Photosynthesis

Photosynthesis is an important activity that occurs in all green plants, whether flowering or non- flowering. It is the only process by which solar energy is converted into chemical energy.  Photosynthesis may be defined as the synthesis of carbohydrates by the green organs (containing chlorophyll) of a plant in presence of sunlight from carbon dioxide ( CO₂) obtained from the air and water ( H₂O ) obtained from the soil with the evolution of oxygen (O₂). Photosynthesis may also be defined as a biochemical process by which living cells of plants containing chlorophyll manufacture their own food (glucose and starch) using carbon dioxide and water as raw materials in the presence of sunlight. Oxygen is released as a by-product of photosynthesis. Photosynthesis may also be defined as the capture of photons of light and conversion of their energy into chemical energy locked in high energy bonds of organic compounds by the chloroplasts of the plant.

The carbohydrates formed during photosynthesis, provide food for the plant and for the entire living organism. The formation of carbohydrates, Glucose is represented by the following chemical equation.

Importance of Photosynthesis:

• Photosynthesis is the only process by which solar energy is trapped by autotrophs and converted into chemical energy in the form of food.
• Autotrophs are the plants which prepare their own food. Green plants are primary producers in the food chain. All other organisms directly or indirectly depend on autotrophs for food. It contributes to the symbiotic relationship between plants, humans, and animals.
• Photosynthesis is essential for sustaining life. It is an important source of oxygen and energy for all living organisms.
• Photosynthesis helps in the growth and development of plants.
• It converts atmospheric carbon dioxide (given out during respiration and other activities) back to oxygen. Thus it contributes to the carbon cycle between the earth, the oceans, plants, and animals.
• It is the only method to manufacture food (organic compounds) from inorganic substances.

Development of Knowledge of Photosynthesis:

• In 320 BC, Aristotle proposed that plants, like animals, require food.
  Theophrastus writes that plants obtain their nourishment through the roots from the soil.
• In 1450 A.D. Nicholas of Cusa proposed an experiment in which a plant is weighed and then planted in a container containing a weighed amount of soil. After a period of growth, the final weights of plant and soil, as well as the total weight of water applied, are determined and compared to the initial values. He speculates this will demonstrate that the mass of the plant was derived from water rather than soil. But he didn’t perform this experiment. This experiment was performed by Jean Baptiste van Helmont in 1648 A.D. He concluded that the entire mass of the plant came from water, but ignores a very slight decrease in the weight of the soil.
• In 1679, Edme Mariotte proposed that plants obtain part of their nourishment from the atmosphere.
• In 1727, Stephen Hales proposed that plant leaves “very probably” take in nourishment from the air, and that light may also be involved.
• In 1754, Charles Bonnet observed the emission of gas bubbles by a submerged illuminated leaf.
• In 1771, Joseph Priestley finds that air which has been made “noxious” by the breathing of animals or burning of a candle can be restored (i.e., made to support breathing or combustion again) by the presence of a green plant. He isolates the gas later identified as oxygen.

Joseph Priestley (1733-1804) in 1770 performed a series of experiments that revealed the essential role of air in the growth of green plants. Priestley observed that a candle burning in a closed space – a bell jar, soon gets extinguished. Similarly, a mouse would soon suffocate in a closed space. He concluded that a burning candle or an animal that breathe the air, both somehow, damage the air. Due to which the candle get extinguished and the mouse died.

Then he placed a mint plant in the same bell jar, he found that the mouse stayed alive and the candle continued to burn.

From this experiment, Priestley concluded that the plants restore to the air whatever breathing animals and burning candles remove.

• In1774, Antoine Lavoisier further investigated the above experiment and later named the gas released by the plants as oxygen and gas given out in combustion of candle and respiration of mouse as carbon dioxide. He recognizes that oxygen is consumed and carbon dioxide is released in both animal respiration and combustion. His experiments discarded “phlogiston theory”.
• In 1779 Jan Ingenhousz discovered that only the green parts of plants are responsible for the purification of noxious air and release oxygen and that this occurs only when they are illuminated by sunlight.
• In 1782 Jean Senebier demonstrated that green plants take in carbon dioxide from the air and emit oxygen under the influence of sunlight.
• In 1791 Comparetti observes green granules in plant tissues, later identified as chloroplasts.
• In 1804 Nicolas de Saussure showed the importance of water in the process and also showed that the carbon assimilated from atmospheric carbon dioxide cannot fully account for the increase in dry weight of a plant. He hypothesized that the additional weight was derived from water. At this point, therefore, the basic equation of photosynthesis was established. It was understood as a process in which a green plant illuminated by sunlight takes in carbon dioxide and water and converts them into organic material and oxygen.
• In 1818 Pierre Joseph Pelletier and Joseph Bienaime Caventou gave the name “chlorophyll” to the green pigment in plants.
• In 1837 Rene Dutrochet made the connection between chlorophyll and the ability of plants to assimilate carbon dioxide. Also identified stomata on leaf surfaces.
• In 1842, Matthias Schleiden postulated that the water molecule is split during photosynthesis.
• In 1844 Hugo von Mohl makes detailed observations of the structure of chloroplasts.
• In 1845 Julius Robert von Mayer reported that green plants convert solar energy into chemical energy of organic matter.
• In 1862 Julius von Sachs discovered and demonstrated light-dependent starch formation in chloroplasts.
• In 1864 Jean Baptiste Boussingault makes accurate quantitative measurements of carbon dioxide uptake and oxygen production, a step leading to a balanced equation for photosynthesis.
• In 1873 Emil Godlewski confirmed that atmospheric carbon dioxide is the source of carbon in photosynthesis by showing that starch formation in illuminated leaves depends upon the presence of carbon dioxide.
• In 1883 Theodor Wilhelm Engelmann illuminated a filamentous alga with light dispersed through a prism. He finds that motile aerobic bacteria congregate near the portions illuminated by red and blue wavelengths, thus producing the first action spectrum for photosynthetic oxygen evolution.
• In 1883 Arthur Meyer described the chloroplast grana.
• In 1893 Charles Barnes suggested that the process by which illuminated green plants manufacture carbon compounds be called either “photosyntax” or “photosynthesis.” Although Barnes prefers the former, “photosynthesis” is adopted into common usage.
• In 1905 F. F. Blackman developed the concept of limiting factors, showing that photosynthesis consists of two types of reactions: a rapid light-dependent photochemical process and a slower temperature-dependent biochemical process. These are later termed “light reactions” and “dark reactions,” respectively.
• In 1913 Richard Willstatter and Arthur Stoll published studies on the structure and chemistry of chlorophyll. Willstatter awarded Nobel Prize, 1915.
• In 1920 Wardburg used unicellular green alga, Chlorella to study photosynthesis.
• In 1931 Van Neil Demonstrated after his studies with purple and green sulphur bacteria that photosynthesis is a light-dependent reaction in which hydrogen from oxidizable compound reduces carbon dioxide to form sugar.
• In 1932 Emerson and Arnold by flashing light experiment showed the existence of light and dark reactions.
• In 1937 Robert (Robin) Hill demonstrated that in the presence of an artificial electron acceptor isolated chloroplasts can evolve oxygen in the absence of carbon dioxide.
• In 1941 Cornelis van Niel published a summary of his work showing that photosynthetic bacteria which use H₂S as an electron donor produce elemental sulfur instead of oxygen. He suggests by analogy that the O₂  released in plant photosynthesis is derived from H₂O rather than CO₂.
• In 1941 Samuel Ruben and Martin Kamen used water labeled with the radioactive oxygen to confirm that the oxygen produced in photosynthesis comes from H₂O.
• In 1954 Melvin Calvin traced the path of carbon in the dark phase of photosynthesis and gave the C₃ Cycle (now named as Calvin Cycle). He was awarded a Nobel prize in 1960.
• In 1954 Daniel Arnon, Allen, and Whatley demonstrated light-dependent ATP formation in chloroplasts.
• In 1955 Daniel Arnon demonstrates that isolated chloroplasts are capable of carrying out complete photosynthesis.
• In 1956 Melvin Calvin and coworkers use radioactively labeled radioactive CO₂  to elucidate the pathway of carbon assimilation in photosynthesis. Calvin awarded Nobel Prize in 1961.
• In 1957 Robert Emerson described the “red drop” and “enhancement” effects, the first indication that the light reactions of photosynthesis consist of two separate photochemical systems.
• In 1960 Robert Woodward synthesizes chlorophyll. Awarded Nobel Prize, 1965.
• In 1960 Robin Hill and Fay Bendall, based on the work of Emerson and others, propose the “Z scheme” model for the photosynthetic light reactions.
• In 1961 Louis Duysens provided evidence in support of the Z scheme by demonstrating that exposure to alternating wavelengths of light causes cytochrome f to switch between oxidized and reduced states.
• In 1965 Hatch and Slack discovered the C₄ pathway for CO₂ fixation in certain tropical grasses.
• In 1968 Roderick Clayton isolates reaction center complexes.
• In 1970 Bessel Kok proposes the “S-states” model of charge accumulation to explain the stepwise oxidation of H₂O and release of O₂.
• In 1984 Hans Deisenhofer, Hartmut Michel, and Robert Huber crystallize the photosynthetic reaction center from a purple bacterium and use X-ray diffraction techniques to determine its detailed structure. The three share Nobel Prize, 1988.
• In 2006 Junko Yano, Vittal Yachandra, and co-workers determine the structure of the manganese-calcium water-splitting complex of Photosystem II.