Protists are simple structures that can have either single or multiple cells. The three common forms of protist are plant-like, fungus-like and animal-like protists. Protists form the basis of life in all forms in the animal and plant kingdoms, with the earliest fossil records dating back about 200 million years.
The species of plant-like protists are autotrophic, meaning they make their own food and produce a large amount of oxygen, which plays an important role in maintaining our environment. Protists of plant-like forms form the basis of the aquatic food chain and form in moist environments; they require large amounts of water for survival. Plant-like protist species survive by living in environments such as soils and tree bark; they can also live abundantly in salt water.
Different forms of plant-like protist species divide cells during reproduction through either mitosis, meiosis or both. Mitosis consists of the production of a new set of cells that are an exact reproduction of the existing cells of the protist. In meiosis, each cell divides to form four haploid cells that contain the genetic information of the protist.
Female plants reproduce by holding eggs within their stems; sperm from male plants fertilize these eggs. Some plant-like protist species can release their eggs into an aquatic environment for fertilization. The eggs travel and come into contact with male reproductive organs for fertilization. Only some protist species actually have stem structures capable of holding eggs within them.
Green algae, among the most common forms of plant-like protist, live in marine environments on the surface of the water. Often unicellular, green algae live in colonies. Red algae is found in both salt and freshwater environments; pigment in the algae forms the red or brown color of the protist. The euglenoid species of protist is autotrophic in the daytime, when it forms its own food using sunlight. In the nighttime hours, the euglenoid becomes a heterotrophic species, meaning the protist moves to find food using its light-sensitive receptor to judge the amount of available light.