Photosynthesis is a physico-chemical process by which green plants use light energy to drive the synthesis of organic compounds. It is the basis of life on earth because it is the primary source of all food and is responsible for the release of oxygen.

Early Experiments

• Joseph Priestley (1770): Performed the bell jar experiments with a candle and a mouse. He concluded that a burning candle or an animal that breathes the air, both somehow damage the air. But when he placed a mint plant in the same bell jar, he found that the mouse stayed alive and the candle continued to burn. He hypothesized that plants restore to the air whatever breathing animals and burning candles remove.

• Jan Ingenhousz: Used a similar setup but placed it in sunlight and darkness. He showed that sunlight is essential to the plant process. He observed that in an aquatic plant (Hydrilla), small bubbles were formed around the green parts while in the light, but not in the dark. He identified these bubbles to be of oxygen.

• Julius von Sachs (1854): Provided evidence for the production of glucose when plants grow. Glucose is usually stored as starch. He showed that the green substance (chlorophyll) is located in special bodies (chloroplasts) within plant cells.

• T.W. Engelmann: Using a prism, he split light into its spectral components and then illuminated a green alga, Cladophora, placed in a suspension of aerobic bacteria. The bacteria were used to detect the sites of O2 evolution. He observed that the bacteria accumulated mainly in the region of blue and red light of the split spectrum. This described the first action spectrum of photosynthesis.

• Cornelius van Niel: A microbiologist who made a significant contribution working on purple and green sulphur bacteria. He demonstrated that photosynthesis is essentially a light-dependent reaction in which hydrogen from a suitable oxidisable compound reduces carbon dioxide to carbohydrates. In green plants, H2O is the hydrogen donor and is oxidised to O2. In purple and green sulphur bacteria, the hydrogen donor is H2S, and the oxidation product is sulphur or sulphate, not O2. Hence, he inferred that the O2 evolved by the green plant comes from H2O, not from CO2.

Where does Photosynthesis take place? It takes place in the green leaves and other green parts of the plants. Within the leaves, the mesophyll cells possess a large number of chloroplasts. The Chloroplast contains a membranous system consisting of grana, the stroma lamellae, and the fluid stroma.

• Membrane System (Grana): Responsible for trapping the light energy and also for the synthesis of ATP and NADPH (Light Reactions).

• Stroma: Enzymatic reactions synthesize sugar, which in turn forms starch (Dark Reactions or Biosynthetic Phase).

Pigments involved in Photosynthesis A chromatographic separation of the leaf pigments shows that the colour is not due to a single pigment but due to four pigments: Chlorophyll a (bright or blue-green), Chlorophyll b (yellow-green), Xanthophylls (yellow), and Carotenoids (yellow to yellow-orange).

• Chlorophyll a is the major pigment responsible for trapping light.

• The absorption spectrum of chlorophyll a and the action spectrum of photosynthesis (rate of photosynthesis) overlap, showing that photosynthesis is maximum in the blue and red regions of the spectrum.

• Accessory pigments (Chl b, xanthophylls, carotenoids) absorb light and transfer the energy to chlorophyll a. They also protect chlorophyll a from photo-oxidation.

Light Reaction (Photochemical Phase) Includes light absorption, water splitting, oxygen release, and the formation of high-energy chemical intermediates, ATP and NADPH.

• Photosystems: Pigments are organised into two discrete photochemical Light Harvesting Complexes (LHC) within the Photosystem I (PS I) and Photosystem II (PS II). Each photosystem has all the pigments (except one molecule of chlorophyll a) forming a light harvesting system also called antennae. The single chlorophyll a molecule forms the reaction centre.

  • In PS I, the reaction centre chlorophyll a has an absorption peak at 700 nm (P700).

  • In PS II, the reaction centre has an absorption peak at 680 nm (P680).

The Electron Transport

• Z-Scheme: In PS II, the reaction centre chlorophyll a absorbs 680 nm light causing electrons to become excited and jump into an orbit farther from the atomic nucleus. These 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 redox potential scale). The electrons are then passed to the pigments of PS I. Simultaneously, electrons in the reaction centre of PS I are also excited when they receive red light of wavelength 700 nm and are transferred to another acceptor molecule. These electrons are moved downhill again to a molecule of NADP+ reducing it to NADPH + H+. This whole scheme of transfer of electrons, starting from the PS II, uphill to the acceptor, down the electron transport chain to PS I, excitation of electrons, transfer to another acceptor, and finally down to NADP+ is called the Z-Scheme.

• Splitting of Water: The electrons that were moved from PS II must be replaced. This is achieved by electrons available due to splitting of water. The splitting of water is associated with PS II (2H2O -> 4H+ + O2 + 4e-). This creates oxygen, one of the net products of photosynthesis. The water splitting complex is located on the inner side of the thylakoid membrane.

Cyclic and Non-cyclic Photo-phosphorylation

• Non-Cyclic: Both ATP and NADPH + H+ are synthesised. Involves both PS I and PS II. Occurs in grana lamellae.

• Cyclic: Only PS I is functional. The electron is circulated within the photosystem and the phosphorylation occurs due to cyclic flow of electrons. Only ATP is synthesised (no NADPH). Occurs in stroma lamellae (which lack PS II and NADP reductase enzyme).

Chemiosmotic Hypothesis Explains the mechanism of ATP synthesis. ATP synthesis is linked to the development of a proton gradient across a membrane (thylakoid membrane).

• Protons (ions) accumulate in the lumen (inside of the thylakoid).

• The breakdown of the gradient provides energy to cause a conformational change in the CF1 particle of the ATP synthase, which makes the enzyme synthesise several molecules of energy-packed ATP.

The Biosynthetic Phase (Dark Reaction) This phase uses the products of the light reaction (ATP and NADPH) to synthesise food (CO2 is reduced to carbohydrate).

The Calvin Cycle (C3 Pathway) Melvin Calvin used radioactive C-14 in algal photosynthesis and discovered that the first CO2 fixation product was a 3-carbon organic acid, 3-phosphoglyceric acid (PGA). The cycle proceeds in three stages:

1. 1. Carboxylation: Fixation of CO2 into a stable organic intermediate. CO2 is utilised for the carboxylation of RuBP (Ribulose-1,5-bisphosphate). This reaction is catalysed by the enzyme RuBisCO (RuBP carboxylase-oxygenase). It results in two molecules of 3-PGA.

   2. Reduction: Involves utilisation of 2 molecules of ATP for phosphorylation and 2 of NADPH for reduction per CO2 molecule fixed. The fixation of six molecules of CO2 and 6 turns of the cycle are required for the formation of one molecule of glucose.

   3. Regeneration: Regeneration of the CO2 acceptor molecule RuBP is crucial for the cycle to continue uninterrupted. This step requires 1 ATP for phosphorylation to form RuBP.

      • Total Output: For every CO2 molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required. To make 1 Glucose molecule: 18 ATP and 12 NADPH are consumed.

The C4 Pathway (Hatch and Slack Pathway) Plants adapted to dry tropical regions have the C4 pathway. Though they use the C4 pathway, the main biosynthetic pathway is still the C3 cycle.

• • Kranz Anatomy: The large cells around the vascular bundles of the C4 plants are called bundle sheath cells, and the leaves which have such anatomy are said to have ‘Kranz’ anatomy. ‘Kranz’ means ‘wreath’ or ring. These cells have thick walls impervious to gaseous exchange and no intercellular spaces. They possess chloroplasts (agranal).

  • Mechanism:

    • In Mesophyll cells: The primary CO2 acceptor is a 3-carbon molecule phosphoenol pyruvate (PEP), catalysed by PEP carboxylase (PEPcase). Mesophyll cells lack RuBisCO. The C4 acid Oxaloacetic acid (OAA) is formed. It is then converted to other 4-carbon acids like malic acid or aspartic acid.

    • Transport to Bundle Sheath cells: The C4 acids are broken down to release CO2 and a 3-carbon molecule.

    • The released CO2 enters the C3 (Calvin) cycle in the bundle sheath cells (which are rich in RuBisCO but lack PEPcase).

  • Advantages: C4 plants are photosynthetically more efficient. They tolerate higher temperatures and high light intensity. They lack photorespiration.

Photorespiration In C3 plants, RuBisCO can bind to O2 (oxygenase activity) when CO2 levels are low and O2 is high. RuBP binds with O2 to form one molecule of phosphoglycerate and phosphoglycolate (2 Carbon). This pathway is wasteful: there is no synthesis of sugars, ATP, or NADPH, but it results in the release of CO2 with the utilization of ATP. In C4 plants, photorespiration does not occur because they have a mechanism that increases the concentration of CO2 at the enzyme site (mesophyll cells pump C4 acids to bundle sheath cells to release CO2).

Factors affecting Photosynthesis Law of Limiting Factors (Blackman, 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.

1. Light: Light saturation occurs at 10% of full sunlight. Increase in incident light beyond a point causes the breakdown of chlorophyll and a decrease in photosynthesis.

2. Carbon dioxide Concentration: It is the major limiting factor. The concentration of CO2 is very low in the atmosphere (0.03-0.04%). Increase in concentration up to 0.05% can cause an increase in CO2 fixation rates.

   • Saturation: C4 plants show saturation at about 360 microlitres per litre, while C3 plants respond to increased CO2 concentration and saturation is seen only beyond 450 microlitres per litre. This is why Greenhouse crops (like tomatoes, bell pepper) are grown in CO2 enriched atmospheres to get higher yields (C3 plants).

3. Temperature: The dark reactions being enzymatic are temperature controlled. C4 plants respond to higher temperatures and show higher rates of photosynthesis while C3 plants have a much lower temperature optimum.

4. Water: Water stress causes the stomata to close hence reducing the CO2 availability. It also makes leaves wilt, reducing the surface area.