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Nutrition in amoeba is holozoic. Thus, solid food particles are ingested which are then acted upon by enzymes and digested. It is an omnivore, feeding on both plants and animals. Its diet includes bacteria, microscopic plants like the diatoms, minute algae, microscopic animals like other protozoa, nematodes and even dead organic matter.
Since it is a unicellular organism, amoeba does not have any specialised structure or organ for the process of nutrition. It takes place through the general body surface with the help of pseudopodia.
Ingestion
The food is ingested at the point where it comes in touch with the cell surface with the help of pseudopodia. Pseudopodia engulf the food into the cytoplasm. The process of ingestion takes about two minutes.
Some methods of ingestion reported in amoeba are:
Circumvallation - When the prey is active, a food cup is formed with the help of pseudopodia.
Circumfluence - When the prey is inactive and the amoeba rolls over it.
Import - The food passively sinks into the body on contact.
Invagination - Pseudopodia secretes a sticky and toxic fluid which adheres and kills the prey. It is then taken in by invagination.
Pinocytosis - Also called cell drinking. There are pinocytosis channels at certain points through which the cell ingests the food.
Digestion in amoeba is intracellular taking place within the cell. The food taken in remains in a food vacuole or gastric vacuole formed by the cell membrane and small part of the cytoplasm. The vacuoles are transported deeper into the cells by cytoplasmic movements. Here they fuse with lysosomes that contain enzymes. Two enzymes amylase and proteinase have been reported. Thus, amoeba can digest sugars, cellulose and proteins. Fats, however, remain undigested.
The contents of the vacuole become lighter and the outline of the vacuole becomes indefinite indicating that the digestion is complete.
Since the food on digestion is converted into liquid diffusible form, it is readily absorbed by the cytoplasm. The vacuole becomes progressively smaller as the food is absorbed by diffusion.
All the parts of the cell get the nutrients by the cyclic movement of the cytoplasm called the cyclosis. These nutrients are used to build new protoplasm. In this manner the food is assimilated.
The egestion takes place by exocytosis. There is no particular point from which the egestion takes place. As the amoeba moves forward, the undigested matter is shifted to the back and eliminated as food pellets through a temporary opening formed at any nearest point on the plasmalemma.
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