Photosynthesis in Higher Plants

In this article we will discuss about:- 1. Photosynthesis as Means of Autotrophic Nutrition 2. Magnitude of Photosynthesis 3. History.

Photosynthesis as Means of Autotrophic Nutrition:

Only photosynthesis is a means by which certain organisms can manufacture their own organic food from inorganic raw materials with the help of solar energy. The organisms performing photosynthesis are, therefore, called autotrophs or photoautotrophs.

They include green plants, red algae, brown algae, yellow green algae, several types of protists, cyanobacteria and some bacteria. Other organisms which cannot manufacture their own food are known as heterotrophs. They depend upon autotrophs for obtaining food.

Magnitude of Photosynthesis:

The total carbon dioxide available to plants for photosynthesis is about 11.2 x 10¹⁴ tonnes. Out of this only 2.2 x 10¹³ (2200 billion) tonnes are present in the atmosphere at the rate of 0.03%.

The amount is sufficient to support photosynthesis for a few hundred years even if there is no replenishment. Oceans contain 11 x 10¹⁴ or 110,000 billion tonnes of carbon dioxide. There is still a large quantity of carbon (2.8 x 10¹⁸ tonnes) present in the lithosphere. Of course, it is not employed in photosynthesis.

About 70-80 billion tonnes of carbon dioxide are fixed annually by terrestrial and aquatic photoautotrophs. It produces over 170 billion tonnes of dry organic matter. According to older estimates out of this only 10% (17 billion tonnes) of dry matter is produced by land plants while 90% of it is formed in oceans. The present estimates put the productivity of land plants to be 68% of the total.

The annual fixation of 70 billion tonnes of carbon requires about 1.05 x 10¹⁸ kcal of energy. So much of radiant energy is, therefore, being converted into chemical energy annually. The total solar energy falling on earth is 5 x 10²⁰ kcal/yr. The plants are thus able to utilize only 0.2% of the solar energy received by the surface of earth.

History of Photosynthesis:

1727 Stephen Hales:

Recognised the importance of sunlight, air and green leaves for nourishment of plants.

1770 Joseph Priestley (1733-1804; Fig. 13.1):

Priestley (1770) found that a burning candle would soon get extinguished in closed space of bell jar. Similarly, a mouse kept in closed space would soon get suffocated and die. However, if a mint plant accompanied them in bell jar, neither the candle would be extinguished nor the mouse died.

The plant also grows. Priestley, therefore, hypothesized that foul air or phlogiston produced during burning of candles or animal (mice) respiration could be converted into pure air or de-phlogiston by plants (mint). In 1774, Priestley discovered oxygen.

1779 Jan Ingenhousz (1730-1799):

In his experiment with an aquatic plant, he showed that in bright sunlight, small bubbles were formed around the green parts while in the dark, such bubbles did not form. He found these bubbles to be of oxygen. He thus confirmed that purification of air or formation of de-phlogiston is carried out by green plants only in the presence of sunlight.

1783 Lavoisier:

Phlogiston is carbon dioxide. De-phlogiston is oxygen.

1804 De Saussure:

Water is essential requirement in photosynthesis. In photosynthesis, plants pick up CO₂ and release O₂.

1818 Pelletier and Caventou:

Discovered and named chlorophyll.

1845 Von Mayer:

Green plants convert solar energy into chemical energy during the process of photosynthesis.

1854 Sachs:

Green part in plant produces glucose which is stored as starch. Starch is the first visible product of photosynthesis.

1888 Engelmann (1843-1909):

He split light into its components by the prism and then illuminated Cladophora (a green alga) placed in a suspension of aerobic bacteria. He found that bacteria accumulated in the region of blue and red light of the split spectrum. He thus discovered the effect of different wave lengths of light on photosynthesis and plotted the first action spectrum of photosynthesis.

1925 Blackman:

Propounded the ‘law’ or principle of limiting factors. He also proposed the occurrence of a dark phase in photosynthesis.

1920 Warburg:

First employed Chlorella, a unicellular non-motile green alga, for study of photosynthesis. He studied the effect of high O₂ concentration, poisons like cyanide and light flashes on photosynthesis.

Van Neil (1897-1985):

On his studies with purple and green sulphur bacteria, demonstrated that photosynthesis is a light dependent reaction in which hydrogen from an oxidisable compound reduces CO₂ to form sugar.

In green sulphur bacteria, when H₂S, instead of H₂O was used as hydrogen donor, no O₂ was evolved. He inferred that O₂ evolved by green plants comes from H₂O but not from CO₂ as thought earlier.

1932 Emerson and Arnold:

Performed flashing light experiments at different temperatures and found out the existence of light and dark phases of photosynthesis.

1937 Hill:

Evolution of oxygen occurs in light reaction performed by isolated illuminated chloroplasts in the presence of suitable electron acceptors and the absence of CO₂.

1941 Ruben and Kamen; Ruben, Hassid and Kamen:

Oxygen evolved during photosynthesis comes from water and not from carbon dioxide. For this, they used water with heavy isotope of oxygen, ¹⁸O.

1954 Amon:

Discovered photophosphorylation. Also showed fixation of CO₂ by previously illuminated isolated chloroplasts by using radioactive carbon ¹⁴C in their carbon dioxide.

1954-55 Calvin:

Traced the pathway of carbon fixation by using (¹⁴CO₂) and gave the C₃ cycle, now known after him as Calvin cycle. Calvin was awarded Nobel Prize for this in 1960.

1957 Emerson:

Found red drop and photosynthetic enhancement or Emerson effect.

1960 Hill and Bendall:

Proposed Z-scheme of two photosystems.

1961 Peter Mitchell:

Proposed chemiosmotic theory of ATP synthesis.

1965 Hatch and Slack:

Discovered supplementary mechanism of CO₂ fixation, called C₄ pathway, in certain tropical plants (grasses and non-grasses).

1985 Huber:

Crystallized and analysed the photo-centre of Rhodobacter through X-ray diffraction technique. They were awarded Nobel Prize in 1988.