Protists are divided in three distinct groups as they differ in their mode of nutrition. Is that true?
Answers
Protists are a diverse collection of organisms. While exceptions exist, they are primarily microscopic and unicellular, or made up of a single cell. The cells of protists are highly organized with a nucleus and specialized cellular machinery called organelles.
At one time, simple organisms such as amoebas and single-celled algae were classified together in a single taxonomic category: the kingdom Protista. However, the emergence of better genetic information has since led to a clearer understanding of evolutionary relationships among different groups of protists, and this classification system was rendered defunct. Understanding protists and their evolutionary history continues to be a matter of scientific discovery and discussion.
Characteristics
All living organisms can be broadly divided into two groups — prokaryotes and eukaryotes — which are distinguished by the relative complexity of their cells. In contrast to prokaryotic cells, eukaryotic cells are highly organized. Bacteria and archaea are prokaryotes, while all other living organisms — protists, plants, animals and fungi — are eukaryotes.
Many diverse organisms including algae, amoebas, ciliates (such as paramecium) fit the general moniker of protist. "The simplest definition is that protists are all the eukaryotic organisms that are not animals, plants or fungi," said Alastair Simpson, a professor in the department of biology at Dalhousie University. The vast majority of protists are unicellular or form colonies consisting of one or a couple of distinct kinds of cells, according to Simpson. He further explained that there are examples of multicellular protists among brown algae and certain red algae.
Cells
Like all eukaryotic cells, those of protists have a characteristic central compartment called the nucleus, which houses their genetic material. They also have specialized cellular machinery called organelles that execute defined functions within the cell. Photosynthetic protists such as the various types of algae contain plastids. These organelles serve as the site of photosynthesis (the process of harvesting sunlight to produce nutrients in the form of carbohydrates). The plastids of some protists are similar to those of plants. According to Simpson, others protists have plastids that differ in the color, the repertoire of photosynthetic pigments and even the number of membranes that enclose the organelle, as in the case of diatoms and dinoflagellates, which constitute phytoplankton in the ocean.