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Photosynthesis in Higher Plants

Questions - Answers

Very Short Answer Type Questions


1. Name the processes which take place in the grana and stroma regions of chloroplasts?
A: Trapping the light energy and synthesis of ATP and NADPH take place in grana. This set of two reactions are light dependent. So it is called light reaction. The biosynthetic phase where in CO2 and water are converted to Glucose by utilising end products of light reaction occurs in stroma. This set of reactions are together called dark reactions as it do not directly depend up on light.

 

2. Can chloroplasts be passed on to progeny? How?
A: Yes. chloroplasts can be passed on to progeny. They have genetic material in the form of double stranded circular DNA. They are self reproducing organelles.

 

3. Where does the photolysis of water occur? What is its significance?
A: Photolysis of water occurs in lumen. It is associated with PS II. The electrons removed from PS II are replaced by those donated by water. Protons and electrons reach NADP and oxygen is released into atmosphere.

 

4. Where is the enzyme NADP reductase located? What is released if the proton gradient breaks down?
A: NADP reductase enzyme is located in the grana thylakoids on the stroma side. Proton concentration gradient is established along the thylakoid membrane between lumen and stroma. The break down of this proton gradient creates energy which is utilised to synthesise ATP.

5. Which tissue transports photosynthates? What experiments prove this?
A: Phloem, the living vascular tissue transports photosynthates in the form of sucrose. Phloem transports food from the leaves upwards and downwards. Thus it is bidirectional. A simple experiment called 'Girdling experiment' explains transportation of food.

 

6. How many molecules of ATP and NADPH are needed to fix a molecule of CO2 in C3 plants? Where does this process occur?
A: 3 ATP and 2 NADPH2 are required to fix a molecule of CO2 in C3 plants. This process occurs in stroma.

 

7. Explain the terms.
     a) Hatch slack pathway   b) Calvin cycle
     c) PEP carboxoylase        d) Bundle sheath cells
A: a) Hatch slack pathway: C4 pathway is also named as Hatch-Slack pathway because M.D. Hatch and C.R. Slack worked on sugarcane and discovered the intermediate compounds in it.
     b) Calvin cycle: C3 pathway is also called Calvin cycle because Melvin Calvin worked on Chlorella to find out the intermediate compounds in it. 
     c) PEP carboxoylase: It is present in the chloroplasts of mesophyll in C4 plants. It helps in the fixation of CO2 to form phospho enol pyruvic acid (β-Carboxylation). It is also present in CAM plants.
    d) Bundle sheath cells: These cells in C4 plants have
      1) larger chloroplasts         2) Agranal chloroplasts
      3) RuBP case enzyme        4) Arranged in the form of a garland or wreath

 

8. What is the role of NADP Reductase in the development of proton gradient?
A: NADP reductase is present in the grana thylakoid towards stroma. It breaks down the proton concentration gradient to produce ATP through CF0 − CF1. Photolysis of H2O occurs in lumen and again proton concentration gradient is established along the thylakoid membrane.

 

9. Mention the components of ATP ase enzyme? What is their location? Which part of the enzyme shows confirmational changes?
A: ATP ase has 2 parts 1) F0 2) F1
ATP ase is present in the thylakoid membrane. F0 is towards lumen and F1 is protruded into the stroma. F0 acts as proton channel. F1 shows confirmational changes.

 

10. What products drive Calvin cycle? What process generates them?
A: ATP and NADPH2 (together called assimilatory power) are the driving force for Calvin cycle. They are produced in light reaction by chemiosmotic hypothesis (photophosphorylation).

 

11. What is the basis for designating C3 and C4 pathway of photosynthesis?
A: The first formed stable intermediate compounds of CO2 fixation in C3 pathway and C4 pathway are PGA (Phospho Glyceric Acid) and OAA (Oxalo Acetic Acid) respectively. As they consist 3 carbons and 4 carbons, the respective phases are called C3 and C4 pathway.

 

12. Distinguish between action spectrum and absorption spectrum.
A: A graph showing the rate of photosynthesis at different wave lengths is called Action spectrum. A graph showing the absorption of light by different pigments at different wave lengths is called absorption spectrum.

13. Of  the basic raw materials of photosynthesis, what is reduced? What is oxidised?
A: CO2 is reduced. H2O is oxidised.

 

14. Define the law of limiting factors proposed by Blackman.
A: According to Blackman (1905) "If a process (like photosynthesis) is conditioned as to its rapidity by a number of separate factors the rate of the process is limited by the factor that is present in a relative minimum value".

 

15. What is Joseph Priestley's contribution to the study of photosynthesis?
A: Joseph Priestley discovered that green plants absorb toxic gases released by breathing animals and release O2. He used mint, mouse and candle in the experiment.

 

16. Comment on the contribution of Van Niel to the understanding of photosynthesis?
A: Carmelius Van Niel, a microbiologist in 1924 worked on purple, green bacteria (Chromatium vinosum) and proposed that ''Photosynthesis is a light, dependent
process''. The source of H2 is H2S. This H2 reduces CO2 to Glucose. This photosynthesis in such bacteria is anoxygenic. It is shown as

In sulphur bacteria H2A = H2S. Hence they release sulphur.

17. With reference to the photosystem, bring out the meaning of the terms
       (a) Antennae (b) Reaction centre.
A: Antennae: Pigment system has 2 parts. 1) Light Harvesting system or complex 2) Reaction centre Light harvesting complex has hundreds of pigments. They absorb different wave lengths of  light and thus act as Antennae. PS I has LHC I and PS II has LHC II. They absorb light and convey it to reaction centre by inductive resonance.
Reaction centre: It is P700 for PS I and P680 for PS II. P700 and P680 are Chl a which converts radiant energy to chemical energy.

 

18. Why is photosynthetic electron transport from H2O to NADP+ named as Zscheme?
A: PS II and PS I participate in the electron transport from H2O to NADP+. The two photosystems are connected through an electron transport chain. When all the electron carriers are arranged in a sequence on a redox potential scale (uphill to downhill or higher redox potential to lower redox potential). Z-shape is formed. So it is called Z-scheme.

19. What is the primary acceptor of CO2 in C3 plants? What is the first stable compound formed in Calvin cycle?
A: The initial acceptor of CO2 in C3 plants is Ribolose bisphosphate (RuBP). It is a 5-carbon compound. The first stable compound in C3 pathway is PGA (Phospho Glyceric Acid). It is a 3-carbon compound.

 

20. What is the primary acceptor of CO2 in C4 plants? What is the first compound formed as a result of primary carboxylation in C4 plants  pathway?
A: The primary acceptor of CO2 in C4 is PEP (Phospho Enol Pyruvate), a 3-carbon compound.
        The first formed compound as a result of primary carboxylation in mesophyll cells of C4 plants is Oxalo Acetic Acid (OAA), a 4-carbon compound. It is a dicarboxylic acid.

EXERCISE

1. By looking at a plant externally, can you tell whether a plant is C3 or C4? Why and how?
A: Yes. By looking at a plant externally we can tell whether a plant is C3 or C4.
     C3 plants                              C4 plants
1) Mostly dicots                   1) Mostly monocots
2) Dorsiventral leaves        2) Isobilateral leaves
3) Reticulate venation        3) Parallel venation
4) Plants have thick leaves as they tolerate high temperature.

 

2. By looking which internal structure of a plant can you tell whether a plant is C3 or C4? Explain.
A: By looking at bundle sheath cells we can tell whether a plant is C3 or C4.

3. Even though a very few cells in a C4 plant carry out the biosynthetic - Calvin pathway, yet they are highly productive. Can you discuss why?
A: C3 pathway in C4 plants occurs in bundle sheath cells only. Yet C4 plants are highly productive. because
1) There is no photorespiration, a wastefull process.
2) RuBisCO acts as carboxylase only.
3) CO2 concentration at RuBisCO site increased.
4) Efficient translocation of photosynthates through phloem as bundle sheath cells are adjacent to it.

 

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?
A: RuBisCO enzyme has 2 active sites, one for CO2 and the other for O2. In C3 plants some O2 binds to RuBisCO. Hence Carbondioxide fixation is decreased. In C4 plants RuBisCO acts only as carboxylase as CO2 concentration is more at the enzyme due to the participation of mesophyll cells also in carboxylation. Thus RuBisCO carries out more carboxylation.

5. Suppose there were plants that had a high concentration of chlorophyll-b, but lacked Chl -a, would it carry out photosynthesis? Then why do plants have Chl-b and other accessory pigments?
A: The plants which have high concentration of Chl-b but lack Chl-a cannot carry  out photosynthesis because Chl-a alone converts radiant energy to chemical energy. The plants have Chl-b and other accessory pigments to make the photosynthesis more efficient by absorbing different wave lengths of light and conveying them to Chl-a. They also protect Chl-a from photo oxidation.

 

6. Why is the colour of a leaf kept in dark frequently yellow, or pale green? Which pigment do you think is more stable?
A: The leaves of the plant grown in dark are colourless or pale green due to the absence of light. plants require light to synthesise chlorophyll. Chlorophyll is more stable than other pigments.

7. Look at the leaves of 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 shade. Which of them has leaves that are darker green? Why?
A. Leaves of the plant kept in the sunlight or of the plant on the sunny side are dark green as they produce more chlorophyll.

 

8.


                      
Graph showing effect of light intensity on the rate of photosynthesis. Based on the above graph answer the following questions.
(a) At which point/s (A, B or C) in the curve is light a limiting factor?
A. At B light becomes limiting factor.

(b) What could be the limiting factor/s in the region A?
A. CO2
(c) What do C and D in region A represent on the curve?
A. C - Rate of photosynthesis remains same.

 

9. Give comparison between the following:
(a) C3 and C4 pathways

(b) Cyclic and Non-Cyclic photo phosphorylations

(c) Anatomy of C3 and C4 plants.

 10. Cyanobacteria and some other photosynthetic bacteria do not have chloroplasts.
How do they conduct photosynthesis?
A: They have no chloroplasts. Pigments are present in Chromatophores or scattered thylakoids, so they conduct photosynthesis in chromatophores.

 

11. Does moon light support photosynthesis?
A: No

 

12. Why photorespiration does not occur in C4 plants?
A: Photorespiration do not occurs in C4 plants because..
1) CO2 concentration is increased at RuBP case in bundle sheath cells.
2) RuBP case acts only as carboxylase.
3) Peroxysomes are not present.

 

13. Tomatoes, Chillis and carrots are red in colour due to the presence of one pigment. Name the pigment. Is it a photosynthetic pigment?
A: The pigments in them are Lycopene in tomato, β-carotene in carrot, Capsanthin in Chillies. Capsanthin is xanthophyll and the rest are Carotenes. Yes, they are also photosynthetic pigments. They are accessory pigments, as they trap sunlight and convey it to Chl-a.

14. If a green plant is kept in dark with proper ventilation, can this plant carry out photosynthesis? Can any thing be given as supplement to maintain its growth or survival?
A: It can not carry out photosynthesis because light is not available. Light should be given as supplement to maintain its growth or survival.

 

15. Why do you believe Chloroplast and Mitochondria to be semi autonomous organelle?
A: They have ds circular DNA as genetic material. They reproduce themselves. They produce their own proteins as they have 70 S ribosomes. They can not survive independently, so they are considered as semi autonomous organelles.

 

16. Is it correct to say that photosynthesis occurs only in the leaves of a plant? Besides leaves, what are the other parts that may be capable of carrying out photosynthesis? Justify?
A: It is not correct to say that photosynthesis occurs only in the leaves of the plant. Besides leaves, photosynthesis also occurs in stipules, petiole, young stem, young branches, pedicel, peduncle, calyx, pericarp of young unripened fruit, thalloid body of algae and bryophytes. They too have chloroplasts, so they also can carry out photosynthesis.

17. What can we conclude from the statement that the action and absorption spectrum of photosynthesis overlap? At which wave length do they show peaks?
A: Yes. We can conclude as absorption and action spectra run parallel. They are directly proportional to each other. They show peak in the blue and red regions of the spectrum.

Short  Answer Type Questions

1. Succulents are known to keep their stomata closed during the day to check transpiration. How do they meet their photosynthetic CO2 requirements?
A: Succulents show CAM pathway. They show light reaction during day time and dark reaction in the night. Thus they get photosynthetic CO2 requirements from light reaction that occurs during day time.

 

2. Chl-a is the primary pigment for light reaction. What are accessory pigments? What is their role in photosynthesis?
A: The primary pigment for light reaction is Chl-a. It converts radiant energy to chemical energy The other pigments which include Chl-b and carotenoids (carotenes and Xanthophylls) are accessory pigments. Their functions are..
1) They absorb light and transfer it to Chl-a.
2) They enable a wide range of wavelength of incoming light to be utilised for photosynthesis.
3) They protect Chl-a from photooxidation or photobleaching or Solarisation.

 

3. Does 'dark reaction' of photosynthesis require light? Explain.
A: Dark reaction do not requires light but it needs the product formed in light reaction. Hence it occurs also during day time.

 

4. How are photosynthesis and respiration are related to each other?
A: Photosynthesis is the source of food and oxygen which are essential and form the basis of life. The oxygen released in it is taken by living organisms (animals) and release CO2. This CO2 in turn taken by the plants. Priestly was the first to propose that green plants take toxic gases released by animals in respiration and release pure gas (O2). Photosynthesis is anabolic and respiration is catabolic.

5. . What conditions enable RuBISCO to function as oxygenase? Explain the ensuing process.
A: The conditions which enable RuBISCO to function as oxygenase are..
1) High concentration of  Oxygen
2) Low concentration of  CO2
             RuBISCO has 2 active sites, one for CO2 and the other for O2. CO2 and O2 show competition. When O2 is more, some O2 also binds to active site along with CO2. Then RuBISCO functions as oxygenase, resulting in a process called photorespiration.

 

6. Why does the rate of photosynthesis decrease at higher temperature?
A: Increase in incident of light increases temperature which causes photooxidation of chlorophyll. Dark reaction is controlled by enzymes. These are temperature sensitive. Thus, when temperature is high, the rate of photosynthesis decreases.

 

7. Expalin how, during light reaction of photosynthesis, ATP synthesis is chemiosmotic phenomenon?
A: Osmosis involves a membrane and diffusion of solvent particles from higher concentration to lower concentraion. ATP synthesis during light reaction occurs as follows.
1) Thylakoid membrane acts as differentially permeable membrane like in osmosis.
2) Proton concentraion gradient is established along the membrane due to Quinone cycles.
3) Proton concentration is more in lumen than in stroma.
4) Protons diffuse through F0 concentration of ATP synthase like solvent diffuses through plasmamembrane.
5) This break down of proton gradient supplies radiant energy present with them convert ADP to ATP in F1. Thus ATP synthesis is chemiosmotic.

 

8. Explain how Calvin worked out the complete biosynthetic pathway for the synthesis of sugar?
A: Melvin Calvin with Benson and Baasham worked on Chlorella, a unicellular, eukaryotic, green algal plant. He used radioactive C14, autoradiography, paper chromatography, boiling methanol and discovered the first intermediate stable compound as PGA (Phospho Glyceric Acid). It is a 3-carbon compound. He also discovered the entire steps starting from carboxylation to the synthesis of sugar. Hence dark reaction is also called C3 - pathway or Calvin, Benson and Baasham cycle.

9. Six turns of Calvin cycle are required to generate one mole of Glucose. Explain.
A: One molecule of CO2 in Calvin cycle consumes 3 ATP and 2 NADPH2 to produce 1/6 Glucose. So 6 molecule of CO2 in Calvin cycle produce 1 Glucose by consuming 18 ATP and 12 NADPH2. Hence six turns of Calvin cycle are required to generate one molecule of Glucose.

 

10. With the help of diagram, explain briefly the process of cyclic photophosphorylation.

It was stuided by Frankel in  the isolated chromatophores of photoheterotrophic Rhodospirillum rubrum PS I alone participates. It occurs when light of wavelengths beyond 680 nm are available.
The reaction centre (P700) when excited loses a pair of energetic electrons. The initial e− acceptor is Fd.

The electrons are circulated with in the photosystem and come back to P700. Thus the route of e− is cyclic. Meanwhile photophosphorylation occurs due to the protons accumulated in lumen (brought by Quinone from stroma). It occurs in stroma thylakoids because
1) It do not consists PS II
2) NADP reductase is absent in them. Thus the cyclic flow of e− results in the formation of ATP only.

 

11. In what type of plants do you come across 'Kranz' anatomy? To which conditions are those plants better adapted? How are these plants better adapted than the plants, which lack this anatomy?
A: We come across Kranz anatomy in the plants with following characters.
1) Mostly grasses.
2) The leaves are isobilateral.
3) Mesophyll is undifferentiated.
4) Bundle sheath cells are big and arranged circularly in the form of a wreath.

5) Leaves show chloroplast dimorphism. These are better adapted to high light intensity, high carbondioxide concentration, high temperature. The plants which lack this anatomy are not adapted to such conditions. These plants have both RuBP case (bundle sheath cells) and PEP case (mesophyll cells) and do not show photorespiration.
 

12. Explain the structure of the chloroplast? Draw a neat labelled diagram.

It is a double membrane bounded, self duplicating, semiautonomous organelle and next to nucleus in size. It is seen in eukaryotic, green, plant cells only.

The envelope encloses periplastidial space.
It has two important parts
1) Grana 2) Stroma
     Grana are many in number. Each granum is a pack of grana thylakoids. They consists pigments, grouped as PS I and PS II. It is the site of light reaction.
      Stroma is colourless, jelly like and rich in enzymes. It is the site of dark reaction. Thus there is division of labour. Stroma also consist stroma thylakoids (Stroma lamellae or Frets).


  They have PS I only and do not consist NADP reductase. Chloroplast also consists circular, ds DNA and 70 S of ribosomes.

13. Explain why 12 molecules are used as substrate, instead of 6 molecules of water in the following equation.

A. 1) Oxygen released in photosynthesis must be only from water.
2) Oxygen present in Glucose in only from CO2, so we have to take 12 H2O in the equation.

 

14. Compare and contrast the absorption spectrum of chlorophylls and carotenodis.        

              

A: A graph showing light absorption of different wavelengths by different pigments is called Absorption spectrum. Chl-a shows maximum absorption at 450 nm and 680 nm Chl-b shows a maximum absorption at 480 at 650 nm. Carotenoids show absorption maximum at 450 nm and 480 nm.
 

15. Which group of plants exhibits two types of photosynthetic cells? What is the first produt of carboxylation? What carboxylating enzymes are present in bundle sheath cells and mesophyll cells?
A: C4 plants exhibit 2 types of photosynthetic cells in their leaves.
1) Mesophyll cells. These are green.
2) Bundle sheath cells. They have a agranal chloroplasts. The product of first carboxylation is Oxalo Acetic Acid. It has 4 carbons and 2 carboxylic groups. Bundle sheath cells have RuBP case and mesophyll cells have PEP case for carboxylation. PEP case has more affinity to CO2

 

16. A cyclic process is occuring in a C3 plant, which is light dependent and needs O2. This process does not produce energy rather it consumes energy.

(a) Can you name the given process?
Ans: Yes, It is photorespiration.
(b) Is it essential for survival?
Ans: Though it is a wasteful process, it is essential because it protects C3 plants from photoxidative damage.
(c) What are the end products of this process?
Ans: CO2 is the end product. It produces neither food nor energy. Rather it consumes ATP.
(d) Where does it occur?
Ans: It occurs in chloroplasts, peroxysomes and mitochondria.

 

17. Suppose Euphorbia and Maize are grown in tropical area
(a) Which one of them do you think will be able to survive under such conditions?
A. Euphorbia.
(b) Which one of them is more efficient in terms of photosynthetic activity?
A. Maize

(c) What differences do you think are there in the leaf?
Ans: Euphorbia is a CAM plant. It has only one kind of photosynthetic cells. i.e. mesophyll cells. Maize has 2 kinds of photosynthetic cells.
1) Green mesophyll cells
2) Colourless bundle sheath cells.                                 

Long Answer Type Questions

1. The entire process of photosynthesis consists a number of reactions. Where in the cell do each of these take place?
a) Synthesis of ATP and NADPH.
A: It occurs in the outside of lumen i.e. in the stroma as follows.

 
b) Photolysis of water
A: It occurs in the lumen of grana thylakoids.
c) Fixation of CO2
A: It occurs in the stroma of the chloroplast.
d) Synthesis of sugar molecule
A: It occurs in the cytosol of mesophyll cells (C3 plants) and bundle sheath cells (C4 plants)
e) synthesis of starch
A: 1) It occurs in the storage tissues of fruit, seed or stem, root or leaves. 
    2) It also occurs in chloroplasts when sucrose synthesis does not keep pace with the Calvin cycle.

 

2. Which property of pigments is responsible for its ability to initiate the process of photosynthesis? Why is the rate of photosynthesis higher in red and blue regions of the spectrum of light?
A: Pigments are present in thylakoid membranes. They are natural colouring substances which have the capacity to trap radiant energy at specific wave lengths. Chlorophyll - a receives light from rest of the pigments. It alone has the capacity to convert solar or radiant energy to chemical energy. The rate of photosynthesis is higher in red and blue regions of the spectrum of light because pigments (Chl a & Chl b) show maximum absorption in red and blue and regions of the spectrum of light.

 

 

First graph showing the absorption spectrum of chlorophyll a, b and the carotenoids. Second grpah showing action spectrum of photosynthesis superimposed on absorption spectrum of chlorophyll a.

3. Under what conditions are C4 plants superior to C3?
A: C4 plants are superior to C3 plants under the following conditions:
 Two types of photosynthetic cells are present.
      a) Mesophyll cells               b) Bundle sheath cells
 Rate of CO2 fixation is more as it occurs in both cells.
 They show kranz anatomy.
 They have 2 carboxylation enzymes.
     a) PEP case and                   b) RuBP case
 They do not have peroxisomes. So they do not show photorespiration, a wasteful process.
 They have efficient translocation of photosynthates as bundle sheath cells (where actually food is prepared) are adjacent to vascular tissues.
 They do not show feed back inhibition.
 They grow in tropical regions.
 They tolerate high temperatures (30°C − 40°C) or even above.
 They show response to high light intensity.
 They show lower CO2 compensation point.
 They maintain higher CO2 concentration at RuBP case. So they do not show photorespiration.

4. What are the important events and end products of light reactions?
A: There are 2 kinds of light reactions.
1) Non-cyclic photophosphorylation.
2) Cyclic photophosphorylation.
NON CYCLIC PHOTOPHOSPHORYLATION:
It occurs in grana thylakoids. PS II and PS I participate in it. It occurs as follows.
1) LHC II absorbs sunlight of 680 nm wavelength of red light and conveys it to the reaction centre P680 by resonance transfer.
2) P680 becomes excited and loses a pair of energetic electrons and thus becomes oxidised.
3) The initial acceptor of these electrons is pheophytin (chlorophyll without Magnesium). The movement of electrons from pheophytin to P700 is downhill (interms of redox potential scale).
4) Plasto Quninone accepts the electrons from pheophytin and convey them to plastocyanin through Cyt b 6 - Cyt f complex.

5) Quinone cycle occurs and for every electron 2 H+ are shifted from stroma to lumen.
6) Photolysis of water occurs in lumen and OEC (Oxygen Evolving Complex) transfers the electrons from water to P 680. For every H2O, two protons are added to lumen oxygen formed in it is released into atmosphere.
7) LHC I absorbs sun light of 700 nm wavelength of red light and conveys it to P 700 of  PS I. P 700 becomes excited & oxidised by losing a pair of energetic electrons.
8) The electrons from plastocyanin are accepted by P 700 . The electrons of
P700 are accepted by Ferredoxin (fe - s protein) which conveys them to NADP through NADP reductase.
9) Thus the transport of electrons is not cyclic i.e., the electrons belonging to water finally reach NADP.
10) Proton concentration gradient is established along the thylakoid membrane. According to Peter Mitchell's chemiosmotic model, the protons pass through F0 − F1 from lumen to stroma.
11) For every 3 protons, one ATP is formed and it is called photophosphorylation. For one H2O, 2 ATP and 1 NADPH2 are formed.

CYCLIC PHOTOPHOSPHORYLATION:
PS I alone participates in it. It occurs stroma thylakoids. Photolysis of water do not occurs. It occurs as follows.
1) LHC I absorbs 7th step in Non-Cyclic.
2) The initial acceptor of electrons in it is Ferredoxin (fe-s protein) from where they go to plasto Quinone. Quinone cycle occurs for every electron, 2 H+ are added to lumen.
3) The electrons from PQ reach plasto cyanin through Cyt b6 − Cyt f complex. Plasto cyanin conveys these electrons back to P700. Thus the transport of electrons is cyclic.
4) See step No 10 & 11.
Thus 1 ATP is produced for every pair of electrons released from P700. NADPH2 is not formed, because
a) Photolysis of water do not occurs.
b) PS II and NADP reductase are absent.

5. Explain various aspects of Mitchell's chemiosmotic hypothesis with the help of diagrams. A. Proton concentration gradient is established along the granathylakoids (Non-Cyclic) and stroma thylakoids (Cyclic). Thylakodis are impermeable to protons. Proton concentration is more in lumen and less in stroma. Peter Mitchell (Nobel laureate) proposed chemiosmotic hypothesis (model). Chemiosmosis requires
 Membrane (Grana thylakoid or Stroma thylakoid)
 Proton pump (plasto quinone acts as proton pump). Radiant energy is used to pump electrons from stroma to lumen.
 Proton concentration gradient. It is established due to the Quinone cycle.
 ATP ase. It is F0 − F1. F0 is towards lumen and F1 protrodes into stroma, F0 acts as proton channel.
 F1 shows confirmational changes by which  ATP is formed. It is called photophosphorylation.
Non-Cyclic photophosphorylaton:
For photolysis water molecule, 6 H+ are accumulated in lumen. When pass through F0 and release energy in rotating F1, 2 ATP and one NADPH2 are produced.

Cyclic photophosporylation:
For every pair of electrons, a total of 4 H+ are accumulated in lumen. They produce 1 ATP through ATP ase as follows.

6. Comment on the dual role of RuBISCO. What is the basis for its oxygenation activity?
Why is this activity absent or negligible in C4 plants?
A: RuBISCO is an enzyme richly present in the stroma of chloroplasts in all green plants. It is the most abundant universal protein in the plant kingdom.
It has 2 reaction sites, one for CO2 and the other for oxygen.
         Generally it participates in carboxylation. Then we call it as RuBP Case. When O2 is more, CO2 concentration is less, and light intensity is more it also acts as Oxygenase. Some of the reaction sites on it are occupied by oxygen along with CO2 on other reaction sites.
So the enzyme is called RuBISCO (Ribulose bisphosphate Carboxylase Oxygenase).
Thus it participates in carboxylation and as well as oxidation reactions. Photo respiration occurs in C3 plants where in RuBISCO plays dual role. The basis for its oxygenation activity is
1) The more concentration of O2
2) Less concentration of CO2 and
3) More light intensity
This activity of RuBISCO is absent or negligible in C4 plants, because

Mesophyll cells participate in primary carboxylation and supplies more CO2 to bundle sheath cells through malic acid. Thus concentration of CO2 is always maintained more at RuBISCO.

      NON CYCLIC PHOTOPHOSPHORYLATION

So there is no chance for oxygen to occupy the reaction site RuBISCO.

Posted Date : 26-07-2021

గమనిక : ప్రతిభ.ఈనాడు.నెట్‌లో కనిపించే వ్యాపార ప్రకటనలు వివిధ దేశాల్లోని వ్యాపారులు, సంస్థల నుంచి వస్తాయి. మరి కొన్ని ప్రకటనలు పాఠకుల అభిరుచి మేరకు కృత్రిమ మేధస్సు సాంకేతికత సాయంతో ప్రదర్శితమవుతుంటాయి. ఆ ప్రకటనల్లోని ఉత్పత్తులను లేదా సేవలను పాఠకులు స్వయంగా విచారించుకొని, జాగ్రత్తగా పరిశీలించి కొనుక్కోవాలి లేదా వినియోగించుకోవాలి. వాటి నాణ్యత లేదా లోపాలతో ఈనాడు యాజమాన్యానికి ఎలాంటి సంబంధం లేదు. ఈ విషయంలో ఉత్తర ప్రత్యుత్తరాలకు, ఈ-మెయిల్స్ కి, ఇంకా ఇతర రూపాల్లో సమాచార మార్పిడికి తావు లేదు. ఫిర్యాదులు స్వీకరించడం కుదరదు. పాఠకులు గమనించి, సహకరించాలని మనవి.

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