14 PLANTS AND MAN 



minerals into food. There are some autotrophic bacteria Hving 

 today which may represent this simple type of constructive 

 metabolism, showing the method by which the first living things 

 lifted themselves out of the realm of the non-living into that of 

 organic compounds and protoplasmic colloids. These are the 

 iron and sulphur bacteria found in stagnant pools. Their proto- 

 plasm has the ability to oxidize iron or sulphur compounds and 

 to thus release small amounts of energy. With this energy at their 

 disposal, the bacteria decompose water and carbon dioxide, 

 substances which diffuse easily into the cell. The carbon dioxide 

 of the atmosphere is the sole source of the carbon utilized by the 

 plant cell; and the water is the only source of the hydrogen 

 needed. Out of the atoms of carbon, hydrogen and oxygen thus 

 released from these two compounds, the bacterial protoplasm 

 synthesizes a simple carbohydrate such as sugar. This is the 

 simplest form of constructive metabolism, often referred to as 

 carbon synthesis. It can be represented in equation form as 

 follows: 



carbon dioxide + water + energy (secured from oxidation of 

 sulphur) = food (sugar) + oxygen. 



The sugar is used as a basis for further elaboration into other 

 carbohydrates and fats, or may be transformed into proteins 

 by the addition of nitrogen, phosphorus and sulphur secured by 

 the absorption of their salts. From proteins new protoplasm — 

 and more life — can be constructed. 



As protoplasm increased in complexity and new structures 

 made their appearance, this primitive type of metabolism 

 gradually gave way to the more specialized types characteristic 

 of the majority of living plants and animals. It is in this con- 

 structive phase of metabolism that we find the most basic differ- 

 ence between the two. We have already discovered some minor 

 ones, such as differences in the cell wall and cell contents. The 

 cellulose wall results in the more rigid aspects of plant bodies 

 with their lessened ability to move about; and the cytoplasmic 

 inclusions give vegetation their characteristic colors. But the 

 feature which has determined to a great extent all of the super- 

 ficial differences between the two kingdoms is the result of two 



