Photosynthesis in Plants (Full Explanation with diagram)

Do you know What is Photosynthesis? What is the process of Photosynthesis in Plants? Where does photosynthesis take place? where photosynthesis and respiration take place?

Hello readers, In this article, we will study the complete process of photosynthesis with a full explanation. Let's start.

What is Photosynthesis?

Photosynthesis in Plants-PaisaRaniIt means, formation in the presence of light.

Definition - The formation of organic compounds from inorganic raw materials in the presence of light and chlorophyll. It occurs only in autotrophic organisms.

  • It is a physio-chemical process because physical and chemical changes occur. Absorption of sunlight is a physical process & the formation of organic compounds is the chemical process.
  • It is an energy-absorbing process so also known as the Endergonic process.
  • It is a synthesizing process that's knowns as Anabolic process.

Carbon dioxide and water are used by a plant as raw materials during photosynthesis. Chlorophyll is the green pigment of the leaf which traps sunlight in the process.

Evolution of Photosynthesis

The first photosynthetic organism on the earth is Eubacteria. It is non-oxygenic bacteria because it uses H2S & C2O as raw substances and evolves sulfur gas from H2S.

 

H2S  +  C2O    ⇒   C6H12O6 +  S8

(Reaction of Photosynthesis in Eubacteria)

 

The first oxygenic photosynthetic bacteria are Cyanobacteria & BGA. It uses H2O & C2O as raw materials. Oxygen gas released from water. It is one of the reasons for oxygen on earth.

            

H2O +  C2O    ⇒   C6H12O6 +  O

(Reaction of Photosynthesis in BGA & All Green Plants)

 

The analysis of both reactions concluded that oxygen released from water, not from carbon dioxide.

Where does Photosynthesis take place?

This process of photosynthesis takes place in green leaves and other green parts of the plant. An organelle known as Chloroplast inside the merismatic cell is the basic location of photosynthesis. Cholrooast contains a number of the photosynthetic pigments known as  Chlorophyll and accessory pigments.

During photosynthesis, accessory pigments trap sunlight and transfer to the chlorophyll 'a' molecules which are reaction centers of this process. Raw materials like water and carbon dioxide are utilized inside the chloroplast. Through stomata, carbon dioxide enters the chloroplast from the surroundings.

Water is sucked by the root with the help of root hairs from the soil. Xylem vessels transport water and minerals from roots to meristematic cells of the plant then enter into chloroplast by Osmosis. Finally, water and carbon dioxide enters into the chloroplast and starts a reaction in the presence of sunlight trapped by chlorophyll molecules.

How many pigments are evolved in photosynthesis?

Photosynthetic pigments are a special type of molecules which show absorption, transmission, and reflection of different wavelength of light.

The following type of Pigments is involved in Photosynthesis.

  1. Chlorophyll
  2. Carotenoids
  3. Phycobilins....etc

Mechanism of Photosynthesis

There are only two types of reactions involved in photosynthesis.

  1. Light reaction
  2. Dark reaction

Light Reaction(Photochemical phase)

This rxn is known as a photochemical phase because it occurs in the presence of light.

This reaction includes,

  • Absorption of light
  • Splitting of water
  • Release of oxygen
  • Formation of ATP & NADPH.

This reaction complete on the membranous part of chloroplast like Grana thylakoids, Stroma thylakoids. Group of pigment about 300-400 are present on the membranous part known as Photosystem. This reaction plays an important role during light reaction.

Reaction center and Light-Harvesting Complex(LHC) are combinedly known as photosystem.

PhotoSystem - PaisaRani

  • P700 And P608 are reaction center of photosystem I & Photosystem II respectively.
  • PSI  has an absorption peak at 700 nm wavelength of light.
  • PS I  has an absorption peak at 700 nm wavelength of light.
  • PS II  has an absorption peak at 680 nm wavelength of light.

The reason for the name of photosystem I & II is based on their discovery. PSI discovered first than PSII.

  • Reaction Center contains only chlorophyll 'a'.
  • LHC contains accessory pigment like chlorophyll 'b', 'c', 'd', 'e', carotenoids and some proteins which are present around the Reaction center. Light trapped by the LHC is transferred to the Reaction Center.
  • Grana thylakoids contain both PS I and PS II. PS I present on the non-appressed part and PS II present on the appressed part of thylakoids.
  • Stroma thylakoids contain only PS I.

During the Light Reaction, Photolysis of water takes place which is associated with Photosystem - II. Splitting of water occurs on the lumen side of the thylakoids membrane because PS II contains oxygen involving complex (OEC) which is near the lumen side.

What is Photophosphorylation?

The formation of ATP from inorganic ADP and phosphate is known as Photophosphorylation. 

Photophosphorylation is of two types:-

  • Non-cyclic photophosphorylation
  • Cyclic photophosphorylation

1. Non-cyclic photophosphorylation

 

Non-cyclic photophosphorylation

Non-cyclic photophosphorylation

 

It Includes,

  • PS I and PS II work together.
  • Splitting of water takes place.
  • Releasing of oxygen takes place.
  • Occur on Grana Thylakoids.
  • Formation of ATP and NADPH

2. Cyclic Photophosphorylation. 

Cyclic photophosphorylationCyclic photophosphorylation

 

It Includes

  • The only photosystem-I is activated.
  • Less than 680 nanometers wavelength of light is required.
  • No splitting of water takes place.
  • No Oxygen is released.
  • Only ATP formation takes Place.

 

What is the Chemiosmotic Hypothesis?

It explains the formation of ATP inside the mitochondria and chloroplast. ATPs are formed due to the development of proton gradient across the membrane. Requirements of ATP formation are Proton gradient membrane ATPase and Proton pump.

Chemiosmotic hypothesis-PaisaRani

Chemiosmotic Hypothesis

Dark reaction(Biosynthesis Phase)

In this process synthesis of glucose take place by the reduction of CO2 is known as a dark reaction but does not means it occurs in dark. The dark reaction does not directly depend on light reaction, it depends on the product of the light reaction that's ATP & NADPH. This reaction complete in the stroma of chloroplasts because of the enzyme for dark reaction present in the stroma. 

The pathways of dark creation complete in different steps

  • C3  pathway
  • C4  pathway

C3 pathway

This pathway discovered by Calvin in green algae chlorella. Calvin used the radioisotopes technique. I occur in the stroma of chloroplast of mesophyll cell. RuBisCO is used in this pathway.

  • 3 carbon compound phosphoglyceric acid or phosphoglycerate(3PGA) is 1st stable product of Calvin cycle so its name C3 pathway.  
  • 5 carbon compound Ribulose bisphosphate (RUBP) is the primary CO2 acceptor.
  • RuBisCO does carboxylase activity because of the high concentration of  CO2  present.
  • Plants of the Calvin cycle are Wheat, Rice, Barely.

c3 pathway-PaisaRaniC3 Pathway

 

In one turn of the Calvin cycle demanded 3 ATP & 2 NADPH2. Formation of one Glucose required reduction of 6 CO2  or 6 turns of the Calvin cycle in which 18 ATP and 12 NADPH2 utilized.

C4  pathway

It is also known as:-

  • DCA Cycle (Dicarboxylic acid) because of OAA contains 2 COOH group.
  • Hatch & Slack Pathway because discovered by Hatch & Slack.
  • Co-operative photosynthesis because it occurs in both mesophyll & bundle sheath cells.

Oxaloacetic acid (OAA) is a 1st stable compound that is a 4-carbon compound so this known as the C4 pathway. Phosphenol pyruvate (PEP) is 1st CO2  acceptor in the C4 pathway. PEPcase enzyme present during this pathway. C4 Plants are Maise, Sugarcane, Sorghum. 

Leaf anatomy of  C4 Plants:-

It shows a dimorphic chloroplast.

Mesophyll Cell Bundle Sheath cell
  • Small Chloroplast.
  •  Consist of PS-I & PS-II.
  • Granal (arranged on each other).
  • RuBisCO absent.
  • PEPcase present.
  • Large Chloroplast.
  • Consist of PS-I only.
  • Agranal (distributed).
  • RuBisCO present.
  • PEPcase absent.

 

Kranz Anatomy

Bundle sheath cell present around the vascular system is known as Kranz anatomy.

Character

  • Large no. of chloroplast present.
  • Thick cell wall impermeable for gases.
  • No intracellular space present.

c4 pathways-PaisaRaniC4 Pathways

 

In 1 turn or reduction of 1 CO2 utilized 5 ATP and 2 NADPHand formation of 1 Glucose required 6 COor 6 turns of C4 pathway and it utilized 30 ATP and 12 NADPH2.

The character of the C4 pathway.

  • Show Kranz Anatomy 
  • Dimorphic Chloroplast.
  • More no. of the chloroplast.
  • Tolerate high temperature (30-40 degree Celcius)
  • Respond at a high light intensity.
  • Photorespiration absent.
  • High productivity.
  • C4 plants are more efficient than C3 plants.

Photorespiration 

It also is known as Glycolate metabolism because 1st stable product is 2 carbon compound is Glycolate. When RuBisCO shows the Oxygense activity at day time then occurs. Just like respiration Ouse and CO2 release in some step of photorespiration. Chloroplast & peroxisome use oxygen then from mitochondria release of carbon dioxide take place. This cycle differentiates C3 & C4 pathways.

This process is known as a wasteful process because the formation of Sugar, ATP, NADPH does not take place only consumption of ATP & NADPH occurs.

Factor affecting the rate of photosynthesis

Blackman's law of limiting factor used for determining the rate of photosynthesis.

The photosynthesis process affected by many factors but the process is limited by which those are in the least amount.

Light:-

Light is directly proportional to the Rate of photosynthesis.

Increasing the light intensity rate of photosynthesis increases or vice-versa.

Up to limit high light intensity causes photo-oxidation of chlorophyll 'a' molecules. Light saturation occurs at 10% of the incidence of light in a day. Light compensation point means, at which light intensity rate of photosynthesis equal to the rate of respiration.

Carbon dioxide:-

By increasing CO2 concentration up to 0.05% rate of photosynthesis increases.

Pathway CO2compensation point CO2 saturation point
C3 25-100 PPM >450 PPM
 C4 0-10 PPM 360 PPM
  • CO2 is not a limiting factor for C4 plants and aquatic submerged plant.

Water:-

A trace amount of water leads to -

  1. Closing of stomata.
  2. Wilting of leaves.
  3. Decreases surface area & metabolism.

 

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