122 Living Things Are Basically Alike unit ii 



then in another. In liquids the molecules [T ~ ~J 



are closer together; they move, but move l^^^^^^^M/ C 



less actively than in gases. If two gases l^^^^^^^^l 



are put together, the molecules of both f^^^^^^^^f 



eases move actively and the gases inter- 1^^^^^^^/ B ItSni^^^-c^^^ 



mingle rapidly. If two liquids are put l^^^^^^^/--- —wi^" *"" 



together, the molecules of both liquids 1^^^^^^/ 



-1 i^MrolMWIal 



usually move about and intermmgle, nQ9KR0l| 



though more slowly. This intermingling i jlMMW^/ l^g^jgiil^/ II 



of substances through the motion of their ^^^ ^^^ ^^^^^^ .noleades arc sbov^^n as nian- 



molecules is called diffusion. Even in g/^^^ water ?nolecules as circles, hi 1, sugar mole- 



solids the molecules intermingle or dif- cules {molasses) have just been put in ivith a 



r 1 1 11. -.^A^^A dropper. II shows what has happened after a 



fuse, but they move very slowly mdeed. " ^^ . ^Tr; ; i i ^L j 



■ • •' short tt7/re. Why have sugar vwlecuies appeared 



Do Exercise io. at level B? Compare the mmiber of water viole- 



By using liquids which differ in color czdes at Level A in I and II. Explain. Draw the 



you can actually watch them diffuse nmibler as it would look after longer standing. 



' Draw it as it woidd look if half as viuch sugar 



though, of course, you cannot see the j^^^ y^^^ ^^^^ ^.„ 



molecules. If you carefully put warm 



molasses with a medicine dropper into oxygen are on the other side of the mem- 



the bottom of a tumbler of warm water brane. The membrane seems to have no 



and allow it to stand quietly you will openings. How can substances pass 



soon see these liquids intermingling, through the cell membrane? 



Each substance spreads or diffuses from Diffusion through a membrane.-'' A 



the region where its molecules are close number of simple experiments can be 



to one another (highly concentrated) to set up to find out whether liquids can 



where its molecules are farther apart pass through a membrane which has no 



(less concentrated). After some time the visible pores. To find out whether water 



molasses molecules are no longer close can pass through a membrane, a sausage 



together at the bottom; they have spread casing may be used as the membrane. 



or become less concentrated. The same This is the wall of a pig's intestine; it is 



is true of the water molecules; they have made of cells, although they are no 



also spread, and eventually the two longer living. If sausage casing is not 



liquids will have completely intermin- available a thin cellophane membrane 



gled. Both liquids have moved. may be substituted. This also has no 



Now food and oxygen move into all visible pores. The bowl of a thistle tube 



the living cells by diffusion. But you may may be filled with a mixture of water 



raise this objection: in the timibler there and molasses with the membrane tied 



was nothing to separate the molasses securely over the mouth of the tube. 



from the water; around the cell body The thistle tube may then be inverted 



there is a membrane and the food and and its mouth placed into a tumbler of 



Osmosis is often defined as diffusion through a membrane. However, osmosis 

 is variously defined and therefore, the authors think it better not to use the term at 

 all, especially since by any definition osmosis is diffusion under special conditions. 



