You are a multicellular organism. You have many specialized cells that

work together to carry out all of life’s functions. However, many living

things are composed of just one cell. These unicellular organisms,

referred to as microorganisms or microbes because they are only visible

under a microscope, must also carry out all of life’s functions. The

single cell is responsible for feeding, digestion, excretion, and

reproduction.

The Importance of MicroorganismsMost people become aware of microorganisms when they get sick.

However, it is unfair to think of microorganisms just in terms of

disease. It’s true that they cause many diseases, but most are harmless

and many are even helpful, as you can see in Figure 1. Dairy products

such as buttermilk, cottage cheese, and yogurt are produced by the

action of microorganisms.

BacteriaBacteria (singular form is bacterium) are among the most primitive and

also the most plentiful organisms on the planet. They are said to be

very successful because they have survived and changed little over

several billion years (Figure 2). Some, like plants, can make their own

food. Others are parasites. (Parasites can live by invading the body of

an animal or a plant.) Some bacteria can even live with little or no

oxygen. There are bacteria in every Earth environment, even in hot

springs. Bacteria are different from animal and plant cells in that

they have no nucleus, no mitochondria, and no ribosomes.

42 Unit 1

Unicellular Organisms

1.131.13

Figure 1

Some microorganisms makeus sick, but without theothers we could not survive.

Figure 2

A typical bacterium cell wall: Providesrigid support

cell membrane: Regulatesmovement of materialsinto and out of the cell

c h romosome: The geneticmaterial of bacteria is org a n i z e din one chromosome. There is non u c l e u s .

pili: These hairlike stru c t u res help bacteriato attach to each other and to surf a c e s .They also help bacteria to move.

flagellum: Some bacteriahave whiplike tails to helpthem move.

capsule: A sticky coating surrounds disease-causingbacteria. The capsule makes it difficult for animals’white blood cells to destroy these bacteria.

a Each droplet sprayed intothe air during a sneezecould contain thousandsof microorganisms.

b Microorganismsdecompose dead plantsand animals into chemicalbuilding blocks that canbe recycled by plants intofood for humans andother animals.

Cells, Tissues, Organs, and Systems 43

ProtistsIf you look into a drop of pond water, you

will find an incredible collection of protists.

Almost anywhere there is water, even in moist

soil or in rotting leaves, you will find protists.

Unlike bacteria, protists have a nucleus and

contain organelles such as mitochondria,

ribosomes, and lysosomes.

Plantlike ProtistsDiatomsDiatoms are found in both fresh and salt

water. They contain chlorophyll and can make

their own food. Diatoms are encased in two

thin shells joined together. Figure 3 shows

some diatoms.

EuglenaEuglena (Figure 4) is like both a plant and an

animal cell. If there is lots of sunlight, euglena

acts like a plant and makes its own food. With

reduced sunlight, euglena acts like an animal

and begins feeding upon smaller cells.

Figure 3

Each species of diatom has a unique shape.They all have symmetrical grooves and pores.

Figure 4

Euglena has features of bothplant and animal cells.

flagellum: Helps the cell to move

mitochondria: Provide the energyfor the flagellum and other needs

chloroplasts: Contain chlorophyll,the green pigment used to makefood by photosynthesis

nucleus:Contains thegenetic material

pellicle: A firm covering thatsurrounds the cell membrane.Unlike plant cell walls, thiscovering is flexible.

contractile vacuole:Expels excess water

Golgi apparatus:Stores chemicals

eyespot: Directs euglenatoward the light

44 Unit 1

Animal-like ProtistsAnimal-like protists cannot make their own

food and must feed on things that are living

or were once alive. They have all of the

organelles of an animal cell, and, like

euglena, they have a contractile vacuole.

AmoebaAs the amoeba moves, it changes shape

(Figure 5). These bloblike organisms move by

stretching out a branch of cytoplasm, called a

pseudopod (false foot). The pseudopod

anchors to an object, and the rest of the cell is

dragged toward it. This method of movement

is also used by animal white blood cells,

including the ones in your blood vessels. The

crawling motion of the amoeba is also used

for feeding.

ParameciumThe paramecium (plural paramecia), like

the amoeba, uses structures designed for

movement to help it feed (Figure 6). Tiny

hairlike structures, called cilia, beat together

to create water currents that move the

paramecium. There are also cilia around the

paramecium’s oral groove. These cilia draw

food into the groove. Bacteria and other

smaller cells are the main food source

for paramecia.

nucleus

contractilevacuole

pseudopod

foodvacuole

a The amoeba is a singleanimal cell. It feeds bywrapping itself around food.

b Amoeba slowly engulfs foodparticles by extending itspseudopods.

c The membrane around the food formsa food vacuole. Digestion takes placeinside the vacuole.

Figure 5

The amoeba crawls and feedsat the same time.

ciliaHairlike structuresthat beat togetherto move water

oral grooveLined with cilia thatsweep in food

anal poreExpels waste

food

Cells, Tissues, Organs, and Systems 45

FungusFungi (singular fungus) include many organisms that

are multicellular. Bread mould, mushrooms, and puff

balls are well-known fungi. Harmful fungi include those

that cause ringworm, Dutch elm disease, and athlete’s

foot. However, there are some unicellular fungi.

Yeast, the Unicellular FungusYeast is one of the few unicellular fungi (Figure 7).

There are many different species of yeast. Like animal

cells, yeast cells do not have chlorophyll and must rely

on other organisms for their source of energy.

Figure 7

Using a microscope,it is difficult to tellthat yeast are living.

There are cells in the tubes that leadto your lungs that have cilia much likethose of paramecium. Human whiteblood cells, like amoeba, engulf anddigest foreign particles. Examine thestructures of the unicellular organismscarefully. Would any of these featuresbe useful in your model cell? Considerlooking to primitive cells forassistance with your Challenge.

Figure 6

The paramecium is also a single animalcell that must perform most of thefunctions that your body performs.

contractile vacuolePumps out excess water;prevents the parameciumfrom bursting

gulletA cavity at the end of the oralgroove. Here the food entersa food vacuole.

food vacuoleWhere food isslowly digested

micronucleusContains genetic informationthat is exchanged betweenparamecia during reproduction

macronucleusContains genetic materialthat regulates cell function

Understanding Concepts1. Why do you think bacteria are

considered to be more primitivethan other cells that you havestudied?

2. Why are diatoms called plantlikeprotists?

3. Compare euglena to a plant cell.Make a list of similarities anddifferences.

4. Compare the process of feeding in the paramecium and amoeba.

Making Connections5. Why do many people associate

microorganisms with disease?

6. Using the information that you havegained about osmosis, indicate whyeuglena, paramecia, and amoebasneed contractile vacuoles.

7. Penicillin is an antibiotic thatweakens the cell wall of bacteria.The sugars and proteins in thecytoplasm of the bacteria are inhigher concentration than they arein their environment. Draw a seriesof diagrams showing how penicillinkills bacteria.

Exploring8. What effect do chemicals such

as caffeine have on unicellularorganisms? State your hypothesis,and design an experiment to test it.With your teacher’s approval,conduct your experiment.