13. PHOTOSYNTHESISPhotosynthesis is a physico-chemical process by which green plants use light energy (solar energy) to synthesise organic compounds. So they are autotrophs.
It is the basis of life on earth.
Ultimately, all living forms depend on sunlight for energy.
Importance of Photosynthesis
- It is the primary source of all food on earth.
- It releases oxygen into the atmosphere.
1. Variegated leaf experiment
Take a variegated leaf (or leaf partially covered with black paper) that was exposed to light.
Test the leaves for starch. It shows that photosynthesis occurs only in green parts of the leaves in presence of light.
2. Half-leaf experiment
A part of a leaf is enclosed in a test tube containing KOH soaked cotton (which absorbs CO2).
The other half of leaf is exposed to air.
Place this setup in light for some time.
Test the leaf for presence of starch. Exposed part shows positive for starch and portion in the tube shows negative. This proves that CO2 is required for photosynthesis.
Experiments by Joseph Priestley (1770)
Priestley performed experiments to prove the role of air in the growth of green plants.
He discovered oxygen in 1774.
He observed that a candle burning in a closed bell jar gets extinguished. Similarly, a mouse suffocated in closed jar. He concluded that a burning candle or a breathing animal damage the air.
He placed a mint plant in the same bell jar. He found that the mouse stayed alive and the candle continued to burn.
He hypothesised that plants restore to the air whatever breathing animals and burning candles remove.
Experiments by Jan Ingenhousz (1730-1799)
He conducted the same experiment by placing in darkness and sunlight.
He showed that sunlight is essential to the plant for purifying the air fouled by burning candles or animals.
He repeated this experiment with an aquatic plant. It showed that in bright sunlight, small bubbles were formed around green parts while in the dark they did not.
Later he identified these bubbles to be of oxygen. Thus he showed that only the green part of plants release O2.
Experiments by Julius von Sachs (1854)
He proved that
- Glucose is produced when plants grow and it is usually stored as starch.
- Chlorophyll is located in special bodies (chloroplasts).
- Glucose is made in the green parts of plants.
Experiments by T.W Engelmann (1843 – 1909)
He split the light using a prism into its spectral components and 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.
It was a first described action spectrum of photosynthesis. It resembles the absorption spectra of chlorophyll a & b.
By the middle of 19th century, it is discovered that plants use light energy to make carbohydrates from CO2 & H2O.
Empirical equation of the process of photosynthesis is
Where, [CH2O] represents a carbohydrate (e.g. glucose).
Experiments by Cornelius van Niel (1897-1985)
Van Niel (microbiologist) conducted some studies in purple and green bacteria.
He demonstrated that photosynthesis is a light-dependent reaction in which hydrogen from an oxidisable compound reduces CO2 to carbohydrates.
In plants, H2O is the hydrogen donor and is oxidised to O2.
Purple & green sulphur bacteria use H2S as H-donor. So the ‘oxidation’ product is sulphur or sulphate and no O2 is produced.
Thus, he inferred that the O2 evolved by the green plant comes from H2O, not from CO2. This was later proved by using radio isotopic techniques.
Therefore overall correct equation for photosynthesis is: