Animal-like Protists

    Around 65,000 kinds of animal-like Protists have been known and studied. They are more usually called Protozoan (Greek: proto = first; zoa = animals). Their characteristics are unicellular and heterotrophic.

 

Body Characteristics

    Protozoan characteristics include size and shape, as well as body structure and function.

Size and shape

    Protozoans are microscopic in size, around 10 to 200 microns (µm). Their cell shape is quite variable; some are fixed, while others are changeable. Most Protozoan have some sort of motion appendage, such as pseudopodia (fake foot), cilia (soft hair), or flagella (whip-hair). Some also have a protective shell.

Structure and function

    Basically, Protozoa are composed of a cell membrane, cytoplasm, food vacuole, contractile vacuole, and nucleus. The cell membrane acts as a protective barrier and regulates food and gas exchange. A food vacuole is formed by ingesting or 'swallowing' molecules using the cell membrane or a spesific structure called cytostome (cell's mouth). The digested products of the food vacuole then enter the cytoplasm by diffusion. Food remains are exported through thr cell membrane by the vacuole. The contractile vacuole is a pump that excretes liquid food remains out of the cell by controlling and expanding, hence its name. The nucleus function is to control cell metabolic activity.

 

Life Styles

    Protozoans are heterotrophic. They prey on bacteria, other Protists, and organic waste. As a bacterial predator, Protozoa play important role in controlling bacteria population in the wild.

 

Habitat

    Protozoans are solitary or form colonies in a diverse range of habitats. Most of them live freely in ocean or freshwater, such as gutters, ponds, or rivers. Other live inside the soil. Some live symbiotically inside the bodies of animals or humans.     

 

Reproduction

    Most Protozoan reproduce asexually by binary fission. Cell division is started by karyokinesis (nuclear division) followed by cytokinesis (cytoplasm division). Those that reproduce sexually do so by combining gametes cells or vegetative. Sexual reproduction by the union of vegetative nuclear is called conjugation.

    In their life cycle, several Protozoans can produce inactive form of cells called a cyst. A cyst is covered by a polysaccharide capsule that protects the Protozoan from unfavorable environments, such as droughts. Once conditions are favorable again for example when food and water available, the cyst will break apart and new protozoa are alive again.

 

Classification

    There are more than 60,000 species of protozoan already identified. They are classified into four groups based on their method of motility; Rhizopoda, Ciliata, Flagellata, and Sporozoa.