Question

Write a short note on an amoeba.

Answer 1

This is a unicellular organism that can be seen under an electron microscope. This organism was first discovered by R. V. Rosenhof in 1755. It moves with the help of pseudopodia.

Complete answer:
Amoeba belongs to the phylum protozoa and class Rhizopoda. The name amoeba is taken from the Greek word 'amoeba' which means change.
- Habit and habitat: It is found in freshwater saltwater wet soil and also in animals as a parasite.
- Structure: The shape of amoeba is irregular and its size varies between 250 microns to 600 microns.
- The body of amoeba is surrounded by a thin layer of the elastic plasma membrane called ectoplasm which is non-granular.
- Inside the ectoplasm there presents an inner granular endoplasm.
- The endoplasm consists of a nucleus, contractile vacuole, food vacuoles, water vacuole, pseudopodium, Golgi complex, mitochondria, lysosomes, and ribosomes.
- Locomotion: Pseudopodium is the locomotory organelle of amoeba. It moves from one place to another very slowly but with the help of pseudopodia. New pseudopodia are formed in the direction of movement and the previous one disappears slowly.
- Nutrition: Amoeba feeds on bacteria, diatoms, small ciliates, algae, etc. Amoeba shows the holozoic mode of nutrition which involves the following four steps:

1. Ingestion: It means taking in food.
2. Digestion: It means the breakdown of large food molecules into smaller ones so that they can be absorbed easily. This breakdown occurs with the help of enzymes.
3. Assimilation: It means changing digested food and other materials into protoplasm.
4. Egestion: It means the removal of undigested food materials out of the body.
   -Reproduction: In amoeba, reproduction takes place by three different processes. These are
5. Binary fission: The process of binary fission occurs under favorable conditions. Here pseudopodia are withdrawn and the nucleus is divided by mitosis.
6. Multiple fission: The process of multiple fission occurs under unfavorable conditions. Here also pseudopodia are withdrawn and the Amoeba becomes round. Then it gets surrounded by a three-layered cyst secreted by cytoplasm. Inside the nucleus continues to divide repeatedly forming a large number of nuclei each surrounded by a little protoplasm. The cyst is blown away by the wind and under favorable conditions, the cyst absorbs water and bursts, then each nucleus starts growing.
7. Sporulation: The process of sporulation occurs under unfavorable conditions. Here also pseudopodia are withdrawn but then the nuclear membrane breaks at one or more points. Now each part of the membrane gets surrounded by a little protoplasm forming a spore. Each spore is covered by a tough spore case for its protection. Later the cytoplasm breaks and the sports are set free which may be blown away by the wind. Then under favorable conditions, each spore starts feeding and growing.

Note:
- For locomotion, the body of the amoeba is dragged forward by the new pseudopodia.
- The entire process of binary fission takes about 33 minutes to complete.
- Multiple fission and sporulation both processes occur under unfavorable conditions.