1. 21.2 Protist Structure and Function
Lesson Overview
21.2 Protist Structure and Function

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

3. 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.

4. Cilia and Flagella
Many protists move by means of cilia and flagella, structures supported by microtubules. Cilia and flagella have almost identical internal structures but they produce cellular motion differently.
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.

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

6. 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.

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

8. 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.

9. 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.

10. 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.

11. Conjugation

12. Conjugation

13. Conjugation

14. Conjugation

15. Conjugation

16. Conjugation

17. Conjugation

18. 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 through meiosis.
In a large population, conjugation helps produce and maintain genetic diversity, the raw material for evolution.

19. 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.

20. Sexual Reproduction
A water mold is an example of a protist that undergoes alternation of generations. Water molds, or oomycetes, thrive on dead and decaying organic matter in water or as parasites of plants on land.

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

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

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

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