Notes-Part-1-Class-11-Science-Biology-Chapter-12-Photosynthesis-Maharashtra Board

Topics to be Learn : Part-1 Introduction Chloroplasts Nature of Light Mechanism of photosynthesis Light Reaction Photophosphorylation Topics to be Learn : Part-2 Dark Reaction Photorespiration C4 Pathway or Hatch — Slack Pathway CAM — Crassulacean Acid Metabolism Factors affecting photosynthesis

Introduction :

Need of energy for life processes :

• Life requires energy as a fundamental component.
• Nothing can be done if there is no energy.
• Energy is a necessity for all living things in order to reproduce and survive.
• The primary energy source is the sun, and this energy needs to be converted into forms that living things can use to perform basic functions.

As a result, energy is necessary for many aspects of existence.

Source of energy :

• The sun is the primary source of energy.
• To make sugars, plants need sunlight, carbon dioxide, and water in a process known as photosynthesis.
• Animals use the carbohydrates supplied by plants in their own cellular energy generators known as mitochondria.
• As a result, energy is obtained.

Photosynthesis : Photosynthesis is defined as synthesis of carbohydrates (glucose) from inorganic materials like CO₂ and H₂O with the help of solar energy trapped by pigments like chlorophyll.

6CO₂  + H₂O \(\frac{light}{Chloroplast}>\) C₆H₁₂O6 + 6O₂ + 6H₂O

Because of the following factors, photosynthesis is regarded as the most essential process in the biosphere.

• All plants (primary producers) manufacture food through the biochemical process of photosynthesis.
• It is in charge of the release of oxygen into the atmosphere.
• Heterotrophs rely on autotrophs for energy and other supplies, either directly or indirectly.
• As a result, photosynthesis is regarded as the most important process in the biosphere.

Know This : A typical mesophyll cell has about 30-40 chloroplasts, each measuring about 2—4 um by 4-7 um. Internal space of each thylakoid has ultra-microscopic particles called Quantasomes. The DNA of chloroplast is a circular, closed, naked ring and is called as plastidome. Outer membrane and inner membrane of chloroplast together is called as Peristromium which is about 40 to 60 A in thickness. Sugar synthesis occurs only in the stroma, which contains all of the enzymes essential for sugar synthesis. Photosynthetic lamellae cannot synthesize sugar even when CO2, light and other nutrients are provided. In photosynthetic lamellae only light reactions occur.

Chlorophyll molecule :

• Chemically chlorophyll molecule consists of two parts head of tetrapyrrole the porphyrin ring and a long hydrocarbon tail called phytol attached to the porphyrin group.
• Both chlorophyll-a and chlorophyll-b are similar in their molecular structure, except that the methyl group (-CH₃) in chlorophyll-a is replaced with an aldehyde group (-CHO) in chlorophyll-b.

• Chlorophyll is a light absorbing pigment. It absorbs light in red and blue regions of the visible light spectrum. Absorption spectrum of chlorophyll for red light is maximum so chlorophyll appears red in transmitted light. Green light is not absorbed but reflected so chlorophyll appear green in reflected light.

Carotenoids :

• Carotenoids are lipid compound present universally in almost all the higher plants and several microorganisms.
• They are usually red orange yellow brown and are associated with chlorophyll. They are of two types – the carotenes and xanthophylls.
• The carotenes (C₄₀H₅₆) are orange red and xanthophylls contain oxygen.
• The light energy absorbed by the carotenoids is transferred to chlorophyll-a to be utilized in photosynthesis.
• All photosynthetic plants have these pigments that absorb light between the red and blue region of the spectrum.
• Carotenoids found mainly in higher plants, absorb primarily in the violet to blue regions of the spectrum.
• They not only absorb light energy and transfer it to chlorophyll but also protect the chlorophyll molecule from photo-oxidation.

Pigments :

• Pigments are the molecules which reflects only certain wavelengths of visible light
• Chromatography is the technique used to separate the chloroplast pigments.
• Tomatoes, carrots and chillies are red in colour due to presence of lycopene pigment.
• Accessory pigments are light absorbing molecules which are found in photosynthetic organisms
• They transfer the absorbed light to chlorophyll-a and thus increasing the photosynthetic rate.

Nature of Light :

• Light is a form of energy.
• It travels as stream of tiny particles called photons.
• A photon contains a quantum of light.
• Light has different wavelengths having different colors.
• One can see electromagnetic radiation with wavelengths ranging from 390 mm to 730 mm. This part of the spectrum is called the Visible light.
• It lies between wavelengths of ultraviolet and infra-red.

Mechanism of Photosynthesis :

• In 1931, Van Niel proved that bacteria used H₂S and CO₂ to synthesize carbohydrates.

6CO₂ + 12H₂S \(\frac{light}{Chloroplast}>\) C₆H₁₂O₆ + 6H₂O + 12S ↓

• This led Van Niel to postulate that in green plants, water is utilized in place of H₂S and O₂ is evolved in place of sulphur.
• Ruben used heavy isotope of oxygen (¹⁸O₂) to confirm that the source of oxygen evolved in photosynthesis is water. This leads to the currently accepted general equation of photosynthesis.

6CO₂  + H₂O  \(\frac{light}{Chloroplast}>\) C₆H₁₂O6 + 6O₂ + 6H₂O

Hill Experiment :

• Robert Hill proved that the source of oxygen evolved during photosynthesis is water and not carbon dioxide. It is called Hill’s Reaction.
• In this experiment, Hill suspended isolated chloroplasts in a medium free of CO₂
• Ferric salts and haemoglobin were added in the medium as hydrogen and oxygen acceptors respectively.
• When the suspension was exposed to sunlight, he observed that ferrous salts and oxyhaemoglobin were formed.
• This indicates that water molecules split into H⁺ and OH^- ions, which are accepted by ferric salts and haemoglobin.
• Ferricyanide is reduced to ferrocyanide by photolysis of water
• This process of splitting up of water molecules under the influence of light in the presence of chlorophyll is called Photolysis of water or Hill Reaction.

Hill Reaction :

Hill’s reaction can be represented as follows:

2H₂O + 2A \(\frac{Sunlight}{Chloroplast}>\)  2AH₂ + O₂

Thus, Hill reaction proves that:

• In photosynthesis, oxygen is released from water.
• Electrons for the reduction of CO₂ are obtained from water

Photosynthesis is a redox reaction :

• Photosynthesis is considered as a redox reaction as it involves both oxidation and reduction reactions.
• Water is oxidized by the removal of H⁺ while CO₂ is reduced by the addition of H⁺ to form sugar.
• The redox reactions of photosynthesis are necessary for the conversion of light energy into chemical energy.

Photosynthesis consists of two successive series of reactions :

• The first reaction requires light and is called Light or Hill reaction
• Second reaction does not require light and is called Dark or Blaekman reaction.

• Of the two reactions, the former is a photochemical reaction, while the latter is a biochemical reaction.

Light reaction :

Light reaction : The light reaction is a reaction in which solar energy is trapped by chlorophyll and stored in the form of chemical energy as ATP and in the form of reducing power as NADPH₂. Oxygen is evolved in the light reaction by splitting of water.

Quantum energy : A certain minimum quantity of energy a photon must have to boost an electron is called quantum energy.

Excited state : A molecule that has absorbed a photon is in energy rich excited state. An excited state of an atom means that the valence electron has moved from its ground state orbital to high energy orbital.

Ground state : When the light source is turned off, the high energy electrons return rapidly to their normal low energy orbitals as the excited molecule reverts to its original stable condition, called the ground state.

Structure of reaction centre :

Solar energy is trapped by chlorophyll and stored in the form of chemical energy as ATP and in the form of reducing power as NADPH₂. Oxygen is evolved in the light reaction by splitting of water. The components of light reaction are as follows:

Reaction centre:

• The light absorbing pigments present in thylakoid membranes are arranged in clusters of chlorophyll and accessory pigments.
• P-680 and P-700 are the special type of chlorophyll molecules which form the reaction centres or photocentres.
• Solar energy is harvested by accessory pigments and other chlorophyll molecule and is passed on to the reaction centre.
• These (accessory pigments) are known as light harvesting or antenna molecule. Their function is to absorb light energy and transmit at a very high rate to the reaction centre where the photochemical reactions occur.

Photosystems I and II :

Photosystems :

• Two kinds of photosystems are present in thylakoid membranes of chloroplasts.
• Each has its own set of light harvesting chlorophyll and carotenoid molecules.
• Chlorophyll and accessory pigments help to capture light over large area and pass it on to the photocenters.
• Thus, a photon absorbed anywhere in the harvesting zone of P-680 center can pass its energy to the P680 molecule.
• The cluster of pigments which transfer their energy to P-680 absorb at or below 680nm.
• Together with P-680 they form Photosystem — II.
• Likewise, P-700 forms Photosystem — I along with pigment molecule which absorbs light at or below 700 nm.

Photosystem II:

• Photolysis of water and release of oxygen takes place in this system. -
• In this process, when PS-II absorbs light, electrons are released and chlorophyll molecule is oxidized.
• Electrons emitted by P680 (PS-II) are ultimately trapped by P700 (PS — I).
• Oxygen is the byproduct by the photosynthesizing plants.
• Protons accumulate inside the thylakoid resulting in a Proton gradient.
• When the protons diffuse across the thylakoid membrane into stroma against the H+ gradient, energy is released.
• This is used to produce ATP.

Photosystem I:

• Upon absorption of light quanta by PS-I (P700) reaction center emits energy rich electrons.
• These flow down a chain of electron carriers to NADP along with the proton generated by splitting of water.
• This result in the formation of NADPH.
• Hydrogen attached to NADPH is used for reduction of CO₂ in dark reaction also called as reducing power of the cell.

Oxidized P₆₈₀ regains its electrons by the photolysis of water :

Reactions related to  oxidized P₆₈₀ regains its electrons by the photolysis of water as follows :

4H₂O → 4H⁺ + 4OH^-

4OH^- →4(OH) + 4e^-

4OH → 2H₂O + O₂

4H₂O → 2H₂O + O₂ ↑ + 4H⁺ + 4e^- (overall reaction)

Know This : ATP synthasis : ATP synthase is the enzyme that makes ATP by chemiosmosis. It allows protons to pass through the membrane using the kinetic energy to phosphorylate ADP making ATP. Splitting of water molecule on the inner side of the membrane results in the accumulation ofhydrogen ions within the lumen of thylakoids. The enzyme, NADP reductase, is located in the stroma side of the membrane. For reduction of NADP to NADPH2, protons are required along with electrons that come from ferredoxin. Thus, within the chloroplast, the protons in the stroma decrease in number, while in the lumen, the number of protons increases. This creates a proton gradient across the thylakoid membrane. Energy generated by the subsequent spontaneous movement of protons is used for the synthesis ofATP.

Photophosphorylation :

Formation of ATP in the chloroplasts in presence of light is called photophosphorylation

It takes place in the two forms : Cyclic and non-cyclic photophosphorylation.

Link between light-dependent and dark reactions: -

• The two phases of photosynthesis, i.e. light and dark reactions are dependent on each other, because neither of them can continue the process of photosynthesis alone.
• Light reaction of photosynthesis generates ATP and NADPH which are necessary for the fixation of carbon dioxide into glucose that occurs during the dark phase.
• ATP and NADPH₂ molecules function as vehicles for transfer of energy of sunlight into dark reaction leaving to carbon fixation. In this reaction CO₂ is reduced to carbohydrate.
• During dark reaction, ATP and NADPH₂ are transformed into ADP, iP and NADP which are transferred to the grana in which light reaction takes place.