MONOCELLULAR ANIMALS-THE PROTOZOA 



m 



accumulate the bulk of the protoplasm, 

 causing the others to retract and follow the 

 cell body in a specific direction. 



Ingestion of food 



The activity of amoeba is about the same 

 during locomotion and in food-getting, with 

 one or two minor exceptions. When it ap- 

 proaches a motionless particle of food, such 

 as an immotile alga, the pseudopods are 

 spread around and over the plant cell in 

 close proximity until they meet, forming a 

 food vacuole. If the food is active, as in the 

 case of a small protozoan, the amoeba 

 spreads its pseudopods over a larger area 

 in order to capture the organism first before 

 closing in on it to form the vacuole (Fig. 

 7-5). 



Once the food vacuole is within the cell 

 body, digestion begins, much the same as 

 it begins in man's stomach when a morsel of 

 food is swallowed. The surrounding proto- 

 plasm secretes enzymes into the vacuole 

 where they proceed to digest the captured 

 food. This interesting process can be ob- 

 served under the microscope. A captured 

 protozoan, for example, slows down in its 

 activity and finally ceases movement alto- 

 gether. It then begins to disintegrate and 

 after a time there is very little left of the 

 original organism, except the undigested 

 parts which, incidentally, are left behind 

 as the amoeba moves on its way. The di- 

 gested food passes into the protoplasm 

 where it is metabolized. This is the source 

 of energy which enables the amoeba to 

 keep continuously on the move and which 

 also provides the materials from which it 

 is able to grow and reproduce. 



Respiration and excretion 



The exchange of oxygen and carbon di- 

 oxide is a simple business in amoeba. Oxy- 

 gen moves into the protoplasm of the cell 

 by diffusion whenever the concentration of 

 the gas is greater outside than inside. Like- 

 wise carbon dioxide diffuses out of the cell 

 into the surrounding water when it forms 



as a result of metabolism within the organ- 

 ism. Since this little creature demands an 

 abundance of oxygen continuously, you 

 would not expect to find it in stagnant 

 water where there is little or no oxygen. 



As a result of the metabolism of nitrogen 

 containing compounds (amino acids), poi- 

 sonous wastes tend to accumulate in the 

 cell. These are removed by diffusion to the 

 outside fluid world also. They are not al- 

 lowed to accumulate because of their toxic 

 effects. 



There is a prominent organelle (little 

 organ) located variously in the cytoplasm 

 of the amoeba which requires some expla- 

 nation. It is the contractile vacuole. As the 

 amoeba is watched, a clear, spherical area 

 forms, which, while small at first, soon 

 grows to maximum size, then suddenly dis- 

 appears. It forms and disappears at regular 

 intervals. Those who have studied this care- 

 fullv believe that the contractile vacuole 

 probably functions only as a device for get- 

 ting rid of excess water that accumulates 

 inside the cell. In other words, it acts like 

 a bailer who works to get rid of the water 

 that is constantly flowing into a leaking 

 boat. Because of the hypertonicity of the 

 amoeba, water is constantly flowing into 

 its protoplasm. If it were not removed the 

 little animal would soon become water- 

 logged. Furthermore, if the concentration 

 of dissolved substances in the surrounding 

 water is increased (hypertonic), as it is in 

 sea water or in a solution with high salt 

 content, the contractile vacuole disappears. 

 If the marine amoeba, which has no con- 

 tractile vacuole, is placed in fresh water, 

 vacuoles form very soon. All evidence 

 points, therefore, to the fact that the con- 

 tractile vacuole functions merely as a hy- 

 drostatic organelle, a mechanism that con- 

 trols the flow of water out of the amoeba. 



Reproduction 



Amoeba, like all other living things, re- 

 produces itself. It does this in the simplest 

 way, namely, by dividing into two equal 



