C02 in the cytoplasm. Thus, concentration of C02 increases in the bundle sheath cells.
  4. Chloroplasts of these cells contain enzymes of Calvin cycle.
  5. Because of high concentration of C02, RuBP carboxylase participates in Calvin cycle and not photorespiration.
  6. Sugar formed in Calvin cycle is transported into the phloem.
  7. Pyruvic acid generated in the bundle sheath cells re-enter mesophyll cells and regenerates
    phosphoenolpyruvic acid by consuming one ATP.
  8. Since this conversion results in the formation of AMP (not ADP), two ATP are required to regenerate ATP from AMP. Thus, C4 pathway needs 12 additional ATP.
  9. The C3 pathway requires 18 ATP for the synthesis of one glucose molecule, whereas C4 pathway requires 30 ATP.
  10. Thus, C4 plants are better photosynthesizers as compared to C3 plants as there is no photorespiration in these plants.

Question 24.
With the help of labelled diagram explain Kranz anatomy.
Answer:

  1. Anatomy of leaves of C4 plants is different from leaves of C3 plants.
  2. C4 plants show Kranz anatomy.
  3. In the leaves of such plants, there is a bundle sheath around the vascular bundles.
  4. The chloroplasts in the bundle – sheath cells are large and without or less developed grana, where as in the mesophyll cells the chloroplasts are small but with well-developed grana.

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 25.
Give reason. Why C4 plants are favoured in tropical regions?
Answer:

  1. C4 plants are favoured in tropical regions as they require 30 ATP to produce 1 molecule of glucose.
  2. High temperature in tropical regions leads to closure of stomata to reduce rate of transpiration. Due to this availability of C02 decreases.
  3. PEP carboxylase present in mesophyll cells can fix C02 even at low concentration. This helps the plant in efficient assimilation of atmospheric carbon dioxide.
  4. C4 plants contain a special leaf anatomy called Kranz anatomy which minimizes the losses due to photorespiration.
  5. It helps C4 plants to survive in conditions of high daytime temperatures, intense sunlight and low moisture.

Question 26.
With the help of suitable flowchart explain CAM.
Answer:
Crassulacean Acid Metabolism (CAM).

  1. It is one more alternative pathway of carbon fixation found in desert plants.
  2. It was first reported in the family Crassulaceae, therefore called as CAM (Crassulacean Acid Metabolism).
  3. In CAM plants, stomata are scotoactive i.e. active during night, hence initial C02 fixation occurs in night.
  4. Thus, C4 pathway fix C02 at night and reduce C02 in day time via the C3 pathway by using NADPH formed during the day.
  5. PEP caboxylase and Rubisco are present in the mesophyll cell (no Kranz anatomy).
  6. Formation of malic acid during dark is called acidification (phase I).
  7. Malate is stored in vacuoles during the night.
  8. Malate releases C02 during the day for C3 pathway within the same cell is called deacidification (phase II).
  9. Examples of CAM plants: Kalanchoe, Opuntia, Aloe etc.
  10. The Chemical reactions of the carbon dioxide fixation and its assimilation are similar to that of C4 plants.

Question 27.
Describe ‘any two’ factors affecting the rate of photosynthesis.
Answer:
External factors which affect photosynthesis are as follows:
1. Light:
a. It is an essential factor as it supplies the energy necessary for photosynthesis.
b. Quality and intensity of light affects the photosynthesis.
c. Highest rate of photosynthesis takes place in red light followed by blue light.
d. The rate of photosynthesis considerably decreases in plants which are growing under a forest canopy.
e. In most of the plants, photosynthesis is maximum in bright diffused sunlight.
f. Uninterrupted and continuous photosynthesis for a very long period of time may be sustained without any visible damage to the plant.

2. Carbon dioxide:
The main source of C02 in land plants is the atmosphere, which contains only 0.3% of the gas.
b. Under normal conditions of temperature and light, carbon dioxide acts as a limiting factor in photosynthesis.
c. Increase in concentration of CO2 increases the photosynthesis.
d. Increase in C02 to about 1% is advantageous to most of the plants.
e. Higher concentration of the gas has an inhibitory effect on photosynthesis.

3. Temperature:
a. Like all other physiological processes, photosynthesis also needs a suitable temperature.
b. The optimum temperature at which the photosynthesis is maximum is 25-30 °C. Except in plants like Opuntia, photosynthesis takes place at as high as 55 °C.
c. This is the maximum temperature. Minimum temperature is temperature at which photosynthesis process just starts.
d. In the presence of sufficient light and CO2, photosynthesis increases with the rise of temperature till it becomes maximum. After that there is a decrease or fall in the rate of the process.

4. Water:
a. Water is necessary for photosynthetic process.
b. An increase in water content of the leaf results in the corresponding increase in the rate of photosynthesis.
c. Thus, the limiting effect of water is not direct but indirect.
d. It is mainly due to the fact that it helps in maintaining the turgidity of the assimilatory cells and the proper hydration of their protoplasm.

Internal factors which affects photosynthesis are as follows:
1. Chlorophyll:
a. Though presence of chlorophyll is essential for photosynthesis but rate of photosynthesis is proportional to the quantity of chlorophyll present.
b. It is because of the fact that chlorophyll merely acts as a biocatalyst and hence a small quantity is quite enough to maintain the large bulk of the reacting substances.
2. Sugar:
The final product in the photosynthesis reaction is sugar and its accumulation in the cells slow down the process of photosynthesis.
3. Internal structures:
The thickness of cuticle and epidermis of the leaf, the size and distribution of intercellular spaces and the distribution of the stomata and the development of chlorenchyma and other tissues also affects the rate of photosynthesis.

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 28.
Enlist the factors that affect the rate of photosynthesis.
Answer:
External factors which affect photosynthesis are as follows:
1. Light:
a. It is an essential factor as it supplies the energy necessary for photosynthesis.
b. Quality and intensity of light affects the photosynthesis.
c. Highest rate of photosynthesis takes place in red light followed by blue light.
d. The rate of photosynthesis considerably decreases in plants which are growing under a forest canopy.
e. In most of the plants, photosynthesis is maximum in bright diffused sunlight.
f. Uninterrupted and continuous photosynthesis for a very long period of time may be sustained without any visible damage to the plant.

2. Carbon dioxide:
The main source of C02 in land plants is the atmosphere, which contains only 0.3% of the gas.
b. Under normal conditions of temperature and light, carbon dioxide acts as a limiting factor in photosynthesis.
c. Increase in concentration of CO2 increases the photosynthesis.
d. Increase in C02 to about 1% is advantageous to most of the plants.
e. Higher concentration of the gas has an inhibitory effect on photosynthesis.

3. Temperature:
a. Like all other physiological processes, photosynthesis also needs a suitable temperature.
b. The optimum temperature at which the photosynthesis is maximum is 25-30 °C. Except in plants like Opuntia, photosynthesis takes place at as high as 55 °C.
c. This is the maximum temperature. Minimum temperature is temperature at which photosynthesis process just starts.
d. In the presence of sufficient light and CO2, photosynthesis increases with the rise of temperature till it becomes maximum. After that there is a decrease or fall in the rate of the process.

4. Water:
a. Water is necessary for photosynthetic process.
b. An increase in water content of the leaf results in the corresponding increase in the rate of photosynthesis.
c. Thus, the limiting effect of water is not direct but indirect.
d. It is mainly due to the fact that it helps in maintaining the turgidity of the assimilatory cells and the proper hydration of their protoplasm.

Internal factors which affects photosynthesis are as follows:
1. Chlorophyll:
a. Though presence of chlorophyll is essential for photosynthesis but rate of photosynthesis is proportional to the quantity of chlorophyll present.
b. It is because of the fact that chlorophyll merely acts as a biocatalyst and hence a small quantity is quite enough to maintain the large bulk of the reacting substances.
2. Sugar:
The final product in the photosynthesis reaction is sugar and its accumulation in the cells slow down the process of photosynthesis.
3. Internal structures:
The thickness of cuticle and epidermis of the leaf, the size and distribution of intercellular spaces and the distribution of the stomata and the development of chlorenchyma and other tissues also affects the rate of photosynthesis.

Question 29.
Give the Blackman’s law of limiting factors.
Answer:

  1. The Blackman’s law of limiting factors states that when a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is controlled by the pace of the “slowest factor”.
  2. The slowest factor is that factor which is present in the lowest or minimum concentration in relation to others.
  3. The law of limiting factor can be explained by taking two external factors such as carbon dioxide and light.
  4. For example, a plant photosynthesizing at a fixed light intensity sufficient to utilize 10mg of C2 per hour only.
  5. Photosynthetic rate goes on increasing when concentration of CO2 increases.
  6. Further increase in CO2 concentration will not increase the rate of photosynthesis. In this case light becomes the limiting factor. Therefore, under such circumstances rate of photosynthesis can be increased only by increasing the light intensity.
  7. This proves that the rate of photosynthesis responds to one factor alone at a time and there would be a sharp break in the curve and a plateau formed exactly at the point where another factor becomes limiting.
  8. If any one of the other factors which is kept constant (e.g. Light) is increased, the photosynthetic rate increases again reaching the optimum where again another factor becomes limiting.

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 44.
Multiple Choice Questions:

Question 1.
How many ATP molecules are required for synthesis of one glucose molecule using C4 pathway?
(A) 30
(B) 18
(C) 6
(D) 2
Answer:
(A) 30

Question 2.
Photosynthesis is ________ reaction.
(A) oxidation
(B) reduction
(C) redox
(D) electrochemical
Answer:
(C) redox

Question 3.
Photosynthesis is minimum in _______ light.
(A) green
(B) blue
(C) red
(D) yellow
Answer:
(A) green

Question 4.
From the visible spectrum of light, which component is reflected by the green leaves?
(A) Blue
(B) Red
(C) Green
(D) Orange
Answer:
(C) Green

Question 5.
Which of the following is not an accessory pigment?
(A) Chlorophyll-b
(B) Xanthophyll
(C) Chlorophyll-a
(D) Carotene
Answer:
(C) Chlorophyll-a

Question 6.
The reaction centre of PS-II is ________ .
(A) Chi-a, 700
(B) Chi-a, 680
(C) Chi- a, 673
(D) Chi-a, 650
Answer:
(B) Chi-a, 680

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 7.
Light reactions occur in ______________ .
(A) stroma
(B) grana
(C) matrix
(D) fret
Answer:
(B) grana

Question 8.
Dark reaction takes place in _____________ .
(A) Stroma
(B) Grana
(C) Matrix
(D) Thylakoid
Answer:
(A) Stroma

Question 9.
In dark reaction, the first compound to accept C02 is ___________ .
(A) RUMP
(B) RUBP
(C) PGAL
(D) PGA
Answer:
(B) RUBP

Question 10.
Which of the following is a photochemical reaction?
(A) Light reaction
(B) C3 pathway
(C) C4 pathway
(D) CAM pathway
Answer:
(A) Light reaction

Question 11.
Which of the following is a biochemical reaction?
(A) Light reaction
(B) Cyclic electron transfer
(C) Photolysis of water
(D) Dark phase
Answer:
(D) Dark phase

Question 12.
How many Calvin cycles are required to produce one molecule of glucose?
(A) 3
(B) 4
(C) 5
(D) 6
Answer:
(D) 6

Question 13.
The first C02 acceptor in C4 pathway is _______ .
(A) Pyruvic acid
(B) PEPA
(C) OAA
(D) Malic acid
Answer:
(B) PEPA

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 14.
In C4 pathway, fixation of metabolic C02 occurs in which of the following cells?
(A) Bundle sheath cells
(B) Mesophyll cells
(C) Epidermal cells
(D) Cortical cells
Answer:
(A) Bundle sheath cells

Question 15.
Which one of the following is C4 plant?
(A) Sunflower
(B) Soyabean
(C) Sugarcane
(D) Spinach
Answer:
(C) Sugarcane

Question 16.
Due to photorespiration, approximately __________ of photosynthetically fixed C02 is lost.
(A) 25%
(B) 50%
(C) 60%
(D) 80%
Answer:
(A) 25%

Question 17.
CAM plants are mostly ________________ .
(A) Tropical plants
(B) Succulents
(C) Monocots
(D) Mangroves
Answer:
(B) Succulents

Question 18.
Which of the following factors is not limiting?
(A) C02 concentration
(B) Light intensity
(C) Temperature
(D) Oxygen
Answer:
(D) Oxygen

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 45.
Competitive Corner

Question 1.
In Hatch and Slack pathway, the primary C02 acceptor is _______________ .
(A) Rubisco
(B) Oxaloacetic acid
(C) Phosphoglyceric acid
(D) Phosphoenol pyruvate
Answer:
(D) Phosphoenol pyruvate

Question 2.
One scientist cultured Cladophora in a suspension of Azotobacter and illuminated the culture by splitting light through a prism. He observed that bacteria accumulated mainly in the region of:
(A) Blue and red light
(B) Violet and green light
(C) Indigo and green light
(D) Orange and yellow light
Hint: Cladophora is green alga and Azotobacter is aerobic bacteria. Theodor Engelmann split light into its spectral components using a prism and detected that aerobic bacteria accumulated mainly in the region of blue and red light.
Answer:
(A) Blue and red light

Question 3.
Clil-a and Chl-b shown maximum absorption in ________ regions of visible light.
(A) blue, violet and red
(B) red, indigo and green
(C) yellow, blue and red
(D) blue, violet and green
Answer:
(A) blue, violet and red

Question 4.
The co-enzyme which acts as hydrogen acceptor during light reaction is ________ .
(A) PQ
(B) FAD
(C) COQ
(D) NADP
Answer:
(D) NADP

Question 5.
During cyclic photophosphorylation, formation of ATP occurs between which of the following two compounds?
(A) FRS → Ferredoxin
(B) Cytochrome b6 → Cytochrome f
(C) Cytochrome f → Plastocyanin
(D) Plastocyanin → Ionised Chi – a
Answer:
(B) Cytochrome b6 → Cytochrome f

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 6.
Which of the following is NOT a product of light reaction of photosynthesis?
(A) NADPH
(B) NADH
(C) ATP
(D) Oxygen
Answer:
(B) NADH

Question 7.
A part of photosynthetically fixed CO2 goes back to the atmosphere due to _______.
(A) cyclic photophosphorylation
(B) noncyclic photophosphorylation
(C) dark reaction
(D) photorespiration
Answer:
(D) photorespiration

Question 8.
The highest rate of Photosynthesis in green plants are in _____ and ______ region of light spectrum.
(A) yellow and orange
(B) green and violet
(C) red and blue
(D) violet and blue
Answer:
(C) red and blue

Question 9.
Which one of the following is an essential factor for photophosphorylation?
(A) Sunlight
(B) Carbohydrate
(C) Oxygen
(D) Water
Answer:
(A) Sunlight

Question 10.
Cyclic photophosphorylation will NOT take place in the absence of ________ .
(A) carotenoids
(B) chlorophyll-a
(C) xanthophylls
(D) phycoerythrin
Answer:
(B) chlorophyll-a

Question 11.
Dark reaction of photosynthesis is a cyclic process as _______ is regenerated.
(A) RuBP
(B) C02
(C) Glucose
(D) PGA
Answer:
(A) RuBP

Question 12.
Phosphoenol pyruvate (PEP) is the primary CO2 acceptor in:
(A) C3 plants
(B) C4 plants
(C) C2 plants
(D) C3 and C4 plants
Answer:
(B) C4 plants

Question 13.
In members of family Crassulaceae ________ is regenerated from starch during night.
(A) Phosphoenol pyruvic Acid
(B) Pyruvic Acid
(C) Malic Acid
(D) Oxalo Acetic Acid
Answer:
(A) Phosphoenol pyruvic Acid

Maharashtra Board Class 11 Biology Important Questions Chapter 12 Photosynthesis

Question 14.
With reference to factors affecting the rate of photosynthesis, which of the following statements are NOT correct?
(A) Light saturation for CO2 fixation occurs at 10% of full sunlight
(B) Increasing atmospheric CO2 concentration upto 0.05% can enhance C02 fixation rate
(C) C3 plants responds to higher temperatures with enhanced photosynthesis while C4 plants have much lower temperature optimum
(D) Tomato is a greenhouse crop which can be grown in CO2 enriched atmosphere for higher yield.
Answer:
(C) C3 plants responds to higher temperatures with enhanced photosynthesis while C4 plants have much lower temperature optimum.

Leave a Comment