1st PUC Biology Question Bank Chapter 13 Photosynthesis in Higher Plants

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Karnataka 1st PUC Biology Question Bank Chapter 13 Photosynthesis in Higher Plants

1st PUC Biology Photosynthesis in Higher Plants NCERT Text Book Questions and Answers

Question 1.
By looking at a plant externally can you tell whether a plant is C₃ or C₄? Why and how?
Answer:
C₄ plants are adapted to the xerophytic climatic conditions they can grow well in high temperature. It cannot be said conclusively that the plant is a C₃ or C₄ by looking at external appearance, some guess can be made by looking at fleshy leaf structure of C₄ plants.

Question 2.
By looking at which internal structure of a plant can you tell whether a plant is C₃ or C₄? Explain.
Answer:
The particularly large cells around the vascular bundles of the C₄ pathway plants are called bundle sheath cells, and the leaves which have such anatomy are said to have ‘Kranz1 anatomy. ‘Kranz’ means ‘wreath’ and is a reflection of the arrangement of cells. The bundle sheath cells may form several layers around the vascular bundles, they are characterized by having a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces.

Question 3.
Even though very few cells In a C₄ plant carry out the biosynthetic – Calvin pathway, yet they are highly productive. Can you discuss why?
Answer:

1. In C₄ plants, the biosynthetic Calvin cycle occurs only in bundle sheaths. Despite few cells performing the Calvin cycle in C₄ plants, they are highly productive due to minimum photorespiration losses.
2. They are adopted to diverse climatic conditions as C₄ plants can synthesize at very low CO₂ concentration while for C₃ plants CO₂ concentration is the limiting factor.
3. C₄ plants can synthesize at high temperatures while C₃ plants cannot.
4. Rapid withdrawal of photosynthates from the bundle sheath cells as they lie over the vascular bundles.
5. Photosynthesis continues even when stomata are closed due to the fixation of CO₂ released through respiration.

Question 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?
Answer:
RuBisCo has a much greater affinity for CO₂ than for O₂. It is the relative concentration of O₂ and CO₂ that determines which of the two will bind to the enzyme. In C₃ plants some O₂ does bind to RuBisCo and hence CO₂ fixation is decreased.

In C₄ plants photorespiration does not occur. This is because they have a mechanism that increases the concentration of CO₂ at the enzyme site. This takes place when the C₄ acid from the mesophyll is broken down in the bundle cells to release CO₂. This results in increasing the intracellular concentration of CO₂. In turn, this ensures that the RuBisCo functions as a carboxylase minimizing the oxygenase activity.

Question 5.
Suppose there were plants that had a high concentration of Chlorophyll b but lacked chlorophyll a, would It carry out photosynthesis? Then why do plants have chlorophyll b and other accessory pigments?
Answer:
No, photosynthesis occurs in plants having a high concentration of chlorophyll ‘b’ but lacks chlorophyll ‘a’ because chlorophyll ‘a’ molecule forms reaction center in both photosystems I and II which converts light energy into electrical energy and excites the electrons for photolysis of water.

Maximum photosynthesis occurs at the wavelengths which is absorbed by chlorophyll ‘a’ molecule i.e. blue and red regions.

Though chlorophyll a is the major pigment responsible for trapping light, other thylakoid pigments like chlorophyll b, xanthophylls, and carotenoids, which are called accessory pigments, also absorb light and transfer the energy to chlorophyll a.

Indeed they not only enable a wider range of wavelengths of incoming light to be utilized for photosynthesis but also protect chlorophyll from photo-oxidation.

Question 6.
Why is the colour of a leaf kept In the dark frequently yellow, or pale green? Which pigment do you think Is more stable?
Answer:
We can look for an answer to these questions by trying to separate the leaf pigments of any green plant through paper chromatography. Chromatographic separation of the leaf pigments shows that the color that we can see in leaves is not due to a single pigment but due to four pigments: Chlorophyll (a) (bright or blue-green in the chromatogram), Chlorophyll (b) (yellow-green), xanthophylls (yellow) and carotenoids (yellow to yellow-orange). Chlorophyll (a) is the chief pigment associated with photosynthesis.

Question 7.
Look at leaves of the same plant on the shady side and compare it with the leaves on the sunny side. Or, compare the potted plants kept in the sunlight with those in the shade. Which of them has leaves that are darker green? Why?
Answer:
In sunny plant colour of leaves is darker green because in sunny plant photosynthesis takes place while in shady plant rate of photosynthesis is low.

Question 8.
The figure shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions

(a) At which point/s (A, B, or C) in the curve is light a limiting factor?
Answer:
Points B-C of the curve, the rate did not increase with an increase in its concentration because under these conditions, the light becomes a limiting factor.

(b) What could be the limiting factor/s in region A?
Answer:
The rate of photosynthesis shows a proportionate increase up to a certain CO₂ concentration (In region A of the curve), beyond which the rate again becomes constant, not showing any increase by increasing CO₂ concentration.

(c) What do C and D represent on the curve?
Answer:
If the light intensity is doubled, i.e., the plants are exposed to 2 Units of light, CO₂ concentration again becomes a limiting factor beyond this concentration (Points C and D represent on the curve.

Question 9.
Give a comparison between the following:

1. C₃ and C₄ Pathways
2. Cyclic and non-cyclic photophosphorylation
3. Anatomy of leaf in C₃ and C₄ plants.

Answer:
(1) Difference between C₃ and C₄ pathways:

C₃ pathway or Calvin:

1. The primary acceptor of CO₂ is RUBP, a 5 carbon compound
2. The first stable product is a 3-PGA a 3 carbon compound.
3. It operates under a low concentration of CO₂ in mesophyll cells.
4. CO₂ once fixed is not released back.
5. There is a net grin of one molecule of glucose with consumption of 6 CO₂, molecules.
6. Fixation of one molecule of CO₂ needs 3ATP and 2NADPH₂, molecules. Thus C₃, pathway requires 18 ATP for the synthesis of one molecule of glucose.
7. C₃ cycle operates in all categories of plants.
8. Rate of CO₂ fixation in slow
9. The optimum temperature for the operation of the Calvin cycle is 10-25° C.

C₄ Pathway or Hatch & Slack:

1. The primary acceptor of CO₂ is a PEP, a 3 carbon compound
2. The first stable product is oxaloacetic acid, a 4 carbon compound.
3. It can operate under very low CP₂, concentration in mesophyll cells.
4. CO₂ once fixed is released back in bundle sheath cells which are finally fixed and reduced by the Calvin cycle.
5. There is no net gain. It rather involves additional consumption of 12 ATP molecules per glucose molecule synthesized.
6. Fixation of one molecule of CO₂ needs 2ATP molecules in addition to that required in the C₃ cycle. The C₄ pathway requires 30 ATP for the synthesis of one molecule of glucose.
7. C₄ cycle operates only in C₄ plants.
8. The rate of CO₂, fixation is faster.
9. The optimum temperature for the operation of C₄, the cycle is 30-45° C

(2) Differences between Cyclic and Non-cyclic photophosphorylation:

Cyclic photophosphorylation:

1. It is performed by photosystem I independently in stromal or thylakoids.
2. In this, an external source of electrons is not required because the same electrons get recycled.
3. It is not connected with the photolysis of water and thus no oxygen is evolved.
4. In this an electron expelled by the exciting photo center P700 is returned to it after passing through a series of electron carriers in an ETS, hence it is called cyclic photophosphorylation.
5. It is activated by light of 700nm wavelength.
6. It generates ATP only. There is no formation of NADPH,
7. Chlorophyll does not receive any electrons from water.
8. It operates under the low intensity of light, anaerobic conditions, or when CO₂, availability is poor.
9. The system does not take part in photosynthesis except in certain bacteria.

Non-cyclic photophosphorylation:

1. It is carried out collectively both by PSI & PSII photosystems in the grana thylakoids.
2. This process required an external electron donor.
3. It is connected with the photolysis of water and thus oxygen is evolved in it.
4. In this an electron expelled by the exciting photo center P₆₈₀ is not returned to it after passing through a series of electron carriers, but reaches NADP; hence it is called non-cyclic photophosphorylation. In this water is the ultimate source of electrons and NADP⁺ is the final acceptor.
5. In this, the photo centers absorb the light of 680 nm as well as 700nm wavelength.
6. It produces both ATP as well as NADPH₂
7. The ultimate source of electrons is the photolysis of water.
8. It operates under optimum light, aerobic conditions, and in the presence of CO₂
9. This system is connected with CO₂ fixation and is dominant in green plants.

(3) Differences between the anatomy of leaf in C₃ and C₄ plants:

C₃, leal anatomy:
The rate of photosynthesis is influenced by leaf anatomy as it greatly affects the availability of sunlight, the rate of diffusion of CO₂, into the mesophyll cells, and the translocation of end products of photosynthesis. The important anatomical features that influence the rate of photosynthesis are the thickness of cuticle, number and distribution of stomata, degree of opening of stomata, the size and number of chloroplasts, size and distribution of intercellular spaces, and number and distribution of vascular strands.

C₄ Kranz anatomy:
In some tropical grasses, the cells of bundle sheath around the vascular strand in the leaves are large green, and barrel-shaped. They are surrounded by one or more concentric layers of mesophyll cells. The mesophyll and bundle sheath cells are connected by plasmodesmata. The chloroplasts in the bundle sheath cells are large but do not have well-defined grana.

1st PUC Biology Photosynthesis in Higher Plants Additional Questions and Answers

1st PUC Biology Photosynthesis in Higher Plants One Mark Questions

Question 1.
What is the name of the green plastid?
Answer:
Chloroplast (Oct. 83)

Question 2.
Where does oxygen liberated during photosynthesis come from? (Oct. 90)
Answer:
Water

Question 3.
Which one is the most important limiting factor in photosynthesis?
Answer:
Carbon dioxide.

Question 4.
Who proposed the Law of Limiting Factors?
Answer:
F.F. Blackman (Apr. 1991,1999)

Question 5.
In which reaction of photosynthesis oxygen is released? (Oct. 1994)
Answer:
Photolysis of water

Question 6.
Mention the two ways in which Ca++ is involved in cell division in plants. Where are the photosynthetic pigments located in a chloroplast?
Answer:
In the thylakoid membrane.

Question 7.
What are Quantasomes? (Apr. 1997)
Answer:
Quantasomes is a functional unit (photosynthetic units) made of a group of pigment molecules required for carrying out a photochemical reaction.

Question 8.
Define Photophosphorylation. (Oct. 1997, Apr. 2000)
Answer:
The synthesis of ATP in the presence of light is called Photophosphorylation.

Question 9.
What is a C₃ plant? (M.Q.P.)
Answer:
A C₃ plant is one in which the first stable compound obtained during the dark reaction is a three-carbon compound.

Question 10.
Define photolysis of water. (Oct. 1998, 2000, M.Q.P.)
Answer:
Photolysis of photoionization is the splitting of water into protons, electrons, and oxygen in presence of light.

Question 11.
What does the variegated leaf experiment of photosynthesis prove?
Answer:
It proves that chlorophyll is necessary for photosynthesis.

Question 12.
Name the first stable product of the Calvin cycle. (Oct. 2003, July 2007, 2009)
Answer:
PGA – Phosphoglyceric Acid.

Question 13.
What is the CAM pathway?
Answer:
The fixation of CO₂ obtained from organic acid like malic acid in members of the family Crassulaceae is called the CAM pathway.

Question 14.
What are C₄ plants?
Answer:
The plants produce 4 – carbon compounds as the first stable substances during the dark reaction are called C₄ plants.

Question 15.
Expand NADP. (April 2004)
Answer:
Nicotinamide Adenine Dinucleotide Phosphate.

Question 16.
CAM plants close their stomata during day time. Give reason. (April 2007)
Answer:
These plants are mainly xerophytes which open their stomata at night when temperatures are low and close stomata during daytime when temperatures are high as a mechanism to conserve water.

Question 17.
Who discovered the C₄ cycle?
Answer:
Hatch and Slack.

Question 18.
Give reason: (March 2008)
Very high temperature decreases the rate of photosynthesis.
Answer:
Chlorophyll undergoes photo-oxidation/ solarization at high temperature reducing photosynthesis.

Question 19.
Give reason: (July 2008, April 2009)
Carotenoid and Xanthophyli are called accessory photosynthetic pigments.
Answer:
Carotenoid and Xanthophyli transfer the absorbed light to chlorophyll ‘a1, hence are called accessory pigments. They cannot release electrons on their own and require chlorophyll.

1st PUC Biology Photosynthesis in Higher Plants Two Marks Questions

Question 1.
Write any two differences between Light and Dark Reactions of Photosynthesis. (Mar. 1988)
Answer:
(i) Light reaction  – (a) Dark reaction

• Takes place in presence of light.
  (a) Independent of Light.
• Mainly Photolysis and ATP synthesis with O₄ evolution.
  (b) Mainly concerned with carbon fixation.

Question 2.
Why does chlorophyll appear red in reflected light and green in transmitted light? Explain the significance of these phenomena in terms of photosynthesis.
Answer:
In reflected light, the chlorophyll appears red because of fluorescence. The light absorbed by chlorophyll molecules loses its energy and emits light of wavelengths corresponds to red colour. In transmitted light, chlorophyll appears green because it absorbs only light of wavelengths corresponds to green colour.

Question 3.
Mention the stages of light reaction. (M.Q.P.)
Answer:
The stages of light reaction are;

• Photoexcitation of chlorophyll
• Photolysis of water
• Photophosphorylation
• Reduction of NADP

Question 4.
How does temperature influence the biosynthetic phase of photosynthesis?
Answer:
Influences of temperature on the biosynthetic phase of photosynthesis:

• At higher temperature enzymes become inactive as it gets denatured.
• At low temperatures also enzyme becomes inactive.
• (Affinity of the enzymes for C0₂ decreases with increasing temperature.

Question 5.
Mention any two differences between Photosystem I and Photosystem II. (Oct. 1999)
Answer:
(i) Photosystem I  – (a) Photosystem II

1. The reaction centre is P 700 and absorbs red light of 700 nm efficiently.
   (a) Reaction centre is P₆₈₀ and absorbs light of 680 nm efficiently.
2. Located on the unstacked stroma thylakoids and regions of grana facing the stroma.
   (b) Located mostly on the stacked membranes of the thylakoids

Question 6.
Define Blackman’s law of limiting factors. (Apr. 2000, Oct. 2002)
Answer:
When “a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest factor”.

Question 7.
Why do scientists expect faster growth and more yield by C₃ plants, if the atmospheric CO₂ increases?
Answer:
If the concentration of CO₂ in the atmosphere increases, the rate of photosynthesis by C₃ plants will increase for the following two reasons.
(a) High availability of substrate (CO₂) for carboxylation.
(b) Photorespiration is reduced due to more availability of CO₂ as the enzyme will function only as a carboxylase.

Question 8.
List out any two differences between photo-phosphorylation and oxidative phosphorylation. (Apr. 2003)
Answer:

1. Photophosphorylation takes place in the presence of light, whereas Oxidative phosphorylation takes place in the presence of oxygen.
2. Photophosphorylation occurs in the chloroplast during photosynthesis, whereas Oxidative occurs in the mitochondria during respiration.

Question 9.
Expand PEP. Where is it produced in C₄ plants? What is its role in the biosynthetic process?
Answer:
PEP – phosphoenolpyruvate. It is produced in the mesophyll cells of the leaves of C₄ plants. It is the primary acceptor of carbon dioxide and is converted into oxaloacetic acid (OAA). Thus it helps in carbon fixation in these plants. By this pathway, the carbon dioxide concentration in the bundle sheath increases, and photorespiration is prevented from occurring.

Question 10.
Draw a neat labelled diagram of the ultrastructure of T.S. of the chloroplast. (March 2008)
Answer:

Chloroplast is lens-shaped cell organelles located in the mesophyll cells. The chloroplast is bounded by a double membrane separated by the intermembrane space. The centre encloses a homogeneous gel-like fluid called stoma or matrix. It has 50% of the chloroplast proteins, Ribosomes, DNA, and enzymes of dark reaction.

The matrix has a system of lamellae differentiated as grana lamellae made of flattened sacs called thylakoids arranged as a stack of coins. The thylakoid membrane has an outer stromal surface and an inner luminal surface in contact with the thylakoid lumen. Some of the lamellae are unstacked and connect the grana called stroma lamellae or inter grana lamellae. The grana lamellae form the site for light reaction and stroma from the site for a dark reaction.

1st PUC Biology Photosynthesis in Higher Plants Five Mark Questions

Question 1.
Define Photosynthesis and explain the light reaction of Photosynthesis.
(Apr. 1983, 1987, 1993, 1995, 1997, 1999, 2006, Oct. 1985, 1991, 1992, M.Q.P.)
Answer:
Photosynthesis is defined as the process in which carbohydrates are synthesised from CO₂ and H₂O by green plants using radiant energy of the sun, O₂ being a byproduct.

Light Reaction: Also called Photochemical reaction and is light-dependent.
The light reaction can be summarized in 4 steps;

• Photoexcitation of chlorophyll
• Photolysis of water
• Photophosphorylation
• Photoreduction of NADP

Photoexcitation of chlorophyll: When a chlorophyll molecule is exposed to light it absorbs radiant energy and the electron in its structure picks up energy and becomes a high energy electron. The energy moves rapidly through the light-harvesting pigment molecules to reach the trie reaction centre.

It causes an electron to acquire a large amount of energy and escapes from the reaction centre leaving the chlorophyll with a net positive charge.

Photolysis or Photoionisation of water:
The source of Oxygen was earlier thought to be CO₂ but by the work of Van Neil, Ruben, and Kamen using O¹⁸ it was proved that it comes from H₂O.

Water splits up in the Manganese containing Oxygen evolving complex under the influence of light to produce protons, electrons, oxygen and water. The overall equation is represented as

\(2{ H }_{ 2 }o\rightarrow 4{ H }_{ 2 }^{ + }+4{ e }^{ – }+{ O }_{ 2 }\)

Theoretically, 8 quanta of light are required to produce one molecule of oxygen from water.

Photophosphorylation: It is the process by which ATP is synthesised in the presence of light. It was first discovered by Arnon. The process is further differentiated into cyclic and non-cyclic depending on the path taken by the electron. In cyclic the path traversed is cyclic and in non-cyclic, the path traversed is non-cyclic.

Non-cyclic Photophosphorylation:
Non-cyclic Photophosphorylation is initiated by the absorption of a photon of light by PS 11 whose reaction centre is P680 and results in the ejection of an electron creating a hole. The ejected electron is trapped by pheophytin, passes to plastoquinone (PQ) and then takes a downhill path along electron carriers cyt b₆ cyt f and plastocyanin and reaches PSI with reaction centre P700.

Absorption of light by PSI now causes the ejection of the electron released from PSII which is trapped by FRS moves to Ferredoxin and using the protons released by photolysis NADP is reduced to NADPH₂.

The hole created in PSII due to loss of the electron is filled by electrons produced during photolysis. Photolysis is aided by the Mn-containing oxygen-evolving complex. The path of electrons is from water to PSII, PSII to PSI and PSI to NADP in a zig-zag manner called a pathway.

In non-cyclic photophosphorylation, the ejected electron does not come back to PSII hence the path is non-cyclic, ATP is synthesised between cyt b₆ and cyt f, photolysis takes place, and photoreduction of NADP to NADPH₂ takes place. (July 2006)

Cyclic Photophosphorylation:
Cyclic Photophosphorylation occurs when additional ATP molecules are required and is a supplementary process. Only PSI with reaction centre P700 is activated. The ejected electron of PSI is captured by Ferredoxin and cycled back through a series of electron carriers cyt b₆, cyt f, and plastocyanin. During the downhill jour

A schematic diagram of the Non cyclic photophosphorylation and Non cyclic ETS. Abbrevations: Ph = Pheophytin, Q = Quinone, PQ = Plastoquinone, PQH₂ = Plastohydro quinone, cytb6 = Cytochrome b₆, cytb₆= Cytochrome f, PC = Plastocyanin, A1, A2, A3 = electron acceptors of PSI, Fd.= Ferridoxin, NADP = Nicotinamide adenine dinucleotide phosphate.

ney of the electron; free energy is utilised for ATP synthesis at 2 places i.e., between ferredoxin and cyt b₆ and cyt b₆ and cyt f. The process does not require photolysis of water and NADPH₂ formation.

Photoreduction of NADP:
This takes place during non-cyclic Photoohosphorylation and NADPH₂ is an excellent reducing agent. ATP and NADPH₂ are called the assimilatory powers which are utilized during the dark reaction.

Question 2.
Draw labelled diagrams and describe the structure and functions of the chloroplast.
(Apr. 1983, Oct. 2001, April 2004, 2006, March 2011)
Answer:

The chloroplast is lens-shaped cell organelles located in the mesophyll cells. The chloroplast is bounded by a double membrane separated by the intermembrane space. The centre encloses a homogeneous gel-like fluid called stoma or matrix. It has 50% of the chloroplast proteins, Ribosomes, DNA, and enzymes of dark reaction.

The matrix has a system of lamellae differentiated as grana lamellae made of flattened sacs called thylakoids arranged as a stack of coins. The thylakoid membrane has an outer stromal surface and an inner luminal surface in contact with the thylakoid lumen. Some of the lamellae are unstacked and connect the grana called stroma lamellae or inter grana lamellae. The grana lamellae form the site for light reaction and stroma from the site for a dark reaction.

Question 3.
Explain the Calvin cycle of photosynthesis.
OR
Describe the essential steps of the dark reaction of photosynthesis. (April 84, 94, 96, 98, 03, Oct. 84, 90, 2001)
OR
Give the schematic representation of the Calvin cycle. (March 2009, 2010)
Write the schematic representation of the C₃-pathway. (July 2011)
Answer:
Calvin cycle or dark reaction or C₃ cycle or thermochemical reaction forms the second step of photosynthesis which takes place independent of light. The steps involved here studied by Calvin using C₁₄.

Dark reaction mainly includes the fixation of carbon to produce the carbohydrate. This occurs in the stroma and utilises the assimilatory powers produced during the light reaction.
The steps involved may be summarised as follows.

1. First Phosphorylation: The starting compound RuMP(Ribulose Mono Phosphate) is converted to RuBP(Ribulose biphosphate) utilizing 6 molecules of ATP from the light reaction. 6 molecules of RuBP are obtained from 6 molecules of RuMP.
2. Carbon fixation by RUBP: The CO₂ is accepted by RuBP the primary acceptor to give 12  molecules of PGA(phosphoglycerate) a 3-carbon compound; The 6-carbon compound is highly unstable and dissociates to form the 3-carbon compound.
3. Second Phosphorylation: The 12 molecules of PGA utilize 12 ATP molecules to yield 12 Di PGA. The ATP used is produced during the light reaction.
4. Reduction: 12 Di PGA combine with NADPH₂ of the light reaction and are reduced to 12 PGAL(phosphoglyceraldehyde) a-3-carbon compound. The other products are 12iP, 12NADP, and H₂O.

Out of the 12 PGAL molecules, only 2 are transported to the cytoplasm and used for the formation of a sugar molecule (hexose) and the remaining two molecules are used to regenerate RuBP.

Synthesis of sugar involves condensation of 2PGAL to give fructose 1-6-diphosphate which by dephosphorylation forms Fructose 6-Phosphate and by isomerization forms, glucose and finally is stored as sucrose.

Regeneration of RuBP involves the formation of intermediates like Erythrose monophosphate, xylulose monophosphate, Sedoheptulose monophosphate, Ribose monophosphate, and finally Ribulose monophosphate-RuMP which gives RuBP.

The overall equation of dark reaction is

Question 4.
Give the importance of C₄ plants.
Answer:
The importance of C₄ plants are:

1. C₄ plants have little photorespiration.
2. C₄ plants are more efficient in picking up CO₂ even when it is found in low concentration because of the high affinity of phosphoenolpyruvate (i.e. PEP).
3. The concentric arrangement of mesophyll cells produces a smaller area in relation to volume for better utilization of available water and reduce the intensity of solar radiation.
4. They are adapted to high temperatures and intense radiation.
5. It prevents photorespiration.

Question 5.
Schematically explain non-cyclic photophosphorylation. (Apr. 2002,2007)
Answer:
Non-cyclic photophosphorylation is the ATP synthesis in the presence of light following a non-cyclic pathway. It requires both PSI and PSII+ work in sequence and is linked by electron carriers of the electron transport system. The path of electrons is from H₂O to PSII, PSII to PSI, and PSI to NADP.

When a photon of light is absorbed by PSII its electron is boosted to a high energy level P680* and transferred to pheophytin which acts as the primary electron acceptor. The electron is then transferred to quinone on the stromal surface of the thylakoid membrane which by accepting a second electron and 2H+ from stroma forms H₂. The H₂ dissociates releasing 2e^– on the luminal side to cyt b complex and 2H+ to the lumen to add to the proton pool. Electrons released by cyt b/f complex move to PC which diffuses through the lumen to P700 of PSI.

The PSI is now activated and the electron from PSII is boosted to P700* and is passed along A₀, A_1, F_x, F_ab and is transferred to Fd on the stromal surface and gets reduced. Reduced ferredoxin transfers its electrons to NADP+ which absorbs 2e- and H+ from stroma to produce NADPH and 1. excellent reducing agent by photoreduction. The electron gap formed in P680* is filled by electrons produced by photolysis through OEC. The electron flow is non-cyclic, H₂ is used to produce NADPH, ATP is formed from ADP, O₂ from H₂O, and water.

A schematic diagram of the Non cyclic photophosphorylation and Non cyclic ETS. Abbrevations : Ph = Pheophytin, Q = Quinone, PQ = Plastoquinone, PQH₂ = Plastohydro quinone, cytb₆ = Cytochrome b₆, cyt f = Cytochrome f, PC = Plastocyanin, A1, A2, A3 = electron acceptors of PSI, Fd = Ferridoxin, NADP = Nicotinamide adenine dinucleotide phosphate.

Question 6.
Explain Photoexcitation and photolysis.
Answer:
The absorption of light energy and ejection of electrons by the chlorophyll is called photoexcitation. The chlorophyll and accessory pigments of LHC of both PSI and PSII absorb radiant energy in the form of photons and transfer their absorbed energy to the reaction centre. The reaction centre of PSI and PSII gets excited and ejects its outer valance electron and the chlorophyll becomes oxidized. The electron given out by the chlorophyll is accepted by the electron carrier molecule of each photosystem. So by giving out an electron, chlorophyll becomes oxidized and the electron carrier molecule is reduced. The electron that escaped from the excited chlorophyll is a high-energy electron, so that it carries a lot of energy.

Photolysis of water (Photooxidation of H₂O) Lysis of water into a proton (H⁺), electrons (e ), and molecular O₂ in the presence of light is called photolysis of water. Water by donating electron becomes oxidized.

When light falls on PSII, its reaction centre (Chla₆₈₀) gets excited by ejecting an electron. Water splits in the Mn²⁺ associated Z – protein-containing oxygen-evolving complex under the influence of the light entering photosystem II. The protons (H⁺) released into the thylakoid lumen, electrons (e^–) are used to fill a gap created in P₆₈₀ and molecular O₂ is released.

Question 7.
Write a note on photosynthetic pigments.
Answer:
The pigments which they absorb light during photosynthesis are called photosynthetic pigments. They are,
(1) Chlorophyll: Chlorophyll pigments are of various types. Chlorophyll a, b, c, d, e, bacteriochlorophyll etc.

Chlorophyll:
It is the primary photosynthetic pigment. The chemical formula of chlorophyll is C₅₅H₇₂O₅N₄Mg. It contains a methyl group (CH₃). Chlorophyll is called primary photosynthetic pigment because it not only absorbs light but it converts absorbed light energy into chemical energy. Chi absorbs red light and blue light.

(2) Accessory pigments: The pigments absorb certain wavelengths of light and they pass that absorbed energy to chi a (primary photosynthetic pigment). They do not convert absorbed energy into chemical energy. The transfer of absorbed energy to chi a is called resonance transfer. Chlorophyll b and carotenoids are accessory pigments.

(3) Chlorophyll: It is an accessory pigment and its formula is C₅₅H₇₀O6N₄Mg. It differs from chi a in having an aldehyde group (CHO). It absorbs red light of 644nm and blue light of 455nm. It transfers absorbed light energy to chla.

(4) Carotenoids:
They are accessory pigments, they are of two types – carotenes (orange) and Xanthophylls (yellow) carotenes are insoluble in water and soluble in organic solvents. Carotenoids are always found associated with chlorophylls and in thylakoids, they are present as chromoproteins. Carotenes absorb blue and green lights and transmit red and yellow lights. Carotenoids absorb light of visible spectrum between 450 – 500nm. The formula of carotene is C₄₀H₅₆ (Carotene is a hydrocarbon compound). The formula of Xanthophyll is C₄₀H₅₆O₂N₄

Question 8.
Describe Mohl’s half leaf experiment. (April 2006)
Answer:
Aim: To show that CO₂ is necessary for photosynthesis.
Procedure: A potted plant is placed in the dark for 2 days to research it completely. To one of its leaf, a wide-mouthed bottle with a split cork is fixed in such a way that half the leaf is inside the bottle and the other half outside the bottle. The bottle contains KOH solution. The set up is placed under sunlight for a few hours, the leaf detached and tested for starch.
Result: The leaf shows a lower portion dark blue in colour and the upper part light in colour.

Inference: CO₂ is one of the raw materials of photosynthesis. It is needed to produce carbohydrates. The portion of the leaf inside the bottle does not receive CO₂ hence gives a negative test for starch, but the lower portion which is outside the bottle receives CO₂ hence gives a positive test. The equation of photosynthesis is

Question 9.
Explain the fixation of carbon dioxide during the C₃ pathway.
Answer:
The photosynthetic reaction which does not require light (light-independent) is called a dark reaction. A dark reaction occurs in the stroma portion of chloroplast so it is called a stroma reaction. It is also called Blackman’s reaction. Dark reaction is influenced by temperature. The different chemical reactions of dark reactions are enzymatic. Enzymes of stroma catalyze these reactions. Enzymatic reactions of dark reactions occur in the form of a Cycle and they were studied by Melvin Calvin, Benson, and Bassham in 1949.

So dark reaction is also called Calvin cycle or Calvin  Benson cycle. They used the tracer technique by using C¹⁴O₂ in unicellular alga Chlorella and found a path of CO₂ in each step of a chemical reaction. The ATP and NADPH of the light reaction are used for the fixation of CO₂ during the dark reaction. The main event of dark reaction is the fixation of CO₂. So dark reaction is also called the CO₂ fixation cycle.

The end product of the Calvin cycle or dark reaction is hexose sugar (glucose).
CO₂ acceptor compound: CO₂ diffuses into the mesophyll tissue through stomata and then into the stroma of the chloroplast. CO₂ is first accepted by a 5-carbon compound called Ribulose 1, 5 biphosphates (RUBP). So RUBP is a CO₂ acceptor compound during dark reaction. RUBP is first available in the form of RUMP (Ribulose monophosphate).

First stable intermediate compound of dark reaction: During the dark reaction, the first stable intermediate compound formed is phosphoric- eric acid and it is 3 – carbon compound. So dark reaction studied by Calvin (Calvin cycle) is called C₃ cycle. The plants which produce 3 carbon compound (phosphoglyceric acid – PGA) as their first stable product are called C₃ plants.

The various steps of the C₃ cycle are:
(1) Phosphorylation: 6 molecules of RUMP (Ribulose monophosphate) reacts with 6 molecules of ATP and produce 6 molecules of RUBP (Ribulose 1,5 biphosphate) which is a CO₂ acceptor compound.

(2) Carboxylatlon: Six molecules of CO₂ react with 6 molecules of RUBP in the presence of an enzyme RUBP carboxylase oxygenase or “Rubisco”. RUBP is converted into 12 molecules unstable 6 carbon compounds.

(3) Cleavage (Splitting): 12 mole of 6 carbon unstable compounds splits into 12 molecules of 3 carbon compounds called phosphoglyceric acid (PGA). Since PGA (Phosphoglyceric acid) is a 3 carbon compound and it is the first stable intermediate product of the Calvin cycle. So it is the C₃ cycle.

(4) Phosphorylation: 12 molecules of PGA are phosphorylated in presence of 12 ATP molecules to 12 molecules of 1,3 ‘ diphosphogly ceric acid (12diPGA)

(5) Reduction: 12 molecules of 1,3 diphosphoglyceric acid is reduced to form 12 molecules of 3 – phosphoglyceraldehyde (PG AL) by 12 molecules of NADPH, produced in the light reaction.
12 mols, 1,3-di PGA+ 12 NADPH

(6) Utilization of PGALD for the synthesis of sugar and the regeneration of RUMP.
(A) Out of 12 PGAL, 5 PGAL are isomerized to 5 molecules of dihydroxyacetone phosphate (DHAP).

(B) 3 PGAL undergo condensation with 3 DHAP to form 3 molecules of 6 carbon compound 3 Fructose 1,6 diphosphate (3 F.1,6 diphosphate)

Question 2.
Explain the structure of ATP and write three types of ATP synthesis.
Answer:

Adenosine triphosphate has three components
(a) Adenine (a nitrogen base)
(b) ribose sugar c) three inorganic phosphate.
Adenine + ribose → Adenosine
Adenosine + 3 Pi → Adenosine + triphosphate
ATP can be written as A – (P) ~ (P) ~ (P).
The first phosphate which is linked by an ester linkage to ribose. The two-terminal bonds are energy-rich and the first bond is energy poor. The energy-rich phosphate bonds are represented by a curly line (~) and the energy-poor bond by a straight line (-).
ATP is synthesized by the addition of inorganic phosphate to ADP. This process is called phosphorylation.
n ADP + n Pi → n ATP.
There are three different types of phosphorylations.

• Photophosphorylation.
• Oxidative phosphorylation.
• Substrate level phosphorylation.