354 UNITY AND DIVERSITY IN BIOCHEMISTRY 



the annual production of steel throughout the world, which is 100 million 

 tons per annum. 



According to Loomis, photosynthesis produces 270,000,000,000 tons of 

 glucose per annum and consumes 396,000,000,000 tons of carbon dioxide. 

 Statistically, over 2000 years all the COg molecules in the air will have been 

 incorporated into the biosphere at some time or other. 



In plants and algae, photosynthesis takes place in cytoplasmic inclusions, 

 the chloroplasts. 



(a) The Chloroplasts 



The chloroplasts in green algae and plants are the seat of photosynthessi 

 and producers of glucose- 1 -P. They are disc-shaped particles 3 to 10 (x in 

 diameter and 1 to 2 [i, in thickness. It is possible to isolate them from 

 leaves and show that they are bounded by a definite membrane which is 

 semi-permeable. The chloroplasts contain a number of gratia whose 

 diameter varies, according to the type of cell, from 0-2 to 2 [x. A chloroplast 

 contains from 10 to 100 grana imbedded in a protein matrix. The electron 

 microscope reveals that they have a laminar structure. They contain 33 to 

 50% protein and also contain lipides. The chloroplasts are auto- 

 reproductive and can divide. 



There are two types of pigments in the chloroplasts, chlorophyll and 

 carotenoids. Besides chlorophyll, the chloroplasts of the red algae contain 

 other tetrapyrrole pigments which are active in photosynthesis, these are 

 the phycobilins. The leaves of green plants often contain in the vacuole 

 (i.e. outside the cytoplasm), pigments such as the anthocyanins which do 

 not play any part in photosynthesis. 



(b) History of the Ideas Relative to Photosynthesis 



It was not until 1727 that Stephen Hales expressed doubts about the 

 correctness of the then current view that plants obtained all their nourish- 

 ment from the soil. Hales suggested that air and light also played a part. 

 The next step was due to Priestley who showed, in 1771, that green plants 

 gave out oxygen. The following year Ingenhouz demonstrated the part 

 played by light in this phenomenon. In 1782 Senebier discovered that 

 CO2 was also concerned in the production, in light, of oxygen by green 

 plants. In 1804 quantitative methods of study were applied by Saussure. 

 He showed that the weight gained by a plant over a given period during 

 photosynthesis, plus the weight o:^ oxygen liberated, is greater than the 

 weight of CO2 taken in. He suggested that water played a part. In 1941 

 Van Niel showed that the photosynthetic purple algae, the Thiorhodaceae, 

 can multiply in an anaerobic inorganic medium provided that the medium 

 contains HgS. It was also shown that the amount of growth is proportional 

 to the HgS concentration. So photosynthesis must be considered as being 



