144 



ELEMENTARY BIOLOGY 



influence of certain ferments the fats or carbohydrates in the 

 protoplasm break up, forming simpler compounds. But these 

 processes can continue only to a certain point unless new 



oxygen is constantly supplied. 



175. Breathing. Strictly speaking, 

 breathing is a process of gas ex- 

 change, — the taking in of oxygen 

 and the giving off of carbon dioxid. 

 BveatJiing, or respiration, makes oxi- 

 dation, or energesis, possible ; but 

 they are not the same. In the low- 

 est plants and animals, which get 

 their oxygen directly from the sur- 

 rounding air or water by osmosis, 

 and give off their carbon dioxid 

 directly to the surrounding medium, 

 respiration and oxidation are indeed 

 closely connected in space and in 

 time. But in higher, more complex 

 plants and animals there is sometimes 

 a considerable separation between the 

 two processes, as we shall see. 



176. Cell respiration. In an or- 



each cell receives oxygen, as well g^uism made Up of VCry many CClls 

 as food, by diffusion from the "^^ r j j 



the cells that are farthest from the 

 surface must necessarily get their 

 oxygen supply in some indirect way. 

 In the interior of a leaf we have 

 seen that there is a constant circu- 

 lation of air among the cells, the 

 spaces between the cells being connected with the outside air 

 by way of the stomates (p. 71). In the young twigs the 

 epidermis also carries stomates that connect with the intercel- 

 lular spaces below the surface. In the older twigs, however, 

 in which the bark formation has gone on for some time, the 



Fig. 42. Cell respiration 



In one of the higher animals 

 rece 

 by 

 surrounding fluids, which in turn 

 communicate with the blood 

 stream. Each cell throws out into 

 the blood stream carbon dioxid, 

 urea, and other products of pro- 

 'toplasm activity by diffusion 

 through the membrane 



