Photosynthesis preeti 12th h.sc

PHOTOSYNTHESIS

INTRODUCTION Anabolic process Light energy is converted to chemical energy

(ATP) Redox reaction : H2O gets oxidised and CO2

gets reduced to food i.e. carbohydrate and the chemical energy i.e ATP is stored in the food and during this process O2 is released.

Equation of photosynthesis :

The following events occur during this process –

(i) Absorption of light energy by chlorophyll. (ii) Conversion of light energy to chemical energy and splitting of water molecules into hydrogen and oxygen. (iii) Reduction of carbon dioxide to carbohydrates.

AUTOTROPHS

PHOTOAUTOTROPHSe.g : all green plants,green sulphur bacteria,cyanobacteria

CHEMOAUTOTROPHSe.g :Bacteria like nitrosomonas, sulphur bacteria

SITE OF PHOTOSYNTHESIS

Peristromium Length-4-10 microns Breadth-2 to 3 microns Plastidome Grana- 40 to 60 in no. Intergrana or frets or

stroma lamellae Thylakoids- each granum

has 12-20 thylakoids Quantosomes-each have

230-300 molecules of photosynthetic pigments.It is a photosynthetic unit.(Park and Biggins)

PHOTOSYNTHETIC PIGMENTS DEFTN : They are the pigments present in the

chloroplasts and capture light energy. These pigments absorb light of a particular

wavelength and reflect light of another wavelength. Thus the colour of the pigment indicates the wavelength reflected by the pigment

TYPES : 3 main types are- chlorophyll , carotenoid and phycobilins.

a) Chlorophylls : Most imp. and active pigments in photosynthesis. Green in colour and soluble in organic solvents but insoluble in H2O.(chl-a,chl-b,chl-c,chl-d,chl-e,bacteriochlorophyll,bacterioviridin).

Chla (blue green)-C55H72O5N4MgChlb (yellow green)-C55H70O6N4Mg

STRUCTURE OF CHLOROPHYLL MOLECULE

b) Carotenoids- 2 types

carotenes xanthophyllsCarotenes are orange in colour-C40H56 e.g of a major carotene is beta carotene.Xanthophylls are yellow in colour-C40H56O2 e.g of a major xanthophyll in plants are lutein.c)Phycobilins- They are soluble in H2O.Present only in red algae and blue green algae. 2 types

Phycocyanin (blue) Phycoerythrin (red)

ROLE OF PHOTOSYNTHETIC PIGMENTS

Chl.a –essential pigment and is the reaction centre.Chl.b and carotenoids are the accessory pigmentsCarotenoids protect chl.a from photooxidation

PHOTOSYSTEM OR PIGMENT SYSTEM (P.S) The reaction centre and accessory pigments

together form PS 2 TYPES

PSI PSII Each PS consists of

CC LHC(Reactn centre (few chl.b and 50 carotenoid mols) and electron carriers) PSI-P700 PSII-P680

EXPERIMENTS DONE TO PROVE SOURCE OF O2 IS H2O AND NOT CO2

Van Neil (1930)- green sulphur bacteria which used H2S instead of H2O in photosynthesis.

Robert Hill (1937) –Isolated chloroplasts from spinach placed it in h20 free of co2 added ferric salts and haemoglobin and exposed to light.

Ferric Ferrous ,Haemoglobin oxyhaemoglobin2H2O + 2A 2AH2 + O2 (Hills Reactn) Ruben and Kamen (1941)-isotope of oxygen Dr.Arnon (1954)-NADP is Hydrogen acceptor

Early thinking: O2 released came from the CO2CO2 C + O2 C + H2O CH2O

sugar

NATURE OF LIGHT Light behaves as a wave while propagating and on

interaction with matter it behaves as small packets of energy called as photons.

Amount of energy in photon is called a quantum. Quantum is inversely proportional to wavelength of

light. Visible light violet (390nm) to red (760nm). PAR Photosynthetically active radiation Max.absorption in red and blue region and little in

orange and yellow regn and nothing in green.

PHOTOLYSIS OF H2OH2O H+ + OH-4H2O 4H+ + 4OH- 4OH- 4OH + 4e-4OH 2H2O22H202 2H2O + O24H20 4e- +4H+ +2H2O +O2

There are 2 phases in photosynthesis :

Light reactn Dark reactn (Site-grana (Site-stroma Photophosphorylation) light independent reactn)

LIGHT REACTION OR PHOTOPHOSPHORYLATION

Photo-light and Phosphorylation-addition of phosphate

Light ADP+iP ATP Chl.a Photophosphorylation

Cyclic Non Cyclic

CYCLIC PHOTOPHOSHORYLATION

NON CYCLIC PHOTOPHOSPHORYLATION

Fd-NADP reductase

CHEMIOSMOTIC HYPOTHESIS Dr.Peter Mitchell-1961 Chemiosmosis-Movement of ions across a selectively

permeable membrane down their electrochemical gradient Chemiosmosis is the source of ATP in photophosphorylation

and ATP synthase is the enz. which makes ATP by chemiosmosis.

Photolysis of H2O takes place in thyakoid memb. Hence more protons in thylakoid memb.

Stroma has the enz. required for reduction of NADP to NADPH2 and proton is required for this reduction.

Protons are more in lumen of thylakoid and less in stroma hence an electrochemical gradient of proton or a proton gradient is formed across the thyl.memb.

Movement of protons take place to stroma and ATP synthase uses this energy of proton diffusion to form ATP from ADP

DARK REACTION 2nd phase of photosynthesis. Site-Stroma This reactn is independent of light but it requires the

products of light reactn i.e. ATP and NADPH2 Its called the blackmans reactn (Blackman) CO2 +2NADPH2 + 2ATP

CH2O+H20+2NADP+2ADP+2iP

CALVIN CYCLE OR C3 PATHWAY Dr. Melvin Calvin-1954 carried expt on unicellular

green algae-chlorella and scenedesmus to find out path of carbon and used C14.

1st stable product is a 3 C compd and hence its called a C3 cycle or calvin cycle or calvin-benson cycle

CO2 is fixed by a CO2 acceptor in stroma and converted to a stable compd and this stable compd. undergoes changes and final product glucose is produced in 90 secs.

Initial CO2 acceptor is regenerated and cycle continues.

3 phases in C3 cycle: a)Carboxylation b)Reduction c)Synthesis and Regeneration.

A) CARBOXYLATION RUBP-1,5-Ribulose bisphosphate or RUDP-1,5-

Ribulose diphosphate RUBP carboxylase-RUBISCO PGA-3-Phosphoglyceric acid

2 Mol. Of 3-PGA

B) REDUCTION Utilisation of assimilatory power 1,3-di PGA-1,3-diphosphoglyceric acid 3PGAL-3-Phosphoglyceraldehyde DHAP-Dihydroxyacetonephosphate (isomer of

3PGAL)

triose phosphate isomerase Some 3PGAL DHAP

C)SYNTHESIS For synthesis of 1mol. of glucose 6 Turns of

calvin cycle or 6mol. Of RUBP and 6 mol. of CO2 are required.

Thus out of 12 mol. of PGAL only 2mol. Is used in synthesis of glucose.

1mol.of PGAL +1 mol. Of DHAP

Fructose-1,6-diphosphate DephosphorylationFructose-6-phosphate IsomerisationGlucose-6-phoshate Dephosphorylation Glucose

C)REGENERATION RUBP is regenerated in this process called sugar

phosphate interconversions where 10mol. of PGAL is used in regeneration.

The intermediates are sugar phoshates e.g-erythrose-4-phosphate(4C),Xylulose-5-phosphate(5C),Ribose-5-phosphate(5C),Sedoheptulose-7-phosphate(7C) and 6mol.of RUMP are formed and 6ATP will phosphorylate to form 6RUBP.

6 mol.of ATP are used in regeneration and 12 ATPs and 12NADPH2 are used in reduction.Thus,18ATP and 12NADPH2 are used in synthesis of 1 mol. of glucose.

E.g of C3 plants – Hibiscus , sunflower, rice, potato etc.

PHOTORESPIRATION C3 plants show photorespiration. RUBISCO can act as carboxylase and oxygenase

because its active site can bind to both CO2 and O2 which is competitive and is seen during day time when temp.are more or high.

At high temp. the stomata closes and CO2 concntrtn

And O2 and instead of carboxylation,oxidation of RUBP takes place. Photorespiration reduces the photosynthetic

efficency of plants by 25% It is the respiration initiated in chloroplast and

takes place only during day time.

oxidation

Phosphorylation

DIVERSITY IN PHOTOSYNTHETIC PATHWAY

A] C3 PATHWAY(CALVIN)

B]C4 PATHWAY(HSK)

C]CAM PATHWAY (CRASSULACEAN ACID METABOLISM)

C4 OR HSK PATHWAY 1st stable product-OAA(Oxalo acetic acid) 4C C4 plants show Kranz anatomy. Kranz anatomy-mesophyll are not differentiated Bundle sheath cells are radially arranged around

the vascular bundle Dimorphic chloroplasts-bundle sheath cells have

large,less in no. agranal chloroplasts and mesophyl cells have small,more in no. granal chloroplasts.

Mesophyll have PEP carboxylase and bundle sheath cells have RUBISCO.

E.g of C4 plants-sugarcane, maize, jowar etc

MECHANISM OF HSK PATHWAY

CAM PATHWAY C4 and C3 pathway take place in mesophyll

cells C4 takes place during night time and C3 during

day time.

Thus during night time the cocentration of organic acid increases and during day time it becomes zero.

This day and night fluctuation in acid concentration is an imp. feature of CAM plant

E.g of CAM plants-cactus , pineapple , bryophyllum etc.

INTERDEPENDENCE OF LIGHT AND DARK REACTION

SIGNIFICANCE OF PHOTOSYNTHESIS Very imp. process by which food is synthesized and

heterotrophic org. depend on them for food Reduce air pollution by maintaining a balance of

CO2 and O2 in air Protects us from harmful U.V rays of sun Releases O2 Plant products like timber,gums,alkaloids,rubber

etc. are products of photosynthesis and are economically imp.