Photosynthesis in higher plants Notes For NEET : Class 11 Biology Notes

Photosynthesis – NEET Notes

Definition of Photosynthesis

Photosynthesis (Gk. photon = light, synthesis = putting together) is the most important anabolic process on earth by which green plants (autotrophic organisms) synthesize complex carbohydrates from simple substances like carbon dioxide and water with the help of light energy and purify the atmospheric air by consuming carbon dioxide and evolving oxygen.

It can also be defined as the transformation of photonic energy (light or radiant energy) into chemical energy stored in the high-energy bonds of carbohydrate molecules by the green parts of plants.

Important Features of Photosynthesis

Most important anabolic process on earth
Occurs in green plants and algae
Uses:
- Carbon dioxide (CO₂)
- Water (H₂O)
- Light energy
- Chlorophyll
Produces:
- Carbohydrates
- Oxygen (O₂)
Converts light energy into chemical energy
Helps in purification of atmospheric air by:
- Consuming CO₂
- Releasing O₂

Equation of Photosynthesis

The simplest equation for photosynthesis in higher plants and algae is:

6CO₂ + 12H₂O + Light + Chlorophyll -> C₆H₁₂O₆ + 6O₂ + 6H₂O

Word Equation

Carbon dioxide + Water + Light energy + Chlorophyll → Glucose + Oxygen + Water

Photosynthesis and Photoautotrophism

Photosynthesis is called photoautotrophism.
Plants that perform photosynthesis are called photoautotrophs.

Photoautotrophs

Organisms that manufacture their own food using light energy.

Examples

Green plants
Algae

Autotrophs and Heterotrophs

Autotrophs

Organisms that manufacture their own food are called autotrophs.

Types Mentioned

Photoautotrophs
Chemoautotrophs

Heterotrophs

Organisms that depend on autotrophs for their food requirements are called heterotrophs.

Herbivores

Heterotrophs which depend completely on photoautotrophs for survival are called herbivores (vegetarians).

Chemosynthesis

Definition

The process of manufacture of organic substances by certain bacteria utilizing directly the chemical energy released in the oxidation of inorganic substances is called chemosynthesis.

Chemoautotrophs

Bacteria performing chemosynthesis are called chemoautotrophs or chemosynthetic bacteria.

Example

Nitrifying bacteria

Difference Between Photosynthesis and Chemosynthesis

Feature	Photosynthesis	Chemosynthesis
Energy Source	Light energy	Chemical energy
Organisms	Green plants and algae	Certain bacteria
Pigment Required	Chlorophyll required	Chlorophyll not mentioned
Product Formation	Carbohydrates synthesized	Organic substances synthesized
Type of Organisms	Photoautotrophs	Chemoautotrophs

Key Terms for NEET

Term	Meaning
Photosynthesis	Conversion of light energy into chemical energy
Photoautotrophs	Organisms using light to make food
Chemosynthesis	Food synthesis using chemical energy
Chemoautotrophs	Organisms using chemical energy to make food
Autotrophs	Organisms making their own food
Heterotrophs	Organisms depending on others for food
Herbivores	Heterotrophs depending on plants

NEET Quick Revision Points

Photosynthesis is the most important anabolic process.
Chlorophyll is essential for photosynthesis.
Photosynthesis converts light energy into chemical energy.
Oxygen is evolved during photosynthesis.
Photosynthesis purifies atmospheric air.
Green plants and algae are photoautotrophs.
Nitrifying bacteria are chemoautotrophs.
Herbivores depend completely on photoautotrophs.

One-Line NCERT/NEET Facts

Photosynthesis = Photoautotrophism
Chemoautotrophs perform chemosynthesis
Autotrophs manufacture their own food
Heterotrophs depend on autotrophs
Main feature of photosynthesis → conversion of light energy into chemical energy

Heterotrophs and Saprophytes

Saprophytes

Heterotrophs which can survive on dead organic matter are called saprophytes.

Importance of Saprophytes

Green plants can survive in the absence of:
- Herbivores
- Carnivores
- Omnivores
But green plants cannot survive in nature without saprophytes because:
- Raw materials required by plants are formed by the action of saprophytes.

Dependence of Organisms on Photoautotrophs

All heterotrophic organisms depend directly or indirectly on photoautotrophs.
Oxygen required for respiration of aerobic organisms is liberated during photosynthesis.

Role of Cyanobacteria

Initially, cyanobacteria were the main source of oxygen in the atmosphere through photosynthesis.
Atmospheric oxygen is balanced continuously by:
- Photosynthesis
- Respiration

Nature of Photosynthesis

Anabolic Process

Photosynthesis is an anabolic or constructive process.

Increase in Dry Weight

Dry weight of the plant increases due to photosynthesis.

Endergonic Reaction

Photosynthesis is an endergonic reaction.

Source of Energy

The ultimate source of energy for all organisms is sunlight.

Photosynthesis as an Oxidation–Reduction Process

Photosynthesis is an oxidation-reduction process where:

Water is oxidized
Carbon dioxide is reduced

Important Discoveries Related to Photosynthesis

Scientist	Discovery
Ingen-Housz	Light is essential for photosynthesis
Theodore de Sassure	CO₂ and H₂O are essential for photosynthesis
Senebier	Absorption of CO₂ during photosynthesis
Joseph Priestly	Oxygen is liberated during photosynthesis
Senebier (1782)	Oxygen evolved in food manufacture comes from CO₂

NEET Quick Revision Points

Saprophytes survive on dead organic matter.
Plants cannot survive in nature without saprophytes.
Oxygen for aerobic respiration comes from photosynthesis.
Cyanobacteria were the initial source of atmospheric oxygen.
Photosynthesis is:
- Anabolic
- Constructive
- Endergonic
- Oxidation-reduction process
Water is oxidized during photosynthesis.
Carbon dioxide is reduced during photosynthesis.
Sunlight is the ultimate source of energy for all organisms.

One-Line NCERT/NEET Facts

Saprophytes decompose dead organic matter.
Photosynthesis increases plant dry weight.
Light is essential for photosynthesis.
Oxygen is liberated during photosynthesis.
Cyanobacteria were the earliest oxygen producers.
Photosynthesis is an endergonic process.

Important Discoveries Related to Photosynthesis

Scientist	Discovery
Dutrochet (1937)	Chlorophyll is essential for photosynthesis
Liebig (1840)	CO₂ is the sole source of carbon in plants
Julius Sachs	Chloroplast is the seat of photosynthesis and starch is the first visible product
Robert Mayer	Recognized light as a source of energy

Site of Photosynthesis

Photosynthesis mainly occurs in the mesophyll cells of leaves.
It occurs to a very small extent in:
- Green stems
- Floral parts (sepals)

Mesophyll Cells

Mesophyll cells contain chloroplasts.
Chloroplasts are arranged along the outer margin of the cells.

Chloroplast

Definition

Chloroplasts are green plastids present in all green parts of plants.

Greek:
- chloros = green
- plastos = moulded

Importance of Chloroplasts

Chloroplasts are called:
- Food production centres of the cell
- Assimilatory centres
- Photosynthetic factories

Main Function

Chloroplasts are the actual sites of photosynthesis.

Pigments Present in Chloroplasts

Chloroplasts contain:

Chlorophyll pigments
Carotenoid pigments

Function

These pigments trap light energy required for photosynthesis.

Occurrence and Shape of Chloroplasts

Majority of chloroplasts are present in the mesophyll cells of leaves.

Shapes

Chloroplasts may be:

Lens-shaped
Oval
Spherical
Discoid
Ribbon-like

Size

Length: 5 – 10 mm
Width: 2 – 4 mm

Structure of Chloroplast

Double Membrane

Chloroplasts are double membrane-bound organelles.

Stroma

Definition

The space enclosed by the inner membrane of the chloroplast is called the stroma.

Function

The stroma contains enzymes required for the synthesis of:

Carbohydrates
Proteins

Thylakoids and Grana

Thylakoids

Flattened membranous sacs present in the stroma are called thylakoids.

Grana

Thylakoids are arranged in stacks like piles of coins.
These stacks are called grana.

Photosynthetic Apparatus

The photosynthetic apparatus consists of:

Photosynthetic pigments
Light

Photosynthetically Active Pigments

Major Pigments

Chlorophylls
Carotenoids
Phycobilins (Biliproteins)

NEET Quick Revision Points

Photosynthesis mainly occurs in mesophyll cells.
Chloroplast is the actual site of photosynthesis.
Chloroplasts are double membrane-bound organelles.
Stroma contains enzymes for carbohydrate and protein synthesis.
Grana are stacks of thylakoids.
Chlorophyll traps light energy.
Starch is the first visible product of photosynthesis.
Robert Mayer recognized light as a source of energy.

One-Line NCERT/NEET Facts

Chlorophyll is essential for photosynthesis.
CO₂ is the source of carbon in plants.
Chloroplasts are called photosynthetic factories.
Grana are formed by stacks of thylakoids.
Stroma contains important enzymes.
Chlorophyll and carotenoids absorb light energy.

Types of Chlorophyll

As many as eight major types of chlorophyll are known in the plant kingdom:

Chlorophyll-a
Chlorophyll-b
Chlorophyll-c
Chlorophyll-d
Chlorophyll-e
Bacteriochlorophyll-a
Bacteriochlorophyll-b
Chlorobium chlorophyll

Location of Photosynthetic Pigments

Photosynthetic pigments are located in the membranes of thylakoids.
They are present in specific areas called quantasomes.

Quantasome

The term quantasome was coined by Park and Biggins.

Contents of Quantasomes

Quantasomes contain:

Chlorophyll-a
Chlorophyll-b
Carotenoids:
- Xanthophylls
- Carotene

Location

Quantasomes are mainly present in grana lamellae.

Chlorophyll

Main Photosynthetic Pigment

Chlorophyll is the main pigment useful in photosynthesis.

Structure of Chlorophyll

Porphyrin Structure

Chlorophyll shows a porphyrin structure.

Components of Chlorophyll

Four pyrrole rings forming a tetrapyrrole head
Magnesium atom present at the center
Phytol tail containing numerous carbon atoms

Chlorophyll-a and Chlorophyll-b

Difference Between Chlorophyll-a and Chlorophyll-b

Feature	Chlorophyll-a	Chlorophyll-b
Functional Group	Methyl group	Formyl (CHO) group
Importance	Main pigment	Accessory pigment

Ratio of Chlorophyll-a and Chlorophyll-b

Usually, the ratio of chlorophyll-a to chlorophyll-b is:

2.5 – 3.5 : 1

In Different Plants

Plant Type	Ratio of Chlorophyll-a : b
Heliophytes	5.5 : 1
Sciophytes	1.4 : 1

Carotenoids

Definition

Carotenoids are accessory pigments present in chloroplasts.

Function

They absorb light and transfer it to the chlorophyll reaction center.
They protect chlorophyll from photo-oxidation caused by high-intensity light.

Shield Pigments

Because of their protective role, carotenoids are called shield pigments.

Absorption by Carotenoids

Carotenoids mainly absorb:
- Violet light
- Indigo light
- Blue light

Maximum Absorption

Maximum absorption occurs in the blue region of the spectrum.

Structure of Carotenoids

Carotenoids are unsaturated polyhydrocarbons.
They are made up of eight isoprene (C₅H₈) units.

Solubility of Carotenoids

Carotenoids are fat-soluble pigments.
Therefore, they are also called lipochromes.

Types of Carotenoids

Carotenoids are of two types:

Carotenes
Xanthophylls

NEET Quick Revision Points

Eight major types of chlorophyll are present in plants.
Quantasomes are present in thylakoid membranes.
Park and Biggins coined the term quantasome.
Chlorophyll contains:
- Tetrapyrrole head
- Central magnesium atom
- Phytol tail
Chlorophyll-b contains a CHO group instead of a methyl group.
Carotenoids are accessory pigments.
Carotenoids protect chlorophyll from photo-oxidation.
Maximum absorption by carotenoids occurs in the blue region.

One-Line NCERT/NEET Facts

Chlorophyll is the chief photosynthetic pigment.
Quantasomes are located in grana lamellae.
Chlorophyll has a porphyrin structure.
Carotenoids are called shield pigments.
Carotenoids are fat-soluble pigments.
Carotenes and xanthophylls are types of carotenoids.

Types of Carotenoids

Carotenes

Orange to red colored carotenoids are called carotenes.
Carotenes were first isolated from the roots of carrot; hence they were named carotenes.

Lycopene

Lycopene is a red pigment.
It is found in:
- Ripe tomato
- Red pepper fruits

β-Carotene

β-carotene gives vitamin A on hydrolysis.
Therefore, it is called provitamin A.

Xanthophylls

Definition

Yellow colored carotenoids are called xanthophylls or carotenols.

Common Xanthophyll

The most common xanthophyll in green plants is lutein.

Formation

Xanthophyll formation occurs in the dark under aerobic conditions.

Ratio

In young leaves, the ratio of xanthophyll to carotene is 2 : 1.

Phycobilins

Structure

Phycobilins consist of a bile pigment attached to protein.
They possess a linear tetrapyrrole structure.

Solubility

Phycobilins are water-soluble pigments.

Occurrence

They are mostly present in:
- Red algae
- Blue-green algae

Important Phycobilins

Phycoerythrin
Phycocyanin

Meaning of Prefixes

r → Rhodophyceae
c → Cyanophyceae

Light and Photons

Electromagnetic radiation from the Sun is received by plants in the form of photons.

Quantum

A particle of light (photon) containing a packet of energy is called a quantum (hv).

Electromagnetic Spectrum

The electromagnetic spectrum includes:

Cosmic rays
Gamma rays (γ-rays)
X-rays
Ultraviolet rays
Visible light
Infrared rays
Radio waves

Visible Light

Wavelength Range

Visible light ranges from 350 nm to 750 nm.

Colors of Visible Light

Visible light consists of 7 colors:

VIBGYOR
- Violet
- Indigo
- Blue
- Green
- Yellow
- Orange
- Red

Absorption of Light by Plants

Plants mainly absorb:
- Blue light
- Red light

Energy and Wavelength

The energy value of light is inversely proportional to its wavelength.

Behavior of Light on a Leaf

When a ray of light falls on a leaf:

Process	Percentage
Reflected	12%
Transmitted	5%
Absorbed	83%

Absorption by Chlorophyll

Out of the total absorbed light, only 4% is absorbed by chlorophyll.

Light Absorption by Chlorophyll

Chlorophyll absorbs blue wavelengths more strongly than red wavelengths.

Absorption Spectrum

Definition

A graph showing the absorption of light by chlorophyll-a and chlorophyll-b at different wavelengths is called an absorption spectrum.

NEET Quick Revision Points

Carotenes are orange to red pigments.
Lycopene is the red pigment of tomato.
β-carotene is provitamin A.
Lutein is the most common xanthophyll.
Phycobilins are water-soluble pigments.
Visible light ranges from 350–750 nm.
Plants mainly absorb blue and red light.
Energy of light is inversely proportional to wavelength.
Chlorophyll absorbs blue light more strongly.

One-Line NCERT/NEET Facts

Carotenes were first isolated from carrot roots.
β-carotene is a precursor of vitamin A.
Phycocyanin occurs in blue-green algae.
Visible light contains VIBGYOR colors.
83% of light falling on a leaf is absorbed.
Absorption spectrum shows light absorption at different wavelengths.

Absorption Peaks of Chlorophyll

Chlorophyll-a

Chlorophyll-a shows maximum absorption peaks at:

430 nm
662 nm

Chlorophyll-b

Chlorophyll-b shows maximum absorption peaks at:

453 nm
642 nm

Action Spectrum

Definition

A graph showing the rate of photosynthesis at different wavelengths of light is called an action spectrum.

Reflection and Absorption of Light

White Appearance

When radiant energy is completely reflected from the surface of a substance, the substance appears white.

Dark Appearance

Substances appear dark when they completely absorb radiant energy and do not reflect it.

Colored Appearance

Partly reflected light appears colored.

Green Color of Leaves

Chlorophyll absorbs all colors of light except the green portion.
Green light is reflected.

Result

Chloroplasts containing chlorophyll impart green color to plant parts.
Therefore, leaves appear green.

Role of Chlorophyll in Photosynthesis

The physical change in chlorophyll after absorbing radiant energy causes changes in the chemical substances present in the stroma.

Fluorescence of Chlorophyll

Definition

Fluorescence is the property of immediate emission of long-wave radiations by substances after attaining an excited state on receiving light energy.

Chlorophyll Fluorescence

Chlorophyll shows fluorescence.
Isolated chlorophyll in pure form emits red color.
Therefore, chlorophyll shows red fluorescence.

Electron Transfer in Green Plants

In green plants, excited electrons are accepted by electron carriers.
Later, these electrons are transferred back to chlorophyll.

Energy Utilization

The liberated free energy is utilized in the synthesis of:

ATP
NADPH + H⁺

Ground State of Chlorophyll

The normal energy level of electrons in chlorophyll is called:
- Singlet state
- Ground state

Fraction-1 Protein

Location

Fraction-1 protein is present in the stroma.

Function

This protein contains the enzyme RuBP carboxylase.

Sites of Light and Dark Reactions

Reaction	Site
Light reaction	Grana region
Dark reaction	Stroma

NEET Quick Revision Points

Chlorophyll-a absorbs maximum light at 430 nm and 662 nm.
Chlorophyll-b absorbs maximum light at 453 nm and 642 nm.
Action spectrum shows the rate of photosynthesis at different wavelengths.
Leaves appear green because chlorophyll reflects green light.
Chlorophyll shows red fluorescence.
ATP and NADPH + H⁺ are formed during photosynthesis.
Fraction-1 protein contains RuBP carboxylase.
Light reactions occur in grana.
Dark reactions occur in stroma.

One-Line NCERT/NEET Facts

Chlorophyll-a is the chief photosynthetic pigment.
Action spectrum represents photosynthetic activity.
Completely reflected light appears white.
Completely absorbed light appears dark.
Chlorophyll reflects green light.
Chlorophyll shows fluorescence.
RuBP carboxylase is present in fraction-1 protein.
Grana are the sites of light reactions.

Pigment Systems

Two pigment systems are involved in photosynthesis:

Photosystem I (PS I)
Photosystem II (PS II)

Photosystem I (PS I)

Location

PS I is located on:
- Outer surface of non-appressed parts of grana thylakoids
- Fret channels

Components of PS I

PS I consists of:

200 to 400 chlorophyll molecules
50 carotenoid molecules
One molecule of P700

Color

PS I is light green in color.

Function of PS I

PS I is not directly involved in:
- Photo-oxidation of water
- Evolution of molecular oxygen

Main Role

It produces a strong reductant that reduces NADP⁺ to NADPH.

Electron Transport

PS I participates in:

Cyclic electron transport
Non-cyclic electron transport

Absorption Range

Pigment molecules of PS I absorb light at or below 700 nm wavelength.

Photosystem II (PS II)

Location

PS II is located on the inner surface of appressed parts of grana thylakoids.

Components of PS II

PS II consists of:

About 200 chlorophyll molecules
50 carotenoid molecules
One molecule of P680

Color

PS II is dark green in color.

Function of PS II

PS II is directly involved in:
- Photo-oxidation of water
- Evolution of molecular oxygen

Electron Donation

PS II donates electrons to PS I when NADP⁺ is reduced.

Electron Transport

PS II participates only in:

Non-cyclic electron transport

Absorption Range

Pigment molecules of PS II absorb light at or below 680 nm wavelength.

Difference Between PS I and PS II

Feature	Photosystem I (PS I)	Photosystem II (PS II)
Location	Outer surface of non-appressed grana	Inner surface of appressed grana
Reaction Center	P700	P680
Color	Light green	Dark green
Water Splitting	Not involved	Directly involved
Oxygen Evolution	Does not occur directly	Occurs directly
Electron Transport	Cyclic and non-cyclic	Only non-cyclic
Absorption Peak	700 nm	680 nm

Temperature Coefficient (Q10)

Definition

The temperature coefficient (Q10) is the ratio of the velocity of a reaction at a particular temperature to the velocity at a temperature 10°C lower.

Q10 Values

Physical Process

Q10 value is slightly greater than 1.

Photochemical Reaction

Q10 value is 1.

Chemical Reaction

Q10 value is 2 or more.

Effect of Temperature

With a rise of 10°C temperature, the rate of a chemical reaction becomes double.

NEET Quick Revision Points

PS I contains P700 reaction center.
PS II contains P680 reaction center.
PS I is involved in cyclic and non-cyclic electron transport.
PS II participates only in non-cyclic electron transport.
Water splitting occurs in PS II.
Oxygen evolution is associated with PS II.
PS I reduces NADP⁺ to NADPH.
Q10 for photochemical reactions is 1.
Q10 for chemical reactions is 2 or more.

One-Line NCERT/NEET Facts

PS I absorbs light at 700 nm.
PS II absorbs light at 680 nm.
PS II is responsible for photolysis of water.
P700 is the reaction center of PS I.
P680 is the reaction center of PS II.
Cyclic electron transport involves PS I only.
Non-cyclic electron transport involves both PS I and PS II.
Q10 indicates the effect of temperature on reaction rate.

Mechanism of Photosynthesis

Light Reaction

Definition

The reactions of the light phase require light energy and are therefore called photochemical reactions.

Important Events of Light Reaction

The light reaction involves:

Transfer of energy
Emerson effect
Photosystem I (PS I)
Photosystem II (PS II)
Cyclic photophosphorylation
Non-cyclic photophosphorylation

Products of Light Reaction

Two important products formed during the light reaction are:

NADPH
ATP

Site of Light Reaction

Light reaction occurs in the grana of chloroplast.
This was observed by Arnon.

Arnon’s Observations

Important Findings

Isolated chloroplasts are capable of fixing CO₂.
This results in the formation of carbohydrates.

Limitation

Isolated chloroplasts cannot carry out CO₂ reduction because enzymes required for CO₂ reduction are leached out during isolation.

Condition for CO₂ Reduction

Isolated chloroplasts can reduce CO₂ if the leached substances are supplied externally.

Hill and Bendall Concept

According to Hill and Bendall:

Light reaction is a two-step electron transport system.

First Step of Light Reaction

The first step involves:

Photolysis of water
Photophosphorylation

Photolysis of Water

Definition

Splitting of water and liberation of oxygen by chlorophyll in the presence of light and hydrogen acceptor is called photolysis of water.

Hill Reaction

Hill reaction or photolysis of water was proposed by Robert Hill.

Hill Oxidants

Definition

Electron or hydrogen acceptors that oxidize water are called Hill oxidants.

Examples

Ferredoxin
Quinone
2,6-dichloro endophenol
Ferric salts (Ferric oxalate)

Natural Hydrogen Acceptor

NADP⁺ is the natural hydrogen acceptor in plant cells.

Proof for Evolution of Oxygen from Water

Ruben and Kamen (1941) provided experimental proof using radioactive oxygen (O¹⁸).
They proved that oxygen released during photosynthesis comes from water and not from CO₂.

Modified Equation of Photosynthesis

6CO_2 + 6H_2O + \text{Light} + \text{Chlorophyll} \rightarrow C_6H_{12}O_6 + 6O_2

Products Used for CO₂ Reduction

The following products formed during light reaction are used in CO₂ reduction:

ATP
NADPH + H⁺

Photophosphorylation

Definition

Formation of ATP from ADP and inorganic phosphate (Pi) in the presence of light is called photophosphorylation.

ADP + P_i \rightarrow ATP

Mechanism of Photophosphorylation

During electron transfer from Cytochrome b to Cytochrome f, inorganic phosphate is converted into pyrophosphate (~P) because of the potential gradient.
ADP reacts with Pi to form ATP.

Discovery of Photophosphorylation

Photophosphorylation was discovered by Arnon.

Types of Photophosphorylation

Arnon identified two types:

Cyclic photophosphorylation
Non-cyclic photophosphorylation

Cyclic Photophosphorylation

Features

Only PS I participates.
It is not concerned with photolysis of water.
Excited electrons return back to PS I.
Electron transport is cyclic.
Only ATP is produced.
Occurs when NADPH accumulates.

Non-Cyclic Photophosphorylation

Features

Both PS I and PS II participate.
Water undergoes photo-oxidation.
Water splits into:
- H⁺
- e⁻
- O₂

Electron Flow

NADP⁺ is reduced by PS I.
PS I is reduced by PS II.
PS II is reduced by water.

Result

Electron transport becomes non-cyclic.
ATP and NADPH are produced.
Occurs under normal conditions.

Reduction of NADP⁺

Second Step

In the second step of light reaction:

NADP⁺ reacts with:
- Electrons
- Hydrogen ions

Product

NADPH + H⁺ is formed.

Emerson Experiments

Emerson and his associates conducted experiments on the alga Chlorella.
They studied photosynthetic yield (evolution of O₂).

Red Drop

Definition

A sudden decrease in photosynthetic yield above 680 nm wavelength of far-red light is called red drop.

Discovery

Red drop was discovered by Emerson and his associates.

Emerson Enhancement Effect

Definition

Increase in the rate of photosynthesis when far-red light and red light of shorter wavelength are supplied together is called Emerson enhancement effect.

Quantum Yield

Definition

Quantum yield is the number of oxygen molecules produced per quantum of light.

Value

Quantum yield = 0.125

Difference Between Cyclic and Non-Cyclic Photophosphorylation

Feature	Cyclic Photophosphorylation	Non-Cyclic Photophosphorylation
Photosystem Involved	Only PS I	PS I and PS II
Photolysis of Water	Absent	Present
Oxygen Evolution	Absent	Present
ATP Formation	Present	Present
NADPH Formation	Absent	Present
Electron Movement	Cyclic	Non-cyclic

NEET Quick Revision Points

Light reaction occurs in grana.
ATP and NADPH are formed in light reaction.
Robert Hill proposed photolysis of water.
Oxygen released during photosynthesis comes from water.
Photophosphorylation was discovered by Arnon.
Cyclic photophosphorylation involves only PS I.
Non-cyclic photophosphorylation involves both PS I and PS II.
Red drop occurs above 680 nm.
Emerson enhancement effect increases photosynthetic yield.
Quantum yield value is 0.125.

One-Line NCERT/NEET Facts

Light reaction is a photochemical reaction.
NADP⁺ is the natural hydrogen acceptor.
ATP synthesis in light is called photophosphorylation.
Hill oxidants oxidize water.
Cyclic electron flow produces only ATP.
Non-cyclic electron flow produces ATP and NADPH.
Red drop was discovered by Emerson.
Chlorella was used in Emerson’s experiments.

Dark Reaction

Definition

Dark reaction can occur even in the presence of light, but it does not directly require light.
It depends upon the light reaction for assimilatory powers.

Nature

Dark reaction is a thermochemical reaction.

Site

Dark reaction takes place in the stroma of chloroplast.

Other Names

CO₂ fixation
Carbon assimilation

Phases of Dark Reaction

Dark reaction consists of three phases:

Carboxylation
Reduction
Regeneration of CO₂ acceptor

Types of Photoautotrophic Plants

Depending on the number of carbon atoms in the first stable product, photoautotrophic plants are divided into:

C₃ plants
C₄ plants

Calvin Cycle or C₃ Pathway

Discovery

Carbon assimilation in C₃ plants was explained by Melvin Calvin along with Benson and Bassham.
Experiments were performed on:
- Chlorella
- Scenedesmus
Radioactive CO₂ was used.

Techniques Used

Chromatography
Autoradiography

Nobel Prize

Melvin Calvin received the Nobel Prize in 1961.

Other Names of Calvin Cycle

Calvin-Benson cycle
C₃ pathway
Reductive pentose phosphate pathway

C₃ Plants

Plants showing C₃ pathway are called C₃ plants.

Number of Cycles

Calvin cycle occurs 6 times for the formation of one glucose molecule.

CO₂ Acceptor

RuBP (Ribulose bisphosphate), a 5-carbon compound, acts as the CO₂ acceptor.

Important Enzyme

RuBP carboxylase (RUBISCO) catalyzes carboxylation.

Formation of PGA

6 molecules of CO₂ react with 6 molecules of RuBP.
An unstable hexose compound is formed.
It immediately breaks into:
- 12 molecules of PGA (Phosphoglyceric acid)

First Stable Product

PGA is the first stable product of C₃ plants.
PGA contains 3 carbon atoms.

Therefore

This pathway is called the C₃ pathway.

Reduction Phase

Activation of PGA

PGA + ATP \rightarrow DPGA

12 PGA molecules react with 12 ATP molecules.
DPGA (Diphosphoglyceric acid) is formed.

Enzyme

Phosphoglyceric phosphotransferase

Formation of PGAL / GAP

DPGA + NADPH + H^+ \rightarrow GAP

DPGA is reduced into:
- GAP (Glyceraldehyde-3-phosphate)
- 3-PGAL
- G-3-P

Enzyme

Triose phosphate dehydrogenase

Triose Phosphate

5 molecules of GAP are converted into DHAP.

Enzyme

Triose phosphate isomerase

Important Point

PGAL and DHAP together are called triose phosphate.

Regeneration Phase

Net Gain

1/6 of total PGAL forms one hexose molecule.
5/6 PGAL regenerates RuBP.

Formation of Fructose Diphosphate

Six triose molecules condense to form:
- 3 fructose-1,6-diphosphate molecules

Enzyme

Aldolase

Formation of Fructose-6-Phosphate

Fructose diphosphate is dephosphorylated to fructose-6-phosphate.

Enzyme

Fructose phosphatase

Formation of Glucose

One fructose phosphate converts into glucose by reversal glycolytic reactions.

Fate of Glucose

Glucose is converted into:
- Sucrose
- Starch

Regeneration of RuBP

Remaining fructose-6-phosphate molecules react through a series of reactions.
Finally:
- 6 molecules of ribulose-5-phosphate are formed.
These are phosphorylated into:
- 6 RuBP molecules

Enzyme

Phosphopentokinase

ATP Requirement

6 ATP are utilized.

Overall Calvin Cycle Reaction

6CO_2 + 18ATP + 12NADPH + H^+ \rightarrow C_6H_{12}O_6 + 18ADP + 12NADP^+

Energy Requirement

For one glucose molecule:

6 CO₂
18 ATP
12 NADPH + H⁺

are required.

Hatch and Slack Pathway (C₄ Cycle)

Discovery

Kortschak and coworkers observed formation of C₄ acids in sugarcane.
Later, Hatch and Slack established the pathway in maize and grasses.

Other Name

Cooperative photosynthesis

Site

Occurs in:
- Mesophyll cells
- Bundle sheath cells

Primary CO₂ Acceptor

PEP (Phosphoenol pyruvic acid)

Carboxylation Enzyme

PEP carboxylase (PEPcase)

Type

β-carboxylation

First Stable Product

OAA (Oxaloacetic acid)

Formation of Malic Acid

OAA is reduced into malic acid.

Enzyme

NADP-specific malic dehydrogenase

Dicarboxylic Acids

The following are dicarboxylic acids:

OAA
Malic acid
Aspartic acid

Transport of Malic Acid

Malic acid moves from mesophyll cells to bundle sheath cells through plasmodesmata.

Decarboxylation

Malic acid undergoes oxidative decarboxylation.
CO₂ is released.

Calvin Cycle in C₄ Plants

Released CO₂ is accepted by RuBP.
Therefore, Calvin cycle occurs in bundle sheath chloroplasts.

Regeneration of PEP

Pyruvic acid returns to mesophyll cells.
It is phosphorylated into PEP.

Enzyme

Pyruvate dikinase

Kranz Anatomy

C₄ plants show Kranz anatomy.

Two Types of Chloroplasts

Mesophyll Chloroplasts

Grana-rich
Starch absent

Bundle Sheath Chloroplasts

Lack grana
Starch-rich

Energy Requirement in C₄ Plants

For fixation of one CO₂ molecule:

5 ATP
2 NADPH

are required.

For One Glucose

30 ATP
12 NADPH

are required.

Advantages of C₄ Plants

Can absorb CO₂ at very low concentrations.
High photosynthetic rate even when stomata are nearly closed.
Better adapted to tropical and desert areas.
Show absence of photorespiration.
More efficient than C₃ plants.

CAM Pathway

Occurrence

CAM occurs in plants of:

Crassulaceae
Cactaceae
Agavaceae
Orchidaceae
Portulacaceae

Special Feature

Stomata remain closed during daytime.
Stomata open during night.

This is called:

Scotoactive opening

CAM Plants

Such plants are called CAM plants.

Formation of OAA and Malic Acid

OAA is formed first.
OAA is reduced into malic acid.
Malic acid accumulates in vacuoles.

Acidification

Definition

Absorption of CO₂ at night and storage as malic acid is called acidification.

Deacidification

Definition

Release of CO₂ from malic acid during daytime is called deacidification.

Site of CAM Reactions

All CAM reactions occur in mesophyll cells.
Bundle sheath chloroplasts are absent.

Importance of CAM

Important for survival of succulent plants.

Photosynthetic Factors

Law of Minimum

Proposed by Justus von Liebig in 1843.

Law of Limiting Factors

Proposed by Frederick Frost Blackman in 1905.

Definition

When a process depends on many factors, the rate is controlled by the slowest factor.

Limiting Factor

The factor present at minimum level is called limiting factor.

Types of Factors

External Factors

Light
CO₂ concentration
O₂ concentration
Temperature
Water
Mineral salts

Internal Factors

Chlorophyll content
Accumulation of carbohydrates
Stomatal opening

Compensation Point

Light intensity at which photosynthesis rate equals respiration rate is called compensation point.

Photo-oxidation and Solarization

Photo-oxidation

Destruction of chlorophyll due to high light intensity.

Solarization

Severe photo-oxidation under very intense light.

Effect of Light Color

Light	Rate of Photosynthesis
Red	Maximum
Blue	High
Green	Minimum

Sciophytes and Heliophytes

Sciophytes

Shade-loving plants

Heliophytes

Sun-loving plants

Effect of CO₂

Increasing CO₂ from 0.03% to 1% increases photosynthesis.
Above 1%, photosynthesis decreases due to stomatal closure.

Most Common Limiting Factor

CO₂ is the most common limiting factor.
Light is the second most important factor.

Warburg Effect

Definition

Inhibitory effect of oxygen on photosynthesis is called Warburg effect.

Effect of Temperature

Temperature mainly affects dark reaction.
Q10 = 2 between 0°C and 35°C.

Role of Water

Water provides:
- Electrons
- H⁺ ions

Role of Minerals

Element	Function
Mg	Chlorophyll synthesis
Fe	Present in ferredoxin
Cu	Present in plastocyanin
Mn²⁺, Cl⁻	Photolysis of water

Albino Plants

Chlorophyll-less mutant plants are called albinos.
They cannot survive for long.

Important Experiments

Requirement	Experiment
CO₂ necessity	Moll’s half-leaf experiment
Light necessity	Light screen experiment
Chlorophyll necessity	Croton leaf experiment
O₂ evolution	Hydrilla funnel experiment

Starch Test

Presence of starch is tested by iodine test.
Starch is the osmotically inactive form of photosynthetic product.

Importance of Photosynthesis

Fundamental Process

Photosynthesis is the fundamental synthetic process present in green plants.
It is an anabolic or constructive process.

Source of Food and Energy

All organisms depend directly or indirectly on photosynthesis for their food and energy requirements.

Source of Atmospheric Oxygen

Entire atmospheric oxygen is derived from photosynthesis.
This oxygen is essential for respiration in living organisms.

Maintenance of Atmospheric Balance

Photosynthesis helps in maintaining a constant balance of O₂ and CO₂ in the atmosphere.

Conversion of Light Energy

Photosynthesis is the only biochemical process in which:
- Light energy is converted into chemical energy.
- Chemical energy is stored as potential energy in carbohydrates.

Role in Ozone Layer Formation

Molecular O₃ released during photosynthesis helps in the formation of the ozone layer in the upper atmosphere.
The ozone layer acts as a protective screen against harmful ultraviolet (UV) rays.

Formation of Important Biomolecules

Carbohydrates produced during photosynthesis are utilized in the synthesis of:

Organic acids
Proteins
Fats
Nucleic acids
Hormones
Pigments
Vitamins
Alkaloids
Other metabolites

Purification of Atmosphere

Photosynthesis consumes atmospheric CO₂.
CO₂ is continuously added to the atmosphere by:
- Respiration
- Burning of organic fuels
Thus, photosynthesis acts as a purifier of the atmosphere.

Photorespiration

Definition

Light-induced liberation of CO₂ from a C₂ compound (glycolic acid) during the dark phase is called photorespiration.

Occurrence

Photorespiration occurs only in green cells.

Presence in Plants

Plant Type	Photorespiration
C₃ plants	Present
C₄ plants	Absent

Cell Organelles Involved

Photorespiration involves three cell organelles:

Peroxisomes
Chloroplasts
Mitochondria

Effect of Photorespiration

No ATP or NADPH is produced during photorespiration.
Therefore, it is considered a wasteful process that reduces the efficiency of photosynthesis.

Loss of Carbon

During photorespiration, carbon is lost in the form of CO₂.

Role of RuBP Oxygenase

When O₂ concentration becomes higher than CO₂ concentration, RuBP oxygenase adds O₂ to RuBP.

Products Formed

RuBP is converted into:

One molecule of PGA
One molecule of phosphoglycolic acid

Formation of Glycolic Acid

Phosphoglycolic acid undergoes dephosphorylation to form glycolic acid.

Entry into Peroxisome

Glycolic acid enters the peroxisome.
It acts as the substrate for photorespiration.

Oxidation of Glycolic Acid

Glycolic Acid -> Glyoxylic Acid + H₂O₂

Enzyme

Glycolic oxidase

Breakdown of Hydrogen Peroxide

2H_2O_2 \rightarrow 2H_2O + O_2

Enzyme

Catalase

Formation of Glycine

Glyoxylic acid undergoes transamination to form glycine.

Enzyme

Glyoxylate aminotransferase

Reactions in Mitochondria

Glycine enters the mitochondrion.

Reaction

Two molecules of glycine combine to form:
- One molecule of serine
- CO₂
- NH₃

Enzyme

Glycine decarboxylase

Liberation of CO₂

Liberation of CO₂ occurs in mitochondria during the conversion of glycine into serine.

Formation of Hydroxypyruvate and Glyceric Acid

Serine undergoes deamination to form hydroxypyruvate.
Glyceric acid is also formed.

Glycolate Pathway

Photorespiration is also called the glycolate pathway.
Formation of PGA from glycolic acid is called the glycolate pathway.

Regeneration of PGA

Glyceric acid moves from the peroxisome into the chloroplast.
With the help of ATP, it forms 3-PGA.

Glyceric\ Acid + ATP -> 3PGA

Utilization of PGA

The formed PGA enters the photosynthetic carbon reduction cycle.

NEET Quick Revision Points

Photosynthesis is an anabolic process.
Atmospheric oxygen is produced during photosynthesis.
Photosynthesis maintains the balance of CO₂ and O₂.
The ozone layer protects Earth from UV rays.
Photorespiration occurs only in green cells.
Photorespiration is present in C₃ plants and absent in C₄ plants.
Peroxisomes, chloroplasts, and mitochondria participate in photorespiration.
No ATP or NADPH is produced during photorespiration.
Glycolic oxidase and catalase are important enzymes of photorespiration.
Photorespiration is also called the glycolate pathway.