Among the forms of energy liberated by protoplasm are motion (as in 

 muscles), heat, electricity, light, and the processes that are confined (so far 

 as we know) to nerve and brain cells, such as thinking, wishing, suffering, 

 enjoying. In glowworms and fireflies, as well as in certain bacteria, slow 

 oxidation liberates much of the energy in a sugar as light. 



Air as Raw Material Although carbon dioxide is but a fraction of 

 1 per cent of the atmosphere, it is a very important factor in the life of the 

 world. For this fraction is a considerable part of the raw material out of 

 which the green plants make sugars and starches (see pages 137-138). And 

 these in turn are the beginnings of all foods, for us and other animals, as well 

 as for the plants. 



How Does Exchange of Materials Take Place between Living Cells 



and Their Surroundings? 



Diffusion^ If a bottle of perfume or ammonia is opened in a corner of 

 a room, the odor will become perceptible in all parts of the room. Sugar 

 left in the bottom of your coffee, without stirring, will in time spread 

 throughout the liquid. Every portion of the now cold coffee will become 

 equally sweet. The process by which a liquid or gas penetrates another 

 liquid or gas is called diffusion, a "spreading apart". 



When salt or sugar gradually diffuses from the bottom of a vessel of 

 water to all levels, "work" is going on. For material is being raised against 

 gravity and distributed through space. It helps us to understand what hap- 



Sugar molecules •Semipermeable membrane separating 



* tvfo liquids 



K^' 



.H^/ vi^. ' *•* * ^!.o ! -%^^\ * :• % 







— Water 

 molecules 



-Wall of 

 containing 

 vessel 



DIFFUSION THROUGH A MEMBRANE 



We may think of the molecules in any liquid or gas as in constant motion. Some 

 molecules are smaller than others. In the diagram the sugar molecules are repre- 

 sented as too large to pass through the pores of the semipermeable membrane. 

 Since more water molecules bombard a given area on the right side of the mem- 

 brane than on the left side, more water moves toward the left side than in the reverse 

 direction 



^See No. 6, p. 94. 



85 



