The amoeba is the simplest unicellular organisms that is heterotrophic. It takes in complex food; digest it in a food vacuole; uses the soluble food for its activities and growth; and gets rid of the undigested food material. It is sensitive to the presence of food in it'd environment and moved towards it quickly. The following features help it obtain food:
• a sensitivity to the presence of organic substances in its environment;
• ability to move, using pseudopodia;
• ability to capture food using pseudopodia, and ingest it into its body in a food vacuole.
An amoeba gets rif kg excess water that enters its cells using a contractile vacuole. The amoeba cells grows to a certain size. Then it stops growing, and reproduces by dividing to form two daughter cells.
The paramecium is also a unicellular organism. However, its cell body is more complex than that of the amoeba. It had cilia that help it to move, waft food into its gullet, and strain food. It had a fixed mouth through which food enters and a fixed anal pore for passing out undigested food. The food vacuoles circulate along a definite path in the cytoplasm. It has special structures for anchoring itself while feeding. It has sensory cilia that help it to move to areas where food is plentiful. Anterior and posterior contractile vacuoles, with radiating channels draining into them, help to get rid of excess water from the body.
Another common unicellular organism is the euglena. This protist has both plant like and animal like features: it has chloroplast for making food and a gullet for taking in food.
By observing the behavior and mode of life of each of these unicellular protists, we can conclude that a cell is a living unit.
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