Biology Now, 2e

288 ■ CHAPTER 16 Plants, Fungi, and Protists

BIODIVERSITY

protists are single-celled and microscopic, but

some protists have evolved from free-living

single cells into multicellular associations, such

as slime molds and kelp, large seaweed such as

kelp. Certain single-celled protists are bound by

nothing more than a flexible plasma membrane,

while others are covered in protective sheets,

heavy coats, or other types of armor.

Most protists are motile and can swim with

the help of one or more flagella, or by waving

a carpet of tiny hairs called cilia. Others can

crawl on a solid surface with the help of cellular

projections called pseudopodia (false feet).

Many protists are heterotrophs and eat other

organisms. Some of these heterotrophs function

as decomposers, breaking down waste material

and releasing nutrients into the environment to

be taken up by producers and cycled back into

the food chain. Other protists are nutritional

Peculiar Protists

Amoebas are single-celled eukaryotes. Most

single-celled eukaryotes are protists. Protista is

a strange kingdom that is defined by exclusion

instead of inclusion: although all protists are

eukaryotes, they are grouped together in this

kingdom simply because they are not plants,

animals, or fungi (Figure 16.3). Researchers

have proposed several classification schemes

to split the protists into multiple kingdoms;

however, no consensus exists on the best way to

do so. For now, protists remain divided into two

traditional, broad categories: the protozoans,

which are nonphotosynthetic and motile (capa-

ble of moving); and the algae, which are photo-

synthetic and may or may not be motile.

Protists are diverse in size, shape, cellu-

lar organization, and mode of nutrition. Most

Domain

Bacteria

Domain

Archaea

Kingdom

Protista

Kingdom

Fungi

Domain

Eukarya

Kingdom

Animalia

Common

ancestral

cell or

universal

ancestor

Kingdom

Plantae

Diplomonads,

others

Brown algae,

diatoms, others

Ciliates,

others

Forams,

others

Red algae

Green

algae

Amoebas,

others

Euglenoids,

others

Figure 16.3

Protista is a diverse kingdom

of eukaryotes

This Protista phylogeny is based on the best

evidence available about the evolutionary

relatedness of the different groups of protists.

It will change as scientists learn more about this

intriguing eukaryotic kingdom.

Q1: Are ciliates more closely related to

euglenoids or to diatoms? To euglenoids or

to forams?

Q2: Are all the algae groups (red, green, and

brown) equally related?

Q3: Which protist group do you think is most

closely related to plants? Justify your answer.