1. Early Experiments in Photosynthesis
Landmark Discoveries
- Joseph Priestley (1770): Conducted the bell jar experiment using a mouse, candle, and mint plant. Discovered that plants restore to the air whatever breathing animals and burning candles remove (discovered Oxygen in 1774) AIPMT 1992 1993.
- Jan Ingenhousz (1779): Used an aquatic plant in sunlight and darkness. Proved that sunlight is essential for the plant to purify air. Showed that only the green parts release oxygen bubbles.
- Julius von Sachs (1854): Provided evidence for production of glucose when plants grow. Glucose is usually stored as starch. Showed that the green substance (chlorophyll) is located in special bodies (chloroplasts).
- T.W. Engelmann (1843-1909): Split light using a prism and illuminated a green alga, Cladophora, placed in a suspension of aerobic bacteria. Bacteria accumulated mainly in the blue and red light regions. This described the first action spectrum of photosynthesis NEET 2020.
- Cornelius van Niel (1897-1985): A microbiologist who studied purple and green sulphur bacteria. Demonstrated that photosynthesis is essentially a light-dependent redox reaction.
- Crucial detail: Van Niel proved that hydrogen from a suitable oxidisable compound reduces CO2 to carbohydrates. In plants, H2O is the hydrogen donor and is oxidised to O2. Therefore, the O2 evolved by green plants comes from H2O, not from CO2 AIPMT 1993 2001 NEET 2020 2023.
2. Site of Photosynthesis and Pigments
Chloroplast Structure
- Mesophyll cells: Contain a large number of chloroplasts, usually aligning themselves along the walls to receive optimum light.
- Membrane System (Grana & Stroma Lamellae): Responsible for trapping light energy and synthesizing ATP and NADPH (Light Reactions or Photochemical phase) AIPMT 2003 NEET 2018 2021.
- Stroma: The fluid matrix where enzymatic reactions synthesize sugar, which forms starch (Dark Reactions or Biosynthetic phase) NEET 2021.
Photosynthetic Pigments
Separation of leaf pigments is done via paper chromatography NEET 2017.
| Pigment | Colour in Chromatogram | Primary Function |
|---|---|---|
| Chlorophyll a | Bright or blue green | Chief pigment associated with photosynthesis NEET 2017. |
| Chlorophyll b | Yellow green | Accessory pigment; broadens the spectrum of light absorbed. |
| Xanthophylls | Yellow | Accessory pigment; protects chl a from photo-oxidation. |
| Carotenoids | Yellow to yellow-orange | Accessory pigment; protects chl a from photo-oxidation. |
3. Light Reaction (Photochemical Phase)
Photosystems
- Light reactions include light absorption, water splitting, oxygen release, and formation of high-energy chemical intermediates (ATP and NADPH).
- Pigments are organized into two discrete photochemical Light Harvesting Complexes (LHC) within Photosystem I (PS I) and Photosystem II (PS II).
- Antennae: Made of hundreds of pigment molecules bound to proteins, transferring light energy to the reaction centre.
- Reaction Centre: Formed by a single chlorophyll a molecule.
- In PS I, the absorption peak is at 700 nm (P700) AIPMT 1990 NEET 2019.
- In PS II, the absorption peak is at 680 nm (P680) AIPMT 1990 1995 NEET 2017.
The Electron Transport (Z-Scheme)
- Step 1: PS II absorbs 680 nm red light, causing electrons to become excited and jump to an outer orbit.
- Step 2: Electrons are picked up by an electron acceptor and passed to an electrons transport system consisting of cytochromes. This movement is downhill in terms of an oxidation-reduction (redox) potential scale AIPMT 1998 2011 NEET 2020.
- Step 3: Electrons are passed to PS I. Concurrently, PS I (P700) absorbs red light (700 nm) and excites electrons to another acceptor with a greater redox potential.
- Step 4: These electrons move downhill again to reduce NADP+ to NADPH + H+. The characteristic Z-shape is formed when all carriers are placed in sequence on a redox potential scale.
Splitting of Water (Photolysis)
- The electrons moved from PS II must be replaced. This is achieved by water splitting.
- Reaction: 2H2O → 4H+ + O2 + 4e-
- Water splitting complex is associated with PS II and is physically located on the inner side of the thylakoid membrane AIPMT 1992 1997 2001 2012 NEET 2020 2024.
- Essential elements required: Manganese (Mn) and Chlorine (Cl).
Photophosphorylation (Cyclic vs Non-Cyclic)
Synthesis of ATP by cells (in mitochondria and chloroplasts) is named phosphorylation.
| Feature | Non-Cyclic Photophosphorylation | Cyclic Photophosphorylation |
|---|---|---|
| Photosystems Involved | PS II and PS I | Only PS I AIPMT 1994 2003 NEET 2019 2023 |
| Products | ATP and NADPH AIPMT 2011 | Only ATP (No NADPH, no O2) NEET 2019 2022 2023 |
| Location | Grana lamellae | Stroma lamellae AIPMT 2009 2015 |
| Flow of Electrons | Unidirectional (Z-scheme) | Cyclic (electrons returned to PS I) |
| Crucial Exception | N/A | Stroma lamellae membrane lacks PS II and NADP reductase enzyme AIPMT 2015 NEET 2019 2022. |
4. Chemiosmotic Hypothesis
Mechanism of ATP Synthesis
- Proposed by Peter Mitchell to explain ATP synthesis driven by a proton (H+) gradient across the thylakoid membrane AIPMT 1996 2007 NEET 2016 2021 2022.
Proton Accumulation in Lumen
- Water Splitting: Occurs on the inner side of the membrane, releasing protons into the thylakoid lumen.
- Electron Transport: As electrons move through the photosystems, carriers (plastoquinone) remove protons from the stroma and release them into the lumen.
- NADP Reductase Activity: Located on the stroma side, it removes protons from the stroma to reduce NADP+ to NADPH.
- Result: High proton concentration in the lumen (low pH) and low proton concentration in the stroma (high pH).
ATP Synthase Enzyme
- Consists of two parts:
- CF0: Embedded in the thylakoid membrane, forms a transmembrane channel for facilitated diffusion of protons across the membrane NEET 2022.
- CF1: Protrudes on the outer surface of the thylakoid membrane (stroma side). The breakdown of the gradient provides energy to cause a conformational change in CF1, synthesizing ATP.
5. Biosynthetic Phase (Dark Reactions)
Calvin Cycle (C3 Pathway)
- Discovered by Melvin Calvin using radioactive 14C in algal photosynthesis.
- The first stable product of CO2 fixation is a 3-carbon organic acid: 3-phosphoglyceric acid (3-PGA) AIPMT 1990 1993 2014 NEET 2018.
- The primary acceptor of CO2 is a 5-carbon ketose sugar: Ribulose bisphosphate (RuBP).
Stages of the Calvin Cycle
- Carboxylation: Most crucial step NEET 2020. RuBP binds with CO2 to form two molecules of 3-PGA, catalyzed by RuBP carboxylase-oxygenase (RuBisCO).
- Reduction: Series of reactions leading to glucose formation. Involves utilization of 2 ATP for phosphorylation and 2 NADPH for reduction per CO2 molecule fixed. One molecule of G3P leaves the cycle.
- Regeneration: Crucial for continuous cycle operation. Requires 1 ATP to regenerate RuBP from G3P.
Calvin Cycle Balance Sheet (For 1 Glucose Molecule)
6 turns of the Calvin cycle are required to synthesize 1 molecule of glucose AIPMT 2000 NEET 2017 2019.
| In | Out |
|---|---|
| 6 CO2 | 1 Glucose |
| 18 ATP | 18 ADP |
| 12 NADPH | 12 NADP+ |
- Crucial detail: The ratio of ATP to NADPH required for the synthesis of one glucose molecule in C3 plants is 18:12 (or 3:2 per CO2) NEET 2021.
6. The C4 Pathway (Hatch and Slack Pathway)
Anatomy and Characteristics
- Adapted by plants in dry tropical regions (e.g., Maize, Sorghum, Sugarcane).
- Kranz Anatomy: Special leaf anatomy where large Bundle Sheath (BS) cells form several layers around vascular bundles.
- Crucial detail: BS cells have thick walls impervious to gaseous exchange, no intercellular spaces, and a large number of agranal chloroplasts (no grana) AIPMT 2005 2011 2012 NEET 2017 2021 2023. Mesophyll cells have granal chloroplasts.
- They lack photorespiration, hence possess higher yields and productivity compared to C3 plants NEET 2017 2019 2022.
The Pathway Mechanism
- Primary CO2 Acceptor: A 3-carbon molecule, Phosphoenolpyruvate (PEP), present in mesophyll cells AIPMT 1999 2001 2014 NEET 2017 2022.
- Enzymes:
- Mesophyll cells contain PEP carboxylase (PEPcase) but completely lack RuBisCO NEET 2022.
- Bundle sheath cells contain RuBisCO but lack PEPcase.
- First Stable Product: A 4-carbon acid, Oxaloacetic acid (OAA), formed in the mesophyll AIPMT 1997 1998 NEET 2021.
- Transport & Decarboxylation: OAA is converted to Malic/Aspartic acid and transported to the bundle sheath, where it is broken down to release CO2 and a 3-carbon molecule. The CO2 enters the Calvin cycle inside the bundle sheath.
- Energy Cost: Requires 30 ATP and 12 NADPH to produce one glucose molecule (2 extra ATP per CO2 compared to C3).
7. Photorespiration (C2 Cycle)
- Mechanism: The active site of RuBisCO can bind to both CO2 and O2. In C3 plants, at high temperature and high O2 concentration, RuBisCO acts as an oxygenase.
- RuBP binds with O2 to form 1 molecule of Phosphoglycerate (3C) and 1 molecule of Phosphoglycolate (2C) AIPMT 2008 2012 NEET 2020 2023.
- Wasteful Process: It does NOT synthesize sugars, nor does it synthesize ATP or NADPH. Instead, it results in the release of CO2 with the utilization of ATP NEET 2021.
- Occurs sequentially in three organelles: Chloroplast → Peroxisome → Mitochondria AIPMT 2000 2006.
- Exception: C4 plants have a mechanism that increases CO2 concentration at the enzyme site (decarboxylation of C4 acids in bundle sheath), completely preventing photorespiration.
8. Factors Affecting Photosynthesis
Blackman's Law of Limiting Factors (1905)
"If a chemical process is affected by more than one factor, then 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." AIPMT 1995 NEET 2017 2023.
Specific Factors
- Light: At low intensities, light limits the rate. Light saturation occurs at 10% of full sunlight. Hence, light is rarely a limiting factor in nature (except for plants in shade/dense forests). High light breaks down chlorophyll (photo-oxidation).
- Carbon Dioxide Concentration: The major limiting factor in nature.
- C3 and C4 plants respond differently. At low light, neither responds to high CO2. At high light, both show increased rates.
- Saturation points: C4 plants saturate at about 360 μl/L. C3 plants saturate at 450 μl/L NEET 2017 2022. Current atmospheric CO2 level (300-400 μl/L) is limiting for C3 plants.
- Greenhouse effect: C3 crops like tomatoes and bell peppers are grown in CO2 enriched atmospheres to yield higher production AIPMT 2011 NEET 2021.
- Temperature: Dark reactions (enzymatic) are highly temperature sensitive. Light reactions are less sensitive. C4 plants have a much higher temperature optimum than C3 plants.
- Water: Water stress causes stomata to close, reducing CO2 availability. It also causes leaves to wilt, reducing the surface area for light absorption.