📝 Chapter Notes & Revision

Photosynthesis in Higher Plants

🏫 MP BoardClass 11Biology

📐 Formula & Cheat Sheet (English)

Quick Revision Notes: Class 11 Biology

Chapter: Photosynthesis in Higher Plants (प्रकाशसंश्लेषण)

### 1. Introduction & Early Experiments

  • Photosynthesis (प्रकाशसंश्लेषण): A physico-chemical process by which plants use light energy to synthesize organic compounds (glucose) from carbon dioxide and water. It is the primary source of all life on earth.
  • Important Scientists & Experiments:
    • Joseph Priestley (1770): Showed that plants restore to the air whatever breathing animals and burning candles remove. Discovered Oxygen in 1774.
    • Jan Ingenhousz (1779): Showed that sunlight is essential to the plant for purifying foul air (releasing $O_2$).
    • Julius von Sachs (1854): Provided evidence that green parts in plants produce glucose, which is usually stored as starch.
    • T.W. Engelmann (1888): Using a prism and green alga (Cladophora), described the first action spectrum of photosynthesis.
    • Cornelius van Niel (1931): Demonstrated that photosynthesis is a light-dependent reaction where hydrogen from an oxidizable compound reduces carbon dioxide to carbohydrate.
      • General Equation for Purple/Green Sulphur bacteria: $2H_2A + CO_2 \xrightarrow{Light} 2A + CH_2O + H_2O$
      • Equation for Green Plants: $6CO_2 + 12H_2O \xrightarrow{Light} C_6H_{12}O_6 + 6H_2O + 6O_2$

### 2. Site of Photosynthesis & Pigments

  • Site: Photosynthesis takes place in the Chloroplast (हरितलवक), primarily in the mesophyll cells of leaves.
  • Pigments involved (Chromatography separation):
    • Chlorophyll a: Bright or blue-green (Chief pigment)
    • Chlorophyll b: Yellow-green
    • Xanthophylls: Yellow
    • Carotenoids: Yellow to yellow-orange
  • Action Spectrum vs. Absorption Spectrum:
    • Action Spectrum: Rate of photosynthesis plotted against different wavelengths of light (matches chlorophyll a & b absorption).
    • Absorption Spectrum: Graph showing the fraction of light absorbed at each wavelength by a pigment.

### 3. Light Reaction (Photochemical Phase)

  • Location: Thylakoid membranes (Grana).
  • Components: Involves two photosystems—Photosystem I (PS I) and Photosystem II (PS II).
    • Reaction Centre:
      • PS I reaction centre is P700 (absorbs 700 nm wavelength of light).
      • PS II reaction centre is P680 (absorbs 680 nm wavelength of light).

Electron Transport System (Z-Scheme):

  1. Excitation: PS II absorbs light, electrons get excited and accepted by an electron acceptor.
  2. Photolysis of Water (जल का प्रकाशीय अपघटन): Water splits to supply electrons to PS II and releases $O_2$ and protons ($H^+$).
    • $2H_2O \rightarrow 4H^+ + O_2 + 4e^-$
  3. Electron Flow: Electrons pass through an electron transport chain (Plastoquinone $\rightarrow$ Cytochrome $b_6f$ $\rightarrow$ Plastocyanin) to PS I.
  4. Reduction: Electrons from PS I are accepted by ferredoxin and finally reduce $NADP^+$ to $NADPH + H^+$.

Photophosphorylation (फास्फोरिलीकरण):

Synthesis of ATP from ADP and inorganic phosphate using light energy.

  • Non-Cyclic Photophosphorylation: Involves both PS II and PS I. Produces both ATP and $NADPH + H^+$.
  • Cyclic Photophosphorylation: Involves only PS I. Electrons circulate within PS I; produces only ATP (no $NADPH$, no $O_2$ evolution). Occurs when only longer wavelengths of light (>680 nm) are available.

Chemiosmotic Hypothesis (कीमीऑस्मोटिक हाइपोथesis):

Explains the mechanism of ATP synthesis in chloroplasts.

  • Key Steps:
    1. Proton accumulation inside the thylakoid lumen (due to splitting of water on the inner side of the membrane).
    2. Movement of electrons through the ETS pumps protons ($H^+$) across the membrane into the lumen.
    3. $NADP^+$ reductase enzyme removes $H^+$ from the stroma, creating a proton gradient.
    4. Protons flow back into the stroma across the membrane through the $F_0$ channel of the ATP synthase enzyme.
    5. This gradient breakdown releases energy to catalyze the formation of ATP: $ADP + Pi \xrightarrow{ATP\ synthase} ATP$

### 4. Dark Reaction (Biosynthetic Phase)

  • Location: Stroma of the chloroplast.
  • Process: Does not directly depend on light, but depends on the products of the light reaction (ATP and $NADPH$).

$C_3$ Pathway (Calvin Cycle / केल्विन चक्र):

Operates in all plants. Occurs in 3 major phases:

  1. Carboxylation (कार्बोक्सीकरण): Fixation of $CO_2$ into stable 3-carbon organic acid.
    • $CO_2 + RuBP \xrightarrow{RuBisCO} 2 \times (3\text{-PGA})$
    • Enzyme: RuBisCO (Ribulose Bisphosphate Carboxylase-Oxygenase).
  2. Reduction (अपचयन): Utilization of 2 ATP and 2 $NADPH$ for reduction per $CO_2$ fixed to form Triose phosphate.
  3. Regeneration (पुनरुद्भवन): Regeneration of $RuBP$ to keep the cycle running (requires 1 ATP).
  • Net requirement to fix 1 molecule of glucose ($C_6H_{12}O_6$):
    • 6 Turns of Calvin Cycle
    • 18 ATP + 12 $NADPH$

$C_4$ Pathway (Hatch and Slack Pathway):

  • Adaptation: Found in plants adapted to dry tropical regions (e.g., Maize, Sugarcane).
  • Anatomy: Exhibits Kranz Anatomy (bundle-sheath cells surrounding vascular bundles, multiple chloroplasts, thick walls impervious to gas exchange).
  • Primary $CO_2$ Acceptor: Phosphoenolpyruvate (PEP - 3C) in mesophyll cells.
  • First Stable Product: Oxaloacetic Acid (OAA - 4C).
  • Mechanism: $CO_2$ is fixed in mesophyll cells by PEPcase to form OAA $\rightarrow$ converted to malic acid $\rightarrow$ transported to bundle sheath cells $\rightarrow$ decarosylated to release $CO_2$ into the Calvin cycle.

### 5. Photorespiration (प्रकाश-श्वसन)

  • Definition: A wasteful process occurring in $C_3$ plants under high light, high temperature, and low $CO_2$ conditions.
  • Role of RuBisCO: RuBisCO has an affinity for both $CO_2$ and $O_2$. When $O_2$ binds to RuBisCO, it acts as an oxygenase:
    • $RuBP + O_2 \rightarrow 3\text{-PGA} + \text{Phosphoglycolate (2C)}$
  • Characteristics:
    • No ATP or $NADPH$ is synthesized.
    • CO2 is actually released, consuming energy.
    • Involves three organelles: Chloroplast, Peroxisome, and Mitochondria.
    • Absent in $C_4$ plants because they have a mechanism to increase $CO_2$ concentration at the enzyme site.

### 6. Factors Affecting Photosynthesis (Blackman's Law of Limiting Factors)

  • Blackman’s Law of Limiting Factors (1905): "If a chemical process is affected by more than one factor, its rate will be determined by the factor which is nearest to its minimal value: it is the factor which directly affects the process if its quantity is changed."

  • Key Factors:

    1. Light: Linear relationship at low light intensities; higher light saturates the rate (except at very high light where solarization/photo-oxidation of pigments occurs).
    2. Carbon Dioxide ($CO_2$): Concentration is very low in the atmosphere (0.03 - 0.04%). Increase up to 0.05% can cause an increase in $CO_2$ fixation rates ($C_3$ responds to higher $CO_2$, leading to higher yields—known as $CO_2$ fertilization effect).
    3. Temperature: Dark reactions are temperature-controlled (enzymatic). $C_4$ plants have a higher temperature optimum than $C_3$ plants.
    4. Water: Water stress causes stomatal closure, reducing $CO_2$ availability, and also wilting of leaves, reducing surface area.