Photosynthesis For NEB

Photosynthesis For NEB

Photosynthesis For NEB

Metabolism :- The whole biochemical processes that occurs within a living organisms inorder to maintain life is called metabolism. Metabolism consists of :

  • Anabolic Process :- It refers to construction or building of organs and tissues. This process produce growth and differentiation of cells and increase in body size, a process that involves synthesis of complex molecules. These reactions require energy so, also known as an endergonic process.
    Example:- Photosynthesis
  • Catabolic Process :- Catabolic process or catabolism refers to the breaking down of large molecules into smaller ones that are oxidized to release energy.
    Example:- Respiration

Photosynthesis is the process of synthesis of food material in presence of light and photosynthetic pigment by using carbondioxide and water molecules. O_{2}O2 is developed as biproduct. It is anabolic, reductive & endergonic process. The overall reaction of Photosynthesis is given below:-
6CO_{2} + 12H_{2}O \xrightarrow{chlorophyll, Light} C_{6}H_{12}O_{6}+6H_{2}O+6O_{2}6CO2+12H2Ochlorophyll,LightC6H12O6+6H2O+6O2

Reduction of Carbondioxide into glucose occurs. In this process solar energy is converted into chemical energy. About only 0.5% – 3% ( ~1%) of sunlight energy reaching plants is used in actual plant production. The life is sustained from this small fraction of available energy. Those plants which performs the photosynthesis is called as autotrophs. The first photosynthetic plant is blue green algae which performs oxygenic photosynthesis. The fungi is non-photosynthetic plant while cuscutta is non-photosynthetic angiosperm plant.

According, to Ruben & Kammen photosynthesis is an oxido-reductive process in which the CO_{2}CO2 is reduced and water is oxidised in the presence of light & photosynthetic pigment. He conducted their experiment of oxygen ( O_{16} or O_{18}O16orO18 ).

Site of Photosynthesis (Photosynthetic Apparatus)

Photosynthesis is carried out by both prokaryotic and eukaryotic cells. In prokaryotic cell Eg:- Blue Green Algae(BGA) members, the photosynthesis takes place in the photosynthetic lamellae or chromatophores.
In the eukaryotic organism, photosynthesis takes part in chloroplast, site of photosynthesis. Chloroplast is bounded by the outer & inner membrane. The space enclosed by inner membrane is known as stroma/ matrix. In the matrix, DNA, RNA, ribosomes, enzymes & number of thylakoid are present. These thylakoids are arranged one above another forming piles known as grana. The grana are interconnected by intergranal lamella. The membranes of thylakoid contain different photosynthetic pigments which help to trap the sunlight to perform the light reaction of photosynthesis.The stroma contains different enzymes to perform the dark reaction of phtosynthesis. Due to presence of most of the enzymes, 70S ribosome and double stranded circular DNA it is also known as semi autonomous cell organelle.


Photosynthetic Pigments

There are different types of photosynthetic pigments present inside the chloroplast which helps to trap the light energy to perform the process of photosynthesis they are:-

  1. Chlorophyll:- Chlorophyll are the green color pigment which is the most abundant pigment in green plants. They are insoluble in water and are fat soluble/organic solvent soluble. They composed of magnesium prophyrin and are of different types of chlorophyll pigment molecules. They are chl’a’, chl’b’, chl’c’, chl’d’, chl’e’ , bacteriochlorophyll and bacterioviridin. The chl’a’ & chl’b’ are present in all the higher plants. These chlorophyll pigment molecules trap the sunlight to perform the process of photosynthesis. Among these chlorophyll molecules, chl’a’ is known as the primary pigment molecules. Since, it takes part in primary reaction of photosynthesis. The other chlorophyll pigment molecules are known as accessory pigment molecules. In all algae, the photosynthesis pigment is chl-a + beta-karyotene while in all algae and in all green plant, the photosynthetic pigment is chl-a + chl-b + beta-karotene. Chlorophyll present in different plants are given below:-
    • chl’a’= Universal green pigment
    • chl’b’= Abundant in Green Algae & Green Plants
    • chl’c’=Diatom Brown Algae
    • chl’d’=Red algae
    • chl’e’=Xanthophyceae
  2. Carotenoid:- Carotenoids are present inside the chloroplast. They are also known as accessory pigment molecules. Caratenoids can be classified into two groups:-
    • Carotenes :- Carotenes are red orange coloured pigments. This consists of open chain conjugated double bond system ending on both sides with ionone rings. It is common in plants.
    • Xanthophyll:- xanthophyll and carotenes are are similar but xanthophylls contains oxygen along with carbon and hydrogen C_{40}H_{56}O_{2}C40H56O2). Caretenoids are accessory pigments.
  3. Phycobilins :- Phycobilin is composed of phycocyanin + Allophycocyanin & phycoerythein. It is also called accessory pigment molecule. It is water soluble and is present in Blue Green Algae(BGA) & Red algae.

Mechanism Of Photosynthesis

It is the complicated oxidation reduction process in which water molecules gets oxidised and carbondioxide is reduced into carbohydrate.

CO_{2} + 12H_{2}O \xrightarrow{sunlight, chlorophyll} C_{6}H_{12}O_{6}+6H_{2}O +6O_{2}↑CO2+12H2Osunlight,chlorophyllC6H12O6+6H2O+6O2

It involves two different steps:-

  1. Light Reaction
  2. Dark Reaction (Blackman’s Reaction)
  1. Light Reaction (Primary Photochemical reaction or Hill Reaction)

    It was given by Robert Hill. It takes place in the presence of light energy. It occurs inside the chromoplast in the grana portion. It is faster than dark reaction. In the light reaction assimulatory power i.e. ATP & NADPH_{2}NADPH2 is produced which are utilized in the dark reaction for the reduction of CO_{2}CO2 into carbohydrate.


Photosynthetic Pigment System or Photosystem & Reaction centre:- They are different types of photosynthetic pigment system involved in the light reaction of photosynthesis:-

  • Photosynthetic pigment system I or Photosystem I or PS I
  • Photosynthetic Pigment System II or Photosystem II or PS II.

Each photosynthetic pigment system consists of about 300 chlorophyll pigment molecules. These pigment molecules trap the light energy. In each pigment there is the presence of primary pigment molecule i.e. chl’a’ which initiates the primary reaction of the photosynthesis. Hence, it is also known as reaction centre. The other In PS I, chl’a’ pigment molecules is P_{700}P700 In PSII, chl’a’ pigment molecule is P_{680}P680


  1. Absorption of light energy
  2. Transfer of light energy to chl’a’ molecule
  3. Photo excitation of chl’a’ pigment molecule
  4. Photolysis of water
  5. Electron transport and photophosphorylation
  1. Absorption of light energy:- All the photosynthetic pigment present inside the chlorophyll absorb the light energy from the visible part of light spectrum.
  2. Transfer of light energy to chl-‘a’ molecule :- All the photosynthetic pigment molecule except the chl-‘a’ tranfers the light energy to chl-‘a’ pigment molecule.
  3. Photo excitation of chl-‘a’ pigment molecule :- When the chl-‘a’ pigment molecule receives the extra light energy it becomes excited having extra energy than ground state. It is known as Photo excitation of chl-‘a’ molecule and in this condition, the chl-‘a’ pigment molecule ejects the electron which represents the light energy so that the electron deficiency occurs in chlorophyll ‘a’ molecule and this chl-‘a’ molecule is known as the io nised chl-‘a’. The ejected electron which represents light energy is utilized in the break down of water molecules in H^{+}H+ & OH^{-}OH ion and some part of light energy is utilized in the formation of ATP.
  4. Photolysis of water:- The oxidation of water molecules or break down of water molecules ( H^{+} OH^{-}H+OH ions) in the presence of light energy.
  1. Electron Transport and photophosphorylation
    Photophosphorylation:- The process of formation of ATP from ADP in the presence of light energy. According to Arnon there are 2 types of photophosphorylation. They are:-
    1. Non-cyclic photophosphorylation and Electron Transport:- In this type, the movement of electrons occurs in unidirectional manner i.e. the initial electron donar is not the final acceptor of electron. It involves 2 pigment system i.e PSI and PSII. Non-cyclic photophosphorylation

F.R.S= Ferredoxin Reducing Substance, P.Q= Plastoquinone, cyt.= Cytochrome, NADP= Nicotinami de Adenine Dinucleotide Phosphate, ATP= Adenosine Triphosphate

Non-cyclic electron transport initiates from the pigment molecule P_{680}P680 of PSII which absorbs as well as receives sufficient quantum of energy and expels electrons. The ejected electron is trapped by a compound of unknown Identity (Q). Electron from the Q passes electron carriers such as plastoquione, cytochrome complex and plastocyanin. From plastocyanin electron passes to P_{700}P700 of photosystem I. Electron deficiency energy in PSI is fulfilled by the electron produced during photolysis of water. P_{700}P700 also expelled electron to primary acceptor. The expelled us utlimately accepted by NADP to for NADPH_{2}NADPH2. The expelled electron when passes over cytochrome complex looses sufficient energy for the synthesis of ATP. The net result of light reaction. Assimitatory powers is ATP & NADPH_{2}NADPH2 are produced with the release of O_{2}O2.

  1. Cyclic Electron Transport:- It involves only one pigment system i.e. PSI. The movement of electron is in cyclic manner. The initial electron donor is the final acceptor of electron.

fig:- cyclic electron transport

P_{700}P700 receives energy by accessory pigments and expels electrons. The electron is received by primary acceptor and then passes to ferrodoxin, plastoquinone, cytochrome complex, plastocyanin and finally back to P_{700}P700. There is cyclic movement of electron. Cyclic phosphorylation is common in bacteria. Here, energy is produced without photolysis of water and formation of NADPH_{2}NADPH2. During the transformation of electron from the ferrodoxin to plastoquonine and in cytochrome complex, there are production of ATP molecules.

DARK REACTION :- It was established by Blackman. Light reaction is independent on this step. It is enzymatic reaction. It takes place in stroma portion of chloroplast. It utilities the assimulatory powers i.e. ATP and NADPH_{2}NADPH2 generated in light reaction for the reduction of CO_{2}CO2 into carbohydrates. The carbondioxide present in the atm. is accepted by certain compounds present in the chloroplast & forms the initial products which leads to the formation of carbohydrates. On the basis of initial product formed, the dark reaction is categorized as:-

  1. Calvin cycle (C_{3}(C3 cycle)
  2. Hatch and Stack Cycle ( C_{4}C4 cycle)
  3. CAM cycle
  1. Calvin Cycle ( C_{3} cycleC3cycle) :-
    1. Carboxylation
    2. Glycolytic reversal
    3. Regeneration of Rebulose-1,5-diphosphate

1). Carboxylation:-

The addition of carbondioxide to a compound in the presence of enzyme carboxylase. This is known as carboxylation. In the calvin cycle, the CO_{2}CO2 is accepted by Ribulose-1,5-diphosphate in the presence of enzyme RUDP carboxylase (Rubisco) & form the 6 carbon compound which is unstable and soon splits into two molecules of 3 compound i.e. Phosphoglyceric acid(PGA)

2).Glycolytic reversal (Formation of Glucose):- The PGA is reduced into phosphoglyceraldehyde(PGAL) by utilising assimilatory power generated by light reaction in the presence of enzymes triophosphate dehydrogenase.


Some molecules of phosphoglyceraldehyde results into the formation of glucose molecule & some molecules regenerate RUDP.

PGAL is isomerised into dihydroxyacetone phosphate (DiHAP) in the presence of enzyme triophosphate isomerase.


DiHAP(3C) & PGAL(3C) unites in the presence of enzymes aldolase to form fructose-1,6-diphosphate.

Fructose-1,6-diphosphate is converted into the fructose-6-phosphate in the presence of enzyme phosphatase.

3). Regeneration of Ribulase-1,5-diphosphate:- some molecules are diverted back to regenerate RUDP.


2. Hatch and Slack Cycle (C_{4} cycle)

  1. C_{4}C4 Plants:- They are those plants in which the first stable product after the CO_{2}CO2 fixation in 4 carbon compound (i.e. oxaloacetic acid, aspartic acid, malic acid etc.) Eg:- Sugarcane, Maize, Cynodon, Amaranthus. The C_{4}C4 plants have special type of leaf anatomy known as Kranzt anatomy. The vascular bundles remain surrounded by 2 layers of cells one is of bundle of sheath cell and other is mesophyll cells. The cells of bundle of sheath are radially arranged like ring and hence known as Krantz sheath. The chloroplasts are of different type. One type of chloroplast is present in mesophyll cella which are small and contain grana. The other type of chloroplast is present in the bundle of sheath which are large, lacks grana and contain the enzymes of the C_{4}C4 cycle. The C_{4}C4 plants undergoes two carboxylation. One carboxylation takes place in chloroplast cell while another carboxylation takes place in bundle of sheath cells.

In Mesophyll cells

In Chloroplast of bundle of sheath cells


Some molecule of PGAL produces the glucose molecules & other diverted back.

3. CAMP Cycle:- The CAM cycle, was firstly observed in Bryophyllum as a member of Crassulaceae family. So, it is known as CAM cycle. It is also found in sedium, Agave, Opuntia, Cactus.


fig:-CAM cycle

These CAM plants have scotomatic stomata (open during night and closes during day) and are sucken. In CAM cycle, during night malic acis is synthesized from oxalo acetic acid and which is shifted towards vacuole and stored in night. During day these malic acid gradually breaks down into CO_{2}CO2 and pyruvic acid. It is called as deacidification. In CAMP cycle as similar as C_{4}C4 30ATP and 12NADPH_{2} is required for the synthesis of Glucose molecule. The CAMP cycle also takes place in high temperature about or more than 45°C and have low CO_{2}CO2 compensation point.

Photorespiration or C_{2}C2 cycle or Glycolate Pathway:-

It is the light dependent intake of oxygen and output of carbondioxide. It takes place in the presence of light and differs from normal respiration. It involves 3 different organelles i.e. chloroplast, peroxisomes & mitochondria. It takes place when the concentration of O_{2}O2 is high & CO_{2}CO2 is low. At the high concentration of oxygen the carbondioxide fixing enzyme instead of accepting the carbondioxide accepts the oxygen and undergoes photorespiration. The energy rich compound i.e. ATP and NADPH_{2}NADPH2 are utilised. It is the special type of respiration shown by green plants when they are exposed to light.

Factors Affecting Photosynthesis

External factors

  • Light
  • CO_{2}CO2
  • Temperature
  • Water
  • O_{2}O2
  1. Light :- It is one of the most important factor without which the plant cannot perform the photosynthesis. The Chief source of light is sunlight. The moon light is also effective for some marine algae. The artificial light can also induce the process of photosynthesis. The light affected the photosynthesis in different ways and they are quality of light, intensity of light and duration of light.
    1. Quality of light:- The process of photosynthesis takes place in visible part of light spectrum. Maximum photosynthesis takes place in red light and minimum in green light. The process of photosynthesis donot occur in ultraviolet rays, infared rays as there rays are harmful.
    2. Intensity of light:- Usually rate of photosynthesis increases with the increase in intensity of the light upto the certain saturation point beyond which it decreases because due to solarization, the photoxidation of photosynthetic component take place. But the plants growing in shady place require low intensity of light. Such plants afmre known as sciophytes. The plants growing in sunny place require high intensity of light is known as heliophytes.
    3. Duration of light:- A brief flash of light is enough for the process of photosynthesis. The intermittent light us favourable for photosynthesis than the continuous light because in continuous light high amount of assimulatory powers are accumulated in the same rate which is produced in light reaction.
  2. Carbondioxide( CO_{2}CO2):- The air contain 0.03% of CO_{2}CO2. Increase in CO_{2}CO2 concentration upto 1% is favourable but high concentration CO_{2}CO2 is toxic to the plants.
  3. Temperature:- The different plants have different requirement of temperature for photosynthesis. Optimum temperature is 6.37°C for photosynthesis. But for the desert plant is 55°C.
  4. Water:- Water is one of the most important raw material to carryout the process of photosynthesis. If the plant is inadequately supplied with water it causes the closing of stomata which affects the exchange of gases.
  5. Oxygen:- When the concentration of O_{2}O2 is high then photorespiration takes place and the photosynthesis gets decreased

Internal Factors

Internal factors are plant factors such as:-

  • chlorophyll content:- The chorophyll content in the plant affects the process of photosynthesis since the chlorophyll traps the light energy.
  • Proper hydration of prtoplasm:- The proper hydration of protoplasm is essential to work the enzyme properly.
  • Anatomy of leaf:- The different anatomical structure of leaf like:- distribution of stomata, presence of chlorophyll cell & distribution of vascular tissues etc. These affects the process of photosynthesis.
  • Accumulation of photosynthetic products:- The quick translocations of photosynthetic products is favourable for the plants.
  • Age of the leaf:- The process of photosynthesis donot immediately start in very young leaves. But the rate of photosynthesis increases with the maturity of the leaves & falls down in old leaves.