Lesson Overview
Protist Structure and Function
Lesson Overview
21.2 Protist Structure
and Function
Lesson Overview
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 neverending struggle for life on Earth. Protists are winners in that struggle.
Lesson Overview
Protist Structure and Function
How Protists Move
How do protists move in the environment?
Lesson Overview
Protist Structure and Function
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.
Lesson Overview
Protist Structure and Function
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.
Lesson Overview
Protist Structure and Function
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.
Lesson Overview
Protist Structure and Function
Cilia and Flagella
Protists that move using cilia are known as ciliates, and those that move
with flagella are called flagellates.
Lesson Overview
Protist Structure and Function
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.
Lesson Overview
Protist Structure and Function
Protist Reproduction
How do protists reproduce?
Lesson Overview
Protist Structure and Function
Protist Reproduction
How do protists reproduce?
Some protists reproduce asexually by mitosis. Others have life cycles that
combine asexual and sexual forms of reproduction.
Lesson Overview
Protist Structure and Function
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.
Lesson Overview
Protist Structure and Function
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.
Lesson Overview
Protist Structure and Function
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.
Lesson Overview
Protist Structure and Function
Conjugation
Lesson Overview
Protist Structure and Function
Conjugation
Lesson Overview
Protist Structure and Function
Conjugation
Lesson Overview
Protist Structure and Function
Conjugation
Lesson Overview
Protist Structure and Function
Conjugation
Lesson Overview
Protist Structure and Function
Conjugation
Lesson Overview
Protist Structure and Function
Conjugation
Lesson Overview
Protist Structure and Function
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.
Lesson Overview
Protist Structure and Function
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.
Lesson Overview
Protist Structure and Function
Sexual Reproduction
A water mold is an example of a protist that undergoes alternation of
generations.
Lesson Overview
Protist Structure and Function
Sexual Reproduction
Water molds grow into long branching filaments consisting of many cells
formed by mitotic cell division.
Lesson Overview
Protist Structure and Function
Sexual Reproduction
Water molds reproduce asexually by producing spores in a structure
called a sporangium.
In water molds the spores are flagellated.
Lesson Overview
Protist Structure and Function
Sexual Reproduction
Water molds also reproduce sexually by undergoing meiosis and
forming male and female structures.
Lesson Overview
Protist Structure and Function
Sexual Reproduction
The male and female structures produce haploid nuclei that fuse during
fertilization, forming a zygote that begins a new life cycle.