21.2 Protist Structure and Function Lesson Overview 21.2 Protist Structure and Function

THINK ABOUT IT Protists move, sense the environment, digest food, and even reproduce—all within the confines of a single cell. Imagine what such cells would have to be like to succeed in the never-ending struggle for life on Earth. Protists are winners in that struggle.

How Protists Move How do protists move in the environment?

How Protists Move How do protists move in the environment? Some protists move by changing their cell shape, and some move by means of specialized organelles. Other protists do not move actively but are carried by wind, water, or other organisms.

Amoeboid Movement Many unicellular protists move by changing their shape, a process that makes use of cytoplasmic projections known as pseudopods. The cytoplasm of the amoeba, for example, streams into the pseudopod and the rest of the cell follows. This type of locomotion is called amoeboid movement and is found in many protists. Amoeboid movement is powered by a cytoskeletal protein called actin. Actin also plays a role in the muscle contractions of animals.

Cilia and Flagella Many protists move by means of cilia and flagella, structures supported by microtubules. Cilia are short and numerous, and they move somewhat like oars on a boat. Flagella are relatively long and usually number only one or two per cell. Some flagella spin like tiny propellers, but most produce a wavelike motion from base to tip.

Cilia and Flagella Protists that move using cilia are known as ciliates, and those that move with flagella are called flagellates.

Passive Movement Some protists are nonmotile—they depend on air or water currents and other organisms to carry them around. These protists form reproductive cells called spores that can enter the cells of other organisms and live as parasites. Spore-forming protists include Plasmodium, which is carried by mosquitoes and causes malaria, and Cryptosporidium, which spreads through contaminated drinking water and causes severe intestinal disease.

Protist Reproduction How do protists reproduce?

Protist Reproduction How do protists reproduce? Some protists reproduce asexually by mitosis. Others have life cycles that combine asexual and sexual forms of reproduction.

Cell Division Amoebas, and many other protists, reproduce by mitosis: They duplicate their genetic material and then divide into two genetically identical cells. Mitosis enables protists to reproduce rapidly, especially under ideal conditions, but it produces cells that are genetically identical to the parent cell, and thus limits the development of genetic diversity.

Conjugation Paramecia and most ciliates reproduce asexually by mitotic cell division. However, under stress, paramecia can remake themselves through conjugation—a process in which two organisms exchange genetic material. After conjugating, the cells then reproduce by mitosis.

Conjugation Paramecium has two types of nuclei: a macronucleus and one or more smaller micronuclei. The micronucleus holds a “reserve copy” of every gene in the cell. The macronucleus has multiple copies of the genes the cell uses in its day-to-day activities.

Conjugation

Conjugation

Conjugation

Conjugation

Conjugation

Conjugation

Conjugation

Conjugation Conjugation is not a type of reproduction because no new individuals are formed. Conjugation is, however, a sexual process because new combinations of genetic information are produced. In a large population, conjugation helps produce and maintain genetic diversity.

Sexual Reproduction Many protists have complex sexual life cycles in which they alternate between a diploid and a haploid phase, a process known as alternation of generations.

Sexual Reproduction A water mold is an example of a protist that undergoes alternation of generations.

Sexual Reproduction Water molds grow into long branching filaments consisting of many cells formed by mitotic cell division.

Sexual Reproduction Water molds reproduce asexually by producing spores in a structure called a sporangium. In water molds the spores are flagellated.

Sexual Reproduction Water molds also reproduce sexually by undergoing meiosis and forming male and female structures.

Sexual Reproduction The male and female structures produce haploid nuclei that fuse during fertilization, forming a zygote that begins a new life cycle.