Single-celled organisms - Protozoans

These one-celled organisms belong to the Kingdom Protista, which includes algae and lower fungi. Although most species of protozoans are invisible to the naked eye, they dominate the Earth's environment, occurring everywhere and in an amazing diversity of forms and functions.

Watch these videos to see how these organisms behave which we normally cannot see just with our eyes!

Paramecium - a slipper-shaped ciliate which is commonly found in freshwater ponds around the world.

Amoeba - a primitive organism characterized by its flowing movements, extending cytoplasm outward to form pseudopodia (false feet); this type of movement is considered to be the most primitive form of animal locomotion. Pseudopodia are also used for feeding. In response to chemical stimuli from smaller organisms, pseudopodia are induced to envelop the organism, at the same time forming a vacuole (cavity). Digestive enzymes are secreted into the cavity and the nutrients diffuse from the vacuole into the cytoplasm. Undigested food and wastes are eliminated through the ectoplasm. The ectoplasm also performs respiration, absorbing oxygen from the water and eliminating carbon dioxide. Amoebas reproduce asexually by binary fission.
Another video clip of amoeba

Euglena - a member of the protozoan order Euglenida, a remarkable group of single-celled creatures, many of which exhibit characteristics of both plants and animals. Like many protozoans, it is free-living, using a whiplike flagellum to move about. Euglena is one of the euglenoid genera that contain chlorophyll, allowing them to create their own food through photosynthesis. Euglena live in a variety of aquatic habitats, both freshwater and marine.
Some species that don't have a rigid cellulose wall have a flexible pellicle (envelope) that allows them to change their shapes. Food is absorbed directly through the cell surface or produced by photosynthesis; then it is stored as a complex carbohydrate. Euglena reproduce asexually, by longitudinal cell division; they are not know to reproduce sexually. Several species produce resting cysts that can withstand drying. Euglena live in fresh and brackish water habitats rich in organic matter. Some species develop tremendous populations as green or red "blooms" in ponds or lakes. Several colorless species are used to study cell growth and metabolism at high temperatures.

Closterium - a desmid, a microscopic green algae that occurs in all types of freshwater habitats. Desmids are typically one-celled, though sometimes filamentous or colonial, and are divided symmetrically into semicells connected at a central point. Closterium is characterized by a sickle shape and sometimes contains gypsum crystals.

Frustulia - one of 16,000 species of diatoms, one of the many groups of organisms that make up the algae. Diatoms are photosynthetic, but have rigid cell walls reinforced with silicon rather than cellulose. They can be found in all aquatic environments and, although they're unicellular, often live in large colonies.