Chapter Twelve 

 FOOD MAKING 



Photosynthesis, commonly called food making, Is a plant 

 process by which, when light falls on active chlorophyll, 

 sugars are made from water and carbon dioxide, with oxygen 

 given off as a by-product. Since It is the only source of 

 supply of two essential substances, food and oxygen, it is 

 impossible to explain in terms too extravagant the impor- 

 tance of this function of plants. Without photosynthesis 

 we can see no way for any form of life to exist over a period 

 of time. Energy from the sun is caught and stored in the 

 sugar or other carbohydrate material, and can later be used 

 as food by animals for their growth or as a source of their 

 energy. The oxygen, without which life cannot exist, is re- 

 leased to the atmosphere in quantities large enough for all 

 its many uses on this planet. 



Chemically it is expressed: 



6 H2O + 6 CO2 + Sun's Energy = CeHi^Oe + 6 O2 



Water Carbon dioxide Sugar Oxygen 



The water enters the plant through the roots and reaches 

 the leaf through the conducting system of the stem and leaf 

 petiole. The carbon dioxide enters the leaf from the air by 

 diffusion through the open stomata and then enters the leaf 

 cell by absorption into water from intercellular spaces. The 

 sun's energy is absorbed by the chlorophyll, but the exact 

 way in which the energy is used to make sugar is not known. 

 Only about 3 per cent of the energy falling on a leaf is used 

 in photosynthesis. Some of the light is reflected and some is 

 transmitted, but only the light absorbed by the chloroplasts 

 (shown in the leaf, Fig. 15) can be used by them. The oxy- 

 gen, shown in the chemical formula to be equal to the amount 



79 



