PROBLEM 2. How Cclls Keep Alive 



cells {absorptiov)\ the protoplasm acts 

 on some of the food, making it usable 

 (digestion)] the protoplasm makes more 

 protoplasm from food {assiiiiilation); 

 some of the food unites chemically with 

 oxygen, releasing energy which keeps 

 the cell alive (oxidatioji which is a part 

 of respiration); some of the energy is 

 used in movement of the protoplasm 

 (motio?2); or in moving the whole cell 

 from place to place {locomotio?i)\ the 

 protoplasm makes materials useful to it 

 or to other cells close by {secretion); 

 waste or unused substances diffuse out 

 of cells {excretion); as protoplasm grows 

 a cell may become separated into two 

 parts, making two new cells in the place 

 of one old one {reprodiictio?i); the pro- 

 toplasm is sensitive to its changing sur- 

 roundings and its activities are frequently 

 changed when the surroundings change 

 {irritability)^ Through all these activi- 

 ties cells remain alive. Taken together 

 these activities are "life." 



The cell theory or cell doctrine. When 

 Hooke discovered cells in the second 

 half of the 17th century other men 

 studied the parts of many different ani- 

 mals and plants and saw cells of various 

 kinds. But for almost a hundred years 

 little was added to the simple idea that 

 cells could be found in organisms. No 

 one knew what cells were or how they 

 were related to the life of the plant or 

 animal; the importance of the cell was 

 not understood. About 1820 a French 

 physician and biologist Dutrochet (Dew'- 

 tro-shay), published a statement in which 

 he said that it was clear that all living 

 things were made of cells and that what- 

 ever activities a living thing performs 

 must be performed by its cells. This 



y^^V^^"^ 



Strong salt solution 



Fig. 153 The upper cell has been placed in dis' 

 tilled Vfater, the lower cell in a salt solution. 

 Why does the cell wall in the upper cell bidge? 

 Why did the distilled water diffuse inward? 

 What has happened to the protoplasm of the 

 lower cell? Why did water diffuse out of it? 



important statement which seems so evi- 

 dent to us was apparently not accepted 

 at that time. Then in 1838 two German 

 biologists, Schleiden, who was a botanist, 

 and Schwann, who was a zoologist, 

 stated that living things are made up of 

 cells and that the cells are the important 

 part of the living thing. Schleiden and 

 Schwann were so well known that biolo- 

 gists everywhere began to accept the 

 cell theory, as it was called, the belief 

 that all living things are made of cells 

 and their products. But Schleiden and 

 Schwann still had not understood the 

 true structure of the cell. Little was 

 known about protoplasm; it had not even 

 been named. 



From that time to this, many millions 

 of plants and animals have been studied 

 under the microscope and very much 

 has been added to our knowledge of 

 cells. Soon after Schleiden and Schwann 

 made their contributions another botanist 



