Photosynthesis in Higher Plants

Photosynthesis in Higher Plants

• Photosynthesis is a process used by plants to produce food using light energy.
• Photosynthesis derives from the Greek words "phos" (light) and "synthesis" (putting together).
• Photosynthesis occurs in chloroplasts, which contain chlorophyll.
• Chlorophyll absorbs light energy, mainly in the blue and red regions of the visible spectrum.
• Photosynthesis is a physico-chemical process, involving both physical and chemical changes.
• Photosynthesis is an endergonic reaction that requires light energy input.
• The overall equation for photosynthesis is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
• Plants are autotrophs, producing their own food.
• Photosynthesis produces glucose (C₆H₁₂O₆) and oxygen (O₂).
• Glucose powers the plant, and oxygen is essential for animal respiration.

Important Experiments

• Variegated Leaf Experiment: A leaf partially covered prevents starch production in the covered sections, demonstrating light's role.
• Molisch Half Leaf Experiment: One leaf half, deprived of CO₂, cannot perform photosynthesis, highlighting CO₂'s necessity.
• Priestley’s Experiment: A mint plant restored breathable air, suggesting photosynthesis releases oxygen.
• Ingenhousz’s Experiment: Light-exposed aquatic plant released oxygen, proving light's role in photosynthesis.
• Sachs’ Experiment: Glucose from photosynthesis is stored as starch.
• Engelmann’s Experiment: Aerobic bacteria clustered around blue and red light, signifying these wavelengths' highest photosynthetic contribution.

Absorption Spectra and Action Spectra

• Absorption Spectra: A graph showing light absorption by a pigment at varying wavelengths.
• Action Spectra: A graph displaying photosynthesis rates at different light wavelengths.
• Chlorophyll a absorbs strongly in the blue and red regions, correlating with photosynthesis's action spectrum.

Photosynthetic Pigments

• Chlorophyll a is the primary pigment in photosynthesis, absorbing light energy.
• Accessory pigments (chlorophyll b, carotenoids, xanthophylls) absorb other wavelengths, assisting chlorophyll a and protecting it from excessive light.
• Photo-oxidative damage occurs when pigments are overexposed to light, decreasing efficiency. Accessory pigments prevent this by transferring excess light energy more gradually.

Cornelius van Niel's Experiment

• Van Niel studied purple and green sulfur bacteria, using hydrogen sulfide (H₂S) as a reducing agent instead of water.
• He proposed a general equation illustrating water as the oxygen source in plant photosynthesis.

Site and Steps of Photosynthesis

• Photosynthesis occurs within the chloroplasts of plant cells.
• Chloroplasts contain chlorophyll, capturing light energy.
• Thylakoid membranes house the pigments.
• Photosynthesis has two stages: Light-dependent reactions in thylakoids and light-independent reactions (Calvin cycle) in the stroma.

Light Reactions

• Absorption of light energy by photosystems (PSII and PSI) drives electron transfer.
• Water splitting releases oxygen, protons, and electrons, replenishing PSII.
• Electron transport generates ATP from the released energy.
• Electrons reduce NADP+ to NADPH.
• Non-cyclic photophosphorylation involves both photosystems, yielding ATP, NADPH, and oxygen.

Dark Reactions (Calvin Cycle)

• ATP and NADPH fuel CO₂ reduction to glucose.
• The cycle fixes carbon, reduces it, and regenerates the initial substrate RuBP.

Paper Chromatography Technique

• Separates plant leaf pigments based on their different solubilities in a solvent.

Photosystems

• Photosystem II (PSII): Reaction center (P680) absorbs light at 680 nm.
• Photosystem I (PSI): Reaction center (P700) absorbs light at 700 nm.
• PSII initiates the light reactions, and PSI receives electrons.

Pigment Systems

• Pigment systems comprise proteins and pigments (chlorophyll, carotenoids, xanthophylls) within photosystems.
• Antenna molecules absorb light and transfer energy to the reaction center.
• The reaction center excites electrons.
• The electron transport chain transfers high-energy electrons, generating ATP and NADPH.

Summary of Photosynthesis

• Photosynthesis uses sunlight to convert CO₂ and water into glucose and oxygen.
• Chloroplasts are the site of the process.
• Photosynthesis is vital for Earth's life and atmosphere.

Z Scheme

• Redox potential guides the electron transport chain in noncyclic photophosphorylation.
• PSII absorbs light to excite electrons, releasing higher-energy electrons.
• PSII's electron loss is replenished by water splitting, releasing oxygen, protons, and electrons.
• Protons from water splitting create a proton gradient, crucial for ATP production.

Cyclic Photophosphorylation

• Cyclic photophosphorylation uses only photosystem I, creating a closed electron loop, resulting in ATP only.
• It occurs in stroma lamellae and does not require water splitting.

Photosynthesis Structures

• Grana: Stacks of thylakoids.
• Thylakoids: Flattened sacs within chloroplasts.
• Stroma lamellae: Interconnecting tubules connecting grana.
• Stroma: Fluid-filled space surrounding the thylakoids.

Electron Transport Chain

• Thylakoid membranes hold electron transport proteins.
• Plastoquinone, cytochrome b6f complex, and plastocyanin are electron carriers.
• Plastoquinone accepts electrons from PSII.
• Cytochrome b6f complex is part of the electron transport chain.
• Plastocyanin carries electrons to PSI.
• Ferredoxin and ferredoxin-NADP reductase are further electron carriers.

Proton Gradient

• Water-splitting releases protons.
• Protons accumulate inside the thylakoid lumen.
• This builds a proton gradient across the thylakoid membrane.

ATP Synthesis

• CF0-CF1 is a chloroplast ATP synthase similar to F0-F1.
• CF0 allows protons to pass, driving ATP synthesis by chemiosmosis, needing electron transport.

Dark Reactions (Calvin Cycle)

• Light reactions provide the ATP and NADPH.
• The Calvin cycle (C3 pathway) converts CO₂ into glucose.
• The cycle uses RuBisCO as a key enzyme.

The Calvin Cycle Steps

• Carboxylation: RuBP combines with CO₂ to form a 6-carbon compound, splitting into two 3-PGA molecules.
• Reduction: PGA receives a phosphate from ATP and is reduced by NADPH to G3P.
• Regeneration: G3P is used to regenerate RuBP.

C4 Plants

• C4 plants use both C4 and C3 pathways, exhibiting specialized anatomy.
• Mesophyll cells and bundle sheath cells contain chloroplasts.
• PEP carboxylase in mesophyll cells fixes CO₂ initially, forming a four-carbon compound (OAA).
• OAA is converted to malate or aspartate, and transported to bundle sheath cells.
• CO₂ is released in bundle sheath cells, entering the Calvin cycle.
• Higher optimal temperatures, less water loss via stomata.
• Examples include sugarcane, corn, and sorghum.

C4 Pathway

• PEP carboxylase catalyzes the initial CO₂ fixation.
• PEP (a 3-carbon compound) and CO₂ form OAA (a 4-carbon compound).
• OAA transforms to malate or aspartate, moving to bundle sheath cells where CO₂ is released to enter the Calvin cycle.
• C4 plants use PEP carboxylase, with higher CO₂ concentrations in bundle sheath cells.

C3 vs. C4

• C4 plants require more ATP to fix CO₂ per molecule compared to C3 plants.
• C3: 3 ATP + 2 NADPH per CO₂ fixed.
• C4: 5 ATP + 2 NADPH per CO₂ fixed.

Photorespiration

• Photorespiration occurs in C3 plants, where RuBisCO binds oxygen when CO₂ is scarce.
• A wasteful process that releases CO₂ without producing sugars.
• Involves three organelles: chloroplasts, peroxisomes, and mitochondria.

Photorespiration (C2 Pathway)

• Involves oxygen instead of CO₂, producing glycolate, later processed in peroxisomes and mitochondria.

Factors Affecting the Rate of Photosynthesis

• Internal Factors:
  • Genetic traits (leaf number, growth potential)
  • CO₂ concentration (C4 plants store more)
  • Leaf structure and age.
  • Leaf orientation.
• External Factors:
  • CO₂ Levels: Crucial for photosynthesis, and elevated levels benefit many plants.
  • Water: Essential reactant; deficiency causes stomata closure, decreasing photosynthesis.
  • Temperature: Influences enzyme activity; C4 plants thrive at higher temperatures.
  • Light quality, duration, and intensity: Affect light-dependent reactions; light intensity eventually saturates photosynthesis.

Blackmann's Law of Limiting Factors:

• Photosynthesis's rate hinges on the least available factor. Increasing other factors won't significantly impact the rate if one is limiting.

C4 Photosynthesis

• C4 plants store CO₂ in four-carbon compounds.
• This allows them greater tolerance of high temperatures and reduced water loss.

Rubisco

• Rubisco, a key enzyme, binds both CO₂ and O_2, with a higher preference for CO₂.

Photorespiration (C2 Pathway)

• Occurs when the concentration of oxygen is higher than carbon dioxide.
• A wasteful process where oxygen binds to RuBisCO instead of CO₂.

This quiz explores the process of photosynthesis in higher plants, detailing its definition, components, and chemical equations. Understand how light energy is utilized to produce food in plants, and the significance of chlorophyll in this essential process. Test your knowledge on the autotrophic nature of plants and the byproducts of photosynthesis.