Photosynthesis

PHOTOSYNTHESIS :

The process in which carbohydrate type of food material is formed by using Carbon dioxide and water in the presence of chlorophyll pigment & sunlight is called as photosynthesis.
or

The process performed by green plants for food synthesis is called as photosynthesis.

-The organism perform photo synthesis are called as photo autotrophs.

Photosynthesis process

Want to Know about Cellular Respiration:

https://uksirnotes.blogspot.com/2021/05/plant-physiology-respiration-by-uk-sir.html

Want to Know about Phyto Hormones:

https://uksirnotes.blogspot.com/2021/05/phyto-hormones-by-uk-sir.html

Want to Know about Mineral Nutrition:

https://uksirnotes.blogspot.com/2021/06/Mineral-nutrition-plants-UKSir.html

HISTORY:

- Van Helmont (1648) increase in plant substance is due to water.

- Stephen Hales: (1727) father of plant physiology - green plants require sunlight.

- Priestley (1772) plant can survive and restore air by burning of candle

- Nicolas T. de Saussure (1804) Total weight of organic matter produced is the by raw material fixed Air and water.

- Palletier & caventou (1818) - discovered and named chlorophyll pigment.

- Von Mayer (1845) - green plant transfer light energy into chemical energy,

Site of photosynthesis:

Photosynthesis site

- Chloroplast is the site of photosynthesis.
- Present in mesophyll cell of the green plants.
- It's a double membrane bound semiautonomous cell organelle.
- Membrane system form- thylakoids & grana.
- Pigments mostly present in grana region.

Pigments:

1.Chlorophyll: –

- Green pigment, convert solar energy to chemical energy.
- Molecular structure- porphyrin head ( 4 pyrrol ring with one central Mg atom.) & a phytol tail ( hydrocarbon tail)

Chlorophyll -a and b

- Types - chlorophyll = a, b, c, d, e ( bacterial chlorophyll a & b.)

Chlorophyll- a: ( C₅₅ H ₇₂ 0₅ N₄ Mg )
They are bluish-green in colour, primary photosynthetic pigment, here methyl group bounded to porphyrin.
Chlorophyll-b:( C₅₅ H ₇₀ 0₆ N₄ Mg )
These are Olive green color, accessory pigment, here aldehyde group present in place of methyl group.
- both Chl- a & b soluble in organic solvent like alcohol, acetone etc.

2. Carotenoids.: ( yellow, brown, orange color )
- They prevent photo oxidation and also called as lipochrome. (fat soluble)
- They act as accessory pigment.
They are of 2 type:
- Carotene: only made up of hydro carbons, C₄₀ H ₅₆,Mostly Orange colour, e.g- lycopene of tomato. Beta- carotene of carrot.
- Xanthophylls - Yellow colour pigment, here 2 additional oxygen C₄₀ H₅₆ O₂ present. Ex- Lutein, Zeaxanthin.

3. Phycobillins- These are Proteinaceous pigments, red and blue color. Found in BGA and Red algae. Soluble in hot water. Ex- Phycocyanin, phycoerythrin

ABSORPTION SPECTRUM

Chlorophyll-Absorption-Spectrum

- It is a graph showing amount of absorption of different light wavelength by pigments.
- Pigments can only absorb at the visible part of light wavelength ( PAR- photosynthetically active region) VIBGYOR.
- Chlorophylls mostly absorb at - 400-500, 700 nm wavelength
- Carotenoids at- 500 nm, and Phycocyanin at - 600 nm

ACTION SPECTRUM

absorption-and-action-spectrum

- It’s a graph showing rate of photosynthesis against different wavelength of light.

However Number of Oxygen molecule produced, is considered as rate of photosynthesis.

Emerson’seffect and Red drop :

Emerson effect red drop

- Emerson (1957) – more than 680 nm light cause photosynthesis rate reduction called- Red drop.
- But when two wavelength of light use alternatively, It cause photosynthetic rate increase- called as- Emerson’senhancement effect.

Pigment System/ Photosystem :( Photosynthetic Unit)

- Photosynthetic unit - minimum number of pigment require for photosynthetic light reaction.
- Thylakoid membrane contain Quantasome (or Photosynthetic units)
- Each Photosystem (LHC) contain - Chlorophylls, carotenoids, proteins etc.

Pigment-system

Two Types of PS present-
PS- 1 : Reaction center is P700, present in outer surface of thylakoid, involved in both cyclic and non cyclic photophosphorylation.
PS- 2: Reaction center is P 680. at inner surface of thylakoid membrane, involved in non cyclic photophosphorylation.

Quantum Required – The number of Photon or Quantum require to release 1 molecule of Oxygen by a plant during photosynthesis. (Mostly 8)

QuantumYield – The number of Oxygen molecule released per photon of light during photosynthesis.

Compensation Point: Rate of Photosynthesis is equivalent to rate of Respiration.

Mechanism ofPhotosynthesis: (Process)

- It’s a Oxidation reduction process. (Water Oxidized, CO₂ reduced)
- Contain Two Phase:
1) Photochemical phage (Light reaction) 2) Bio synthetic phage (Dark reaction)

Photosynthesis-process

1) Light Reaction:

-It’s a light dependent reaction.
-Takes place inside grana thylakoid membrane.
-It starts with absorption of light energy by PS.
- Antenna molecules transfer energy to reaction center.
-Photolysis of water occurs in to H⁺, e^- and Oxygen.
-Photolysis of water occurs near PS-II with the help of MN²⁺, Cl^- and Ca²⁺
-Here evolution of O₂ occurs.
-This Oxygen evolution process also named as Hill reaction (by R.Hill 1937)

Photophosphorylation :

-Here transfer of electron from PS through an ETS occurs.
-It leads to formation of ATP and NADPH₂ by using light energy.

It is of 2 types :
-Non Cyclic and Cyclic photophosphorylation.

NON CYCLIC photophosphorylation

Non-Cyclic-photophosphorylation

Also called Z -Scheme
-Here ATP formation coupled with non cyclic transfer of electron (produced from photolysis of water).

- The electrons transfers through a series of carriers and at last reduce NADP⁺ to NADPH₂
- Transfer of Electron occurs from -

PS II → Phaeophytin →Plasto Quinones →Cytochrome b6,f complex → Plastocyanin →PS I → Fes center→ NADP⁺ → NADPH₂
- Here ATP production occurs by pumping of proton ion by Cytochrome complex.
- Here both ATP and NADPH₂ produced.

CYCLIC photophosphorylation

Cyclic-photophosphorylation

- Here production of ATP occurs by a cyclic flow of electron.
- Here e^- donor and final acceptor is same.
- In absence of the final acceptor NADP⁺, the e^- bounce back to ETS.
- From PS-I it again transfer to Fd and Cytochrome complex.
- Here only ATP is produced.
- It do not involve Photolysis of water.

Product From light reaction
- ATP + NADPH₂ (assimilatory power) and O₂

Chemiosmotic Hypothesis

- It was proposed by P. Mitchell (1961).

- Here both Osmotic and Chemical gradient is used for production of ATP

- Due to proton pumping, A proton gradient develops across the thylakoid membrane.

- H+pump from outer side to inner lumen in Chloroplast.

- But proton can’t pass through the membrane as it is impermeable.

- Again Those H+ move through the F0-F1 particle for maintain the equilibrium.

H⁺only pass through the ATPase / F0-F1 Particle.

Chemiosmotic-Hypothesis

2) Dark reaction:

- It is light independent reaction.
- Carbon dioxide reduced to form carbohydrate.
- Also called as Black man's Reaction. (by -Blackman 1905)
- Also called C3 Pathway or Calvin cycle.
- Contain 3 stages:
- ( Carboxylations of RuBp , reduction of CO₂ & regeneration of RuBP.)

Calvin-Cycle

Carboxylation:

- Atmospheric CO₂ fixed to a stable organic compound.
- 1st acceptor of carbon dioxide is RuBP.
- With the help of an enzyme RuBisCO.
- This enzyme has affinity towards both O₂& CO₂
- It form 2 molecule of phosphoglyceric acid (PGA)