☀️ Photosynthesis in Higher Plants

Photosynthesis is an enzyme-mediated, endergonic oxidation-reduction process in which light energy is harvested by chloroplast thylakoids to reduce atmospheric $C{O}_2$ into carbohydrates while evolving oxygen ($O_2$) via water photolysis.

$$\mathbf{6}{\mathbf{CO}}_{\mathbf{2}}\mathbf{+}\mathbf{12}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\underset{\text{Chloroplast}}{\overset{\text{Light Energy}}{\to }}{\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{12}}{\mathbf{O}}_{\mathbf{6}}\mathbf{+}\mathbf{6}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{+}\mathbf{6}{\mathbf{O}}_{\mathbf{2}}\uparrow$$

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1. 🌈 Photosynthetic Pigments & Action Spectrum

1. Chlorophyll $a$ (Bright/Blue-green): Chief reaction center pigment ($P_{680}$ in PS II, $P_{700}$ in PS I).
2. Chlorophyll $b$ (Yellow-green): Accessory antenna pigment; broadens light absorption wavelengths.
3. Carotenoids (Yellow to Yellow-orange): Accessory pigments; protect chlorophyll $a$ against photo-oxidation (solarization).

  Light Absorption
      ▲
      │    /──\        /──\  <-- Absorption Spectrum of Chl a (Blue & Red Peaks)
      │   /    \      /    \
      │  /      \────/      \
      │ /                    \  <-- Matches Action Spectrum of Photosynthesis (Engelmann's Experiment)
      └────────────────────────► Wavelength (nm: 400nm -> 700nm PAR)

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2. ⚡ The Light Reactions (Photochemical Phase) & The Z-Scheme

                        THE Z-SCHEME PHOTOPHOSPHORYLATION
                                   [Electron Acceptor]
                                          ▲
                                          │
                                       [PS II] (P680)
                                          ▲   \
                                          │    \──► Plastoquinone (PQ) ──► Cyt b6f ──► Plastocyanin (PC)
                  H2O ──► 2H+ + 2e- + 1/2 O2                                                │
                  (Oxygen Evolving Complex: Mn2+, Cl-, Ca2+)                               ▼
                                                                                   [Electron Acceptor]
                                                                                          ▲
                                                                                          │
                                                                                       [PS I] (P700)
                                                                                          │
                                                                                          ▼
                                                                                NADP+ ──► NADPH + H+

Parameter	Non-Cyclic Photophosphorylation	Cyclic Photophosphorylation
Photosystems Involved	Both PS II ($P_{680}$) and PS I ($P_{700}$)	PS I ($P_{700}$) exclusively
Electron Path	Non-cyclic unidirectional flow	Cyclic closed loop back to PS I via Cyt $b_{6}f$
Photolysis of Water ($H_{2}O$)	Present ($O_2$ is evolved)	Absent (No $O_2$ evolution)
Products Formed	ATP + $\text{NADPH}+H^{+}$	ATP only (No NADPH)
Operating Conditions	Optimal light ($\lambda <680\text{nm}$) and normal $C{O}_2$	Wavelengths $>680\text{nm}$ (Red drop) or Stroma lamellae lacking PS II

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3. 🔄 The Calvin Cycle ($C_3$ Pathway)

The universal carbon fixation pathway occurring in the chloroplast stroma:

                       THE THREE MASTER STAGES OF CALVIN CYCLE
                                          CO2 + H2O
                                             │
                                             ▼ [1. CARBOXYLATION: Rubisco Enzyme]
                       RuBP (5C) ◄──────────────────── 3-PGA (3C, First Stable Product)
                           ▲                                │
                           │                                ▼ [2. REDUCTION: 2 ATP + 2 NADPH per CO2]
                    [3. REGENERATION] ─────────────── Triose Phosphate (G3P)
                     (Requires 1 ATP)                       │
                                                            ▼
                                                     Glucose (1/6 molecule)

$$\mathbf{6}{\mathbf{CO}}_{\mathbf{2}}\mathbf{+}\mathbf{18}\mathbf{ATP}\mathbf{+}\mathbf{12}\mathbf{NADPH}\mathbf{+}\mathbf{12}{\mathbf{H}}^{\mathbf{+}}⟶\mathbf{1}\text{ Glucose }\mathbf{(}{\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{12}}{\mathbf{O}}_{\mathbf{6}}\mathbf{)}\mathbf{+}\mathbf{18}\mathbf{ADP}\mathbf{+}\mathbf{18}{\mathbf{P}}_{\mathbf{i}}\mathbf{+}\mathbf{12}{\mathbf{NADP}}^{\mathbf{+}}$$

Energetics Per Molecule of Glucose

For synthesis of 1 Glucose ($6C$):

• $C_3$ Pathway: Requires $18\text{ ATP}$ and $12\text{ NADPH}$.
• $C_4$ Pathway: Requires $30\text{ ATP}$ and $12\text{ NADPH}$ ($2\text{ extra ATP}$ per $C{O}_2$ fixed during PEP regeneration).

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4. 🌾 The Hatch-Slack Pathway ($C_4$ Photosynthesis)

Adapted to hot, arid climates (e.g., Maize, Sugarcane, Sorghum) to eliminate photorespiratory loss:

• Kranz Anatomy: Mesophyll cells surround large, thick-walled bundle sheath cells with agranal chloroplasts and no intercellular spaces.
• First Stable Product: Oxaloacetic Acid (OAA, $4C$) formed via PEP Carboxylase in mesophyll cells (PEPcase has high affinity for $C{O}_2$ and zero affinity for $O_2$).

  [MESOPHYLL CELL]                               [BUNDLE SHEATH CELL]
  Atmospheric CO2 + PEP (3C)                      Malic Acid (4C)
            │                                             │
            ▼ [PEP Carboxylase]                           ▼ [Decarboxylation: Releases CO2]
       OAA (4C) ──► Malate (4C) ──────────────► CO2 ──► Calvin Cycle (Rubisco) ──► Glucose
                                                          │
  Pyruvate (3C) ◄─────────────────────────────── Pyruvate (3C)
        │
        ▼ [Regeneration: Requires 2 ATP]
     PEP (3C)

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5. ⚠️ Photorespiration ($C_2$ Cycle / Glycolate Pathway)

When $[O_2]$ is high and $[C{O}_2]$ is low, Rubisco binds $O_2$ via its oxygenase activity:

$$\text{RuBP }\mathbf{(}\mathbf{5}\mathbf{C}\mathbf{)}\mathbf{+}{\mathbf{O}}_{\mathbf{2}}\stackrel{\text{Rubisco}}{\to }\text{3-PGA }\mathbf{(}\mathbf{3}\mathbf{C}\mathbf{)}\mathbf{+}\text{2-Phosphoglycolate }\mathbf{(}\mathbf{2}\mathbf{C}\mathbf{)}$$

• Occurs sequentially across 3 organelles: Chloroplast $⟶$ Peroxisome $⟶$ Mitochondria.
• Wasteful Process: Consumes ATP and $O_2$ while releasing $C{O}_2$ without producing ATP, NADPH, or sugars.
• $C_4$ plants have a $C{O}_2$-concentrating pump around Rubisco, hence photorespiration is completely absent in $C_4$ plants.