13- PHOTOSYNTHESIS IN HIGHER PLANTS

CHAPTER NO.13 PHOTOSYNTHESIS IN HIGHER PLANTS

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INTRODUCTION PHOTOSYNTHESIS:Photosynthesis may be defined as “the synthesis of simple sugars (glucose)from carbon dioxide and water by the cells of green plants containing chlorophyll by using light energy”.Oxygen gas is given out as by product.The complete equation for photosynthesis is written like this:

Below mentioned are the experiments that were conducted by the early scientists in support of photosynthesis.

EXPERIMENT: To Prove Carbon dioxide is essential for Photosynthesis.

(MOLL’s HALF LEAF EXPERIMENT)

PROCEDURE:Select a healthy potted plant and place it in the darkroom for two to

three days to ensure leaves are free from starch.In a wide-mouthed glass bottle add 10-15 ml of potassium hydroxide

solution and split the cork vertically.

Now carefully insert half part of a leaf into a glass bottle through the split cork and the other half exposed to air.Place the complete unit undisturbed in sunlight for about 3 — 4 hours.After 4 hours detach the leaf from the plant and slowly remove it out from the bottle and test it with the lodine solution for starch presence.

We can observe that the half part leaf which was inside the glass bottle (KOH

solution) did not show any colour change but the other half part exposed to

surroundings turned its color to blue-black, indicating the presence of starch

in it.

CONCLUSION: In this experiment, we can conclude that carbon dioxide is

essential for photosynthesis. Both the portion of leaf received the same

amount of water, chloroplasts, and sunlight but the half part which was inside the glass bottle did not receive carbon dioxide (as CO2 absorbed by KOH).

Later, many improvised experiments were conducted by scientists to analyze the essential components for photosynthesis. Joseph Priestley (1733-1804)was amongst the first to carry out these experiments.Experiment by Joseph Priestley In 1770, after a series of experiments,Joseph Priestley came to a conclusion regarding the ESSENTIALITY OF AIR FOR PHOTOSYNTHESIS AND ALSO FOR THE GROWTH OF PLANTS;

MATERIALS REQUIRED: A bell jar, candle, rat, and a plant.

PROCEDURE:Priestley kept a burning candle and a rat together in the single bell jar.

After some time, candle extinguished and the rat died.For the second time, he kept a burning candle, rat, and a green plant

together in the bell jar.He observed that neither the candle got extinguished, nor did the rat die.

CONCLUSION:Based on his observations, Priestley concluded that in the first case, the air in the bell jar got polluted by the candle and rat.However, in the second case, the plant reinstated the air that was spoiled by the candle and the rat.Other Experiments Followed by Priestley;

Janlngenhousz conducted experiments using the same set-up and showed:That sunlight is essential for plants to reinstate the air that was spoiled by the candle or rat.

Later, the experiments by Julius von Sachs revealed that glucose was produced by plants.T.W. Engelmann discovered the role of chlorophyll.Engelmann used a prism to split light into its spectral components, and then illuminated a green alga, Cladophora, placed in a suspension of aerobic bacteria.(The bacteria were used to detect the sites of oxygen evolution).

He observed that bacteria mainly accumulated in the region of blue and

red light of the split spectrum, thus giving the first action spectrum of

photosynthesis.

Cornelius van Niel uncovered that the release of oxygen by plants is from water (H:O}, not from carbon dioxide,

By studying purple and green sulphur bacteria he was the first scientist

to demonstrate, in 1931, that photosynthesis is a light-dependent redox

reaction in which hydrogen from an oxidizable compound reduces carbon dioxide to carbohydrates. Expressed as:

2H2A + CO2 — 2A + (CH20), + H2O

Where “A” is the electron acceptor. In HzO “A” is oxygen and in H2S “A” is sulphur.His discovery predicted that H2O is the hydrogen donor in green plant photosynthesis and is oxidized to Oz.

¢ Some organisms do not release oxygen during photosynthesis. When H2S, instead is the hydrogen donor purple and green sulphur bacteria.the oxidation product is sulphur or sulphate depending on the organism not oxygen.He inferred that the oxygen evolved by the green plant comes from water, not from carbondioxide. This was later proved by radioisotopic teachniques.(Chlorophyll, light & enzymes)6CO:z + 12H.O ----------> C.eH120¢ + 6H.O + 60.2By the mid-nineteenth century, scientists outlined the process of

photosynthesis. They concluded that light is essential for photosynthesis,and plants use carbon dioxide and water for the preparation of glucose (carbohydrate), where water molecules are the hydrogen donors and oxygen (Oz) is the by-product.

LET US KNOW WHAT WE HAVE LEARNT!

PART: A_ VERY SHORT ANSWER TYPE QUESTIONS:

(A) MCQs:

1. Who demonstrated the importance of sunlight for process of photosynthesis ?

(a) Robert Hill

(b) Englemmann

(d) Joseph Pristley

2. In experiment of photosynthesis, KOH solution is used because:

a) It provides O2 to the leaf

b) it provide moisture to the leaf

c) it helps in CO> fixation

d) it absorbs CO,

3. Van neil hypothesis about the production of oxygen during photosynthesis was based on the study and investigations ?

a) Purple and green sulphur bacteria

b) Fungi

c) Bacteria

d) Plant

(B) TRUE/FALSE:-

1. Julius Von Sachs provided evidence for production of glucose.

2. T.W. Englemann observed bacteria used to detect the sites of oxygen evolution.

(C)FILL THE BLANKS:

1. Joseph Priestley experiment shows essential role of ------in the growth of

green plants.

2. Jan Ingenhousz ,shows that ------—----is essential to the plant processes.

ANSWER KEY: PART -A

(A) MCQs:

1 (D). Hint: Ingenhousz found out that in the presence of light, plants give off

bubbles from their green areas, while in the shade these bubbles stop. He

determined this gas to be oxygen. Ingenhousz continued his work and discovered that plants produce carbon dioxide in the dark.

2 (D). Hints: KOH absorb carbon dioxide. Experiment to prove that carbon dioxide

is required for the process of photosynthesis to make food.

3 (A). Hints: Purple and green sulphur bacteria.

(B) TRUE/FALSE:

1.TRUE Hints : Julius von sachs provided evidence for production of glucose.

2.TRUE Hints : T.W Englemann observed bacteria used to detect the sites of

oxygen evolution.

C) FILL IN THE BLANKS:

1. Air

2. Sunlight

PART: B_ SHORT ANSWER TYPE QUESTIONS:

Q. 1. A pond weed was placed in a test tube containing pond water and was exposed to bright sunlight. Bubbles evolved at the cut end of stem as shown in the figure. In dimlight and in darkness, no bubbles evolved.

a) Name the gas that evolved in the bright light?

b) What is the process in which this gas has evolved?

Q. 2. Explain experiment by Joseph Priestley?

PART: C LONG ANSWER TYPE QUESTIONS:

1. To prove that carbon dioxide is necessary for photosynthesis.

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INTRODUCTION:Where Does Photosynthesis Happen?

The entire plant is not involved in the process of photosynthesis.Photosynthesis mostly takes place in the green parts of the plant that have chloroplasts.

Mesophyll cells in the leaves have a large number of CHLOROPLASTS in them. This is because the chloroplasts can find

the optimum quantity of sunlight they need for photosynthesis at the edges of the mesophyll cells of the leaves.Chloroplasts are a type of membrane-bound plastids that contain a network of membranes embedded into a liquid matrix and harbour the

photosynthetic pigment called chlorophyll. It is this pigment that imparts a green colour to plant parts and serves to capture light energy.

Within the chloroplast, there is a clear division of work. Each

component of the chloroplast has its own function. The main components in the chloroplast are the inner and outer membranes, the stroma lamellae, the grana, and the fluid stroma.The membranes are responsible for trapping sunlight energy. The membrane also synthesizes ATP (Adenosine triphosphate) and NADPH

(nicotinamide adenine dinucleotide hydrogen phosphate). = ®.2:< page dios oS yorve gel eeeiay cect: and are called 0.6 —

The stroma is responsible for the reactions that convert carbon dioxide into sugar, which forms starch. These are called DARK REACTIONS,because they do not directly utilize sunlight.They require by-products of light reaction; ATP and NADPH.

PART: AVERY SHORT ANSWER TYPE QUESTIONS: -

(A) MCQs:

1. Stroma in the chloroplasts of higher plant contains

(a) lignt-dependent reaction enzymes

(b) ribosomes

(d) light-independent reaction enzymes.

2. Stroma is the ground material of which of these

(a) Ribosomes

(b) Lysosomes

(d) Chloroplasts

3. In chloroplasts, where do light reactions occur

(a) outer membrane

(b) inner membrane

(d) stroma

4. Chlorophyll in chloroplasts is present in

(a) Stroma

(b) outer membrane

(d) inner membrane

5. Factory for synthesis of sugars in autotrophic eukaryotes:

(a) Mitochondria

(b) Chloroplast

(d) Endoplasmic reticulum

(B) TRUE/FALSE: -

1. Plastids and chloroplasts are present only in plant cells.

2. Chloroplast is found in chlorophyll.

(C) FILL IN THE BLANKS: -

1. Stroma is the ground substance in................

2. The site of light reaction in the cells of a leaf is.......... .

3. The ............ is responsible for the reactions that convert carbon

dioxide into sugar, which forms starch.

(A) MCQs:

Answer:1 (d) Explanation: The dark reactions of photosynthesis are

purely enzymatic and slower than the primary photochemical reaction.

They take place in stroma portion of the chloroplast and are independent

of light, i.e., they can occur either in presence or in absence of light

provided that assimilatory power is available.

Answer : 2 (d)Explanation: The main components in the chloroplast are

the inner and outer membranes, the stroma lamellae, the grana, and the fluid stroma.

Answer: 3 (c)Explanation: The membranes are responsible for trapping sunlight energy. The membrane also synthesizes ATP (Adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide hydrogen

phosphate).These reactions involve light energy directly and are called light reactions.

Answer: 4 (c)

Answer: 5 (b)Explanation: The stroma of chloroplast is responsible for

the reactions that convert carbon dioxide into sugar, which forms starch.

(B) TRUE/FALSE: -

Answer: 1 True

Answer: 2 False: Chlorophyll is found in chloroplasts.

(C) FILL IN THE BLANKS: -

Answer: 1. Chloroplast

Answer: 2. Grana.

Answer: 3. Stroma

PART: B SHORT ANSWER TYPE QUESTIONS: -

1. Where in the chloroplast is the NADP reductase enzyme located?

2. Expand the following biological abbreviation — NADP.

PART: C LONG ANSWER TYPE QUESTION: -

Q.1. List the location in the cell where the following reactions take place during the process of photosynthesis:

a) Synthesis of NADPH and ATP

b) Photolysis of water

c) CO2 fixation

d) Synthesis of sugar molecule

e) Synthesis of starch

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INTRODUCTION:PIGMENTS are the substances that have an ability to absorb light energy at specific wavelengths.The pigments involves in the process of photosynthesis are called photosynthetic pigments.

Without pigments it's not possible to carry out photosynthesis by plants.It is also a limiting factor to change the light energy into chemical energy.There are many types of pigments in photosynthetic organisms like;Chlorophyll, Carotenoids (carotene and xanthophyll) and Phycobilinis, which are

found in the plants.

LOCATION:Photosynthetic pigments are located in the thylakoids membrane of the

chloroplast.

The above diagrams are showing the location of thylakoids in chloroplast

and arrangement of pigments in thylakoids.

As told earlier there are three major types of pigments;

I CHLOROPHYLLS:They are the main photosynthetic pigments which are green in colour.In plants they are of five types. I .Bacteriochlorophyll and Bacterioviridin found in bacteria.Out of all Chl. a is universal.Chlorophylls are soluble in organic solvents.

STRUCTURE:All the chlorophyll contains four and a es = They have a formula Cs5sH720sNaMg. The Chlorophyll “a” bear Methyl group at carbon 3.

I. CAROTENOIDS:There are two types of carotenoids:

a. Carotene

b. Xanthophyll

Carotene are orange red in colour with general formula is CacHs.Xanthophylls are Yellow in colour having C, H,O with formula CaoHs6O2z.

FUNCTIONS OF CAROTENOIDS:

1 .They act as accessory pigments and absorbs energy and transfer to Chl.

“a”.

2. They protect the chloroplast from oxygen.

3. Colour provided to fruits and flower which attract pollinators and dispersal

of seeds.

I. PHYCOBILINS:These pigments are found in some algal group which are structurally related to bile pigment.These are of two types:

1. phycocyanin which is found in Blue Green Algae.

2. Phycoerythrin found in red algae.

The Above together form Phycobilins and are water soluble and are associated

with small Granules attached with lamellae.

PART: A_VERY SHORT ANSWER TYPE QUESTIONS :

A. MCQs:

1. Which metal ion is constituent of chlorophyll?

(a) Iron

(b) Copper

(d) Zinc

2. Which pigment acts directly to covert light energy to chemical energy?

(a) Chi. a

(b) Chi. b

(d) Carotenoids

3.The most abundant chlorophyll is :

(a) Chia

(b) Chi. b

(d) Chil y

4. Which of the following is/are accessory pigments?

(a) Carotene

(b) Xanthophylls

(d) None

5. Where does pigments are located?

(a) Mitochondria

(b) In stroma of chloroplast

(d) none

B. TRUE / FALSE:

1. The Chlorophyll a bear Methyl group at carbon 3

2. Phycobillins are soluble in organic solvent.

3. Carotenes are yellow in colour.

C. FILL IN THE BLANKS:

1. Xanthophyll is ............ in colour.

2. Chlorophyll is soluble in...............

3. All the pigments are located in......... membrane of chloroplast.

ANSWER KEY: PART -A

A. MCQs:

Answer 1. c - Hint. The chemical formula of chlorophyll is CssH72OsNaMg

Answer 2. a - Hint. Chi. “a” is the main pigment that convert light energy into

chemical energy other are accessory pigments which absorb and transfer to Chi.

Answer 3. c-Hint. As Chi. “a” is the universal pigment found in all

photosynthetic organisms and others are found in select groups.

Answer 4. c - Hint. Both are the accessory pigments as it transfer energy to the

Chi. “a”

Answer 5. c — Hint. Pigments are located in thylakoid membrane.

B. TRUE / FALSE:

Answer 1. True

Answer 2. False; Phycobilins are water soluble.

Answer 3. False; Carotenes are orange red in colour.

C. FILL IN THE BLANKS:

Answer 1. Yellow

Answer 2. Organic solvent.

Answer 3. Thylakoid

PART: B_ SHORT ANSWER TYPE QUESTIONS :

1. What are the photosynthetic pigments?

2. What are accessory pigments and role in photosynthesis?

PART: C LONG ANSWER TYPE QUESTIONS:

1. Discuss about photosynthetic pigments, types and function in Photosynthesis.

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INTRODUCTION:PHOTOSYNTHESIS in higher plants involves additional processes, but

fundamentally it remains the same.

It is a physicochemical process that uses sunlight for the synthesis of Organic compounds. In this process, oxygen is released into the atmosphere. hotosynthesis occurs in the chloroplast, found in the

mesophyll cells of the leaves.

Pigments involved in photosynthesis:

Chlorophyll a

Chlorophyll b

Xanthophylls

PHOTOSYNTHESIS IN HIGHER PLANTS INVOLVES THE FOLLOWING PROCESSES:

Light Reaction

Dark Reaction

LIGHT REACTION

This phenomenon occurs in the presence of light. The pigment absorbs light and

produces energy in the form of ATP. The process involves- absorption of light, water splitting, the release of oxygen, and formation of &7F and N&Or +H. The protein-

bound pigment molecules form the light-harvesting complexes present within two

protosystemns FS- and PS. Each photosystem has a reaction centre consisting

of chlorophyll a molecule, and antennae containing accessory pigments. The

reaction centre for PS- is P-700 because the absorption peak for chlorophyll “a”

is at 700 nm while that for PS-I is P-680 because the absorption peak for

chlorophyll “a” is at 680 nm.

PHOTOPHOSPHORYLATION:The formation of ATP in the presence of sunlight is called photophosphorylation. It is of two types:

CYCLIC PHOTOPHOSPHORYLATION AND NON-CYCLIC PHOTOPHOSPHORYLATION

CYCLIC PHOTOPHOSPHORYLATION The photophosphorylation process which results in the movement of the electrons in

a cyclic manner for synthesizing ATP molecules is called Cyclic

Photophosphorylation. In this process, plant cells just accomplish the ADP to ATP for immediate energy for the cells. This process usually takes place in the thylakoid membrane and uses Photosystem | and the chlorophyll P 700. During cyclic

photophosphorylation, the electrons are transferred back to P700 instead of

moving into the NADP from the electron acceptor. This downward movement of

electrons from an acceptor to P700 results in the formation of ATP molecules. In

cyclic photophosphorylation, only PS4 is involved. The electrons circulate within the photosystem which results in a cyclic flow of electrons. This only forms ATP and not NADPH+ H .

OTOPHOSPHORYLATION:The photophosphorylation process which results in the movement of the electrons in anon-cyclic manner for synthesizing ATP molecules using the energy from excited electrons provided by Photo System and reaction center P- 680is called

Non-Cyclic Photophosphorylation.This process is referred to as non- cyclic photophosphorylation because the lost

electrons by P ¢ao of Photosystem II are occupied by P 709 of Photosystem and

are not reverted to P sso. Here the complete movement of the electrons is in a

UNIDIRECTIONAL or in a non- cyclic manner.

During non-cyclic photophosphorylation, the electrons released by P70 are carried by primary acceptor and are finally passed on to NADP.Here, the electrons combine with the protons — H which is produced by

splitting up of the water molecule and reduces NADP to NADPH:2.

WATER SPLITTING :The light-dependent splitting of water is called photolysis. This process is associated with PS4I in which manganese and chlorine play an important role. The electrons lost from P ego are replaced by the electrons formed in this process.A molecule of water splits to release oxygen upon the absorption of light by P680.

CHEMIOSMOTIC HYPOTHESIS :The chemiosmotic hypothesis was proposed by Peter Mitchell. It is the generation of ATP by ATP synthase in an electron transport chain.Electron Transport Chain is an oxidative phosphorylation reaction that takes

place in the inner membrane of the mitochondria.It begins with NADH and FADH that loses electrons. These electrons are

transferred along the chain.As each complex accepts electrons and passes them on, energy is released.This energy is used to pump the protons against the concentration gradient from the matrix of the mitochondria to the inner membrane space.

Eventually, there is a high concentration of protons build up in the membrane

space and the protons try to move back into the matrix.However, the inner membrane is impermeable to protons.A Proton Motive Force (PMF) is set up and ATP synthase undergoes a conformational change and uses the PMF to make ATP from ADP.ATP synthase allows the proton to diffuse back down their gradient.

ATP +Pi— ATP

LET US KNOW WHAT WE HAVE LEARNT!

PART: A_ VERY SHORT ANSWER TYPE QUESTIONS:

A) MCQs:

1. Which of the following is not an accessory pigment?

(a) Carotene

(b) Chlorophyll a

(d) Xanthophyll

2. Light reactions of photosynthesis occurs in:

(a) stroma

(b) thylakoid

(d) cytoplasm of cell

3. Electrons from the excited chlorophyll molecules of PS4I are first accepted

by:

(a)Pheophytin

(b)Ferredoxin

(c)Cytochrome f

(d)Cytochrome b

4. Non-cyclic photophosphorylation results in the production of:

(a) NADH

(b) NADPH

(d) ATP and NADPH

5. Which one occurs both during cyclic and noncyclic modes of photophosphorylation?

(a) Involvement of both PS | and PS II

(b) Formation of ATP

(d) Formation of NADPH

B) FILL UPS:

1. Cyclic photophosphorylation results in the formation of

2. Rate of photosynthesis is independent of .

3. Photosystem II absorbs light at a wavelength of

C) TRUE FALSE:

1. In the light reaction, the light is absorbed and energy is used to drive electrons from water to generate ATP and to drive protons across the membrane.

2. The light reaction occurs in the stroma of the chloroplast.

3. The end products of light reaction are ATP and NADPH.

ANSWER KEY: PART-A

A)MCQ's:

1. (b) Chlorophyll “a” HINT- Chlorophyll a is a primary pigment.

2. (c) Grana HINT - Light reactions of photosynthesis occurs in grana.

3. (a) Pheophytin HINT. When the pigment in the reaction centre of PSII absorbs the photon , an electron in this molecule gets excited and first transfer to electron acceptor namely Pheophytin.

(b) ATP and NADPH HINT- Emission of a pair of electrons due to excitation of

chlorophyll molecules, using this energy to form ATP from ADP + Pi, the process is referred to as photophosphorylation, a water molecule is split. End products are ATP and NADPH.

4. (b) Formation of ATP HINT- Formation of ATP occurs both during cyclic and

noncyclic modes of photophosphorylation.

B) FILL UPS:

1. ATP HINT - There is no evolution of oxygen takes place because it is not

connected with photolysis of water. Cyclic photophosphorylation produces

ATP only.

2. Light intensity. HINT - Rate of photosynthesis is independent of LIGHT

INTENSITY.

3.680 nm. HINT - Photosystem | absorbs light at a wavelength of 700 nm,

whereas Photosystem II absorbs light at a wavelength of 680 nm.

C)TRUE /FALSE :

1. FALSE HINT - In the light reaction, the light is absorbed and energy is

used to drive electrons from water to generate NADP and to drive protons

across the membrane.

2. FALSE HINT - The light reaction occurs in the grana of the chloroplast.

3. TRUE HINT - The light reaction is a lignht-dependent process which includes

a series of events such as light absorption, hydrolysis, the release of

oxygen, formation of ATP and NADPH.

PART: B SHORT ANSWER TYPE QUESTIONS:

1. Where in the chloroplast is the NADP reductase enzyme located? State its

role in the proton gradient development.

2. What are the important events and end products of light reaction?

3. Define Non - Cyclic Photophosphorylation.

PART: C LONG ANSWER TYPE QUESTION:

1. Differentiate between Cyclic Photophosphorylation and Non- Cyclic

Photophosphorylation.

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INTRODUCTION:CELLULAR RESPIRATION is one of the pivotal processes in an organism. It

directly supports the survival of the organism.

TYPES:Aerobic Respiration (in presence of oxygen) &Anaerobic Respiration (in absence of oxygen).Aerobic respiration is a process by which organisms combine oxygen

with foodstuff molecules, diverting the chemical energy in these substances into life-sustaining activities and discarding, as waste products, carbon dioxide and water.

COMPONENTS OF AEROBIC RESPIRATION:

Aerobic cellular respiration has three components:GLYCOLYSIS,THE CITRIC ACID (KREBS) CYCLE, and

CELLULAR RESPIRATION: (A brief Review):

Let's revise a bit about cellular respiration that you have studied earlier,

In_ glycolysis, glucose metabolizes into two molecules of Pyruvate, with an

output of ATP and Nicotinamide Adenine Dinucleotide (NADH). Each pyruvate oxidizes into acetyl CoA and an additional molecule of NADH and carbon dioxide (COz2).

The acetyl CoA is then used in the citric acid cycle, which is a chain of

chemical reactions that produce COz, NADH, Flavin Adenine Dinucleotide

(FADH2), and ATP.In the final step, the oxidative phosphorylation, three NADH and one FADHz amassed from the previous steps are used to make water and ATP.

Oxidative Phosphorylation has two components:The Electron Transport Chain (ETC) and Chemiosmosis.

ELECTRON TRANSPORT CHAIN The ETC is a collection of proteins bound to the inner mitochondrial membrane and organic molecules.Electrons pass through in a series of redox reactions to release energy.The flow of electrons through the electron transport chain is an exergonic

process.The energy released forms a proton gradient, which is used in chemiosmosis to make a large amount of ATP by the protein ATP-synthase.The electron transport chain is only part of glucose metabolism that

uses atmospheric oxygen.In eukaryotic organisms, the electron transport chain, and site of oxidative phosphorylation, is found on the inner mitochondrial membrane.

Present in multiple copies.

INITIATION:The ETC proteins in a general order are complex I, complex Il, coenzyme

Q, complex Ill, cytochrome C, and complex IV.Each of the four large multi protein complexes in the respiratory chain

spans the inner mitochondrial membrane and contains several prosthetic . groups that participate in moving electrons.These small non peptide organic molecules or metal ions are tightly and specifically associated with the multi protein complexes.

Reduced electron carriers (NADH and FADHz2) from other steps of cellular respiration transfer their electrons to molecules near the beginning of the transport chain and get oxidized for reuse.NADH-CoQ Reductase (Complex 1) Electrons are carried from NADH to CoQ by the NADH-CoQ reductase

complex.

NAD is exclusively a two-electron carrier: it accepts or releases a pair of

electrons at a time.In the NADH-CoQ reductase complex, electrons first flow from NADH to FMN (flavin mononucleotide), a cofactor related to FAD, then to an iron-

sulfur cluster, and finally to CoQ.

FMN, like FAD, can accept two electrons, but does so one electron at a time.

The overall reaction catalyzed by this complex is:

CoQh2-Cytochrome ¢ Reductase (Complex Ill) A CoQHz2 generated either by complex | or complex Il donates two

electrons to the CoQH2-cytochrome c_ reductase complex,regenerating oxidized CoQ.Concomitantly it releases two protons picked up on the cytosolic face

into the intermembrane space, generating part of the proton-motive force.Within complex Ill, the released electrons first are transferred to an iron-sulfur cluster within complex Ill and then to two b- type cytochromes (bi and bu) or Cytochrome c1.

Finally, the two electrons are transferred to two molecules of the oxidized

form of Cytochrome c. The overall reaction catalyzed by this complex is:

Cytochrome c Oxidase (Complex IV) Cytochrome c after being reduced by the CoQH2- cytochrome c

reductase complex, transports electrons, one at a time, to the cytochrome

c oxidase complex.Within this complex, electrons are transferred, again, first to a pair of copper ions called Cu,*, then to cytochrome a, next to a complex of

another copper ion (Cu»**) and cytochrome as, and finally to O2, the ultimate electron acceptor, yielding H2O.

For each pair of electrons transferred, the overall reaction is:

SO, OVERALL, WHAT DOES THE ELECTRON TRANSPORT CHAIN DO FOR THE CELL?

1. REGENERATES ELECTRON CARRIERS:NADH and FAD:2, pass their electrons to the electron transport chain,turning back into NAD* and FAD.This is important because the oxidized forms of these electron carriers are used in glycolysis and the citric acid cycle and must be available to keep

these processes running.

2. MAKES A PROTON GRADIENT:The transport chain builds a proton gradient across the inner mitochondrial membrane, with a higher concentration of H in the intermembrane space and a lower concentration in the matrix. This gradient represents a stored form of energy and used to make ATP.

ATP synthase uses the proton gradient across the mitochondrial membrane

to form ATP. It is made up of Fo and F; subunits which act as a rotational motor

system.Fo portion is embedded in the mitochondrial membrane and is protonated and deprotonated repeatedly causing it to rotate. This rotation catalyzes the

formation of ATP from ADP and Pi

LET US KNOW WHAT WE HAVE LEARNT!

PART: A- VERY SHORT ANSWER TYPE QUESTIONS:

1. Electron transport system (ETS) is present in which of the following parts of

mitochondria?

(a) Inner membrane

(b) Outer membrane

(d) Stroma

2. Which of the following is the first complex (Complex |) of ETS?

(a) NADH dehydrogenase

(b) Cytochrome aa3

(d) ATP synthase

3. Which of the following is the Complex Ill of ETS?

(a) NADH dehydrogenase

(b) Cytochrome aa3

(d) ATP synthase

4. In which part of the cell, oxidative phosphorylation takes place?

(a) Inner mitochondrial membrane

(b) Outer mitochondrial membrane

(d) Stroma of chloroplast

5. Cytochromes are present in:

(a) lysosomes

(b) mitochondrial matrix

(d) outer membrane of mitochondria

TRUE/ FALSE:

1. Given a healthy individual with a normal metabolic rate, NADPH is the most

energy rich.

2. Oxidative phosphorylation phase of cellular respiration has the highest ATP yield

3. Water and energy are end-products of aerobic respiration.

FILL UPS

1. ATP synthesis by ATP synthase is driven by the movement of .

2. products of glucose oxidation are necessary for oxidative

phosphorylation.

3. is the Complex Il of ETS.

ANSWER KEY : PARTA

MCQ’'s:

1. A lnner membrane

2. A( NADH dehydrogenase)

3. C ( Cytochrome bc1)

4. A( Inner mitochondrial membrane)

5. C (cristae of mitochondria)

TRUE/ FALSE:

1. True, During oxidative phosphorylation (the electron transport chain), each 1 ATP

is produced for each GTP, 2 ATP are produced for each FADH2, and 3 ATP are

produced for each NADH.

2. True, Oxidative phosphorylation, which traps energy in a high-energy phosphate

bond and uses an electron gradient and ATP synthase to create ATP, yields the

most ATP. Oxidative phosphorylation is linked with the electron transport chain.

3. False Aerobic respiration is the complete oxidation of sugars in the presence of oxygen.The carbohydrates are broken down into CO, and H.O and this releases energy.This energy is stored in the form of ATP and later used in the body.

FILL UPS:

1. Protons

2. NADH and FADH2

3. Succinate dehydrogenase

PART: B- SHORT ANSWER TYPE QUESTIONS:

1. Why is oxygen necessary in aerobic cellular respiration?

2. Write a note on the role of respiration in our body?

3. Write a note on the effect of proton gradient on ATP production.

PART: C LONG ANSWER TYPE QUESTIONS:

1. Explain the Electron transport chain with the help of suitable diagrams.

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INTRODUCTION:As we have learnt during light reaction ATP, NADPH and O2 are produced. Of

these products’ oxygen diffuses out of the chloroplast while ATP and NADPH

are used in Calvin Cycle to reduce the CO, to sugar leading to synthesis of

food. This is biosynthetic phase of photosynthesis.This process doesn’t directly depend on the presence of light but it is dependent on the products of the light reaction, i.e.: ATP and NADPH besides

CO.and H.O.Let us now see how the ATP and NADPH are used in biosynthetic phase.

We saw earlier that CO2 is combined with H2O to produce (CH2O)n or sugars. Now

what is the first product formed when COzis taken into a reaction or fixed.

Just after the World War II, Melvin Calvin used radioactive C in algal photosynthesis

and led to the discovery that first CO. fixation product was a 3-carbon organic acid.He also contributed to working out the complete biosynthetic pathway, hence it was

called Calvin Cycle after him. The first product formed is 3-phosphoglyceric acid

(PGA).Later the experiments led to the discovery of another group of plants where the first stable product of CO2 fixation was a 4-carbon atom organic acid identified as oxaloacetic acid. Since then, CO2 assimilation during photosynthesis was said to be of two main types i.e.:

1) C3 pathway: It occurs in those plants in which the first product of CO> fixation is

a C3 acid (PGA)

2) C4 pathway: It occurs in those plants in which first product of CO: fixation is a

C4 acid (OAA) The primary acceptor of CO2 is a five-carbon ketose sugar- ribulose biphosphate (RuBP).

THE CALVIN CYCLE:Calvin and his coworkers worked out on the whole pathway and showed that the pathway operated in a cyclic manner, the RuBP was regenerated. Calvin cycle occurs in all photosynthetic plants, it does not matter whether they have C3 or C4 pathways. Calvin cycle is also known as C; cycle or light independent or dark

reaction of photosynthesis. To build organic molecules the plant cells use raw

materials provided by the light reaction. However, it is most active during the day

when NADPH and ATP are abundant.

Energy: ATP provided by cyclic and noncyclic photophosphorylation, which drives the endergonic reactions.

Reducing Power: NADPH provided by the photosystem-! is the source of

hydrogen and the energetic electrons required to bind them to carbon atoms. Much

of the light energy captured during photosynthesis ends up in the energy-rich C-H bonds of sugar.Plants store light energy in the form of carbohydrates, primarily starch and sucrose. The carbon and oxygen required in the process are obtained from CO2and the energy for carbon fixation is derived from the ATP and NADPH

produced during the photosynthesis process.

The Calvin cycle proceeds in three stages:

1) CARBOXYLATION: A CO2 molecule combines with a five-carbon acceptor molecule, ribulose- 1,5-biphosphate (RUBP). This step makes a six-carbon compound that splits into two molecules of three carbon compound, 3-phosphoglyceric acid(3-PGA). This reaction is catalyzed by the enzyme

RuBP carboxylase/oxygenase or rubisco.

2) REDUCTION: In the second stage, ATP and NADPH are used to convert the 3-PGA molecules into molecules of a three-carbon sugar, glyceraldehyde-3-phosphate (G3P). The stage gets its name because NADPH donates electrons to, or reduces a three-carbon intermediate to make G3P.

3) REGENERATION: Regeneration of the carbon di oxide acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted. The regeneration steps require one ATP for phosphorylation to form RuBP.Hence for every CO: molecule entering the Calvin Cycle, 3 molecules of ATP and 2 molecules of NADPH are required. To make one molecule of glucose,six turns of the cycle are required, so 18 ATP and 12 NADPH molecules will be required to make one molecule of glucose through the Calvin pathway.

EQUATION FOR CALVIN CYCLE:

3 CO.+6NADPH+6H*+9ATP-- G3P+6NADP*+9ADP+3 H,0+8P;(G3P=glyceraldehyde-3 phosphate (Pi = inorganic phosphate)The carbohydrate products of the Calvin cycle are three-carbon sugar phosphate molecules, or "triose phosphates", amely, glyceraldehyde-3- phosphate(G3P).

PART: A- VERY SHORT ANSWER TYPE QUESTIONS:

A.MCQ’S

1. The first stable compound formed in Calvin Cycle is:

a) Glyceraldehyde 3 Phosphate

b) 1,3 bi phosphoglycerate

c) 3-phosphoglyceric acid

d) Ribulose 1,5 bi phosphate

2. How many ATP is required for the regeneration of RuBP in Calvin

cycle?

a) 6

b) 12

b) 18

d) 24

3. How many molecules of NADP are produced as a result of 6 Calvin

cycles?

a) 9

b) 12

c) 18

d) 24

4. Which of the following is correct statement regarding the Calvin

cycle?

a) It cannot occur during the daytime

b) Itis lignt dependent

c) It is lignt independent

d) It occurs rapidly at night

5. How many turns of the Calvin cycle are required for the formation of 3 glucose molecules?

a) 15

b) 16

b) 12

d) 18

B. TRUE AND FLASE:

1. Calvin cycle is common in both C3 and C4 plants.

2. RuBP is regenerated in Calvin cycle.

3. Four Calvin cycles are required for the formation of one glucose molecule.

C.FILL UPS:

1. In Calvin cycle the carbon di oxide acceptor molecule is ............

2. Calvin cycle is also known as..........

A.MCQs:

Ans. 1 (c) 3-phosphoglyceric acid

Ans. 2 (a) 6

Ans. 3(b) 12

EXPLANATION: For every carbon di oxide molecule used up in the Calvin

cycle, 2 molecules of NADPH are required, hence two molecules of NADP are produced. So, for 6 Calvin cycles 12 molecules of NADP are produced.

Ans. 4(c) It is light independent.

EXPLANATION: Calvin cycle does not directly depend on presence of lignt but is dependent on the products of light reactions i.e., ATP and NADPH besides Carbon di oxide and water.

Ans. 5 (d) 18

EXPLANATION: The formation of one molecule of glucose requires the fixation of 6 carbon di oxide molecules and hence formation of three glucose molecules will require 18 turns of the cycle.

B. TRUE AND FLASE:

1. True

2. True.

3. False: 6 Calvin cycles are required for the formation of one glucose molecule.

C: FILL UPS:

1. RuBP (Ribulose 1,5- bi phosphate)

2. C3 cycle

Explanation: because the first stable molecule to be produced is a three-

carbon molecule (PGA)

PART B : SHORT ANSWER TYPE QUESTIONS:

1. Name the three steps in the Calvin cycle.

2. Give the equation of Calvin cycle.

PART C: LONG ANSWER TYPE QUESTIONS:

1. Explain Calvin cycle with the help of diagram.

A136

INTRODUCTION:Calvin cycle is the path of carbon assimilation in plant photosynthesis.. It was given by Melvin Calvin, James Bassham, and Andrew Benson. They got the Nobel Prize for this work in 1961.It is also known as C3 cycle and Calvin-Benson—Bassham (CBB) cycle.

DEFINITION: “Calvin cycle” or “C3 cycle” is defined as a set of chemical reactions performed by the plants to reduce carbon dioxide and other compounds into glucose.”

THE CALVIN CYCLE It takes place in the stroma of chloroplast in photosynthetic organisms.Calvin cycle is called as Cs cycle because the major stable intermediate is the 3-carbon molecule, glyceraldehyde-3-phosphate.It is also called the Lightindependent Reactions, or Bio Synthetic Phase, or Dark Reactions because they are not directly driven by light. It only requires the product of light reactions i.e., reduced NADP and ATP.

Calvin cycle is also called Photosynthetic Carbon Reduction (PCR) cycle

because it uses the energy from short-lived electronically excited carriers to

convert carbon dioxide and water into organic compounds that can be used by

the organism (and by animals that feed on it)

STAGES OF CALVIN CYCLE:The Calvin cycle can be described under three stages:

1. Carboxylation

2. Reduction

3. Regeneration

1. CARBOXYLATION- Carboxylation is the fixation of CO2 into a stable

organic intermediate. Carboxylation is the most crucial step of the Calvin

cycle where CQz is utilized for the carboxylation of Ribulose-1, 5-

Bisphosphate (RUBP). This reaction is catalyzed by the enzyme RuBP carboxylase Oxygenase (RuBisCO) which results in the formation of two molecules of 3-Phosphoglycerate (3-PGA).

2. REDUCTION — In the reductive phase, glucose is formed. In this step two

molecule of ATP are required for phosphorylation and two NADPH is

required for reduction to fix one CO2 molecule. For generating one molecule

of glucose, six CO2 molecules are required.

3. REGENERATION OF RuBP - In this step, regeneration of the CO2 acceptor takes place. The regeneration steps require one ATP for phosphorylation to form RuBP.

So, for the synthesis of one molecule of glucose, 18 ATP and 12 NADPH are

required.

Calvin Cycle Equation:3 CO2+6NADPH+6H*+9 ATP — G3P+6 NADP*+9 ADP +3 H20+8P;

(G3P=glyceraldehyde-3 phosphate)

(Pi = inorganic phosphate)The carbohydrate products of the Calvin cycle are three-carbon sugar phosphate molecules, or “triose phosphates", namely, glyceraldehyde-3-phosphate (G3P).

IMPORTANCE OF CALVIN CYCLE:The Calvin cycle is a process that plants and algae use to turn carbon dioxide from the air into sugar, the food autotrophs need to grow. Every living thing on Earth depends on the Calvin cycle. Plants depend on the Calvin cycle for energy and food. This makes the Calvin cycle vital for the existence of most ecosystems, where plants form the base of the energy pyramid.

LET US KNOW WHAT WE HAVE LEARNT!

PART: A VERY SHORT ANSWER TYPE QUESTIONS:

A)MCQs:

1. In Calvin cycle, 1 molecule of glucose is formed from:

a. 6CO2 + 30ATP + 12NADPH

b. 6CO2+ 12ATP

c. 6CO2+ 18ATP + 12NADPH

d. 6CO2+ 18ATP + 30NADPH

2. Calvin cycle is also called as:

a. CBB cycle

b. Photosynthetic carbon reduction (PCR) cycle

c. Dark reaction

d. All of these

3. In Cs plants photosynthesis occurs in:

a. Bundle sheath cells

b. Peroxisomes

c. Stroma

d. Kranz anatomy

4. The carbon dioxide acceptor in Calvin cycle is:

a. Phospho-enol-pyruvate

b. Ribulose 1, 5-bisphosphate

c. Phosphoglyceric acid

d. Ribulose monophosphate

5. Which of the following are products of the light reactions of photosynthesis that are utilized in the Calvin cycle?

a. Glucose

b. ADP and NADPH

c. Electron and H

d. ATP and NADPH

B)TRUE/ FALSE:

1. The Calvin cycle uses light directly.

2. In all photosynthetic plants Calvin cycle occurs.

3. The end product of Calvin cycle is triose phosphate.

C)FILL IN THE BLANKS:

1. The first stable compound in C3 plant is--—

2. Calvin cycle take place in---—--of chloroplast.

ANSWER KEY: PART -A

A)MCQs:

1. c) In Calvin cycle, 6CO2+ 18ATP + 12NADPH are required for formation of

one molecule of glucose.

2. d) Calvin cycle is also called as CBB cycle, photosynthetic carbon reduction

(PCR) cycle and dark reaction.

3. c) In Cs plants photosynthesis occurs in stoma.

4. b) CO acceptor in Calvin cycle is Ribulose 1, 5 -bisphosphate.

5. d) ATP and NADPH are the products of light reaction to be used by calvin

cycle.

B) TRUE/FALSE:

1. False; It is also called the light-independent reactions, bio synthetic

phase, dark reactions because they are not directly driven by light. It only

requires the product of light reactions i.e., reduced NADP and ATP

2. True

3. True

C)FILL IN THE BLANKS:

1. Phosphoglyceric acid

2. Stroma

PART: B SHORT ANSWER TYPE QUESTIONS:

1. Give an account of carboxylation stage in calvin cycle in photosynthesis.

2. Why calvin cycle is called C3 cycle?

3. What are the end products of calvin cycle?

PART: C LONG ANSWER TYPE QUESTIONS:

1. Where does Calvin cycle take place in plants? Explain the cycle.

A137

INTRODUCTION;C4, PATHWAY is the alternate pathway of Cs pathway to fix COz. In this

cycle, the first formed stable compound is a 4 carbon compound viz., Oxalo

Acetic Acid. Hence it is called C4 cycle.

The path way is also called as Hatch and Slack as they worked out the pathway in 1966 and it is also called as C, Di Carboxylic Acid Pathway.This pathway is commonly seen in many Grasses, Sugarcane, Maize,Sorghum and Amaranthus.Hatch & Slack (1966) Studies in detail and proposed pathway for dark reactions in sugarcane & maize First stable product of this reaction is OAA,Which is 4C,DCA (Dicarboxylic Acid), thus Hatch & Slack pathway is called as C4 cycle or DCA cycle.There are differences in leaf anatomy between plants that have a C4 carbon cycle (called C4 plants) and those that photosynthesize solely via the Calvin

photosynthetic cycle (called C3 plants).

CHARACTERISTICS OF C, PLANTS:Kranz( Wreath) Anatomy - present in leaves of C, plants that shows:Green bundle sheath cells present around the vascular bundles.

Dimorphic choroplast present in leaf cells.

Chloroplast of Bundle sheath cells or Kranz’s cells are larger and without grana.

Mesophyll chloroplasts are small and with grana.

STAGES IN C, CYCLE:

1. Fixation of CO. by the carboxylation of Phospho Enol Pyruvate in the mesophyll cells to form a C, acid. (malate and/or aspartate).

2. Transport of the C. acids to the bundle sheath cells.

3. Decarboxylation of the C, acids within the bundle sheath cells and generation of COz, which is then reduced to carbohydrate via the Calvin cycle

4. Transport of the Cs acid (pyruvate or alanine) that is formed by the

decarboxylation step back to the mesophyll cell and regeneration of the COz

acceptor Phospho Enol Pyruvate (PEP).

There are four steps in Hatch and Slack cycle:

1. Carboxylation

2. Breakdown

3. Splitting

4. Phosphorylation

1. Carboxylation: It takes place in the chloroplasts of mesophyll cells. Phospho

Enol Pyruvate, a 3 - carbon compound picks up COz and changes into 4 - carbon

compound Oxaloacetate in the presence of water. This is catalysed by the

enzyme, phosphoenol pyruvate carboxylase.

2. Breakdown: Oxaloacetate breaks down readily into 4 - carbon Malate and

Aspartate in the presence of the enzyme, Transaminase and Malate Dehydrogenase. These compounds diffuse from the mesophyll cells into sheath cells.

3. Splitting: In the sheath cells, Malate and Aspartate split enzymatically to yield

free CO and 3 - carbon pyruvate. The CO: is used in Calvin's cycle in the sheath cell.

The second Carboxylation occurs in the chloroplast of bundle sheath cells.

The COzis accepted by 5 - carbon compound Ribulose Diphosphate in the presence of the enzyme, carboxy dismutase and ultimately yields 3 - Phospho Glyceric Acid. Some of the 3 — Phospho Glyceric Acid is utilized in the formation of sugars and the rest regenerate Ribulose Bisphosphate.

4. Phosphorylation: The Pyruvate molecule is transferred to chloroplasts of

Mesophyll Cells where, it is phosphorylated to regenerate Phospho Enol Pyruvate in the presence of ATP. This reaction is catalyzed by Pyruvate Phosphokinase and The Phospho Enol Pyruvate is regenerated.

SIGNIFICANCE OF C, CYCLE:In Hatch and Slack pathway, the Cs and C, cycles of carboxylation are linked and this is due to the Kranz anatomy of the leaves.The Cy plants are more efficient in photosynthesis than the Cs plants. The enzyme, Phosphoenol Pyruvate Carboxylase of the C4 cycle is found to have more affinity for Carbon dioxide than the Ribulose Bisphosphate Carboxylase of the C3 cycle in fixing the molecular CO, in organic compound during

Carboxylation.

LET US KNOW WHAT WE HAVE LEARNT!

PART: A_ VERY SHORT ANSWER TYPE QUESTIONS:

(a) MCQs:

1. Which of these is not a characteristic of C, plants?

a) Toleration of higher temperatures

b) Response to high light intensities

c) Greater productivity of biomass

d) Photorespiration

2. What is the special leaf anatomy in Cs plants known as?

a) Mesophyll anatomy

b) Vascular anatomy

c) Kranz anatomy

d)Calvin anatomy

3. What is the arrangement of cells in plants showing Kranz anatomy?

a) Tapered

b) Irregular

c) Wreath

d) Triangular

4. Which of the following is false regarding the bundle sheaths of plants

showing Kranz anatomy?

a) They lack chloroplasts

b) They are layered around the vascular bundles

c) They have thick walls

d) They lack intercellular spaces

5. Which feature cannot help us differentiate between C3 and C, plants?

a) Presence of Kranz anatomy

b) Presence of photorespiration

c) Presence of chloroplasts

d) Presence of bundle sheath

(b) TRUE/FALSE:-

1. Calvin cycle is common to C3 and C, plants.

2. PEPCO is a major component of the bundle sheath cells.

3. Plants adapted to dry tropical regions use the C. pathway

(c) FILL THE BLANKS:

1. PEP is present in the cells.

2. Bundle sheath cells are rich in the enzyme

ANSWER KEY: PART-A

(a) Multiple Choice Questions:

1. (Answer: d) Explanation: C. plants show some special adaptations to survive

extreme climate, such as tolerance of higher temperatures, response to high

intensity lights, lack of photorespiration and greater productivity of biomass.

2. (Answer: c) Explanation: C. plants have a special type of anatomy of the leaves. This is Known as Kranz anatomy. It is characterized by the presence of

large cells around the vascular bundles known as bundle sheath cells.

3. (Answer: c) Explanation: C. plants have a special type of anatomy of the leaves. This is Known as Kranz anatomy. It is characterized by the presence of

large cells around the vascular bundles in the form of a wreath.

4. (Answer: a) Explanation: In plants showing Kranz anatomy, the bundle

sheaths surrounding the vascular bundles have characteristics such as thick

wails, a large number of chloroplasts and no intercellular spaces. They are

layered around the vascular bundles.

5. (Answer: c) Explanation: C. plants show Kranz anatomy in which bundle sheath ceils surrounds the vascular bundles. They lack photorespiration and show presence of bundle sheaths. However, chloroplasts are present in both types of plants.

b) True/False:

1. True: In C3 plants, the first stable product of carbon dioxide fixation is 3-PGA.For C, plants, itis OAA or Oxalo Acetic Acid. C, plants lack photorespiration. The

Calvin cycle is common to both.

2. False: PEPcase, also known as PEP carboxylase is not present in bundle

sheath cells. However, bundle sheath cells are rich in the enzyme RuBisCO.

3. True: Plants using the C, pathway have special adaptations which help them

survive extreme climatic conditions. Some such adaptations are toleration of high

temperatures, response to high light intensities and lack of photorespiration.

c) Fill in the Blanks:

1) PEP is present in the mesophyll cells.

2) Bundle sheath cells are rich in the enzyme RuBisCO.

B) SHORT ANSWER TYPE QUESTIONS:-

1) By looking at which internal structure of plant can you tell whether a plant is C3

or C4? Explain.

2) Which plants use the C. cycle?

3) By looking at which external structure of plant can you tell whether a plant is Cs

or C4? Why and how?

C) LONG ANSWER TYPE QUESTIONS:

1} Even though a very few cells in a Ca. plant carry out the biosynthetic - Calvin pathway, yet they are highly productive. Can you discuss, why?

2) RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why

do you think RuBisCO carries out more carboxylation in C4 plants?

3) Give comparison between C: and Cy pathway.

A138

INTRODUCTION:Photosynthesis is a process by which plants prepare their own food in the

presence of water, chlorophyll, sunlight, and CO2. This process occurs mainly

in the leaves of the plants. In some other plants, the stems also perform

photosynthesis. Photosynthesis occurs in the chloroplast ceils of the leaves.

FACTORS AFFECTING HOTOSYNTHESIS:

Photosynthesis is under the influence of several factors, both internal and

external.The plant or external factors include the number, size, age and

orientation of leaves, mesophyll cells and chloroplasts, internal CO2 concentration and the amount of chlorophyll. The external or environmental factors would include the availability of sunlight, temperature, CO. concentration and water.

BLACKMAN’S LAW OF LIMITING FACTORS:When several factors affect any biochemical process, Blackman’s law of limiting factors comes into effect, which states that,

“Ifa chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value: it is the factor which directly affects the process if its quantity is changed.

”For example, if a leaf is exposed to a certain amount of light intensity with constant temperature but the CQO. available is less, the rate of

photosynthesis will not increase with an increase in the light intensity.

Therefore, in this case, CO:z is the limiting factor. The process will start only on the availability of CO2.

1. LIGHT:There are two broad categories of plants:

A) SCIOPHYTES: Shade loving Plants.

B) MHELIOPHYTES: Sun loving Plants.

Itis one of the major factors affecting photosynthesis. Photosynthesis

cannot occur in the dark and the source of light for the plants is sunlight.There is a linear relationship between incident light and CO:> fixation rates at low light intensities.At higher light intensities, gradually the rate does not show further

increase as other factors become limiting.

Increase in incident light beyond a point causes the breakdown of chlorophyll and a decrease in photosynthesis.Light saturation occurs at 10% of full sunlight.Experiments conducted by Engelmann prove that the chlorophyll most effectively absorbs RED and BLUE wavelengths from the entire

spectrum of light.

2. CARBON DIOXIDE CONCENTRATION:The atmosphere contains 0.03%-0.04% of carbon dioxide. Plants take in carbon dioxide from the air. But, since the amount of CO: in the air is very less, it acts as a limiting factor for photosynthesis.It is seen that, when light and temperature are not the limiting factors,increasing CO, concentration leads to an increase in the rate of photosynthesis.But, beyond a certain limit, CO: starts accumulating in the plant and this leads to slowing down of the process.The C3 and C, plants respond differently to CO At low light conditions, neither C3nor C,4 group responds to high

CO2 conditions, whereas at high light intensities, both Cs and Cz plants

show increase in the rates of photosynthesis.C4 plants show saturation at about 360 pIL*, whereas Cs plants

responds to increased CO, concentration and saturation is seen only beyond 450 pil".

3. TEMPERATURE:The dark reactions being enzymatic are more sensitive to

temperature than light reaction The C, plants respond to higher temperatures and show higher rate of photosynthesis, whereas C3 plants have a much lower temperature

optimum Tropical plants have a higher temperature optimum than the plants

adapted to temperate climates Beyond the optimum levels on both sides of the normal range, the enzymes are deactivated or destroyed and photosynthesis stops.

4. WATER:Water stress causes the stomata to close hence reducing the CO: intake

which affects photosynthesis Besides, water stress also makes leaves wilt, thus, reducing the surface area of the leaves and their metabolic activity as well

LET US KNOW WHAT WE HAVE LEARNT!

PART: A VERY SHORT ANSWER TYPE QUESTION:-

(A) Multiple Choice Questions:

1. Which of these is not an internal factor affecting photosynthesis?

(a)Size

(b)Age

(c)Number

(d)Water

2. What happen when incident light on a plant is excessive?

(a)RUBP undergoes oxidation

(b)Vascular bundles lose functionality

(c)Mesophyll cells are destroyed

(d)Chlorophyll breaks down

3. Blackman’s law of limiting factor is applicable to:

(a)Growth

(b)Perspiration

(d)Photosynthesis

4. At what percentage of sunlight does light saturation occurs?

(a)30

(b)20

(d)15

5. Which of these is NOT an effect of water stress?

(a)Closing of stomata

(b)Increase in metabolism

(d)Reduction in surface area of leave

(B) True/False:

1. Optimum temperature is necessary for photosynthesis.

2. Water stress causes stomata to open to reduce CO: availability.

3. The internal factor is dependent on the genetic predisposition.

(C) Fill in the Blanks:-

1.C.4 Plants show saturation at concentration.

2. Law of limiting factors is given by .

3. The concentration of CO2 in atmosphere is .

ANSWER KEY: PART-A

a) Multiple Choice Questions:

1. (d) Water HINT- Size, Age, and number are internal factors but water is

external factor

2. (d) Chlorophyll breaks down HINT- Excess of light reaching photosynthesis parts of plants causes’ damage of cells.

3. (d) Photosynthesis HINT- Law of limiting factor was proposed by

Blackman in 1905.

4 (c) 10% HINT- Light is internal factor that affects photosynthesis at

higher light intensities rate doesnot increase linearly.

5. (b) Increase in Metabolism HINT- Water stress closes stomata, hence CO, availability reduces. Leaves wilt to reduce surface area and metabolism.

b) True/False:

1. True.

2. False: Water stress causes stomata to close to reduce CO: availability

3. True.

c) Fillin the Blanks:

1. 360wit'

2. Blackman

3. .03-.04%

PART B: SHORT ANSWER TYPE QUESTIONS:-

1.Name the internal factors which affect the rate of photosynthesis.

2. State Blackman’s law of limiting factors.

3. How water affects the rate of photosynthesis?

PART C: LONG ANSWER TYPE QUESTION:-

1. Explain the effect of CO2 concentration on plants.

A139

RECAPITULATION:Green plants make their own food by photosynthesis. Photosynthesis takes placeonly in the green parts of the plants, mainly the leaves. Within the leaves,themesophyll cells have a large number of chloroplasts that are responsible for COQz2 fixation. Within the chloroplasts, the membranes are sites for the light reaction.while the chemosynthetic pathway occurs in the stroma.

hotosynthesis as twostages: the light reaction and the carbon fixing reactions. -

In the light reaction the light energy is absorbed by the pigments present in the

antennae, and funnelled tospecial chlorophyll a molecules called reaction centre chlorophylls. There are twophotosystems. PS | and PS Il. PS | has a 700 nm absorbing chlorophyll a P700 molecule at its reaction centre, while PS Il has a P680 reaction centre that absorbsred light at 680 nm. After absorbing light,electrons are excited and transferredthrough PS Il and PS | and finally to NAD

forming NADH: In the carbon fixation cycle, CO2 is added by the enzyme,RuBISCO, to a 5 carbon compound RuBP that is converted to 2 molecules of 3- carbon PGA. Thisis then converted to sugar by the Calvin cycle, and the RuBP is regenerated. Duringthis process ATP and NADPH 2 synthesised in the light

reaction are utilised. RuBisCOalso catalyses a wasteful oxygenation reaction in C3

plants: photorespiration.Some tropical plants show a special type of photosynthesis called C-,pathway.In

these plants the first product of CO:, fixation that takes place in the mesophyll is a 4-carbon compound. In the bundle sheath cells the Calvin pathway is carriedout for the synthesis of carbohydrates.

NCERT QUESTION ANSWERS:

1. By looking at a plant externally can you tell whether a plant is C3 or C4?

Why and how?

Answer-Though plants which are adapted to dry tropical climate have the C. pathway, they do not show any external morphologic characteristics. Hence it is not possible to say whether the plant is C3 and Cz, by looking at its external appearance.

2. By looking at which internal structure of a plant can you tell whether a

plant is C3 or C,? Explain.

Answer-Leaves of C4 plants have Kranz Anatomy which makes them different than Cs

plants. C, plants also have specialised cells called a bundle sheath cell which

surround the vascular bundles. Every cell of the bundle sheath has chloroplasts.

The mesophyll cells of leaves do not differentiate into the spongy and palisade

parenchyma possessing less intercellular spaces while there is normal occurrence

of chloroplasts in the mesophyll cells. But in C3 plants, the bundle shealth does not

exhibit chloroplast and the mesophyll cells of the leaves are differentiated into the

spongy and palisade parenchyma. Hence by examining the internal structure we

can tell whether the plant is C3 or Cy.

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?

Answer-Plant productivity can be estimated by the rate at which photosynthesis takes

place. The amount of carbon dioxide in a plant is directly proportional to the rate ofphotosynthesis. In C4 plants, very few of the cells carry out the biosynthetic

pathway yet they are highly productive for the following reasons.They have a mechanism that increases the CO. concentration at the site of enzyme.Mesophyll cells are broken down in the bundle sheath cells those results in CO2 release which in turn increases the intracellular CO 2 concentration.RuBisCO functions as a carboxylase minimizing the oxygenase activity.Increase in photosynthesis make C4 plants more productive.

4. RuBisCO is an enzyme that acts both as a carboxylase and oxygenase.Why do you think RuBisCO carries out more carboxylation in C4 plants?

Answer-The affinity of RUBISCO is much higher than its affinity for Oxygen. It is

the concentration of Oxygen and CO; that determines the binding of the enzyme.

Mesophyll cells of C4 plants lack this enzyme but are found in the bundle sheath

cells that girdle the vascular bundles where the Calvin cycle occurs.RuBisco functions as oxygenase when the concentration of Oxygen is higher and it

acts as carboxylase when the concentration of CO2 is high. In the mesophyll cells,

the primary carbon dioxide acceptor is a three carbon compound — phosphoenol

pyruvate which is converted into a four-carbon compound, oxaloacetic acid or

OAA. This is converted further into malic acid which is transported to the bundle-

sheath cells where it undergoes decarboxylation and carbon fixation takes place through the Calvin cycle which prevents RuBisCo to serve as an oxygenase.

5. Suppose there were plants that had a high concentration of Chlorophyll b,

but lacked chlorophyll a, would it carry out photosynthesis? Then why do

plants have chlorophyll b and other accessory pigments?

Answer-In the absence of chlorophyll-a photosynthesis will not take place because

chlorophyll-a is a reaction centre responsible for the conversion of solar energy into chemical energy. Although chlorophyll is the primary pigment that traps sunlight but accessory pigments like chlorophyll-b, xanthophylls and carotenoids

also absorb sunlight and transfer energy to chlorophyll-a.

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-Chlorophyll fails to absorb the energy in the absence of light; hence it

loses stability to give the leaf yellow colouration. This proves that carotenoid which imparts yellow colour is more stable.

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-Leaves in the shade get less sunlight to carry out photosynthesis than the plants kept in sunlight. In leaves that get less sunlight, more chlorophyll is present to carry out photosynthesis efficiently. In the presence of strong light, the orientation of the chloroplasts in the mesophyll cells is irregular and in vertical files along the walls. Also, in bright light, Photo Oxidation of chloroplasts takes place while non-oxidation takes place in shaded regions. Hence the plants kept in shade

have dark green leaves due to high concentration of chlorophyll.

8. 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?

(b) What could be the limiting factor/s in region A?

Answer- a) Light is a limiting factor at A and 50% of B which is due to the increase

in the photosynthetic rate with increase in the intensity of light

b) Light, CO2 and H2 O could be the limiting factors in region A.

c) C indicates a stage beyond which light is not a limiting factor and D is the line

beyond which the intensity of lignt does not affect the photosynthetic rate.

A140

INTRODUCTION:PHOTOSYNTHESIS is the process in which energy rich compounds like

carbohydrates (food) is synthesized from simple inorganic compounds like carbon dioxide and water in presence of chlorophyll and sunlight with liberation of oxygen.

SITE OF PHOTOSYNTHESIS:Chloroplasts (Green plastids) are the site of photosynthesis.

Chloroplasts have membranous system consisting of grana,stromal lamellae and the matrix stroma.

PIGMENTS INVOLVED IN PHOTOSYNTHESIS:Chlorophyll a ( bright or blue green in chromatograph) : major pigment responsible for trapping light

Chlorophyll b (yellow green)Xanthophylls (yellow)Carotenoids (yellow to yellow orange)Chlorophyll b, Xanthopylis, Carotenoids are called accessory

pigments as these absorb light and transfer the energy to chlorophyll a.Wavelength at which maximum absorption by chlorophyll a — Blue and Red region One to one overlap between absorption spectrum of chlorophyll a and

action spectrum of photosynthesis

PROCESS OF PHOTOSYNTHESIS IN HIGHER PLANTS involve Light Reaction & Dark Reaction

LIGHT REACTION: This phenomenon occurs in the presence of light.The process involves- absorption of light, water splitting, the release of oxygen, and formation of ATP and NADPH.The protein-bound pigment molecules form the light-harvesting complexes (LHC) present within two photosystems

Photosystem | (PS-) Photosystem II (PS4-l)

Each photosystem has a reaction centre consisting of chlorophyll a molecule, and antennae containing accessory pigments.

The reaction centre for PS-I is P-700 because the absorption peak for

chlorophyll a is at 700 nm while that for PS-II is P-680 because the absorption peak for chlorophyll a is at 680 nm.SPLITTING OF WATER the light-dependent splitting of water is called photolysis. This process is associated with PS4-I.The electrons lost from PS Il are replaced by the electrons formed in this process.2H.0 — 4H* + O2 + de"

PHOTOPHOSPHORYLATION: The formation of ATP in the presence of sunlight is called photophosphorylation. It is of two types

Non-cyclic photophosphorylation & Cyclic photophosphorylation

NON-CYCLIC PHOTOPHOSPHORYLATION is the process when the two photosystems work in a series, first PS Il then PS | Two photosystems

are connected through electron transport chain as shown in Z scheme.Transfer of electron, starting from PS Il, uphill to acceptor, down the electron transport chain to PS I, excitation of electrons, transfer to another acceptor and finally downhill to NADP+ reducing it to NADPH + H.The electrons lost by PS-II do not return to it and hence named non-cyclic photophosphory lation.ATP and NADPH + H* are synthesized.

Membrane or lamellae of grana have both PS | and PS

Ilphotophosphorylation, only PS4 is involved.The electrons circulate within the photosystem which results in a

cyclic flow of electrons.This only forms ATP and not NADPH+ H+.Occurs in stromal lamellae membranes as it lack PS Il and NADP reductase enzyme.

DARK REACTION: This process occurs in the absence of light in the stroma of the chloroplasts.CALVIN CYCLE This cycle involves the following steps:1. Carbon-fixation: Ribulose-1, 5-bisphosphate combines with carbon dioxide to fix it to a 3 carbon compound, 3-phosphoglyceric acid. Theenzyme RuBisCO is involved in the process.

2. Reduction: This involve utilization of 2 molecules of ATP for phosphorylation and two of NADPH for reduction to fix one molecule of carbon dioxide to form glyceraldehyde-3-phosphate.

3. Regeneration: Some glyceraldehyde-3-phosphate molecules undergo a series of reactions to form glucose while the RuBP regenerates to continue the cycle. This step requires one ATP for

The fixation of six CO2 molecules and 6 turns of cycles are involved for formation of one glucose molecule with utilization of 18 ATP and 12 NADPH.

C4, PATHWAY (HATCH AND SLACK PATHWAY:It is a cyclic pathway.The enzymes involved in the C, pathway are located in the Mesophyll

cells and Bundle Sheath cells.In this pathway, the plants convert atmospheric carbon dioxide Phospho Enol Pyruvate is the primary carbon dioxide acceptor and is

located in the mesophyll cells. The reaction is mediated by Phosphor

Enol Pyruvate Carboxylase.After this, aspartic acid and malic acid are formed within the mesophyll cells and transported to the bundle sheath cells. Here,

The C4 acids breakdown to release a 3-carbon molecule and carbon dioxide.The 3-carbon molecule move back to the mesophyll cells where it get converted into Phosphor Enol Pyruvate and complete the cycle.

The carbon dioxide enters the bundle sheath cells and enters the Calvin cycle.

FACTORS AFFECTING PHOTOSYNTHESIS:BLACKMAN’S LAW OF LIMITING FACTORS: If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value: it is the factor which directly affects the process if its quantity is changed.

1. Light- As light intensity increases, the rate of photosynthesis also increases at low light intensities, until light saturation point. At higher light intensities gradually the rate does not show further increase. Incident light

beyond a point causes break down of chlorophyll and a decrease in

photosynthesis.

2. Carbon dioxide concentration— with increase in concentration of COz upto 0.05 percent can cause increase in CO; fixation rates; beyond this the levels can become damaging. However C; & C, plants respond differently to COz concentrations.

3. Temperature — Temperature optimum for photosynthesis depends on habitat but at higher temperature enzyme activity is inhibited due to denaturation of enzymes which affect the dark reaction, decreases the rate of photosynthesis.

4.Water — The effect of water as a factor is more through its effect on the plant rather than directly on photosynthesis. Water stress causes the stomata to close hence reducing carbon dioxide. Water

stress also makes leaves wilt. So reduce the surface area and metabolic activity.

A. MCQs. :

1. First stable product of carbon assimilation is:

(a) Phosphoglycerate

(b) Glucose

(d) Pyruvic Acid

2. Kranz anatomy is found in:

(a) C4 Plants

(b) C3 Plants

(d) None of The Above

3. In C4 plants initial acceptor of carbon dioxide is;

(a) Ribulose 5 Phosphate

(b) Phosphoenolpyruvic Acid

(d) 3PGA

4. For each molecule of glucose formed in plants, the number of molecules of ATP and NADPH2 required are respectively:

(a) 15 and 10

(b) 12 and 18

(d) 33 and 22

5. Rate of photosynthesis is maximum in;

(a) Red And Blue Light

(b) Blue And Yellow Light

(d) White Light

6. Malic acid is found in C4 plants in the cells of:

(a) Epidermis

(b) Bundle Sheath

(d) Mesophyll

7.The site of dark reaction in photosynthesis is:

(a) Grana

(b) Cytoplasm

(d) Thylakoid

8. What will be the number of Calvin cycles required to generate one molecule of hexose?

(a) 4

(b) 6

(d)9

ANSWER KEY:

(MCQs):

Answer 1: (a) Phosphoglycerate Explanation: Coz combines with RuBP results in formation of two molecules of 3-phosphoglycerate.

Answer 2: (a) C4 plants Explanation: Leaves of C, plants have large bundle sheath cells around vascular bundles have Kranz anatomy.

Answer 3:(b) phosphoenolpyruvic acid Explanation: PEP, a 3-carbon molecule is primary acceptor of CO2 in mesophyll cells of C4 plant.

Answer 4:(c) 18 And 12 Explanation: for formation of one glucose molecule with utilization of 18 ATP and 12 NADPH.

Answer 5: (a) Red and Blue Light Explanation: Wavelength at which maximum absorption by chlorophyll a — Blue and Red region

Answer 6: (d) Mesophy!l Explanation: During C4 pathway OAA is formed in mesophyll cells which then form four carbon compounds like malic acid or aspartic acid in mesophyll cells which then transported to bundle sheath cells.

Answer 7: (c) Stroma Explanation: Enzymatic reactions which synthesise sugar occur in stroma during dark reactions.

Answer 8: (b) 6 Explanation: The fixation of six CO2 molecules 6 turns of cycles is involved for formation of one hexose molecule.

B. VERY SHORT ANSWER TYPE QUESTIONS:

Q.1. Observe the given figure:

a) ls this composition present in a plant cell or animal cell?

b) Can it be inherited by the offspring? How?

c) Write the metabolic processes that are occurring at the places marked

as (1) and (2) in the figure

A.1. a) It is a chloroplast, green in colour, found in plant cells.

b)Because of the presence of extranuclear DNA, it has the potential to self-

replicate.

c) At site 1 — Dark reactions of photosynthesis takes place at the

stroma of the chloroplasts. At 2 —It is the structure of extracellular DNA that is responsible for replication of chloroplasts when necessary in the photosynthesizing cells.

Q.2. Answer the following questions based on the equation given below:

2H:20 ------ 2H* + O2 + 4e7

a) Where in plants does this reaction occur?

b) What is the importance of this reaction?

A.2. a) In the reaction center PS II that situated in the inner surface of the

thylakoid membrane. Here electrons are extracted from water and are known as water splitting center.

b) The process releases molecular oxygen as a by-product of photosynthesis and is an important source of oxygen in the air, for living entities on earth. The ions of hydrogen hence produced are used in

reducing NADP to NADPH which is a strong reducing agent. It creates a gradient for the synthesis of ATP when electrons released are transferred from PS Il to PS I through electron carriers.

Q.3. How do photosynthetic bacteria such as Cyanobacteria conduct photosynthesis in the absence of chloroplasts?

A.3. They are prokaryotes containing photosynthetic pigments ina

membranous form that are primitive in nature, that can trap and use solar

energy. Hence they can carry out photosynthesis.

Q.4. a) Where is the enzyme NADP reductase located?

b) What does the breakdown of proton gradient cause the release of?

A.4. a) It is located on the outer side of the thylakoid membrane. b) ATP molecules. In the presence of enzyme ATP synthase, it is coupled with the movement of ions across the membrane and formation of ATP synthesis.

Q.5. Does moonlight promotes photosynthesis?

A.5. No, it does not as it does not carry enough energy to excite chlorophyll molecules — reaction centers PS | and PS Il. Hence light-dependent reactions are not initiated.

Q.6. Where in the chloroplast is the NADP reductase enzyme located?State its role in the proton gradient development.

A.6. On the outer side of lamella or thylakoid of the chloroplast. It breaks

down the proton gradient to release energy i.e. NADPH.

Q.7. Name the two parts in ATPase enzyme. State their arrangement in the thylakoid membrane. Which part of the enzyme shows conformational changes?

A.7. The two parts areCF: & CFo embedded in the thylakoid membrane and form a transmembrane channel and CF, protrudes on the outer surface of thylakoid membrane on side that faces stroma. The conformational

changes occur in the CF, part of the enzyme.

Q.8. Name the products used to drive the dark reaction that is formed during the light reaction of photosynthesis.

A.8. ATP and NADPH. They are used to fix COzand to form a glucose molecule.

Q.10. On what basis is the C3; and C, pathways of photosynthesis selected?

A.10. On the basis of carbon atoms in the first stable product of carbon

dioxide fixation.

C. SHORT ANSWER TYPE QUESTIONS:

Q.1. How is succulents able to meet their photosynthetic CO2 requirements

as they are known to keep their stomata closed during the day to check

transpiration?

A.1. During day time, the succulent plants have to shut their stomata as

they grow in dry and xeric conditions, for prevention of water loss during transpiration, the gaseous exchange does not take place. Hence plants have adapted to fix CO2 in the form of malic acid during night time.Malic acid is a four carbon compounds that stores CO2 which is liberated during the day within the photosynthetic cells.

Q.2.Define accessory pigments. State their significance in photosynthesis.

A.2. They are chlorophyll-b, xanthophyll, and carotenoids and are also referred to as photosynthetic pigments. They aid in accumulating solar radiations and passing them onto chlorophyll-a. They are not directly involved in the emission of excited electrons. While doing so, this pigment itself absorbs maximum radiations at regions — blue and green. Hence

chlorophyll-a is the main pigment and other pigments are accessory pigments (chlorophyll b, xanthophyll, and carotenoid)

Q.3. Do photosynthetic reactions such as dark reactions require light?Explain.

A.3. Dark reactions are light-independent reactions. Through various processes, carbon dioxide is reduced to glucose which does not require light. However, they rely upon the products formed during the light

reactions (NADPH2 and ATR).

Q.4. What is the relationship between photosynthesis and respiration?

A.4. They are related. In both mechanisms, plants gain energy in photosynthesis from solar radiations while in respiration, the breakdown of glucose molecule takes place to get energy in the form of ATP

molecules. Both processes are dependent on each other. Respiration uses the end product of the photosynthesis (glucose in the form of food),to produce ATP for energy. In the process, carbon dioxide and water are released which photosynthesis uses to produce more sugars.

Q.5. Can green plants kept in dark with proper ventilation carry out photosynthesis? Can they be administered with supplements to maintain survival or growth?

A.5. No, they cannot carry out photosynthesis as light is essential for

green plants to make their own food. For survival, plants need to be watered properly.

Q.6. How are short plants growing below thick canopies and receiving filtered light able to carry out photosynthesis in tropical rain forests?

A.6. In the presence of light, plants carry out photosynthesis. The rate of

photosynthesis is the only factor that gets affected by the quality and intensity of light. Hence only the rate could vary amongst plants growing in different canopy regions.

Q.7. The rate of photosynthesis decreases at higher temperatures. Why?

A.7. The process of photosynthesis is an enzyme specific process. All the

enzymes act at an optimum temperature. If the temperature increases beyond 35 °C, enzymes denature, which affects the rate of photosynthesis.

Q.8. Ina C3 plant, a light dependent cyclic process is occurring that requires oxygen. Instead of producing, it consumes energy.

a) Name the process

b) Is it necessary for survival?

c) Write the end products of this process.

d) Where does it take place?

A.8. a) Photorespiration

b) Not necessary for the survival of a C3 plant.

c) The end product — H2O2

d) Three cell organelles are involved — chloroplast, peroxisome,mitochondria.

Q.9.Explain why is the colour of a leaf in the dark changes? Which pigment do you think is more stable?

A.9. The colour of leaf stored in dark changes from green to pale green or

yellow in colour. This is mainly because of the absence of sunlight and

chlorophyll pigment. In the dark or during the absence of light,photosynthesis does not take place, therefore there is no synthesis of starch and chlorophyll pigment. The most stable pigment is chlorophyll a.

D. LONG ANSWER TYPE QUESTIONS:

Q.1. Does photosynthesis occurs in leaves only? If no, what are the other

parts that are capable of carrying out photosynthesis? Justify.

A.1. The process of photosynthesis primarily occurs in leaves of all green

plants. The leaves are designed in such a way that they capture sunlight

and convert solar energy into chemical energy effectively. However, in

some plants, modified parts other than leaves carry out photosynthesis. In

some plants, roots develop chlorophyll and initiate photosynthesis; they

are referred to as assimilatory roots. Examples:Tinospora, Trapa. The

stem in some plants such as the Opuntia gets modified and adapts to

perform the functions of leaves, be turning thick, succulent and flattened,

performing photosynthesis; these structures are referred to as phylloclade. In Australian Acacia, the petiole takes the shape and function of photosynthesis when lamina of leaf fails.

Q.2. List the location in the cell where the following reactions take place

during the process of photosynthesis.

a) Synthesis of NADPH and ATP

b) Photolysis of water

c) CO: fixation

d) Synthesis of sugar molecule

e) Synthesis of starch

A.2. a) Outer side of the thylakoid membrane

b) The inner side of the thylakoid membrane

c) Stroma of chloroplast

d) Chloroplast

e) Cytoplasm

a) Cs and C4 pathways

Q.3. Give a comparison between the following:

(a) C3 and C4 pathways

(b) Cyclic and non-cyclic photophosphorylation

Q.4. What are the important events and end products of light reaction?

A.4. The following are the events: Emission of a pair of electrons due to excitation of chlorophyll molecules, using this energy to form ATP from ADP + Pi, the process is referred to as photophosphorylation, a water molecule is split 2H,O—4H+ + 4e- + Oct NADP+2H+—NADPH,End products are ATP and NADPH. The light reaction produces the

reducing power i.e., the ATP and NADPH2z molecules which are utilized in the dark reaction. A by-product of splitting water is O2.

Q.5. Why photorespiration does not take place in C, plants?

A.5. Photorespiration is related to Cs cycle, where plants lose carbon

dioxide fixation because of the increase in the concentration ion of oxygen and modification in the nature of the activity of RuBP carboxylase oxygenase. C, plants have developed a mechanism to avoid the loss of carbon dioxide. There is not a direct contact of RUBP carboxylase

oxygenase as C; cycle functions in bundle sheath cells. The enzyme PEP carboxylase in mesophyll cells carries out carbon dioxide fixation, wherein oxaloacetate is formed which is converted to malic acid which is passed to bundle sheath cells where carbon dioxide is utilized in the Calvin cycle

that functions in bundle sheath cells of Cs plants.