🎨 Pigment

Photosynthesis

• About 90 % of the world’s photosynthesis is carried out by marine and fresh water algae.
• From Aristotle’s time to 17th century, it was generally believed that plant and animal debris of the soil was the source of plant nutrition.
• According to Von Helmont in early 17th century, it was water and soil which contributed to the plant growth.
• Stephan Hales (1727): Green plants may get part of their nourishment through their leaves and sunlight may have to do something with it.
• Priestley (1772): Idea of gas exchange taking place in photosynthesis.

• Ingenhouz (Austria, 1779): Plants purify the air only in the presence of light. Only the green parts of the plant produce the purifying agent (O₂) while non-green tissue contaminate the air. Thus Ingenhouz recognised the participation of chlorophyll and light in the photosynthetic process.
• Jean Senebier (1800): Oxygen, liberated from the plants in this process, comes directly from CO₂, which was absorbed by plants. Red wavelengths of light is the most effective in this process.
• de Saussure (1840): Confirmed the finding of Ingenhouz regarding the gas exchange one in light and other in darkness (respiration). He also discovered that water was also utilized in the process.
• Dutrochet (1837): Green part of the plant is essential for photo-synthesis.
• Liebig (1840): The sole source of ‘C’ in plants was CO₂ of the air.
• Robert Mayer: Law of conservation of Energy (1845) and idea of organic synthesis and energy transformation (1848).
• Sachs (1887): Green Chloroplast, were the organs where CO₂ was used up and O₂ was liberated. And starch was the first visible product of photosynthesis.
• Moll’s half leaf experiment showed that CO₂ was necessary for photosynthesis.

Photosynthetic Pigments

• Plants appear green because they reflect green light.

A. Chlorophyll Pigments

• Chlorophylls occur mostly in the grana and are associated with the thylakoid membrane.
• At least 7 types of chlorophylls are known viz. chl. a, b, c, d, e, bacterio chlorophyll and bacterio viridin.
• All chlorophyll (Chl a & Chl b) molecules contain a tetrapyrrole skelton formed into ring with ‘Mg’ at the centre. Thus it has five atoms i.e. 4 carbon and one nitrogen. The base unit of the chlorophyll molecule is a porphyrin ring system made up of 4 simple pyrrole nuclei (tetrapyrrole) joined by carbon linkages. The centre of the porphyrine ring is occupied by a single atom (non-ionic) of Magnesium (Mg).
• Only Chl. a & Chl. b contain Magnesium.

• Chl. a and Chl. b are the most abundant ones found in all autotrophic plants except pigmented bacteria. Other chlorophylls (viz Chl. c, Chl. d, Chl. e) are found only in algae and in combination with chl. a.
• Precursor of chlorophyll is protochlorophyll but according to recent view the immediate precursor is chlorophyllide.

  Protochlorophyllide → Chlorophyllide → Chl. a → Chl. b

• In fresh green leave, the proportions of photosynthetic pigments are as follows:

Difference between Chl. a & Chl. b

B. Carotenoid Pigments

• Carotene and Xanthophyll are together called carotenoids.
• These are fat soluble yellow pigments.
• Carotenoids are located in chloroplast and chromoplast.
• YelIow colour of etiolated and variegated leaves (Leaves with yellow or colorless patches) is due to carotenoids.

• Such pigments are composed of two 6-membered rings with a hydrocarbon chain stretched between them.
• Light energy absorbed by carotenoids is shunted to Chl. a and light absorption results in fluorescence of chlorophyll.
• Strong absorption takes place in the blue violet and ultraviolet end of spectrum with almost no absorption in the red end.

Carotene

• Its colour is Orange-yellow having empirical formula C₄₀H₅₆ (i.e. exclusively of C & H).
• It is abundant in roots of carrot hence the name carotene.
• It is insoluble in water.
• Its most common form is β-carotene. β-carotene is the precursor of Vit. A. Other forms of carotene are α-carotene, R-carotene.
• It is quickly oxidized in air and hence the rapid change of colour takes place in the scraped carrot.

Xanthophylls/Carotenols

• It is more abundant than carotenes.
• It occur in many isomeric forms having colour yellow to brown.
• Empirical formula is C₄₀H₅₆O₂
• It is also called carotenol.
• The common form is Luteol (lutein) followed by violaxanthal (violaxanthin).
• The principal yellow pigment of maize is zeaxanthin.

C. Phycobilin pigments

• Phycobilins are found in blue-green algae (BGA) and red algae.
• It has tetrapyrrole rings but in straight chain.
• Light absorbed by phycobilins is transferred to chl. a where is used in photosynthesis.
• It contains no magnesium (Mg).
• Red pigment is called phyco-erytluin and blue pigment is phyco-cyanin found in red algae and blue-green algae respectively.
• It is soluble in hot water while chlorophylls and carotenoids are soluble in organic solvent.
• It masks the green colour like anthocyanin.

D. Anthocyanin

• It is a purple pigment, soluble in water hence it occurs in solution in the water of the cells means it is actually dissolved in the cell sap and not in cytoplasm.
• It does not take part in photosynthesis
• It is present in sugarbeet, apple.

Photosynthesis/CO₂ assimilation/Food Production

👉🏻 It has two phases viz. light phase and a dark phase

• Reaction of the light phase is light sensitive hence called photochemical reaction.
• The reactions of the dark phase are temperature sensitive and don’t require light. These are purely Chemical reaction and called Blackman reactions on the name of F.F. Blackman who first demonstrated its existence.
• These two steps are:

• Reactants: CO₂ + H₂O
• Requirement: Energy (light) + Catalyst (pigments)
• Products: Food (carbohydrate) + O₂

Ancient view

• Source of O₂ is CO₂.
• CO₂ absorbed is equal to O₂ evolved.

Modern View

• Evidences in support of source of O₂ is H₂O are given by following scientists:
  • 1) Von Niel: Experiment on bacteria i.e. purple sulphure bacteria. These bacteria are autotrophic and photosynthetic. (Normally bacteria are heterotrophs).

• 2) Ruben: Experiment by Ruben is more authentic work. His experiment was on Algae (i.e. Chlorella) through isotopic studies (O₁₆ and O₁₈). Conclusion: Source of Oxygen is water.

• 3) Hill: Experiment on cell free or isolated chloroplast. Here CO₂ is not supplied to isolated chloroplast although O₂ is released. This reaction is called Hill’s reaction.

• On the basis of the above evidences, the revised reaction of photosynthesis is:

👉🏻 Empirical molecular reaction:

• Breakdown of water molecule to release of O₂
• 50% Hydrogen from water combines with C = O of CO₂ to produce food
• Remaining 50 % of H₂O combines with [O] of CO₂ to produce water
• Division of labour: Light and Dark reaction:

• The conversion of light energy into chemical energy i.e. ATP is called Photophosphorylation.
  • ATP = Adenosine Triphosphate
  • ADP = Adenosine Diphosphate
• The breakdown of water molecule (H₂O) into hydrogen and oxygen by light energy is called Photolysis of water. (Photo means light and lysis means to break).
• Pigment system is responsible for photolysis of water.
• Origin of Food: Three major reactions are
  • Photolysis of water: For hydrogen (2H) → Light dependent
  • Photophosphorylation: For ATP → Light dependent
  • CO₂-reduction: For carbohydrate → Light independent (Dark)
• Hence there are two phases viz. Light phase and Dark phase.

Leaf Area Index (LAI)

• Leaf area index is a dimensionless quantity that characterizes plant canopies.
• It is defined as the one-sided green leaf area per unit ground surface area.
• LAI = (Leaf Area)/(Ground Area)
• Leaf Area
  • = L X W X A
  • Where, L: leaf length; W: leaf maximum width; A: Constant

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