📖 Introduction
Photosynthesis is a physico-chemical process by which green plants use light energy to drive the synthesis of organic compounds. It is the primary source of all food on earth and is responsible for the release of oxygen into the atmosphere. This chapter focuses on the structure of the photosynthetic machinery and the reactions that transform light energy into chemical energy.
📝 High-Yield NEET Notes
- Photosynthesis is a physico-chemical process by which green plants use light energy to drive the synthesis of organic compounds.
- Green plants carry out photosynthesis and are therefore called autotrophs, while all other organisms that depend on green plants for food are heterotrophs.
- Photosynthesis is important due to two reasons: it is the primary source of all food on earth and it is responsible for the release of oxygen into the atmosphere by green plants.
- Joseph Priestley (1733-1804) in 1770 performed experiments that revealed the essential role of air in the growth of green plants and hypothesized that plants restore to the air whatever breathing animals and burning candles remove.
- Jan Ingenhousz (1730-1799) showed that sunlight is essential for the plant process that purifies the air fouled by burning candles or breathing animals, and demonstrated that only the green parts of plants release oxygen.
- Julius von Sachs provided evidence for production of glucose when plants grow, and found that the green substance in plants (chlorophyll) is located in special bodies later called chloroplasts within plant cells.
- T.W. Engelmann used a prism to split light into spectral components and illuminated a green alga Cladophora placed in a suspension of aerobic bacteria, observing that bacteria accumulated mainly in the region of blue and red light, thus describing the first action spectrum of photosynthesis.
- Cornelius van Niel (1897-1985) demonstrated that photosynthesis is essentially a light-dependent reaction in which hydrogen from a suitable oxidizable compound reduces carbon dioxide to carbohydrates.
- Van Niel inferred that the O₂ evolved by green plants comes from H₂O, not from carbon dioxide, which was later proved using radioisotope techniques.
- The correct equation representing the overall process of photosynthesis is: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂ (in the presence of light), where the O₂ released is from water.
- Within the chloroplast there is a membranous system consisting of grana, the stroma lamellae, and the matrix stroma, with a clear division of labour.
- The membrane system is responsible for trapping light energy and for the synthesis of ATP and NADPH, while enzymatic reactions in the stroma synthesize sugar which forms starch.
- The light-driven reactions are called light reactions (photochemical reactions), while the reactions dependent on products of light reactions (ATP and NADPH) are called dark reactions (carbon reactions), though they are not actually light-independent.
- Four pigments are involved in photosynthesis: Chlorophyll a (bright or blue green), chlorophyll b (yellow green), xanthophylls (yellow) and carotenoids (yellow to yellow-orange).
- Chlorophyll a is the chief pigment associated with photosynthesis, while chlorophyll b, xanthophylls and carotenoids are accessory pigments that absorb light and transfer energy to chlorophyll a and protect it from photo-oxidation.
- Light reactions include light absorption, water splitting, oxygen release, and the 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), named in the sequence of their discovery and not in the sequence of their function.
- In PS I the reaction centre chlorophyll a has an absorption peak at 700 nm and is called P700, while in PS II it has absorption maxima at 680 nm and is called P680.
- The scheme of transfer of electrons starting from PS II, through an electron transport chain to PS I, and finally reducing NADP⁺ to NADPH + H⁺ is called the Z scheme due to its characteristic shape on a redox potential scale.
- The splitting of water is associated with PS II; water is split into 2H⁺, [O] and electrons according to the reaction: 2H₂O → 4H⁺ + O₂ + 4e⁻, creating oxygen as one of the net products of photosynthesis.
- Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light.
- When the two photosystems work in series (first PS II and then PS I), non-cyclic photophosphorylation occurs, synthesizing both ATP and NADPH + H⁺.
- When only PS I is functional, cyclic photophosphorylation occurs where the electron is circulated within the photosystem resulting only in ATP synthesis but not NADPH + H⁺.
- The chemiosmotic hypothesis explains ATP synthesis in chloroplasts: ATP synthesis is linked to development of a proton gradient across the thylakoid membrane with protons accumulating in the lumen.
- Proton gradient across thylakoid membrane develops due to: (a) splitting of water on the inner side releasing protons into lumen, (b) proton transport during electron movement through photosystems, and (c) removal of protons from stroma for NADP⁺ reduction.
- ATP synthase consists of two parts: CF₀ embedded in thylakoid membrane forming a transmembrane channel for proton diffusion, and CF₁ protruding into stroma which synthesizes ATP using energy from proton gradient breakdown.
- Melvin Calvin used radioactive ¹⁴C in algal photosynthesis studies and discovered that the first CO₂ fixation product was a 3-carbon organic acid (3-phosphoglyceric acid or PGA), and worked out the complete biosynthetic pathway called Calvin cycle.
- RuBP (ribulose bisphosphate) is a 5-carbon ketose sugar that acts as the primary CO₂ acceptor molecule in the Calvin cycle.
- RuBisCO (RuBP carboxylase-oxygenase) is the enzyme that catalyzes carboxylation of RuBP with CO₂ to form two molecules of 3-PGA, and is the most abundant enzyme in the world.
- The Calvin cycle operates in three stages: carboxylation (CO₂ fixation), reduction (formation of glucose using ATP and NADPH), and regeneration (of RuBP requiring ATP).
- For every CO₂ molecule entering the Calvin cycle in C₃ plants, 3 molecules of ATP and 2 of NADPH are required; to make one molecule of glucose, 6 turns of the cycle requiring 18 ATP and 12 NADPH are needed.
- C₄ plants require additional ATP for the C₄ cycle: 2 ATP molecules per CO₂ are consumed to regenerate PEP from pyruvate in mesophyll cells, making the total ATP requirement approximately 5 ATP and 2 NADPH per CO₂ fixed (3 for Calvin cycle + 2 for C₄ cycle).
- Carbon fixation occurs once in C₃ plants (via RuBisCO in mesophyll cells) but twice in C₄ plants—first in mesophyll cells via PEP carboxylase forming OAA (4C), and second in bundle sheath cells via RuBisCO in the Calvin cycle.
- C₄ plants are adapted to dry tropical regions and have a special leaf anatomy called Kranz anatomy, where bundle sheath cells form several layers around vascular bundles with large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces.
- In C₄ plants, the primary CO₂ acceptor is phosphoenol pyruvate (PEP), a 3-carbon molecule present in mesophyll cells, and the enzyme responsible is PEP carboxylase (PEPcase).
- The first stable product of CO₂ fixation in C₄ plants is oxaloacetic acid (OAA), a 4-carbon compound, hence the name C₄ pathway or Hatch and Slack pathway.
- In C₄ plants, mesophyll cells lack RuBisCO enzyme while bundle sheath cells are rich in RuBisCO but lack PEPcase; the Calvin cycle occurs only in bundle sheath cells.
- Photorespiration occurs in C₃ plants when RuBisCO binds with O₂ instead of CO₂, forming one molecule of phosphoglycerate and phosphoglycolate, resulting in release of CO₂ with utilization of ATP but no synthesis of sugars, ATP or NADPH.
- RuBisCO has a much greater affinity for CO₂ than O₂ when the CO₂:O₂ ratio is nearly equal, but binding is competitive and determined by relative concentrations of O₂ and CO₂.
- C₄ plants lack photorespiration because they have a mechanism that increases CO₂ concentration at the enzyme site when C₄ acid from mesophyll is broken down in bundle sheath cells, ensuring RuBisCO functions primarily as a carboxylase.
- CAM (Crassulacean Acid Metabolism) plants show temporal separation of carbon fixation: CO₂ is fixed at night into oxaloacetic acid (via PEP carboxylase) which is converted to malic acid and stored in vacuoles; during daytime, malic acid is decarboxylated to release CO₂ for the Calvin cycle.
- CAM plants (e.g., Opuntia, pineapple, Agave) are adapted to extremely xerophytic conditions and keep stomata closed during daytime to minimize water loss, opening them only at night for CO₂ uptake.
- Like C₄ plants, CAM plants use PEP carboxylase for initial CO₂ fixation (high affinity for CO₂, no oxygenation activity), but unlike C₄ plants which show spatial separation (mesophyll vs bundle sheath), CAM plants show temporal separation (night vs day) of the two carboxylation events.
- Blackman's (1905) Law of Limiting Factors states that 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.
- Light saturation occurs at about 10 per cent of full sunlight, hence light is rarely a limiting factor in nature except for plants in shade or dense forests.
- Carbon dioxide is the major limiting factor for photosynthesis as its atmospheric concentration is very low (between 0.03 and 0.04 per cent).
- C₄ plants show saturation at about 360 µlL⁻¹ CO₂ concentration while C₃ plants show saturation only beyond 450 µlL⁻¹, making current CO₂ levels limiting for C₃ plants.
- C₄ plants respond to higher temperatures and show higher rate of photosynthesis while C₃ plants have a much lower temperature optimum (20-25°C for C₃ versus 30-40°C for C₄ plants).
🎯 Key Comparison: C₃ vs C₄ vs CAM
- C₃ Plants: Single CO₂ fixation via RuBisCO; first product is PGA (3C); photorespiration present.
- C₄ Plants: Double fixation (PEPcase then RuBisCO); first product is OAA (4C); no photorespiration; Kranz anatomy present.
- CAM Plants: Temporal separation of fixation; CO₂ fixed at night via PEPcase; stomata closed during day.