106 Inside the Living Cell 



entropy and increase in organization if it is compensated by a greater 

 increase of entropy elsewhere. 



If we examine the matter more carefully, we find that life is not 

 independent of the great processes of the natural world. All life de- 

 pends directly on the great outflowing of energy from the sun, which 

 is a dissipative process, by which entropy is increased. Animal life is 

 entirely dependent on plants for food; and plants make direct use of 

 the energy of the sun's radiation. If the sun's rays were cut off, life 

 could not maintain itself for more than a very short time on this 

 planet. 



Thus we reach the conclusion that the elaboration of complex 

 living structures is directly dependent on the dissipation of solar 

 energy. There is a local decrease of entropy in the living organism, 

 but it is compensated by a great increase of entropy in the dissipation 

 of radiation from the sun. 



PHOTOSYNTHESIS 



The basic process on which all life depends is thus the utilization 

 of solar energy for the elaboration of complex compounds. However 

 this originally occurred, it now takes place almost entirely in the 

 photosynthetic apparatus of plants and algae, which have evolved a 

 highly efficient apparatus for making use of the visible radiations 

 arriving from the sun. 



The photosynthetic apparatus of green plants makes use of a green 

 pigment called chlorophyll, which is present in small granules called 

 chloroplasts, which exist in the green cells. Chlorophyll is a not re- 

 markably complex molecule, and its structure is largely known. 

 Its most distinctive feature is the presence of a magnesium atom 

 surrounded by four indole groups, making a kind of flat plate.^ 

 However, chlorophyll is only capable of bringing about photosyn- 

 thesis when it is in the chloroplast. In recent years much has been 

 learnt about the structure of these bodies from electron microscope 

 photographs of thin sections of them. All the chloroplasts which have 

 been examined show a laminated structure (see PI. 13), i.e. a system 

 like the sheets of a book packed parallel with each other. Sometimes 

 the sheets or lamellae run through the whole chloroplast but in other 

 cases there are a number of more or less separate piles of sheets. 

 Each lamella is found to consist of two membranes joined at the ends 

 and each membrane is 65 A thick^ and the space between them is 

 about the same thickness. This is about the thickness of a small pro- 



M A = 10-8 cm. 



2 Similar to haemoglobin (p. 164) with Mg in place of Fe. 



