Photosynthesis in Higher Plants

Understanding Photosynthesis

Introduction to Photosynthesis:

• Plants make their own food using photosynthesis.
• This process uses light energy to create organic compounds.
• Plants are called autotrophs because they make their own food.
• Animals and humans, who rely on plants for food, are called heterotrophs.

Importance of Photosynthesis:

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

What Do We Know About Photosynthesis?

Key Requirements:

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

Simple Experiments:

• Experiment 1: Variegated Leaf Test:
  • 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.
• Experiment 2: CO2 Requirement Test:
  • Cover 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 the air, making it possible for candles to burn and animals to breathe.

Jan Ingenhousz’s Findings (1790s):

• Built on Priestley’s work.
• Showed sunlight is essential for plants to purify air.
• Experiment with aquatic plants:
  • Observed bubbles forming around green parts in sunlight.
  • Identified bubbles as oxygen.
  • Conclusion: Only green parts of plants release oxygen in sunlight.

Julius von Sachs’s Contributions (1854):

• Showed plants produce glucose, stored as starch.
• Discovered chlorophyll is located in chloroplasts within plant cells.
• Green parts of plants make glucose.

T.W. Engelmann’s Experiment (Late 1800s):

• Used a prism to split light into colors.
• Exposed green algae to different colors.
• Observed bacteria gathering in blue and red light areas.
• Conclusion: These colors are most effective for photosynthesis.

Cornelius van Niel’s Breakthrough (1930s):

• Studied photosynthesis in purple and green 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 CO2.

Overall Understanding of Photosynthesis:

• Plants use light energy to convert CO2 and water into carbohydrates and oxygen.
• General equation for photosynthesis:
  • 6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O
• Explanation:
  • 12 molecules of water are used to ensure the oxygen released comes from water, not CO2.

Where Does Photosynthesis Take Place?

Main Locations:

• Primarily in the green leaves.
• Also in other green parts of plants like stems.

Mesophyll Cells:

• Located in leaves.
• Contain many chloroplasts.
• Chloroplasts align along the cell walls to capture optimal light.

Chloroplast Structure:

• Consists of grana, stroma lamellae, and stroma.
• Grana and Stroma Lamellae:
  • Trap light energy.
  • Synthesize ATP and NADPH.
• Stroma:
  • Enzymatic reactions occur here to synthesize sugar (glucose) and starch.

Types of Reactions:

• Light Reactions (Photochemical Reactions):
  • Occur in grana and stroma lamellae.
  • Directly driven by light.
• Dark Reactions (Carbon Reactions):
  • Occur in stroma.
  • Dependent on ATP and NADPH from light reactions.
  • Do not require darkness, but they do not directly use light.

Types of Pigments Involved in Photosynthesis

Variety of Leaf Colors:

• Leaves have many shades of green.
• Different pigments cause these variations.

Chromatographic Separation:

• Separates leaf pigments to show different colors.
• Four main pigments:
  • Chlorophyll a: Bright or blue-green.
  • Chlorophyll b: Yellow-green.
  • Xanthophylls: Yellow.
  • Carotenoids: Yellow to yellow-orange.

Role of Pigments:

• Pigments absorb light at specific wavelengths.
• Chlorophyll a:
  • Most abundant plant pigment.
  • Chief pigment in photosynthesis.
  • Absorbs light mostly in the blue and red regions.
• Accessory Pigments:
  • Chlorophyll b, Xanthophylls, Carotenoids.
  • Absorb light and transfer energy to chlorophyll a.
  • Enable use of a wider range of light wavelengths.
  • Protect chlorophyll a from photo-oxidation.

Absorption and Photosynthesis:

• Chlorophyll a shows maximum absorption in blue and red light.
• Maximum photosynthesis also occurs in blue and red light regions.
• Some photosynthesis happens at other wavelengths due to accessory pigments.

Conclusion:

• Chlorophyll a is the primary pigment for photosynthesis.
• Accessory pigments help by expanding light absorption range and protecting chlorophyll a.

What is Light Reaction?

Processes Involved:

• Light absorption.
• Water splitting.
• Oxygen release.
• Formation of ATP and NADPH.

Photosystems:

• Photosystem I (PS I) and Photosystem II (PS II):
  • Contain light-harvesting complexes (LHC).
  • Named based on their discovery order, not the sequence of function.
• Light-Harvesting Complexes:
  • Made up of hundreds of pigment molecules bound to proteins.
  • Absorb different wavelengths of light to make photosynthesis efficient.

Reaction Centers:

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

The Electron Transport in Photosynthesis

Photosystem II (PS II):

• Chlorophyll a absorbs 680 nm red light.
• Electrons get excited and move to a higher orbit.
• Electrons are picked up by an electron acceptor.
• Electrons pass through an electron transport chain (cytochromes).
• Movement is downhill on a redox potential scale.
• Electrons move to Photosystem I (PS I).

Photosystem I (PS I):

• Chlorophyll a absorbs 700 nm red light.
• Electrons get excited and move to another acceptor molecule.
• Electrons move downhill to NADP+.
• NADP+ is reduced to NADPH + H+.

Z Scheme:

• Describes the flow of electrons from PS II to PS I and then to NADP+.
• Called the Z scheme due to its characteristic shape on the redox potential scale.

Splitting of Water:

• PS II continuously needs electrons.
• Water splits to provide these electrons.
• Water splitting produces 2H+, [O], and electrons.
• Oxygen is released as a net product of photosynthesis.
• The reaction: 2𝐻2𝑂→4𝐻++𝑂2+4𝑒−2H2O→4H++O2+4e−
• Water splitting complex is associated with PS II, located on the inner side of the thylakoid membrane.
• Protons (H+) and oxygen (O2) are likely released into the lumen of the thylakoid.

Cyclic and Non-Cyclic Photophosphorylation

Energy Extraction and Storage:

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

Phosphorylation:

• Synthesis of ATP from ADP and inorganic phosphate.
• Happens in mitochondria and chloroplasts.

Photophosphorylation:

• ATP synthesis using light energy.

Non-Cyclic Photophosphorylation:

• Involves both Photosystem II (PS II) and Photosystem I (PS I) working in series.
• Connected through an electron transport chain (Z scheme).
• Produces both ATP and NADPH + H+.

Cyclic Photophosphorylation:

• Involves only PS I.
• Electrons circulate within PS I.
• Occurs in the stroma lamellae, which lack PS II and NADP reductase enzyme.
• Electrons cycle back to PS I instead of passing to NADP+.
• Produces only ATP, not NADPH + H+.
• Happens when only light beyond 680 nm is available.

Chemiosmotic Hypothesis

How is ATP Made in Chloroplasts?

• Chemiosmotic Hypothesis: Explains ATP synthesis.
• Similar to Respiration: Both link ATP synthesis to a proton gradient across a membrane.
• Difference in Photosynthesis: Protons accumulate inside the thylakoid membrane (in the lumen).

What Causes the Proton Gradient?

1. Splitting of Water:
   • Happens on the inner side of the membrane.
   • Produces protons (hydrogen ions) that accumulate in the lumen.
2. Electron Movement:
   • Electrons move through photosystems.
   • Primary electron acceptor (outer side of the membrane) transfers electrons to an H carrier, not an electron carrier.
   • H carrier removes a proton from the stroma and releases it into the lumen.
3. NADP Reductase Enzyme:
   • Located on the stroma side.
   • Uses protons and electrons to reduce NADP+ to NADPH + H+.
   • Protons are taken from the stroma, decreasing their number in the stroma and increasing them in the lumen.

Why is the Proton Gradient Important?

• Breakdown of Gradient: Leads to ATP synthesis.
• ATP Synthase: Enzyme with two parts:
  • CF0: Embedded in the membrane, forms a channel for protons to move back to the stroma.
  • CF1: On the outer surface, faces the stroma, changes shape to make ATP.

Requirements for Chemiosmosis:

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

Process Overview:

• Energy is used to pump protons into the lumen, creating a gradient.
• ATP synthase allows protons to flow back, releasing energy.
• This energy helps ATP synthase make ATP.

Use of ATP and NADPH:

• Both are used immediately in the stroma.
• They help in fixing CO2 and synthesizing sugars.

Where Are the ATP and NADPH Used?

Light Reaction Products:

• ATP and NADPH: Used to make food (sugars).
• O2: Diffuses out of the chloroplast.

Biosynthetic Phase of Photosynthesis:

• Uses ATP, NADPH, CO2, and H2O.
• Doesn’t directly need light but relies on products from the light reaction.
• Continues briefly without light: Stops when ATP and NADPH are used up.
• Restarts with light: Shows the importance of light reaction products.

Is “Dark Reaction” the Right Term?

• Discussion Point: The term “dark reaction” might be misleading since it depends on light reaction products.

How ATP and NADPH Are Used:

• CO2 + H2O → Sugars (CH2O)n
• Melvin Calvin’s Research: Used radioactive 14C to study photosynthesis in algae.
• First Product Discovered: 3-phosphoglyceric acid (PGA), a 3-carbon organic acid.

CO2 Fixation Products:

• C3 Pathway: First product is a 3-carbon acid (PGA).
• C4 Pathway: First product is a 4-carbon acid (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.

These discoveries show the different ways plants convert CO2 into organic compounds during photosynthesis.

The Primary Acceptor of CO2

• Question: What molecule accepts CO2 and forms a 3-carbon compound (PGA)?
• Unexpected Discovery: The acceptor is a 5-carbon sugar called ribulose bisphosphate (RuBP).
• 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 process is cyclic and regenerates RuBP.
• Occurs in All Plants: Whether they use C3 or C4 pathways.

Stages of the Calvin Cycle:

1. Carboxylation:
   • Fixation of CO2: CO2 is fixed into an organic intermediate.
   • Key Reaction: CO2 combines with RuBP, catalyzed by RuBP carboxylase-oxygenase (RuBisCO).
   • Result: Two molecules of 3-PGA are formed.
2. Reduction:
   • Formation of Glucose: Series of reactions that lead to glucose.
   • Energy Use: Uses 2 ATPs for phosphorylation and 2 NADPHs for reduction per CO2 molecule fixed.
   • Cycle Turns: 6 CO2 molecules and 6 turns of the cycle are needed to form one glucose molecule.
3. Regeneration:
   • Regenerating RuBP: Crucial for the cycle to continue.
   • Energy Use: Requires one ATP for phosphorylation to form RuBP.

Energy Requirement:

• Per CO2 Molecule: 3 ATPs and 2 NADPHs.
• To Make One Glucose Molecule: 6 turns of the cycle.
• Calculate Energy Needs: For one glucose molecule, multiply the energy requirements by 6.

Summary of Inputs and Outputs:

• Input: CO2, ATP, NADPH.
• Output: Glucose, ADP, 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 regions.
• First Product: They form oxaloacetic acid (a 4-carbon compound) first.
• Main Pathway: Use the C3 pathway (Calvin cycle) for biosynthesis.

Special Features of C4 Plants:

• Leaf Anatomy: Have a special leaf structure called ‘Kranz’ anatomy.
• Temperature Tolerance: Handle higher temperatures better.
• Light Intensity Response: Thrive under high light.
• No Photorespiration: Lack this energy-wasting process.
• High Productivity: Produce more biomass.

Leaf Anatomy Differences:

• Bundle Sheath Cells: Large cells around vascular bundles.
  • Features:
    • Many chloroplasts
    • Thick walls (no gas exchange)
    • No spaces between cells
• Mesophyll Cells: Different arrangement compared to C3 plants.

Hatch and Slack Pathway (C4 Pathway):

1. CO2 Fixation:
   • Primary Acceptor: Phosphoenol pyruvate (PEP), a 3-carbon molecule in mesophyll cells.
   • Enzyme: PEP carboxylase (PEPcase).
   • Product: C4 acid oxaloacetic acid (OAA).
2. Formation of C4 Compounds:
   • In Mesophyll Cells: OAA turns into malic acid or aspartic acid.
   • Transport to Bundle Sheath Cells: These acids move to bundle sheath cells.
3. CO2 Release:
   • In Bundle Sheath Cells: C4 acids break down, releasing CO2 and a 3-carbon molecule.
   • CO2 Enters Calvin Cycle: Common to all plants.
4. Regeneration:
   • 3-Carbon Molecule: Returns to mesophyll cells and converts back to PEP.
   • Cycle Completes: Ready to fix more CO2.

Key Points to Remember:

• Calvin Cycle: Occurs in all mesophyll cells of C3 plants.
• In C4 Plants: Calvin cycle happens only in bundle sheath cells, not in mesophyll cells. e.g.- maize and sorghum.
• Identification: Bundle sheath cells help identify C4 plants.

Photorespiration

Understanding Photorespiration:

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

Role of RuBisCO:

• Most Abundant Enzyme: RuBisCO can bind to both CO2 and O2.
• Binding Affinity: Prefers CO2 when CO2 and O2 concentrations are nearly equal.
• Competitive Binding: Relative concentrations of O2 and CO2 determine which one binds to RuBisCO.

Photorespiration in C3 Plants:

• O2 Binding: Some O2 binds to RuBisCO, reducing CO2 fixation.
• Alternate Reaction:
  • RuBP + O2: Forms phosphoglycerate and phosphoglycolate (2-carbon).
• Photorespiratory Pathway:
  • No sugar or ATP synthesis.
  • Results in CO2 release and ATP use.

No Photorespiration in C4 Plants:

• Mechanism: C4 acid from mesophyll breaks down in bundle sheath cells, releasing CO2.
• Increased CO2 Concentration: Ensures RuBisCO acts mainly as a carboxylase, reducing oxygenase activity.
• Benefits:
  • Higher productivity and yields.
  • Tolerance to higher temperatures.

Comparison of C3 and C4 Pathways:

Feature	C3 Plants	C4 Plants
First Product	3PGA (3-carbon compound)	OAA (4-carbon compound)
Photorespiration	Occurs	Does not occur
CO2 Fixation Enzyme	RuBisCO	PEP Carboxylase (PEPcase)
Leaf Anatomy	No Kranz anatomy	Kranz anatomy
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

Understanding the factors that affect photosynthesis is important for improving plant yields. These factors can be both internal (within the plant) and external (environmental).

Internal Factors:

• Number, size, age, and orientation of leaves.
• Mesophyll cells and chloroplasts.
• Internal CO2 concentration.
• Amount of chlorophyll.
• Dependent on the plant’s genetics and growth.

External Factors:

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

Law of Limiting Factors (Blackman’s Law):

• The rate of photosynthesis is determined by the factor that is nearest to its minimal value.
• Example: Even with a green leaf and optimal light and CO2, low temperature can limit photosynthesis.

a. Light:

• Quality, Intensity, and Duration: All affect photosynthesis.
• Low Light Intensities: CO2 fixation rates increase linearly.
• High Light Intensities: Rate plateaus 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.
• Natural Limiting Factor: Light is rarely limiting except in shaded areas or dense forests.

b. Carbon Dioxide Concentration:

• Major Limiting Factor: CO2 concentration in the atmosphere is low (0.03 – 0.04%).
• Increasing CO2: Up to 0.05% can increase CO2 fixation rates but higher levels can be damaging over time.
• C3 vs. C4 Plants:
  • Low Light: Neither group responds to high CO2.
  • High Light:
    • C4 Plants: Saturation at about 360 µl/L.
    • C3 Plants: Saturation beyond 450 µl/L. Current CO2 levels are limiting for C3 plants.
• Greenhouse Crops: Higher CO2 concentrations used to increase productivity in crops like tomatoes and bell peppers.

c. Temperature

• Dark reactions: Enzymatic and temperature-controlled.
• Light reactions: Temperature-sensitive but less affected.
• C4 plants: Higher temperature optimum, showing higher photosynthesis rates.
• C3 plants: Lower temperature optimum.
• Habitat adaptation: Tropical plants have a higher temperature optimum compared to temperate climate plants.

d. Water

• Effect on plant: More significant than direct impact on photosynthesis.
• Water stress: Causes stomata to close, reducing CO2 availability.
• Wilted leaves: Reduced surface area and metabolic activity.

These factors are essential for optimizing plant growth and improving agricultural yields.

Chapter Summary:

• Green plants make their own food by photosynthesis.
• 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.
• In chloroplasts, membranes are sites for the light reaction.
• Chemosynthetic pathway occurs 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 excites electrons, transferring them through PS II and PS I.
• Electrons are finally transferred to NAD, forming NADH.
• A proton gradient is created across the thylakoid membrane.
• Breakdown of the proton gradient releases energy for ATP synthesis.
• Splitting of water molecules is associated with PS II, releasing O2, protons, and electrons to PS II.
• In the carbon fixation cycle, CO2 is added to a 5-carbon compound RuBP 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.
• Some tropical plants have a special type of photosynthesis called the C4 pathway.
• In C4 plants, the first product of CO2 fixation in the mesophyll is a 4-carbon compound.
• In the bundle sheath cells, the Calvin pathway synthesizes carbohydrates.