Info

Euglenophyta

euglenoids

unicellular

flagella

autotrophic, some heterotrophic

Euglena

Dictyostelida

cellular slime molds

unicellular, multicellular

pseudopodia

heterotrophic

Dictyostelium

Oomycota

water molds

unicellular, multicellular

some with flagella

heterotrophic, some parasitic

Phytophthora

figure 25-2

Unicellular and Multicellular

As shown in Figure 25-2, protists come in a wide variety of body plans. Most protists are unicellular, such as the amoeba shown in Figure 25-2a. Some protists, such as the Volvox in Figure 25-2b, form colonies in which several cells are joined into a larger body. Some of these colonies have a division of labor; certain cells specialize in reproduction, and other cells specialize in obtaining energy.

A few protists, such as the brown algae in Figure 25-2c and the pretzel slime mold in Figure 25-2d, form large multicellular bodies. Some brown algae may grow to more than 60 m in length. These marine giants have specialized regions for reproduction, photosynthesis, and attachment to the ocean floor. However, these regions lack the cellular differentiation found in true tissues and organs.

Nutrition

Protists obtain energy in a number of ways. Many protists are autotrophs, organisms that can make their own food molecules. These protists make food in much the same way that plants do. The protists absorb energy from sunlight with the aid of specialized light-absorbing pigments. Protists often utilize chlorophyll, as plants do, but they may use additional pigments. The protists use the captured light energy, water molecules, and carbon dioxide molecules to make carbohydrates.

Some protists are heterotrophs, organisms that must get their food by eating other organisms or their byproducts and remains. Some heterotrophic protists engulf smaller protists and digest them. Other heterotrophic protists obtain energy in the same way that fungi do. These protists secrete digestive enzymes into the environment. The enzymes break down cells or bits of food into small molecules that the protists can absorb and use.

Motility

Most protists are able to move at some time during their life cycles. Some protists move with the aid of long, whiplike structures called flagella (singular, flagellum). Other protists move with the aid of cilia (singular, cilium), which are shorter than flagella and often form rows. Finally, some protists, such as amoebas, move by temporarily extending structures called pseudopodia (singular, pseudopodium).

figure 25-2

Protists have a variety of body plans.

(a) The single-celled Amoeba proteus constantly changes its body shape.

(b) Cells from Volvox, a colonial protist, have coordinated activity. (c) Brown algae, such as this kelp (Macrocystis pyrifera), are multicellular giants among protists. (d) This pretzel slime mold (Hemitrichia serpula) reproduces similarly to fungi.

Sexual reproduction

Asexual reproduction

Sexual reproduction

Asexual reproduction

figure 25-3

Algae of the genus Chlamydomonas are unicellular green algae that undergo both asexual and sexual reproduction. For many protist producers, the type of reproduction alternates by generation. For example, a parent may reproduce asexually, but its offspring may reproduce sexually. For other protists, sexual reproduction occurs only when environmental conditions are stressful.

figure 25-3

Algae of the genus Chlamydomonas are unicellular green algae that undergo both asexual and sexual reproduction. For many protist producers, the type of reproduction alternates by generation. For example, a parent may reproduce asexually, but its offspring may reproduce sexually. For other protists, sexual reproduction occurs only when environmental conditions are stressful.

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