Protista (Amoeba)

Amoeba belong to the kingdom Protista. It is a microscopic unicellular organism that lives at the bottom of freshwater ponds and streams.

Nutrition: Heterotrophic - gets food from external sources. Omnivorous - eats other single-celled protists, plants and animals.It feeds by enclosing its food with a drop of water inside a temporary food vacuole (phagocytosis).Food vacuoles are small membrane bound spaces containing solid ingested food.Amoeba feed on bacteria, protists smaller than themselves and dead orgainc matter.Digestive enzymes pass into the food vacuole from the cytoplasm, the food is digested and the small, soluble particles of digestion pass out of the vacuole and into the cytoplasm.

Movement:The amoeba move by forming false feet called pseudopodiaCytoplasm flows into the pseudopodia propelling the organism towards or away from stimuli.

Reproduction:Reproduction is by means of binary fission (where the cell splits in two to form two identical daughter cells)

Structure of amoeba:The living material in the cell (protoplasm) is surrounded by a flexible membrane.The outer cytoplasm  (ectoplasm) is clear and relatively stiff. The inner cytoplasm (endoplasm) is grainy due to food vacuoles and waste materials. The endoplasm is more fluid than the ectoplasm.The endoplasm contains a variable number of food vacuoles, fat droplets, and crystals of waste products. There is usually a single, large contractile vacuole present. Sometimes a number of these vacuoles are found.

Functions of cell structures:The cell membrane retains the cells contents in place. In addition, gases diffuse in and out through the cell membrane.The nucleus is the control centre of the cell.Food vacuoles secrete acids to kill prey. They then produce enzymes to digest the prey. Digested material is absorbed from the food vacuole into the cytoplasm.The pseudopodia extend in the direction the amoeba wants to move. The cytoplasm (especially the endoplasm) moves or flows into the pseudopodia causing the cell to move. The pseudopodia are aslo used to surround (engulf) its prey.Amoeba live in fresh water. This means its cytoplasm is more concentrated than its external environment. As a result, water enters the amoeba by osmosis.The contractile vacuole collects water that enters the amoeba. It then expands, touches the cell membrane, and bursts to expel the water. It is said to be responsible for osmoregulation. Without it, amoeba waould expand and burst.

* The endoplasm supports the nucleus, contractile vacuoles and mitochondria --> usually in motion, this is referred to as cytoplasmic streaming (the driving force for the formation os pseudopodia)*Amoeba proteus (harmless)Entamoeba histolytica (amoebic dysentry) --> this species exists in the Tropics, can prove fatal (vomiting, abdominal pain)Infection is through faecal contamination of food or water through very poor hygeine. Treatment is by means of anti-amoebic drugs.*Ectoplasm: Outer, stiff, gel-like solid part of the cytoplasm, slows down the inflow of water and plays a role in the formation of pseudopodia.*Osmoregulation: Regulating salt and water. The contactile vacuole maintain the amoeba's cytoplasm at a correct concentration for efficient metabolism. Marine and parasitic amoeba often lack a contractile vacuole beacuse the cytoplasm and external environment are close or identical in concentration (isotonic).

Amoeba

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