BUILDING OF LIVING MATTER 113 



the sun shines." Sunlight energy is taken, 

 water broken up, its hydrogen built up with 

 the carbon of carbon-dioxide into an organic 

 substance. Then, on reversal of the process, 

 this organic substance can be oxidized again, 

 yielding once more its hydrogen as water, its 

 carbon as carbon-dioxide, and its energy for 

 the life-processes. The process by which the 

 green plant first builds up carbohydrate may 

 be represented very simply ; if the carbon- 

 dioxide from the air passes into union with the 

 water in the plant juices there is formed 

 carbonic acid, which may be represented, thus 



OH 



I If now, sunlight acts upon this 



HO-C=0 



splitting up one of the O H groups and 

 replacing it by H, the oxygen being given off 

 by the plant to the air, the formula becomes 



H 



I 

 HO-C=0 



This is formic acid, and contains the 

 characteristic carbohydrate grouping, carbon, 

 hydrogen, and hydroxyl united together. 

 When it is again reduced and united up with 

 others to form a six carbon chain, a carbo- 

 hydrate is the result. 



