Photosynthesis (Part-1) - Photosynthesis in Higher Plants, Biology, Class 11 | Science for ACT PDF Download

PHOTOSYNTHESIS

HISTORY OF PHOTOSYNTHESIS

• Aristotle and Theophrastus (320 BC) :– State that plants absorb all food matter from soil (Humus theory).

• Van Helmont (1648) :– By weighing the Willow plant, concluded that plant take up their food mostly from soil water.

• J. Woodbard (1699) :– Besides water, soil also increases the weight of plants.

• Stephen Hales (1727) :– Recognised the importance of air (CO2) and light for photosynthesis (nourishment) in plants. He is considered as discoverer of photosynthesis and "Father of plant physiology".

• J. Priestley (1772) :– He carried out very interesting experiment on Bell jar, Rat, Pudina & Candle. He came to conclude that plants purify air (burning of candles) and gaseous exchange occurs during photosynthesis. (Phlogiston r Bad air from candles)

• Jan Ingenhousz (1779) :– He explained the importance of light and green colour and also suggested the O2 releases in the presence of light by green parts.

• Senebier (1782) :– Green plants absorb CO2 from atmosphere and when the concentration of CO2 increases the rate of O2 evolution also increases.

• N. De–Saussure (1804) :– Clarified that released O2 is equal to the absorbed CO2. He realised the significance of H2O in this process. De–Saussure stated that O2 comes from CO2 during photosynthesis. (Later on it was disproved by Van Niel)

• Pallatier & Caventou (1818) :– They named green pigment as 'Chlorophyll' and isolated the chlorphyll with the help of alcohol.

•  Englemann (1888) :– Described action spectrum of photosynthesis with the help of Spirogyra/Cladophora and aerobic bacteria experiment.

•  Mayer (1845) :– Green plants convert solar energy into chemical (potential) energy in the form of organic substance. He gave law of conservation of energy. Formation of organic matter recognised by Mayer.

•  Liebig (1845) :– Organic matter are derived from CO2 and H2O, during the process of photosynthesis.

•  J. V. Sachs (1862) :– Recognised the relation among photosynthesis, chloroplast and starch. First visible product of photosynthesis is starch. Founder of modern concept of photosynthesis. Some people consider Sachs as father of plant physiology. Three cardinal point concept wal also given by him.

•  Willstater, Stall Fisher :– Chemistry, structure and properties of Chl–a, and nobel prize winner.

•  F. F. Blackman (1905) :– Dark reaction associated with light reaction in photosynthesis and law of limiting factors.

•  Warburg (1920) :– Intermittent or flash light experiment on Chlorella and proved that dark reaction exists in photosynthesis.

•  Emerson and Arnold (1932) :– Concept of two pigment system (photosystem) in light reaction. Red drop & Emerson enhancement effect.

•  Van Niel :– O2 releases from water and O2 of glucose comes from CO2.

•  Robert Hill & Bendal (1937) :– Detailed study of light reaction in isolated chloroplast of Stellaria.

•  Photolysis of H2O is the chief role of chloroplast and evolution of O2 only in the presence of suitable e– acceptor, from water in photosynthesis.(Hill–reaction)

•  Ruben, Hassid & Kamen (1941) :– Used O18 to experimentally show that O2 in photosynthesis released from water.

•  Arnon :– ATP formation in presence of light (photophosphorylation) and cyclic and non–cyclic electron transport system.

•  M. Calvin and Benson (1954) :– Biochemical cyclic pathway of dark reaction and recognised PGA is Ist stable product in dark reaction. (It is formed from unstable 6C Keto Acid) .

.C3 – cycle or Calvin – Benson – cycle discovered.

. Chromatography and Radioisotopy (C14) techniques used in Chlorella and Scenedesmus algae. (Nobel Prize 1960).

•  Arnon, Allen & Whitley (1954) :– CO2 fixation demonstrated in isolated chloroplast by C14O2 isotope.

•  Hatch & Slack (1967) :– C4 pathway dicarboxylic acid cycle (DCA cycle) in sugarcane and maize. Ist stable product is oxaloacetic acid (OAA 4C).

•  Moll :– CO2 is essential for photosynthesis by half leaf experiment. * Govindji & Rabinowitch :– Studied ultrastructure of pigment system in detail.

•  Kok & Clayton :– Chl–a, P–700 discovered

•  Bussingault :– Photosynthetic Quotient (PQ) or Assimillatory coefficient =

•  Park & Biggins :– Photosynthetic units as Quantasome in chloroplast.

•  Huber Michel and Dissenhofer :– Crystalization & X–ray crystallography of reaction center in Rhodobacter. (Nobel–1988)

INTRODUCTION

"Photosynthesis is a photo–biochemical process (anabolic & endergonic) in which organic compounds (carbohydrates) are synthesised from the inorganic raw material (H2O & CO2) in presence of light & pigments.
O2 is evolved as a by product".

•  Light energy is conserved into chemical energy by photosynthesis.

•  90% of total photosynthesis is carried out by aquatic plants (85% algae) & 10% by land plants. 

• First true & oxygenic photosynthesis started in cyanobacteria. (BGA)

• In the Cuscuta & fungi photosynthesis is absent. Euglena is photosynthetic organism & is link between animal & plants.

•  Roots of Tinospora & Trapa are assimilatory or photosynthetic.

•  Absorption spectrum of photosynthesis is blue & red light. (maximum absorbed part of spectrum)

•  Action spectrum of photosynthesis is red & blue light. (most effective in reaction) *

• Rate of photosynthesis is higher in red wavelength of light, but highest in white light (Full spectrum), than monochromatic light.

IMPORTANT SCIENTIFIC CONTRIBUTION :

•  According to Van Niel, oxygen comes from water in photosynthesis.

•  Ruben, Hassid and Kamen (1941) :– Used O18 to show experimentally that O2 in photosynthesis comes from water.

Existence of two steps in Photosynthesis –

•  Blackman discovered dark reaction (By study of Q10 value or tempereture coefficient). Calvin and Benson gave cyclic pathway for this, thus dark reaction is called as Calvin cycle OR C3–cycle.

•  Q10 (Tempereture coefficient) for light reaction is one, while Q10 for dark reaction is between 2-3. (By Vont Hoff).

•  Q10 means the doubling of rate of reaction, which involves chemicals, on 10°C rise in temperature in it's optimum range.

•  Experimental evidences for Blackman findings were given by Warburg 1920. He carried out intermittent light experiment on Chlorella.(by using flash light)

•  The product of photosynthesis has been found greater in intermittent light (i.e., light given after intervals of dark periods) than in continuous light.

•  This is due to the fact that light reactions are faster than the dark reaction.

In continuous light product of light reactions (ATP and NADPH2) are not consumed at the same rate as in subsequent dark reaction. Thus dark reaction is rate limiting step of photosynthesis.

•  Photosynthesis – (i) Light reactn/Hill reactn (ii) Dark reaction/Blackman reactn.

• Hill Reaction – Experiment on isolated chloroplast (Stelaria plant) study of light reaction, which is called as Hill Reaction.

O₂ gas liberated fromphotolysis of H2O, only in the presence of suitable e– acceptor. (DCPIP (Dichlorophenol Indophenol- a dye), ferricyanide, NADP+ – Hill reagents)

•  Emerson & Arnold – worked on Chlorella and gave the concept of two photosystem or two pigment systems.

When they gave only monochromatic light, longer than 680 nm wavelength, then quantum yield is suddenly dropped down, this event is called as red drop.

• When Emerson gave light, shorter and greater than 680 nm (combined light) then photosynthetic, activity increases, this is called as Emerson effect or enhancement effect.

(i) 680 nm­ r PS – I (cyclic process) red drop appears.

(ii) 680 nm¯ + 680­ nm (Mixed light) rBoth cyclic & non cyclic operates. (Emerson effect)

Quantum requirement – The number  of light Quanta or photons required for the evolution of 1 mol. of O2 in photosynthesis =8

Quantum Yield – The number of oxygen molecule evolved by one quantum of light in photosynthesis is called as Quantum yield.
Emerson calculated that the quantum requirement is 8. Hence the quantum yield is 0.125 or 12.5%)

•  Arnon's experiment (Chlorella) –

Discovered cyclic and non–cyclic photophosphorylation  

E.T.S. in photosynthesis was proposed by Arnon.

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Chloroplast and Pigments - Photosynthesis in Higher Plants, Biology, Class 11

CHLOROPLAST AND PIGMENTS 

Photosynthetic pigments –

• Many pigment present in photosynthetic cells. PSU (Photosynthetic units) presents on thylakoid membranes, are made up of 230-400 molecules of various pigments, called Quantasomes by Park & Biggins.

• The PS II is located in the appressed region of granal thylakoids and PS I in non  appressed region of grana and stroma thylakoids.

• PS I located - on both granum & intergranum (Stroma thylakoid), (P-700, 680 nm­, Cyclic ETS).

• PS II located - on only granum, (P-680, 680 nm¯, non cyclic ETS).

• Chlorophyll – a C 55 H 72 O 5 N 4 Mg ® CH3 grp. at IIIrd C of IInd pyrrole ring. 

• Chlorophyll – b C 55 H 70 O 6 N 4 Mg ® CHO group at IIIrd C of IInd pyrrole.

• Chlorophylls are magnesium porphyrin compounds. Porphyrin ring consists of four–pyrrole rings (Tetrapyrrole).

• Chlorophyll molecule has a Mg–porphyrin head and alcoholic phytol tail. Head is hydrophilic and phytol tail is lipophilic (hydrophobic).

• Phytol tail is alcoholic with one double bond. Phytol part embeded in lipid layer.

• Chl–a and carotenes are universal pigment, which are found in all O2 liberating cells.

• Chlorophylls are soluble only in organic solvents like ketons, ethers etc.

• Stroma lamellae/stroma thylakoids lack PS II and enzyme NADP reductase .

• In paper chromatography/chromatogram –

Chlorophyll 'a' appears bright or blue-green.

Chlorophyll 'b' as — yellow green/grass green.

Xanthophyll as — yellow

Carotenoids as — Yellow to yellow - orange

DISTRIBUTION & LIGHT ABSORPTION OF PIGMENTS