II.

Anatomy and Physiology

Most protists contain many mitochondria, membrane-bounded organelles (cellular parts) that break down complex organic molecules and, in the process, release the chemical energy that powers the rest of the cell. The physiology of these protists is very similar to the cellular physiology of plants, animals, and fungi. Mitochondria require oxygen. Protists that live in environments without oxygen—for instance, in black muds or inside the digestive tract of animals—generally do not have mitochondria, and they create cellular energy by processes unique to the kingdom Protista.

In addition to mitochondria and nuclei, membrane-bounded organelles called plastids are found in protists that perform photosynthesis. Plastids capture the energy of sunlight and convert the energy into a chemical form available for use in the cell. Plastids are also able to capture inorganic carbon dioxide from the atmosphere and turn it into organic carbon useful for cell growth. Pigments, including chlorophyll, synthesized within plastids capture the sunlight and give photosynthetic protists their distinct colors: The brown algae contain brown plastids; the green algae, green plastids; and the red algae, red plastids.

In all likelihood, plastids and mitochondria were once free-living bacteria that became organelles through the process of symbiosis (an intimate and ongoing association of two or more different species). Once controversial, the theory of the symbiotic origin of plastids and mitochondria is generally accepted today. DNA molecules within present-day plastids and mitochondria are probably relics of the DNA of free-living bacteria ancestors.

Some protists are capable of movement. Protists known as flagellates move by means of flagella, long whiplike structures that the protists beat to propel themselves through water. Other protists use numerous and comparatively short structures called cilia. One group of protists, the ciliates, are distinguished by the great number and unusual arrangement of cilia along their surface. The single-celled amoebas and their multicellular relatives the slime molds are protists that use pseudopodia, temporary protrusions of the cell, both for movement and to engulf prey.

The vast majority of protists reproduce asexually—cells simply grow and divide. Sexual recombination (the production of gametes and the merging of DNA from two individuals to form a zygote) has been observed in some protists. However, many protists have been only poorly studied.