NEET 2022#NCERT XI# STATE BOARD11# Biology# Photosynthesis

Photosynthesis:- Photoexcitation of Chlorophyll-a

 

Q.6:   Why Chl-a is called as essential pigment?               

Ans.: Essential pigment Chlorophyll–a:                  

a) It absorbs light independently and receives energy from non-essential pigments.

b) It shows photo-excitation due to absorption of light.

c) It converts solar energy into biologically usable form of energy i.e. ATP.

d) Chl-a is site of photolysis of water and releases oxygen.

e) It plays major role in the synthesis of NADPHZ; thus, it acts as reaction centre for photochemical phase or light reaction.

Q. 7: Write a note on photoexcitation of chl-a molecule:                 

Ans.: 1)  It is the first step of photosynthesis.

2) It explains expulsion of electron from chl-a molecule after absorption of energy.

3) Photoexcitation results in synthesis of ATP and NADPH2.

4) Photoexcitation brings about photolysis of water and releases free molecular oxygen.

5) The expelled electron, may or may not come back to its original positions.

 

 

Photosynthesis :- Role of water, Arnon's concept and Ruben Kamen Reaction

 

Q. 5: Write a note on Hill’s reaction.           

     Give an account of photolysis of water.

     ‘‘Hill’s experiment does not prove that the source of oxygen evolved during photo synthesis is water.”  This statement is true or false? Explain.

     Give the significance of photolysis of water.

Ans.: 1) Splitting of water molecules into its components i.e. H+ and OH– ions in the presence of light and chlorophyll is called as photolysis of water.

2) It is one of the important reactions in photosynthesis and its experimental proof was given by Robert Hill in 1937.

3) He stated that during photosynthesis water molecule splits up into H+ and OH– ions.

4) He used isolated chloroplast from unicellular algae chlorella in CO2 free water, ferric salt (Fe3+) as hydrogen acceptor and haemoglobin as oxygen carrier.

5) He carried out photosynthesis and observed the splitting of water molecules. Ferric salt (Fe3+) reduced to ferrous salt. (Fe2+) and haemoglobin to oxyhaemoglobin.

i)          4H2O 4H + 4OH– 

ii)    4H+ + 4OH– 4H+ 4e + 2H2O + O2

He used suitable hydrogen accepter for his experiment “A”, where “A” is Hill’s oxidant.

6) Hill’s reaction can be represented as follows;

        2H2O + 2A2AH2 + O2 

Where `A’ is Hill’s oxidant.

7) Significance of photolysis of water: -   

i)  Splitting of water molecules is observed.

ii)     Oxygen is released as by-product.

iii)    Water acts as hydrogen donor.;

iv)    Assimilatory power of photosynthesis i.e. NADPH2 is produced.

v) Loss of electrons of PS-II or P680 in non – cyclic photophosphorylation is compensated by photolysis of water.

D) Arnon’s concept:

(1) Arnon, discovered the nature of Hill’s oxidant as NADP i.e. Nicotinamide Adenine Dinucleotide phosphate:

(2) Arnon coined the term “Assimilatory power’ for photosynthesis as ATP and NADPH2.

    2H2O + 2NADP----------2NADPH2 + O2 

E) Ruben, Kamen and Randall reaction:

(1) These three-scientist confirmed water as a source of oxygen during photosynthesis.

(2) They used isotope tracer technique in their experiment. They used two isotopes of oxygen as O16 and O18

Photosynthesis :- Light, Pigment systems and photosynthetic pigments

 

Q. 4: - Write a note on Nature of light and explain its role in photosynthesis.

Ans.: A) Nature of light.

1) Light is essential for photosynthesis. It shows dual nature i.e. both particle and wave nature.

2) Photon or light (quantum) is a part of electromagnetic radiations.

3) It carries no charge; mass is negligible and shows momentum.

4) Visible range of light is preferred for photosynthesis. (Photosynthetically Active Radiation)

5) The process takes place in red region of visible spectrum between of the range 650 nanometre to 730 nanometres.

6) As the intensity of light increases, rate of photosynthesis also increases. A point comes in red region where there is sharp decline in the rate of reaction even after increasing intensity of light. It takes place in red region hence, called as red drop effect (emersion effect).

B) Role of light:      (2 Marks)

1) Light is source of energy.

2) It is essential for development of chlorophyll.

3) Light plays important role in synthesis of ATP.

4) Light brings about photolysis of water, through which molecular oxygen is released.

5) Light plays important role in synthesis of NADPH2 which acts as reducing power.

6) Light brings about photoexcitation of chlorophyll – a molecule.

Photosynthetic Pigments

1) Different pigments which help in absorption of light in photosynthesis are called photosynthetic pigment.

2) These are present in grana of chloroplast inside quantosomes.

3) In higher plants and green algal two main types of photosynthetic pigments are present namely chlorophylls and carotenoids.

a) Chlorophyll:

1) These are given coloured compounds or pigments which absorb maximum light in red and blue as well as violet region.

2) These are insoluble in water and soluble inorganic solvents like petroleum ether, acetone, ethyl alcohol, etc.

3) There are two types of chlorophyll pigments such as chl-a and chl-b.

I) Chlorophyll-a:

1) It is bluish green pigment.  Its molecular weight in 893 & molecular formula is C55H12O5N4Mg.

2) It can convert the absorbed light energy into chemical energy hence it is called as essential pigment / reaction centre/master molecule.

3) It is known as universal pigment or primary photosynthetic pigment as it is present in all photosynthetic organism except bacteria.

4) It is active in two forms as chl-a 680 and chl-a 700.

II)  Chlorophyll-b:

1) It is yellow-green pigment, molecular wt. is more than 900 (901) and molecular formula is C55H70O6N4 Mg.

2) Chl-b is present in all photosynthetic organisms except blue-green algae and bacteria.

3) It is non-essential pigment and transfers its absorbed light energy to
chl-a molecule.  (Chl-b differs from chl-a in having aldehyde [-CHO] group at third carbon atom instead of methyl (-CH3) group).

Q. 5: - Write an account of pigment system or photosystems.

Ans.: 1) A group of photosynthetic pigment which absorb-light energy and coverts it into chemical energy is called as photosystem as pigment system.

2) In a pigment system a special chlorophyll molecule [(Chl-a680 (PS-II)] or [Chl-a700(PS-I)] in the centre is surrounded by accessory pigments.

3) Depending upon the type of chl-a or reaction centre pigment systems are of two types: -

A) Pigment system-I or Photosystem-I or P700

1) It consists of chl-700 or P700 as reaction centre which absorbs light at 700 nm wavelength.

2) The system is composed of chl-a700 (about 200 molecules) carotenes xanthophylls are about 50 molecules, one cytochrome-f, one plastocyanin, one ferredoxin reducing substance (FRS), one or two ferredoxin molecules etc.

3) Number of chlorophylls is more than carotenoids.

4) Stroma lamellae mainly contains PS-I.

5) Role: - (a) No free molecular oxygen is liberated.

             (b)  It helps PS-II in synthesis of assimilatory power.

B) Pigment system -II / or Photosystem - II / P60:

1) It consists of chl-a-680, or P680 as reaction centre which absorbs light at the wavelength 680 nm.

2) The system is composed of about 200 molecules of chl-a680, chl-a-670. Carotenoids about 50 molecules, chl-6 about 200 molecules, primary electron acceptor plastoquinone, Mn ions, Cl-ions, etc.

3) Grana mainly contains PS-II.

4) Role: -

a) It is involved only in non-cyclic photophosphorylation.

b) It is responsible for photolysis of water and re3lease of molecular oxygen.

c) It accepts the electrons released during photolysis of water.

Photosynthesis :- Chloroplast and functions

 

Q. 2: Describe ultra – structure of chloroplast.  

     Write a note a Grana of chloroplast:               

                Draw a well – labelled diagram of chloroplast:

1)  Chloroplast is found in plant cells only.

2) Chloroplast is considered as site of photosynthesis.

3) It is a double membranous structure and the membranes are made up of lipids and proteins i.e. lipoproteins in nature.

4) The membranes are also called as peristomium.

5) Outer and inner membranes are smooth and continuous in nature.

6) Outer and inner membranes are separated by space called as inter membranous space or periplastidal space which is filled up with watery fluid called as peristomium fluid.

7) Functionally, these two membranes are selectively permeable and allow entry and exit of certain substances only.

8) Chloroplast shows two structural components which are as follows:

A) Grana:          

i)  It is made up to thylakoids or lamellae arranged similar to stack of coins.

ii) Each granum shows presence of 8 – 12 thylakoids.  Each thylakoid is made up of 225 to 230 quantosomes.

iii) Each quantosomes shows presence of 4 photosynthetic pigments namely chl–a chl–b, carotenoids and xanthophyll.

iv) Due to presence of quantosomes appearance is granular, hence the name grana.

v)  Grana is responsible for absorption of light and its conversion into ATP.

vi) Grana acts as site for light reaction or photochemical phase.

B) Stroma:

i)  It is homogenous matrix or ground substance.

ii) It shows presence of its own DNA called as ct DNA or plastidome (Plastids), RNA, ribosomes, ions, enzymes etc.

iii) DNA can synthesize different types of enzymes required for various biochemical reactions.

iv) It acts as site for enzyme dependent reactions of photosynthesis (biochemical phase or dark reaction or light independent phase). 

9) Due to presence of DNA, chloroplast becomes semiautonomous structure but it cannot duplicate itself.

 

 

 Q.3: Give various functions of chloroplast    

OR             

       Give different functions of grana and stroma of the chloroplast.

Ans.: A) Grana:

1.  Different photosynthetic pigments absorb solar energy and chl-a converts it into ATP i.e. biologically usable form of energy.

2. Photolysis of water i.e. splitting of water molecule in the presence of light and chlorophyll into its components is carried out in grana.

It releases molecular oxygen.

        2H2O2H2 +O2­

3) Assimilatory power of photosynthesis i.e. ATP and NADPH2 are synthesized in grana.

4) Grana serves as a site for light reaction.

B. Stroma:

1) It is a site of CO2 fixation and its reduction into stored food i.e.; glucose.

2) It brings about synthesis of different types of enzymes required for CO2 fixation.

3) It acts as a site for dark reaction or biochemical phase.