You are a multicellular organism. You have many specialized cells that work together to car ry 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 Microorganisms Most 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. Bacteria Bacteria (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 1.13 1.13 Figure 1 Some microorganisms make us sick, but without the others we could not survive. Figure 2 A typical bacterium cell wall: Provides rigid support cell membrane: Regulates movement of materials into and out of the cell chromosome: The genetic material of bacteria is organized in one chromosome. There is no nucleus. pili: These hairlike structures help bacteria to attach to each other and to surfaces. They also help bacteria to move. flagellum: Some bacteria have whiplike tails to help them move. capsule: A sticky coating sur rounds disease-causing bacteria. The capsule makes it difficult for animals’ white blood cells to destroy these bacteria. a Each droplet sprayed into the air during a sneeze could contain thousands of microorganisms. b Microorganisms decompose dead plants and animals into chemical building blocks that can be recycled by plants into food for humans and other animals.
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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.