The cells of protists are among the most elaborate of all cells. Multicellular plants, animals, and fungi are embedded among the protists in eukaryotic phylogeny. In most plants and animals and some fungi, complexity arises out of multicellularity, tissue specialization, and subsequent interaction because of these features.
Although a rudimentary form of multicellularity exists among some of the organisms labelled as “protists,” those that have remained unicellular show how complexity can evolve in the absence of true multicellularity, with the differentiation of cellular morphology and function. An example of a complex protist are the Trychonymphas . These are single-celled protists that live in the gut of termites and help the termites digest wood cellulose. And while the Trychonymphas are eukaryotic organisms, like all protists, they lack functional mitochondria (there are several groups of protists that lack mitochondria). The lack of functional mitochondria in some protist groups is another example of the diversity and complexity of these organisms! The image below shows an example of a Trychonympha associated with the gut of a termite species.
A few protists live as colonies that behave in some ways as a group of free-living cells and in other ways as a multicellular organism. Some protists are composed of enormous, multinucleate, single cells that look like amorphous blobs of slime, or in other cases, like ferns. In some species of protists, the nuclei are different sizes and have distinct roles in protist cell function. Single protist cells range in size from less than a micrometer to three meters in length to hectares!
Protist cells may be enveloped by animal-like cell membranes or plant-like cell walls. Others, like the diatoms pictured below, are encased in glassy silica-based shells.
Protists exhibit many forms of nutrition and may be aerobic or anaerobic. Those that store energy by photosynthesis belong to a group of photoautotrophs and are characterized by the presence of chloroplasts. Other protists are heterotrophic and consume organic materials (such as other organisms) to obtain nutrition. There are even some protists that are capable of being primarily photosynthetic when sunlight is available and primarily heterotrophic when sunlight is limiting, and food organisms are more readily available - these organisms are called "mixotrophs ".
Amoebas and some other heterotrophic protist species ingest particles by a process called phagocytosis, in which the cell membrane engulfs a food particle and brings it inward, pinching off an intracellular membranous sac, or vesicle, called a food vacuole. The vesicle containing the ingested particle then fuses with a lysosome containing hydrolytic enzymes to produce a phagolysosome, and the food particle is broken down into small molecules that can diffuse into the cytoplasm and be used in cellular metabolism.
This short video shows an Amoeba engulfing a Paramecium prey:
To watch this video on YouTube (and see closed captioning) - press the arrow icon in the bottom right corner of the video player.