4.1 PHOTOSYNTHESIS

Photosynthesis is the fundamental process by which plants, algae, and some bacteria convert light energy into chemical energy, stored in the form of glucose. It is a critical process for life on Earth, producing organic compounds and releasing oxygen into the atmosphere.

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Definition of Photosynthesis

• Chemical Definition: A redox process where carbon dioxide (${\text{CO}}_2$), an oxidized form of carbon, is reduced into glucose (${\text{C}}_6{\text{H}}_{12}{\text{O}}_6$), a reduced form of carbon. Water (${\text{H}}_2\text{O}$) acts as the reducing agent and is oxidized into oxygen (${\text{O}}_2$).
• Bio-energetic Definition: An energy conversion process where light energy is used to transform energy-poor inorganic molecules (${\text{CO}}_2$, ${\text{H}}_2\text{O}$) into an energy-rich organic molecule (glucose).

Overall Chemical Equation:

$6{\text{CO}}_2+12{\text{H}}_2\text{O}+\text{Light Energy}\stackrel{\text{Chlorophyll}}{\to }{\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{O}}_2+6{\text{H}}_2\text{O}$

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Raw Materials for Photosynthesis

1. Role of Light

• Sunlight is the ultimate source of energy for most ecosystems. It is a form of electromagnetic energy.
• Photosynthesis converts light energy into chemical energy.
• The electromagnetic spectrum represents the full range of electromagnetic radiation.
• Visible light (wavelengths from 380 nm to 750 nm) is the portion of the spectrum effective for photosynthesis.
• The effectiveness of different wavelengths is shown by an action spectrum, which plots the rate of photosynthesis against the wavelength of light. Photosynthesis rates are highest in blue-violet and red-orange light.
• Compensation Point: The light intensity at which the rate of photosynthesis exactly equals the rate of respiration, resulting in no net gas exchange with the environment.

2. Role of Carbon Dioxide (${\text{CO}}_2$)

• ${\text{CO}}_2$ is the carbon source for synthesizing organic compounds (glucose).
• Plants are autotrophs because they use inorganic ${\text{CO}}_2$ to create their own food.
• It is utilized during the light-independent reactions (Calvin cycle).
• Atmospheric Concentration: Air contains about 0.03% to 0.04% ${\text{CO}}_2$.
• Effect of Concentration: Increasing ${\text{CO}}_2$ concentration generally increases the rate of photosynthesis, but levels above 1% cause stomata to close, slowing the rate.
• Aquatic Plants: Use dissolved ${\text{CO}}_2$, bicarbonates, and carbonates from the water.

3. Role of Water (${\text{H}}_2\text{O}$)

• Water acts as a hydrogen and electron donor.

• During the light-dependent phase, water undergoes photolysis (splitting by light):

  • ${\text{H}}_2\text{O}\to 2{\text{H}}^{+}+2{\text{e}}^{-}+\frac{1}{2}{\text{O}}_2$
  • The electrons (${\text{e}}^{-}$) replace those lost by chlorophyll in Photosystem II.
  • The protons (${\text{H}}^{+}$) contribute to the proton gradient for ATP synthesis and are used to reduce ${\text{NADP}}^{+}$ to $\text{NADPH}$.
  • The oxygen ($\frac{1}{2}{\text{O}}_2$) is released as a byproduct.

• Van Niel's Hypothesis (1931): Proposed that plants split water, not ${\text{CO}}_2$, to release oxygen. This was confirmed in 1940 using an isotope of oxygen (${}^{18}\text{O}$).

  • Experiment 1: $6{\text{CO}}_2+12{\text{H}}_2{}^{18}\mathbf{O}\stackrel{\text{Light}}{\to }{\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{H}}_2\text{O}+6{{}^{18}\mathbf{O}}_2$
    • When water was labeled with ${}^{18}\text{O}$, the oxygen gas produced was radioactive — proving oxygen comes from water.
  • Experiment 2: $6\text{C}{{}^{18}\mathbf{O}}_2+12{\text{H}}_2\text{O}\stackrel{\text{Light}}{\to }{\text{C}}_6{\text{H}}_{12}{{}^{18}\mathbf{O}}_6+6{\text{H}}_2{}^{18}\mathbf{O}+6{\text{O}}_2$
    • When ${\text{CO}}_2$ was labeled, the oxygen gas produced was normal — confirming water is the source of evolved ${\text{O}}_2$.

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Photosynthetic Pigments

Pigments are substances that absorb light energy. In plants, they are located in the thylakoid membranes of chloroplasts. For more on chloroplast structure, see .

Chlorophylls

• Absorb mainly violet-blue and orange-red light; they reflect green light, which is why plants appear green.
• Structure: Composed of a hydrophilic porphyrin ring head (with a central Magnesium atom) and a hydrophobic phytol tail that anchors it in the thylakoid membrane.

• Chlorosis: Deficiency of chlorophyll, causing leaves to turn yellow.
• Absorption Spectra Comparison:
  • Chlorophyll a absorbs maximally at ~430 nm (blue-violet) and ~662 nm (red).
  • Chlorophyll b absorbs maximally at ~453 nm (blue) and ~642 nm (red) — shifted slightly compared to chlorophyll a.
  • Together, they broaden the range of wavelengths absorbed for photosynthesis.

Carotenoids

• Accessory pigments that are yellow, orange, or red. They absorb light in the blue-violet range (430–500 nm).
• Functions:
  1. Absorb light energy and transfer it to chlorophyll a.
  2. Protect chlorophyll a from excess light (photo-oxidation).
  3. Attract animals for pollination and seed dispersal.
• Types:
  • Carotenes: Orange-red pigments (e.g., beta-carotene).
  • Xanthophylls: Yellow pigments (e.g., lutein), responsible for yellow autumn leaf colors.

Absorption Spectrum

• A graph showing the relative absorption of different wavelengths of light by a particular pigment.
• Measured using a spectrophotometer.
• Chlorophylls a and b show strong absorption in the violet-blue (400–470 nm) and orange-red (630–660 nm) regions.
• Carotenoids absorb strongly in the 430–500 nm range.

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Photosystems

Photosynthetic pigments are organized into clusters called photosystems within the thylakoid membrane to efficiently capture light energy.

• Structure of a Photosystem:

  • Antenna Complex: Outer part containing accessory pigments (chlorophyll b, carotenoids). It absorbs photons and funnels the energy to the reaction center via resonance energy transfer.
  • Reaction Center: Central part containing a special pair of chlorophyll a molecules and associated proteins. This is where light energy is converted to chemical energy by releasing high-energy electrons.

• Photosystem I (PS I):

  • Reaction center: P700 (absorbs light maximally at 700 nm).
  • Located in the thylakoid membrane (unstacked regions).
  • Receives electrons from PS II via the electron transport chain.
  • Produces NADPH — electrons are used to reduce ${\text{NADP}}^{+}$.

• Photosystem II (PS II):

  • Reaction center: P680 (absorbs light maximally at 680 nm).
  • Located in the stacked grana thylakoids.
  • Splits water (photolysis) to obtain electrons: $2{\text{H}}_2\text{O}\to 4{\text{H}}^{+}+4{\text{e}}^{-}+{\text{O}}_2$
  • Produces ATP via the proton gradient.

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Light-Dependent Reactions

These reactions occur in the thylakoid membranes and require direct light energy. They produce ATP, NADPH, and ${\text{O}}_2$.

Non-Cyclic Photophosphorylation (Z-Scheme)

This is the main pathway, involving both PS II and PS I:

1. PS II absorbs light → P680 is excited → releases high-energy electrons.
2. Photolysis of water → electrons replace those lost by P680; ${\text{O}}_2$ is released.
3. Electrons pass through the Electron Transport Chain (ETC): Plastoquinone (PQ) → Cytochrome b6f complex → Plastocyanin (PC).
   • As electrons move through the ETC, protons (${\text{H}}^{+}$) are pumped into the thylakoid lumen, creating a proton gradient.
   • This gradient drives ATP synthase to produce ATP (photophosphorylation).
4. PS I absorbs light → P700 is excited → releases high-energy electrons.
5. Electrons from PS I reduce ${\text{NADP}}^{+}$ to NADPH (via ferredoxin and NADP reductase).

Net products: ATP, NADPH, and ${\text{O}}_2$.

Cyclic Photophosphorylation

This pathway involves only PS I and produces only ATP (no NADPH, no ${\text{O}}_2$):

1. PS I absorbs light → P700 releases excited electrons.
2. Electrons pass through ferredoxin → Cytochrome b6f → Plastocyanin → back to P700.
3. The cyclic flow of electrons pumps ${\text{H}}^{+}$ ions, driving ATP synthesis.
4. No water is split; no NADPH is produced.

Function: Produces extra ATP when the cell needs more ATP than NADPH.

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Light-Independent Reactions (Calvin Cycle)

These reactions occur in the stroma of the chloroplast and do not directly require light, but depend on ATP and NADPH from the light-dependent reactions.

Three Stages:

Stage 1 — Carbon Fixation:

• The enzyme Rubisco (RuBP carboxylase/oxygenase) fixes ${\text{CO}}_2$ by attaching it to the 5-carbon RuBP (ribulose-1,5-bisphosphate).
• An unstable 6-carbon intermediate forms and immediately splits into two molecules of 3-PGA (3-phosphoglycerate).

Stage 2 — Reduction:

• 3-PGA is phosphorylated by ATP and reduced by NADPH to form G3P (glyceraldehyde-3-phosphate).
• G3P is the first carbohydrate produced — it is used to synthesize glucose and other organic molecules.

Stage 3 — Regeneration of RuBP:

• Most G3P molecules are used to regenerate RuBP using ATP, allowing the cycle to continue.
• For every 3 turns of the cycle (fixing 3 ${\text{CO}}_2$), one net G3P is produced.

Summary per glucose molecule:

• 6 turns of the cycle
• Input: $6{\text{CO}}_2$, 18 ATP, 12 NADPH
• Output: 1 glucose (${\text{C}}_6{\text{H}}_{12}{\text{O}}_6$)

Importance of G3P

• G3P is the key intermediate that links photosynthesis to biosynthesis.
• It can be used to synthesize glucose, sucrose, starch, amino acids, fatty acids, and other organic molecules.
• It represents the point at which inorganic carbon (${\text{CO}}_2$) is converted into organic carbon.

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Summary: Two Stages of Photosynthesis