58 PRINCIPLES OF ANIMAL BIOWGY 



with the process of getting air into the body and out of it again. The 

 hitter process is only incidental to the real respiration which takes place 

 in the cells. 



Respiration results in the production of carbon dioxide and water 

 or of certain other intermediate compounds, and in the release of energy, 

 most commonly in the form of heat and motion. 



In the simple animals and plants dissolved oxygen passes by osmosis 

 directly through the surface of the organism into the protoplasm. Thence 

 by diffusion and protoplasmic currents it is carried to all parts of the 

 cell where it is used in oxidative processes. In many small aquatic 

 multicellular animals and plants with few layers of cells the dissolved 

 oxygen may readily diffuse through the intervening cells to those which 

 lie deeper. The distribution of dissolved oxygen to the various cells in 

 higher plants and animals is discussed in a later chapter. 



Respiration and Photosynthesis. — The absorption of carbon dioxide 

 and release of oxygen by green plants in sunlight are sometimes 

 popularly but erroneously attributed to respiration, and have led 

 to the mistaken notion that respiration in plants is the reverse of 

 the corresponding process in animals. The behavior of the gases 

 in green plants just referred to is, as pointed out above, due to 

 photosynthesis. A comparison of the processes of respiration and photo- 

 synthesis shows that they are entirely distinct processes. Respiration 

 is the same in both plants and animals. It is not a synthetic process 

 but a destructive process yielding for the most part carbon dioxide and 

 water as the end products. Photosynthesis is a constructive process 

 in which carbon dioxide and water, as raw materials, are elaborated into 

 carbohydrates by the chloroplasts which employ the energy of light for 

 this transformation. By this process energy is stored. A by-product 

 of photosynthesis is oxygen. In respiration, contrariwise, oxygen is 

 consumed in the oxidation of elaborated foods, energy is released and the 

 by-products are carbon dioxide and water. From this incomplete com- 

 parison it may be seen that these two processes are^clTstinctly opposed 

 to each other. In plants in the sunlight the two processes go on simul- 

 taneously but when photosynthesis is proceeding energetically the process 

 of respiration is somewhat masked. In the dark photosynthesis stops 

 while respiration continues. 



Excretion. — The end products of the oxidation of carbohydrates and 

 fats in respiration are carbon dioxide and water. When a protein is 

 combined with oxygen there are formed some nitrogen compounds in 

 addition to carbon dioxide and water. These nitrogen compounds differ 

 according to the completeness of the oxidation but one of the principal 

 ones is urea. The process of getting these substances out of the proto- 

 plasm is called excretion and the nitrogenous compounds and the carbon 

 dioxide are waste materials or excretions. These waste substances pass 



