Photosynthesis in Higher Plants

• Introduction & Early Experiments

  • Definition: Physio-chemical process using light energy to synthesize organic compounds.
  • Priestley's Experiment: Discovered oxygen role using bell jar, mint plant, and candle.
  • Jan Ingenhousz: Sunlight is essential; only green parts release oxygen.
  • Julius von Sachs: Glucose is produced and stored as starch.
  • T.W. Engelmann: Described first action spectrum using Cladophora and aerobic bacteria.
  • Cornelius van Niel: Proved water is the hydrogen donor releasing $O_2$, not $CO_2$.
  • Equation: $6CO_2 + 12H_2O \xrightarrow{Light} C_6H_{12}O_6 + 6H_2O + 6O_2$

• Site of Photosynthesis

  • Occurs in Chloroplasts (primarily in mesophyll cells).
  • Grana: Site of Light reaction (photochemical phase).
  • Stroma: Site of Dark reaction (biosynthetic phase).

• Pigments Involved

  • Chlorophyll a: Bright/blue-green, primary photosynthetic pigment.
  • Chlorophyll b: Yellow-green, accessory pigment.
  • Carotenoids: Yellow to yellow-orange, accessory pigments and photoprotection.
  • Absorption Spectrum: Graph of light absorbed at specific wavelengths.
  • Action Spectrum: Graph of rate of photosynthesis at different wavelengths (matches Chl a).

• Light Reaction (Photochemical Phase)

  • Driven by Light Harvesting Complexes (LHC) in Photosystems.
  • Photosystem I (PS I): Reaction center chlorophyll $a$ absorbs 700 nm ($P_{700}$).
  • Photosystem II (PS II): Reaction center chlorophyll $a$ absorbs 680 nm ($P_{680}$).
  • Electron Transport System (ETS):
    • Non-Cyclic Photophosphorylation: Involves both PS II and PS I; produces ATP, $NADPH + H^+$, and $O_2$ (via Photolysis of water).
    • Cyclic Photophosphorylation: Involves only PS I; occurs when only light of wavelengths > 680 nm is available; synthesizes only ATP.
  • Chemiosmotic Hypothesis: Explains ATP synthesis; driven by proton gradient across thylakoid membrane, ATPase enzyme, and proton accumulation in lumen.

• Dark Reaction (Biosynthetic Phase)

  • Does not directly require light, depends on products of light reaction (ATP & NADPH).
  • Occurs in Stroma.
  • $C_3$ Pathway (Calvin Cycle):
    • Carboxylation: Fixation of $CO_2$ by RuBisCO onto RuBP to form 3-PGA.
    • Reduction: Utilization of ATP and NADPH to form Triose phosphate.
    • Regeneration: Regeneration of RuBP consuming ATP.
  • $C_4$ Pathway (Hatch-Slack Pathway):
    • Adapted for plants in dry tropical regions.
    • Exhibits Kranz anatomy in leaves (bundle sheath cells surround vascular bundles).
    • Primary $CO_2$ acceptor is PEP in mesophyll cells, forming OAA.
    • Fixation enzyme: PEPcase.
    • Avoids Photorespiration, making it more efficient at high temperatures.

• Photorespiration

  • Wasteful process occurring in $C_3$ plants.
  • Caused by dual oxygenase activity of RuBisCO ($O_2$ binds instead of $CO_2$).
  • Results in release of $CO_2$ with no ATP or NADPH synthesis.
  • Involves three organelles: Chloroplast, Peroxisome, and Mitochondria.
  • Absent in $C_4$ plants.

• Factors Affecting Photosynthesis

  • Blackman's Law of Limiting Factors: Rate is determined by the factor that is nearest to its minimal value.
  • Light: Linear relationship at low light; saturation at high intensities; photo-oxidation at very high light.
  • Carbon Dioxide Concentration: Major limiting factor; $C_4$ plants saturate at lower concentrations (~360 $\mu$l $L^{-1}$) compared to $C_3$ plants (~450 $\mu$l $L^{-1}$).
  • Temperature: $C_4$ plants have a higher temperature optimum than $C_3$ plants.
  • Water: Water stress leads to stomatal closure, reducing $CO_2$ availability and plant wilting.