Photosynthesis

(Gr photon = light . Synthesis = putting together)

Photosynthesis is the most important anabolic process on earth by which green plant (autotrophic org) synthesized Complex Carbohydrates from simple Substance like CO2 & water with the help of light energy & purify atmospheric air by consuming CO2 & giving Oxygen. So the

So the process by which green plant Synthesized organic food from CO2 + H2O Using energy absorb from sunligh by chlorophyll the overall equation representing photosynthesis is

6 CO2 + 12H2O --(light / chlorophyll)--> C6H12O6 + 6O2 + 6H2O

(Glucose)

Signification

• Preparation of food for all living beings
• Maintain balance of O2 and CO2
• Prodice O2 that make life possible

Site of Photosynthesis :-

Chloroplast :-

Chloroplast are the green plastics which occur in all green part of plant. They are the actual site for photosynthesis. The maximum number of chloroplast occurs in the leaves with over half a million per square mm.

Chloroplast of higher plant are usually discoided, ellipsoidal or biconvex lens shaped. They are usually 2 to 10 μm in dimater and 1 to 2 μ in thickness.

Structure :-

Chloroplast is an organelle that has an outer & inner membran. A inner membran of chloroplast encloses a liquid proteinaceous matrix called stroma & contain all the necessary enzyme of photosynthesis.

The membran System Consist of an flattened fluid filled sack called thylakoids. This thylakoids are stacked in some places to form grana (granum). The Grana stacks are Inter connected membraneous lamella called stroma lamellae or fret.

The pigment are present in Thylakoids membrane so the mambrane System Contain all the Enzyme & is the site of light rexn of photosynthesis. The stroma jelly like Contain Soluble Enzyme in the dark rexn.

The process of photosynthesis is the transformation of photonic Energy into chemical Energy locked into Carbohydrate molecules by the green part of the plant.

[Diagram: chloroplast showing region of photo chemical & biosynthetic reaction]
Labels: Nucleus, Cytoplasm, Vacule, Outer membrane, Stroma lamella or fret channel, Inner membrane, Ribosomes, Stroma => Dark rxn, DNA, Thylakoid.

Light reaction
also called (photophosphorylation)
types:
- Cyclic
- Non-cyclic

Photosynthetic pigment

The pigment which are involved in the process of photosynthesis are know as photosynthetic pigment. pigment are the organic molecule that absorb light of specific wavelength in the visible region due to presence of Conjugated double bond in their structure.

The chloroplast pigment are fat soluble and are located in the lipid part of thylakoid membrane.

Chloroplast are group under 3 main channel Categories -

1. Chlorophyll
2. Carotenoids
3. The other photosynthetic pigment present in Some algea & cynobacterai are phycobilians.

1) Chlorophyll (Greek chlor = Green : phyll = leaf)

Chlorophyll are the green photosynthetic pigment present in chloroplast that convert light energy to chemical Energy. There are about 10 types a chloroplast a, b, c, d & e Bacteriochlorophyll a, b, c & d, Bacterioviridin.

• Out of all i.e chl. a & chl. b are widely distributed in green algea & higher plants.
• chl. a is found in all plants. It is bluish-green in color & is soluble in Organic Solvent - Ex petroleum, ether. They reflect green light hence appear Green.
• whereas chl. b, c, d & e Carotenoids & phycobilin are called accessary pigment as they don't directly part in photochemical reaction.

Structure of chloroplast was first stuides by Wilstatter, Stoll & Fischer in 1912.

Each chloroplast molecule shows a tadpole-like configuration having a porphyrin head phytol - Tail.

Difference between Chemical Formula and Distribution :-

The chlorophyll b differs from chlorophyll a in having aldehyde group instead of methyl group (-CH3) on ring II of Porphyrin head.

2) Carotenoids :-

Carotenoids are yellow, brown or reddish in Colour.
They Occurs in Thylakoids and acts as accessary pigment of photosynthesis.

• They absorb light energy in the mid region of visible Spetrum ie blue to voilet region & Trans fer their absorb Energy to chlorophyll A molecule.
• They protect the chlorophyll molecule from photooxidation by oxygen. i.e Photooxidation. That is why Carotenoid are Called sheild pigment.

It is a 2 types:

1 Carotens:
Carotens are unsaturated hydrocarbons which the general formula of C40H56. They absorb blue & green light and apperas Yellow to red in colour.
Eg:- β-Caroten which is Converted into Vit-A by annual animal & human being.

C40H56 + 2H2O -> 2 C19H27CH2OH
β-Carotene -> Vitamin A

2) Xanthophyll : (Carotenols)
Xanthophyll (ex lutein) are oxygen desilative of Carotene Containg 2-8 O2 atom. Lutein is responsible for yellow colour in autum foilage.

3) Phycobilines

Phycobilins:
It is a major group of photosynthetic pigment occuring in blue green algea (ie Cyanobacteria) & red algea: they are protein linked pigment. They are water Soluble pigment & is present attached to the phycobilisomes.
Like chlorophyll they have tetrapyrrole but dult Contain Mg & phytol tail.

• They absorb light & acts as accessary pigement it is of 2 types -
• i) Phycocyonin -> blue green algea
• ii) Phycoerythrin -> Ex Red algea

Note: The accessary pigment (chlorophyll b by xanthophyll & carotenoids) + the reaction Centre (Chlorophyll a) together form a photosystem.

Photosystem :-

light rexn of photosynthesis involves participation of 2 Seperate pigment called photosystem-I & photosystem-II.

Each photosystem has a pigment protein Complex Composed of chlorophyll a, b, carotenoids & other Compound required in Electron transport.

Each protein pigment Consist of Core Complex (Chlorophyll a) & anti complex.

• Various molecule present in anticomplex absorb different wavelength of light & transfer their Observe energy to Core Complex. ie reaction Centre chlorophyll.
• It is because the Energy trape by a single chlorophyll molecule is not enough to indicate to first chemical rexn that would Occurs in light.

• PS I complex Contain Single molecule of chlorophyll A 700 or P700 (a special chlorophyll molecule that absorve absorbs visible light near 700mm wavelength.
• PS II Complex Contains Single molecule of chlorophyll a 680 or P680 that absorbs visible light near 680mm wavelength.

[Diagram: Antenna Complex -> PSI / PSII -> Chl a Reaction Centre]
chla & accessory pigment -> Chl a -> Reaction Centre

Mechanism of photosynthesis

The photosynthesis Complete in 2 stage.

1. Photochemical phase or light rexn (Occurs in thylakoid)
2. Biosynthetic phase or dark rexn (Occurs in stroma of chloroplast)

Light rexn

The rexn begin as soon as light fall on green plant. The pigment molecule present in PSI and PSII (locate on thylakoides) absorb Specfic wavelength of light become energies A accessary pigment absorb light & transfer their absorb Energy finally to the rexn Centre. The rexn Centre become so exicated that high Energy electron escapes & move to near by electron accepter molecule.

The electron moves through 2 path way - non-cyclic & cyclic. The non-cyclic electron transport envolves particapation of both PS II & PS I where as Cyclic Election envolves only PS I.
Phosphorylation is the process by which ATP is Synthesis from ADP and Phosphate in presenc of light.

1 Non-cyclic :-

The synthesis of ATP is called oxidation phosphorylation.
(Hill reaction)

[Diagram: Z-Scheme / Non-Cyclic Photophosphorylation]
PEA (Phaeophytin) -> PQ -> Cytocrom b6f Complex -> PC -> PSI (P700) -> PEA(FRS) -> Fd -> NADP+ -> NADPH + H+
PSII (P680) <-> H2O -> 2H+ + 1/2 O2 (Photolysis of water)

PQ = Plastoquinone
cy = Cytochrome
PC = Plastocyanine
Fd = Ferredoxin
FRS = Ferredoxin Reducing Substances
Fig: Non-cyclic Photophosphorylation (Hill reaction)

The light Energy of Specific wavelength is absorb by chlorophyll & asessary pigment of PSII. This pigment transfer their absorb Energy to P680 rexn Centre.

• The electron excited which is unstable & return brown state by loosing electron as heat. The electron losed by PSII are taken of by PEA don't get back to PSII hence Called non cyclic photophosphorylation 'Z' scheme.
• The electron pass through the primary acceptor, plastoquinone, cytochrome, plastocyanine and finally to PS I.
• The electron given out by PS I are taken by primary acceptor and are passed NADP.
• The electron Combines with hydrogen ions & reduces NADP+ to NADPH2. The hydrogen ion are also proteins and is made available by splitting of water also Called photolysis of water.
• Non-cyclic photophosphorylation need a constant Supply of water molecule.
• The net result non-cyclic phosphorylation is the Formation of O2, NADPH2 & ATP molecules.
• O2 is produce as a waste of photosynthesis.

Summery

• It involves both PS II to PS I
• Occurs in Granal thylakoids.
• Electrons are efected from PSII donot return back to it.
• These occurs photolysis of water & evolution of oxygen.
• Formation of assimilatory water also ie ATP + NADPH2
• Non-cyclic phosphorylation is also know as Z-Scheme or Hill rexn.

2) Cyclic photophosphorylation :-

Cyclic electron transport occurs only occassionally when Synthesis of Carbohydrate is stoped due to limited supply of CO2 and NADPH accumulating so there no need for NADPH where as cyclic phosphorylation purpose of more production ATP when need by chloroplast.

1. The cyclic electron transport pathway begins when the PSI Complex Complete absorb light & transfer their Energy PSI reaction (P700).
2. The excited election of P700 is raised to higher energy level which is caputred by the primary acceptor of PSI. then Primary electron acceptor transfer electron to ferrodoxin.
3. Reduce ferrodoxin is unable to reduce NADP+, to return the electron to PSI via Cytochrome b6f -> plastoquinone (PQ) -> Cytochromeb6f -> plastocyanine.
4. The electron Transport is Called cyclic because the electron immetate PSI return back to PSI passing through Several intermmidated Carries.

[Diagram: Cyclic Photophosphorylation]
Primary electron Acceptor (FRS) -> Fd -> PQ -> Cyt b6f Complex -> PC -> PSI (P750/P700) -> Light
ADP + Pi -> ATP (at Cyt b6f step)

Summary :-

• It involves only PS-I
• These Occurs cyclic flow of e- with in photosystem
• Photophosphorylation (ATP from ADP + Pi light)
• Possible site of cylic phosphorylation is stromma lamella.
• Doesn't evolve O2.
• There is no splitting of H2O molecule (photolysis of water)

Biosynthetic phase or Dark rexn :-

Dark rexn occurs in the stroma of the chloroplast where the product of light rexn (ie assimilatory power are used to incorporate Carbon from CO2 to Carbonhydride.

The product of light reaction are ATP, NADPH2 and O2. The O2 diffuses of the chloroplast while ATP and NADPH are used in the next phase for the Synthesis of Sugar. NADPH (Nicotin amiade nine dinucleotide phosphate) so this process is not directly dependent on its product ie ATP / NADPH2.

Melvin Benson Calvin was the Scientist who discover the first CO2 fixation was a three Carbonacid acid (PGA) which was 3 phosphoglyceric acid (PGA). Because of its Contribution the biosynthesis cycle is Called Melvin Cycle.
Biosynthetic Cycle is called Melvin cycle. For this work Kelvina was awared with noble prize in 1921. So CO2 in plant is two types:

1. C3 and C3 are first stable Compound.
2. is C4 (which is first stable compound OAA (4C))

1) C3 pathway :-

Kelvin cycle can be studied by Splitting the Complete cycle into three phase.

1. Carboxylation phase
2. Reduction phase
3. Regeneration phase

[Diagram: The Calvin Cycle (C3 cycle)]
1. Carboxylation: Ribulose 1,5 bisphosphate (RuBP) + CO2 + H2O --(Rubisco)--> 3-PGA (3-phosphoglyceric acid)
2. Reduction: 3-PGA --(ATP -> ADP, NADPH -> NADP)--> Triose phosphate (3-phosphoglyceraldehyde)
3. Regeneration: Triose phosphate -> RuBP (ATP -> ADP)
Output: Sucrose, Starch (main product of photosynthesis)

The net rexn of calvin or C3 cycle is;
6 RuBP + 6 CO2 + 18 ATP + 12 NADPH -> 6 RuBP + C6H12O6 + 18 ADP + 12 NADP + 12 Pi

How many CO2 molecules are needed to synthesis 1 mole of Glucose.
-> 6 CO2 molecules
As per CO2 molecules is fixed, turn of C3 cycle. 6 turns of cycle will be required to fix 6 momleculs of CO2 using 18 ATP and 12 NADPH2.

-> Carboxylation phase:
It is an addition of CO2 to a Compound and its needs an enzyme called Carboxylase. It is initial rexn of calvin cycle in which the atmospheric CO2 combines with Ribulose 1,5 bisphosphate to form an unstable compound which splits into 2 molecules of 3 phosphoglyeric acid (PGA). The rexn is Catalysed by the enzyme Ribulose bis phosphate Carboxylase (RUBISCO).
3 CO2 + 3 RuBP + 3 H2O -> (Rubisco / Carboxylase) -> 3C intermediate Compound -> 6 PGA
RuBP (Carboxylase / RUBISCO) -> 3C Compound first produce

Ribulose 1,5 bisphosphate + CO2 -> Rubisco -> 2-Carboxy 3 keto ribitol 1,5 bisphosphate.
2-Carboxy 3 keto-ribitol 1,5 bisphosphate + H2O -> Rubisco -> 3-phosphoglyceric acid.

Reduction rexn :-
It is first rexn. It involves reduction of PGA by Using ATP and formed during photochemical phase. So PGA is reduced to 1,3 Diphosphoglyceric acid molecules by using ATP produced in the light rexn.

later by Using all NADPH2 molecule 3-phosphoglyceraldehyde (PGAL) are obtained. PGAL in equilibrium with an isomer dihydroxy acetone phosphate & both are collecting known as triose phosphate. PGAL is a 3C Sugar & is the first carbohydrate formed in photosynthesis.
PGAL is a
PGA --(ATP / NADPH2)--> PGAL

iii) Regeneration phase :-
The Glycerol dehyde phophate undergoes a series of reaction utimately by utilizing its 5 molecules of PGAL & 3 molecules of ATP & or rubp regenerated. The rest 1 molecules of (3 Carbon sugar) Glycerol dehydye phosphate is used to make other molecules like Glucose, Sucrose, Strach -
-> To make 2 molecules of 3 C sugar the Glyeral dehydephosphate 6 molecules CO2 are required. that means to make 1 molecule of glucose 6 ton of Kalvin Cycle are required.

C4 path way (Hatch & Slack Cycle)

Since the first stable product of CO2 fixation is the 4 carbon Oxalic acid So the cycle is called C4 cycle. The plant which shows this cycle are called C4 plant. In 1966 hatch & Slack studied detalis in maize + Sugarcane. In plant that performed C4 path way.
The plant that perform C4 cycle are found in tropical Celry -

• In high light intensisty to reduce photorespiration.
• C4 cycle is Seen in monocot Such as Sorghum, maize, Sugarcane (sms)

Exception :-
In wheat, Rice & Barley have C3 cyclic.
- C4 cycle in Some dicot such as (Chenopodium, Amaranthus, Salsola, Digitaria brownii)

Characteristic of C4 plant
- Such plant have Special like Seed leaf anatomy known as 'Kranz anatomy' (Kranz means 'Crown' which refers to distinct ring of cells).
- The mesophyll cell are bundle sheat formed to ring form 2 ring out the bundle sheat.
- In both the cell the chloroplast defers in form hence called dimorphic.
- Tolerate high temp.
- They responde to high light intensisty
- They have high productivity of biomass due to presence of pepcase (phospho enol pyruvate carboxylase) as it shows carboxy lation.
- They lacks photorespiration.

Biochemical Path of C4 cycle :-

[Diagram of C4 cycle showing Mesophyll cell and Bundle Sheath cell]
Mesophyll cell: PEP + CO2 -> (Fixation) -> Oxaloacetic acid (4C) -> Malic acid (4C).
Transport to Bundle Sheath.
Bundle Sheath cell: Malic acid -> (Decarboxylation) -> CO2 + Pyruvic acid (3C).
CO2 -> Calvin Cycle (RuBP -> PGA).
Pyruvic acid Transport back to Mesophyll.
Mesophyll: Pyruvic acid -> (Regeneration, uses ATP) -> PEP.

1. The CO2 is fixed in the cytoplasm of Mesophyll cell. The acceptor is phosphoenol pyruvate 'PEP' (3C compound) instead of RUBP. The Enzyme involved is phosphoenol pyruvate Carboxylases which has a much high finity to CO2 are doesn't participate in photorespiration.
   CO2 + PEP --(PEP Carboxylase)--> Oxaloacetic acid (4C) (1st product of photosynthesis in C4 path way)
2. The oxaloacetic is formed and is Converted mallic acid also known as malate which is than transported to bundle sheath cell through plasmodesmata.
3. In the bundle sheath malate is Converted to Pyruvic acid and CO2 is released. They pyruvic acid returns to Mesophyll cell & CO2 enters to C3 cycle in bundle sheath.
4. The CO2 is than accepted by RUBP Carboxylase & C3 cycle countious & sugar is synthesis.
5. In the whole process since CO2 under goes double Fixation Energy utilize is almost the double. C3 path way required 18 ATP while C4 path way required 30 ATP to Fixed 1 molecule of Glucose.
6. No doubt C4 plant need more energy but it is Compensated by the fact that they grow in high light inteneist & hence modified chloroplast are aviable to trap enough light to provide desire energy.

Significance of C4 Cycle :-

• The C4 plant can absorb CO2 even from low CO2 Concentration.
• The C4 plant perform high rate of photosyethesis where the stomata are nearly closed & They can comsume H2O.
• It requires more light Energy (ATP) to fix CO2 as compare to C3 plant.
• Thus C4 plant are better adapated to tropical & desert area where Sunlight is more intense & growing seasons is longer.