One group of protists, the algae, are autotrophic photosynthesizers, while the rest eat bacterial or other protistan cells, or small organic particles suspended or dissolved in water. Due to these feeding habits, and the weakness of non-aquatic locomotion capabilities, protists thrive in aquatic habitats. As with all eukaryotic cells, protists contain membrane-bound nuclei and endomembrane systems, as well as numerous organelles. Movement is often provided by one or more flagella, and cilia are often present on the plasma membrane as sensory organelles. Unlike prokaryotes, protistan nuclei contain multiple DNA strands, though the total number of nucleotides is significantly less than in more complex eukaryotes.
Protists can reproduce mitotically, and some are capable of meiosis for sexual reproduction.
Cellular respiration in the kingdom is primarily an aerobic process, but some protists, including those that live in mud below ponds or in animal digestive tracts, are strict or facultative anaerobes. Protists represent an important step in early evolution, evolving from prokaryotes and eventually giving rise to the entire line of eukaryotes. The first protists probably evolved 1.7 billion years ago, 2.3 billion years after the origin of life, from simple communities of prokaryotic cells. The second major step in the evolution of protists from bacteria was the process of endosymbiosis, which introduced the mitochondrion and chloroplast as organelles of eukaryotic cells. Small prokaryotic cells capable of cellular respiration or photosynthesis entered eukaryotic cells, either as parasites or indigestible food, and these prokaryotes evolved into mitochondria and chloroplasts as they developed a symbiotic relationship with the host cell.