Photosynthesis in Higher Plants

Understanding Photosynthesis

Introduction to Photosynthesis:

• Green plants make their own food using photosynthesis.
• Plants are called autotrophs because they make their own food using photosynthesis.
• Animals and humans, who rely on plants for food, are called heterotrophs.
• Photosynthesis uses light energy to synthesise organic compounds from inorganic raw materials
• Photosynthesis forms the basis of life on Earth.

Importance of Photosynthesis:

• Primary source of all food on Earth.
• Releases oxygen into the atmosphere, which is essential for respiration & survival.

What Do We Know About Photosynthesis?

• Photosynthesis is an enzyme-regulated anabolic process that occurs in chlorophyll-containing cells.
• In this process, carbon dioxide and water are converted into carbohydrates using light energy.

Key Requirements for Photosynthesis:

• Chlorophyll: The green pigment in leaves.
• Light: Necessary for the process.
• CO2: Carbon dioxide from the air.
• Water

Simple Experiments Demonstrating Photosynthesis:

• Experiment 1: Variegated Leaf Experiment:
  • Use a leaf with both green and non-green parts.
  • Expose it to light and test for starch.
  • Result: Only green parts show starch presence, proving photosynthesis happens in green parts in the presence of light.
    • Only the green regions turn blue-black with iodine, showing starch formation.
    • This proves that chlorophyll is necessary for photosynthesis
    • Photosynthesis occurs only in green parts of the leaf in the presence of light.
• Experiment 2: CO2 Requirement Experiment (Moll’s Half-Leaf Experiment):
  • Cover half part of a leaf with a test tube containing KOH-soaked cotton (absorbs CO2).
  • Leave the other part exposed to air and place the setup in light.
  • Result: Exposed part shows starch presence, covered part does not, proving CO2 is needed for photosynthesis.

Conclusion:

• Photosynthesis is crucial for life on Earth.
• It requires chlorophyll, light, and CO2 to produce food and oxygen.
• Simple experiments can demonstrate the necessity of these components.

Early Experiments in Photosynthesis

Joseph Priestley’s Experiments (1770s):

• Discovered oxygen in 1774.
• Experiment:
  • Placed a candle in a closed jar; it went out quickly.
  • Placed a mouse in a closed jar; it suffocated.
  • Added a mint plant to the jar; candle burned longer, mouse survived.
  • Conclusion: Plants restore/purify the air, making it possible for candles to burn and animals to breathe.

Jan Ingenhousz’s Findings (1779):

• Built on Priestley’s work.
• Showed sunlight is essential for plants to purify air.
• Experiment with aquatic plants:
  • Observed bubbles forming only around green parts in sunlight and not in darkness.
  • Identified bubbles as oxygen.
  • Conclusion: Only green parts of plants release oxygen in sunlight.

Julius von Sachs’s Contributions (1864):

• Green parts of plants synthesise glucose during photosynthesis and store it as starch.
• Discovered chlorophyll is located in chloroplasts within plant cells.
• Starch is the first visible product of photosynthesis.

T.W. Engelmann’s Experiment (Late 1884–1888):

• Used a prism to split white light into colors (wavelengths).
• Exposed green algae to different colors, in the presence of aerobic bacteria.
• Bacteria accumulated in regions exposed to blue and red light, indicating maximum photosynthetic activity.
• Conclusion: This led to the discovery of the action spectrum of photosynthesis.

Cornelius van Niel’s Breakthrough (1931):

• Studied photosynthesis in purple and green sulphur bacteria.
• Demonstrated photosynthesis is a light-dependent process.
• Showed hydrogen from water reduces CO2 to form carbohydrates in green plants.
• Conclusion: Oxygen released during photosynthesis comes from water, not from CO₂.

Hill Reaction (1937):

• Hill demonstrated that oxygen evolution occurs during the light reaction of photosynthesis in isolated chloroplasts even in the absence of carbon dioxide, supporting van Niel’s conclusion.

Ruben and Kamen (1941):

• Using heavy oxygen isotope (¹⁸O) labelled water, Ruben and Kamen conclusively proved that oxygen released during photosynthesis originates from water.

Overall Understanding of Photosynthesis:

• Photosynthesis is a light-dependent process.
• Plants use light energy to convert CO₂ and water into carbohydrates, releasing oxygen as a by-product.
• General equation for photosynthesis:
  • 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O

The use of 12 water molecules explains that oxygen released during photosynthesis comes from water and not from carbon dioxide.

Where Does Photosynthesis Take Place?

Main Locations:

• Primarily in the green leaves.
• Also takes place in other green parts such as young green stems.
• The actual site of photosynthesis is the chloroplast.

Mesophyll Cells:

• Leaves contain specialised cells called mesophyll cells.
• Contain a large number of chloroplasts.
• Chloroplasts are green plastids present in all green parts of plants and are the functional units of photosynthesis.
  • In mesophyll cells, chloroplasts usually align themselves along the cell walls.
  • This arrangement helps them receive maximum incident light, increasing the efficiency of photosynthesis.

General Structure of Chloroplast:

• Each chloroplast is a double membrane-bound organelle with an outer membrane and an inner membrane.
• The double membrane acts as a selective barrier, controlling the movement of substances into and out of the chloroplast.
• Inside the inner membrane is a semi-fluid, protein-rich matrix called the stroma.
  • The stroma contains enzymes required for carbon fixation and sugar synthesis.

Internal Membrane System of Chloroplast

• Within the stroma, the chloroplast has a well-developed membrane system made of flattened sac-like structures called thylakoids.
• Thylakoids are arranged in stacks known as grana.
• Individual grana are connected with each other by membranous channels called stroma lamellae or fret channels.

Functional Regions of Chloroplast

• Grana and stroma lamellae
  • These regions contain the photosynthetic pigments and protein complexes involved in light reactions.
  • Light energy is trapped here, leading to the formation of ATP and NADPH.
• Stroma
  • The stroma contains enzymes required for carbon fixation.
  • Carbon dioxide is incorporated into organic molecules here, resulting in the formation of sugars, which are later converted into starch.

Types of Reactions in Photosynthesis:

• Light Reactions (Photochemical Reactions):
  • Light reactions take place on the thylakoid membranes present in grana and stroma lamellae.
  • These reactions are directly driven by light energy.
  • During light reactions, light energy is converted into chemical energy in the form of ATP and NADPH.
• Dark Reactions (Carbon Reactions):
  • Occur in stroma of the chloroplast.
  • These reactions do not directly require light but depend on ATP and NADPH produced during light reactions.
  • Carbon dioxide is reduced to form sugars during these reactions.

Photosystems in Thylakoid Membranes

• The thylakoid membranes contain pigment-protein complexes organised into photosystems.
• Photosystem I
  • Mainly located in the stroma thylakoids and non-appressed regions of grana that face the stroma.
• Photosystem II
  • Present mainly in the appressed regions of granal thylakoids.

These photosystems work together to trap light energy and drive the light reactions of photosynthesis.

Types of Pigments Involved in Photosynthesis

Variety of Leaf Colors:

• Leaves show different shades of green.
• This variation is due to the presence of different photosynthetic pigments in chloroplasts.

Chromatographic Separation:

• Chromatographic separation of leaf pigments shows that leaf colour is not due to a single pigment but a combination of pigments.
• Four main pigments present in green leaves are:
  • Chlorophyll a: Bright or blue-green.
  • Chlorophyll b: Yellow-green.
  • Xanthophylls: Yellow.
  • Carotenoids: Yellow to yellow-orange.

“Chromatography separates pigments based on their differential solubility and movement, allowing individual pigments to appear as distinct coloured bands.”

Role of Pigments:

• Pigments absorb light at specific wavelengths.
• Chlorophyll a:
  • Most abundant pigment in plants.
  • Chief pigment in photosynthesis.
  • Absorbs light mainly in the blue and red regions of the visible spectrum.

“Chlorophyll a is present in all photosynthetic plants except bacteria and is therefore considered the universal photosynthetic pigment.”

Some chlorophyll a molecules act as reaction centres where light energy is converted into chemical or electrical energy.

• Accessory Pigments:
  • includes Chlorophyll b, Xanthophylls, Carotenoids.
  • Absorb light at wavelengths not efficiently absorbed by chlorophyll a.
  • Transfer absorbed energy to chlorophyll a
  • Enable use of a wider range of light wavelengths.
  • Protect chlorophyll a from photo-oxidation.

“Accessory pigments pass absorbed energy to chlorophyll a, ensuring efficient photosynthesis even under variable light conditions.”

Types of Accessory Pigments

• Carotenoids
  • Carotenoids are yellow, orange or reddish pigments present in chloroplasts and chromoplasts.
• They are of two types:
  1. Carotenes: Orange coloured hydrocarbons (example: beta-carotene)
  2. Xanthophylls: Yellow, oxygen-containing pigments
• “Beta-carotene is an important carotene that can be converted into vitamin A in animals.”
• Xanthophylls help in light absorption and protection of chlorophyll molecules from excess light damage.
• Phycobilins
  • “Phycobilins are accessory pigments found in cyanobacteria and red algae.”
  • “They are water-soluble, protein-linked pigments located in phycobilisomes attached to thylakoid membranes.”
• Types include phycocyanin and phycoerythrin.

Absorption and Photosynthesis:

Absorption Spectrum
The absorption spectrum shows the wavelengths of light absorbed by a pigment.

• Chlorophyll a and chlorophyll b show maximum absorption in the blue and red regions of the spectrum.

Action Spectrum
The action spectrum shows the rate of photosynthesis at different wavelengths of light.

• “The action spectrum closely matches the absorption spectrum of chlorophyll pigments.”
• Maximum photosynthesis occurs in blue and red light regions.
• Some photosynthesis also occurs at other wavelengths due to the presence of accessory pigments.

Conclusion:

• Chlorophyll a is the primary pigment responsible for photosynthesis.
• Accessory pigments help by:
  • Increasing the range of absorbed light
  • Transferring energy to chlorophyll a
  • Protecting the photosynthetic apparatus from photo-oxidation

What is Light Reaction?

Light reaction, also called the photochemical phase of photosynthesis, includes the following processes:

Processes Involved:

1. Light absorption.
2. Splitting of water (photolysis)
3. Release of oxygen
4. Formation of ATP and NADPH.

“These reactions take place in the thylakoid membranes of chloroplasts.”

Photosystems:

• Photosynthesis involves two photosystems:
  • Photosystem I (PS I)
  • Photosystem II (PS II)
• These photosystems contain light-harvesting complexes (LHC).

“They are named based on the order of their discovery and not the order in which they function during photosynthesis.”

• Light-Harvesting Complexes (LHC):
  • Made up of hundreds of pigment molecules bound to proteins.
• Functions of LHC:
  • Absorb light of different wavelengths
  • Transfer absorbed energy to reaction centres
  • Increase the efficiency of photosynthesis

“These pigments together form an antenna system that funnels energy towards the reaction centre.”

Reaction Centers:

Each photosystem has a single special chlorophyll a molecule that acts as the reaction centre.

• PS I:
  • Reaction centre chlorophyll a is called P700
  • Maximum absorption at 700 nm
• PS II:
  • Reaction center chlorophyll a is called P680 (absorption peak at 680 nm).

The Electron Transport in Photosynthesis

Photosystem II (PS II):

• Chlorophyll a (P680) absorbs red light of 680 nm
• Electrons get excited and move to a higher energy level
• These electrons are captured by a primary electron acceptor
• Electrons pass through an electron transport chain consisting of cytochromes
• Electron movement is downhill on a redox potential scale

“The electrons are not consumed but are transferred to Photosystem I.”

Photosystem I (PS I):

• Chlorophyll a (P700) absorbs red light of 700 nm
• Electrons get excited again and move to a higher energy acceptor
• Electrons finally move downhill to NADP⁺
• NADP⁺ is reduced to NADPH + H⁺

Z Scheme:

The flow of electrons from:
Photosystem II → Electron transport chain → Photosystem I → NADP⁺

is called the Z scheme of photosynthesis.

“This pathway is named Z scheme because when electron carriers are arranged on a redox potential scale, the pattern resembles the letter ‘Z’.”

• Key Points
  • Light reactions produce ATP and NADPH.
  • Oxygen is released due to splitting of water.
  • Both PS II and PS I work together in non-cyclic photophosphorylation.
  • ATP and NADPH formed in light reactions are used in dark reactions for carbon fixation.

Splitting of Water:

• Photosystem II (PS II) continuously loses electrons during light reactions.
• To replace these electrons, water molecules undergo splitting.
  • Water splitting provides: Protons (H⁺), Oxygen (O₂), and electrons.
  • The reaction is: 2H₂O → 4H⁺ + O₂ + 4e⁻

“Oxygen released during photosynthesis comes from water and not from carbon dioxide.”

• The water splitting complex is associated with Photosystem II.
• It is located on the inner side of the thylakoid membrane.
• “The protons (H⁺) and oxygen (O₂) produced are released into the thylakoid lumen.”
• Oxygen is released as a net product of photosynthesis.

Cyclic and Non-Cyclic Photophosphorylation

Energy Extraction and Storage:

• Living organisms extract energy from oxidizable substances.
• This energy is stored in chemical bonds, mainly in ATP.

Phosphorylation:

• Phosphorylation is the synthesis of ATP from ADP and inorganic phosphate.
• It occurs in mitochondria and chloroplasts.

Photophosphorylation:

• Photophosphorylation is ATP synthesis using light energy during photosynthesis.

Non-Cyclic Photophosphorylation:

• Involves both Photosystem II (PS II) and Photosystem I (PS I).
• Both photosystems work in series and are connected through an electron transport chain (Z scheme).
• Produces both ATP and NADPH + H+.

“Electrons flow from water through PS II and PS I and finally reduce NADP⁺.”

Cyclic Photophosphorylation:

• Involves only Photosystem I (PS I).
• Electrons circulate within PS I and return to the same reaction centre.
  • Electrons cycle back to PS I instead of passing to NADP+.
• Occurs in stroma lamellae.
  • “These regions lack PS II and NADP reductase enzyme.”
• Produces:
  • Only ATP
  • Does not produce NADPH + H⁺
  • Does not release oxygen

“Happens when light of wavelength longer than 680 nm is available or when carbon fixation is inhibited.”

Chemiosmotic Hypothesis

• How is ATP Made in Chloroplasts?
  • The chemiosmotic hypothesis explains ATP synthesis.

“This hypothesis was proposed by Peter Mitchell.”

• Similarity with Respiration
• ATP synthesis in both respiration and photosynthesis depends on a proton gradient across a membrane.

• Difference in Photosynthesis
  • “In photosynthesis, protons accumulate inside the thylakoid membrane, in the lumen.”

What Causes the Proton Gradient?

1. Splitting of Water:
   • Occurs on the inner side of the thylakoid membrane.
   • Produces protons (hydrogen ions) that accumulate in the lumen.
2. Electron Transport:
   • As electrons move through photosystems, protons are transported across the membrane.
   • “The primary electron acceptor transfers electrons to a hydrogen carrier, which removes protons from the stroma and releases them into the lumen.”
3. NADP Reductase Enzyme:
   • Located on the stroma side of the thylakoid membrane.
   • Uses electrons and protons to reduce NADP⁺ to NADPH + H⁺.
   • “Protons used for NADPH formation are taken from the stroma, further increasing the proton concentration difference.”

Why is the Proton Gradient Important?

• The breakdown of the proton gradient releases energy required for ATP synthesis.
• ATP Synthase: ATP synthase enzyme has two parts:
  1. CF₀:
     • Embedded in the thylakoid membrane
     • Forms a channel for proton movement back to the stroma
  2. CF₁:
     • Faces the stroma
     • Undergoes shape changes to synthesise ATP

Requirements for Chemiosmosis:

• A membrane
• A proton pump
• A proton gradient
• ATP synthase

Process Overview:

• Energy is used to pump protons into the thylakoid lumen.
  • This creates a proton gradient.
• ATP synthase allows protons to flow back into the stroma.
  • This step release energy.
• This released energy helps ATP synthase make ATP.

Use of ATP and NADPH:

• Both are used immediately in the stroma.
• They are required for carbon fixation and synthesis of sugars during dark reactions.

Where Are the ATP and NADPH Used?

Light Reaction Products:

• ATP and NADPH are produced during the light reaction.
• O₂ is also produced and diffuses out of the chloroplast.
• ATP and NADPH are used to drive sugar synthesis.
• “They are utilised in the biosynthetic phase of photosynthesis.”

Biosynthetic Phase of Photosynthesis:

• Also called the biosynthetic phase.
• Occurs in the stroma of the chloroplast where all enzymes are present.
• Uses ATP, NADPH, CO2, and H2O.
• Doesn’t directly need light but relies on products from the light reaction.
  • If light is removed, the process continues briefly.
  • It stops once ATP and NADPH are exhausted.
  • It restarts when light reactions resume.

Restarts with light: Shows the importance of light reaction products.

Is “Dark Reaction” the Right Term?

• The term “dark reaction” can be misleading.
• “This phase does not require darkness but depends on ATP and NADPH produced in light.”

How ATP and NADPH Are Used:

• CO₂ is reduced to carbohydrates using ATP and NADPH.
• General equation: CO₂ + H₂O → (CH₂O)ₙ (Sugars)

Melvin Calvin’s Research:

• Melvin Calvin, Benson and Bassham studied this process using radioactive ¹⁴C.
• They traced the pathway of carbon fixation in algae.
• “The first stable product discovered was 3-phosphoglyceric acid (PGA), a three-carbon compound.”

CO₂ Fixation Products:

• C3 Pathway: First stable product is a 3-carbon compound (PGA).
• C4 Pathway: First stable product is a 4-carbon compound (oxaloacetic acid, OAA).

Types of Plants Based on CO₂ Fixation:

• C3 Plants: First product of CO2 fixation is PGA.
• C4 Plants: First product of CO2 fixation is OAA.

The Primary Acceptor of CO2

• Question: What molecule accepts CO₂ and forms a 3-carbon compound (PGA)?
• Unexpected Discovery:
  • The primary acceptor of CO₂ is not a 2-carbon compound.
  • The actual acceptor is a 5-carbon sugar called ribulose bisphosphate (RuBP).
  • “It is a 5-carbon ketose sugar.”

Scientific Journey: Scientists initially thought it was a 2-carbon compound and spent years trying to identify it.

The Calvin Cycle

• Calvin Cycle Discovery: Calvin and his team found that the Carbon fixation process is cyclic and regenerates RuBP.
• Occurs in All Plants: whether they are C₃ or C₄ plants.
• “The cycle regenerates RuBP, allowing continuous CO₂ fixation.”

Stages of the Calvin Cycle:

1. Carboxylation: Fixation of CO₂.
   • CO₂ combines with RuBP.
   • Reaction is catalysed by RuBP carboxylase-oxygenase (RuBisCO).
     • “RuBisCO is the most abundant protein in the biological world.”
   • Result: The unstable intermediate splits to form 2 molecules of 3-phosphoglyceric acid (PGA).
2. Reduction: Formation of Glucose.
   • Series of reactions convert PGA into carbohydrate.
   • Energy requirement per CO₂ fixed:
     • 2 ATP for phosphorylation
     • 2 NADPH for reduction
   • Cycle Turns: 6 turns of the cycle and 6 CO₂ molecules are required to form one glucose molecule.
3. Regeneration: Regenerating RuBP.
   • RuBP is regenerated so the cycle can continue.
   • Energy Use: Requires 1 ATP per CO₂ fixed Or Requires 1 ATP for phosphorylation to form RuBP.

“Regeneration of RuBP is essential for uninterrupted functioning of the cycle.”

Energy Requirement:

• Per CO₂ molecule fixed: 3 ATPs and 2 NADPHs.
• To Make One Glucose Molecule: 6 turns of the cycle .
  • Energy Needs: For one glucose molecule, multiply the energy requirements by 6.
  • i.e. 6 CO₂, 18 ATP, 12 NADPH

Summary of Inputs and Outputs:

• Input: CO2, ATP, NADPH.
• Output: Glucose, ADP, Pi, NADP+.

Understanding this cycle helps explain how plants convert CO2 and energy into sugars.

The C4 Pathway

C4 Plants vs. C3 Plants:

• Adaptation: C₄ plants are adapted to dry, tropical and subtropical regions.
• First Product: The first stable product of CO₂ fixation is a 4-carbon compound, oxaloacetic acid (OAA).
• Main Pathway: Although initial CO₂ fixation differs, biosynthesis finally occurs through the C₃ pathway (Calvin cycle).

Special Features of C4 Plants:

• Leaf Anatomy: C₄ plants possess a special leaf anatomy called Kranz anatomy.
• Temperature Tolerance: They tolerate higher temperatures efficiently.
• Light Intensity Response: They show higher photosynthetic rates under high light intensity.
• No Photorespiration: Photorespiration is absent or negligible.
• High Productivity: Produce more biomass.

Leaf Anatomy Differences (Kranz Anatomy):

• Bundle Sheath Cells:
• Features:
  • Large cells surrounding vascular bundles.
  • Arranged in a wreath-like manner.
  • Have thick cell walls, preventing gas exchange.
  • No intercellular spaces.
  • Contain chloroplasts with RuBisCO enzyme.
  • Chloroplasts are agranal.
  • Well protected from oxygen released by mesophyll cells.
• Mesophyll Cells:
  • Arranged outside bundle sheath cells.
  • Chloroplasts lack starch.
  • Contain PEP carboxylase (PEPcase).
  • RuBisCO enzyme is absent.
  • Specialised for light reactions, oxygen evolution and formation of ATP and NADPH.

Hatch and Slack Pathway (C4 Pathway):

1. CO2 Fixation:
   • Primary Acceptor: Phosphoenol pyruvate (PEP), a 3-carbon molecule present in mesophyll cells.
   • Enzyme: PEP carboxylase (PEPcase).
   • Product: Oxaloacetic acid (OAA), a 4-carbon compound.
2. Formation of C4 Compounds:
   • In Mesophyll Cells: OAA is reduced to malic acid or transaminated to form aspartic acid.
   • Transport: Malic acid or aspartic acid is transported to bundle sheath cells through plasmodesmata.
3. CO2 Release:
   • In Bundle Sheath Cells: C4 acids goes under break down, or decarboxylated.
   • Products: CO₂, A 3-carbon compound (pyruvate).
   • CO₂ Utilisation: Released CO₂ enters the Calvin cycle.
4. Regeneration:
   • 3-Carbon Molecule: Pyruvate returns to mesophyll cells and converts back to PEP.
   • ATP Requirement: ATP is used to regenerate PEP from pyruvate.
   • Cycle Completes: The system becomes ready for the next CO₂ fixation.

Key Points to Remember:

• In C₃ Plants
  • Calvin cycle occurs in mesophyll cells.
• In C₄ Plants
  • Calvin cycle occurs only in bundle sheath cells, not in mesophyll cells.
• RuBP
  • Acts as the final acceptor of CO₂ in the Calvin cycle of C₄ plants.
• Examples of C₄ Plants
  • Maize, Sugarcane, Sorghum, Amaranthus, Atriplex
• Identification
  • Presence of prominent bundle sheath cells helps identify C₄ plants.

C₄ plants show spatial separation of CO₂ fixation:
Initial fixation by PEPcase occurs in mesophyll cells.
Final fixation by RuBisCO occurs in bundle sheath cells.

This separation prevents photorespiration and increases efficiency.

Photorespiration

Understanding Photorespiration:

• First Step of Calvin Pathway: RuBP combines with CO2 to form 2 molecules of 3PGA, catalyzed by RuBisCO enzyme.
• Reaction: RuBP + CO₂ (in presence of RuBisCO) → 2 × 3-PGA

Role of RuBisCO:

• RuBisCO is the most abundant enzyme on Earth.
• Dual nature: It has a dual nature and can bind with both CO₂ and O₂.
• Competitive Binding: Relative concentrations of O2 and CO2 determine which one binds to RuBisCO.
• Binding Affinity: Prefers CO2 when CO2 and O2 concentrations are nearly equal.
  • However, at higher temperatures or lower CO₂ concentration, O₂ competes with CO₂ for RuBisCO.

Photorespiration in C3 Plants:

• O2 Binding: In C₃ plants, RuBisCO sometimes binds O₂ instead of CO₂, reducing CO2 fixation.
• Alternate Reaction:
  • RuBP + O₂ →
    • One molecule of 3-PGA (3-carbon)
    • One molecule of phosphoglycolate (2-carbon)
    • This reaction initiates photorespiration.
• Photorespiratory Pathway:
  • Photorespiration is a cyclic process.
  • It involves three organelles:
    • Chloroplast
    • Peroxisome
    • Mitochondrion
• Key Features of Photorespiration
  • No synthesis of ATP or NADPH
  • No sugar production
  • CO₂ is released
  • ATP is consumed
• Thus, photorespiration leads to loss of fixed carbon and energy.

No Photorespiration in C4 Plants, Why?

• Mechanism: In C₄ plants, CO₂ is first fixed in mesophyll cells and then released in bundle sheath cells.
  • This increases CO₂ concentration around RuBisCO.
• Increased CO2 Concentration: Ensures RuBisCO acts mainly as a carboxylase, reducing oxygenase activity.
• Benefits of Absence of Photorespiration in C₄ Plants:
  • Higher photosynthetic efficiency
  • Higher productivity and yield
  • Better tolerance to high temperature

Comparison of C3 and C4 Pathways:

Feature	C3 Plants	C4 Plants
First Stable Product	3PGA (3-carbon compound)	OAA (4-carbon compound)
Photorespiration	Occurs	Does not occur
CO2 Fixation Enzyme	RuBisCO	PEP Carboxylase (initial fixation)
Leaf Anatomy	No Kranz anatomy	Kranz anatomy present
Productivity	Lower	Higher
Temperature Tolerance	Lower	Higher
CO2 Concentration	Lower at enzyme site	Higher at enzyme site

By understanding these differences, we see why C4 plants are more efficient and better suited for hot, dry environments.

Factors Affecting Photosynthesis

Photosynthesis is influenced by both internal and external factors.

Internal Factors:

• Number, size, age, and orientation of leaves.
• Mesophyll cells and chloroplast number.
• Internal CO2 concentration.
• Amount of chlorophyll.
• Genetic makeup and growth stage of the plant.

External Factors:

• Availability of sunlight.
• Temperature.
• CO₂ concentration.
• Water.
• Oxygen

Law of Limiting Factors (Blackman’s Law, 1905):

• The rate of photosynthesis is controlled by the factor that is present in the minimum amount.
• Even if all other factors are favourable, the process will be limited by the weakest factor.
• Example: Even with a green leaf and optimal light and CO2, low temperature can limit photosynthesis.

a. Light:

• Quality, Intensity, and Duration: of light, all affect photosynthesis.
• At Low Light Intensities: CO₂ fixation increases linearly
• At High Light Intensities: Rate reaches a plateau as other factors become limiting
• Light Saturation: Occurs at 10% of full sunlight.
  • Beyond this, too much light can break down chlorophyll and decrease photosynthesis.
• Light is rarely a limiting factor in natural conditions, except in shade or dense forests.

b. Carbon Dioxide Concentration:

• Major Limiting Factor: Atmospheric CO₂ concentration is low (0.03–0.04%).
• Increasing CO2: Increase up to about 0.05% increases photosynthesis in C₃ plants.
  • But very high concentrations can be harmful over long periods.
• CO₂ Compensation Point:
  • It is the CO₂ concentration at which CO₂ fixation equals CO₂ release.
  • C₃ plants: High compensation point (about 25–100 ppm)
  • C₄ plants: Very low compensation point (0–10 ppm)
• Response to CO₂ with respect to light:
  • Low Light: Neither C₃ nor C₄ responds to high CO₂ at low light.
  • High Light:
    • C4 Plants: Saturation at about 360 µl/L.
    • C3 Plants: Saturation beyond 450 µl/L.
      • Current atmospheric CO₂ levels are limiting for C₃ plants.
• Greenhouse Crops: like tomatoes and bell peppers, use higher CO₂ concentration to increase yield.

Compensation point is technically a point at which rate of photosynthesis = rate of respiration.

c. Temperature

• Dark reactions: are enzyme-controlled and temperature dependent.
• Light reactions: Temperature-sensitive but less affected.
• With a rise in temperature up to optimum
  • Rate of photosynthesis doubles for every 10°C rise.
• Above optimum temperature
  • Rate initially increases
  • Then declines with time (time factor)
• C4 plants: have a higher temperature optimum than C₃ plants.
• C3 plants: Lower temperature optimum.
• Habitat adaptation: Tropical plants have higher optimum temperatures than temperate plants.

d. Water

• Water requirement for photosynthesis is small.
  • However, photosynthesis is highly sensitive to dehydration.
• Water stress causes:
  • Stomatal closure
  • Reduced CO₂ entry
  • Reduced leaf area (wilted leaves)
  • Decreased metabolic activity

e. Oxygen

High oxygen concentration reduces photosynthesis.
This phenomenon is called the Warburg effect.

Conclusion

• Photorespiration reduces photosynthetic efficiency in C₃ plants.
• C₄ plants avoid photorespiration by increasing CO₂ concentration around RuBisCO.
• Photosynthesis is regulated by multiple internal and external factors, and the limiting factor determines the overall rate.

Chapter Summary:

• Green plants prepare their own food by the process of photosynthesis and are therefore called autotrophs.
• Carbon dioxide from the atmosphere is taken in by leaves through stomata.
• CO₂ is used to make carbohydrates, mainly glucose and starch.

• Photosynthesis takes place only in green parts of plants, mainly leaves.
• Mesophyll cells in leaves have many chloroplasts for CO₂ fixation.
• Within chloroplasts, the membrane system is the site of the light reactions,
  • while the enzymatic carbon fixation (Chemosynthetic pathway) reactions occur in the stroma.

• Photosynthesis has two stages: light reaction and carbon-fixing reactions.
• In the light reaction, pigments absorb light energy.
  • Energy is funneled to reaction centre chlorophylls.
  • There are two photosystems, PS I and PS II.
  • PS I has P700 chlorophyll a molecule at its reaction centre.
  • PS II has P680 chlorophyll a molecule that absorbs red light.

• Absorbed light energy excites electrons, transferring them through PS II and PS I.
• Electrons are finally transferred to NADP⁺, forming NADH.
• The movement of electrons leads to the creation of a proton gradient across the thylakoid membrane.
• Breakdown of the proton gradient provides energy for the synthesis of ATP through chemiosmosis.
• Splitting of water molecules, known as photolysis, is associated with PS II.
• This process releases oxygen, protons, and electrons.
• Oxygen is released as a by-product, while electrons replace those lost by PS II.

• In the carbon fixation stage, carbon dioxide combines with a five-carbon compound, ribulose-1,5-bisphosphate (RuBP), in a reaction catalysed by the enzyme RuBisCO.
• RuBP is converted to 2 molecules of 3-carbon PGA.
• PGA is then converted to sugar by the Calvin cycle, and RuBP is regenerated.
  • ATP and NADPH synthesized in the light reaction are used in this process.

• RuBisCO also catalyses a wasteful oxygenation reaction in C3 plants called photorespiration.
  • Photorespiration results in loss of carbon and energy.
• Some tropical plants have a special type of photosynthesis called the C4 pathway to minimise photorespiration.
• In C₄ plants, carbon dioxide is first fixed in the mesophyll cells into a four-carbon compound.
• This compound is transported to bundle sheath cells, where carbon dioxide is released and refixed through the Calvin cycle to synthesise carbohydrates.

• Thus, photosynthesis is a coordinated process involving light energy absorption, electron transport, ATP and NADPH synthesis, and carbon dioxide fixation, forming the basis of food production and life on Earth.