What characteristics of protists differentiate them from bacteria? What do protists do that bacteria and archaea do not do? How do you think these differences have affected their spread?
Physically, the cell structure of protists and prokaryotes differ. First, the external structures of prokaryotes consist of cell walls made of peptodoglycan. Protists do not have a cell wall, and instead have a semi-permeable plasma membrane in the form of a phospholipid bilayer. This membrane allowed specialized organelles to form within protists, including nuclei, chloroplasts and mitochondria. Protists also contain distinct structures that create a cytoskeleton for support within their cells.
Both protists and prokaryotes may be unicellular or colonial, but only protists have the ability to be multicellular organisms. A multicellular organism differs from a colony by consisting of specialized cells carrying out specific functions. Interactions between these cells are essential because, as cells differentiate, not all genes are transcribed in each cell, making them dysfunctional as an individual. Colonial cells, conversely, are made up of independent cells with the ability to transcribe all genes.
These differences have allowed protists to function differently from bacteria and archaea. The presence of a plasma membrane allows some protists to ingest food through membrane enfolding. This is an advantage because the organism can move away from the food source after engulfing large amounts to perform digestion elsewhere. The ability for protists to be multicellular has also caused them to be larger than other protists in many cases. Larger sizes mean that these organisms may prey on larger organisms.
In general, these adaptations have made protists more efficient in the way that they acquire organic compounds. Chloroplast allows for greater productivity while larger sizes and the presence of a plasma membrane increase consumption success. Protists can, therefore, out-compete other organisms in favorable areas, and are usually found in places with good environmental conditions as opposed to harsh habitats.