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LZ Unici
Strange as it might seem, a cell in your ringer has characteristics in common with a
microscopic organism and with the cells in an oak leat. One way to understand the
structure and function of cells in multicellular (many-celled) organisms, such as
human beings, is to investigate the characteristics of unicellular (single-celled)
organisms, like the ones shown in Figure 1.6.
T h e photographs in Figure 1.6 show a variety of microscopic pond organisms.
Although unicellular, they are not simple. Each has a way of moving, obtaining
food, and carrying out all other functions essential for life.
Pause&
Reflect
In your Science Log, list
or sketch some of the different types of homes
that humans live in or
have lived in. Include
Chlamydomonas (180x) Makes its own food
through photosynthesis, and moves by means of
two long, whiplike structures called flagella.
Euglena (100x) A common pond organism that
.ilso photosynthesis, and moves by means of a
single flagellum.
homes used in past periods of history, and
homes used in different
areas of the world. What
parts do all these homes
have in common? What
are the functions of these
parts? In what ways do
you think a human home
may be like a cell? Write
your ideas in your
Science Log.
Paramecium (160x) Paramecia obtain their own
food from the external environment. They are
covered with short, hairlike structures called cilia
that are used both for movement and to sweep
food into a tiny groove that is similar to a mouth.
Stentor (125x) Stentor and some other
unicellular organisms produce stalks to attach
themselves to the bottom of ponds and streams.
Stentor, like Paramecium, has cilia, but these
structures are used to bring in food rather than
for movement.
Diatoms (100x) Varied in shape and beautiful.
Diatoms produce shells around themselves,
make their own food through photosynthesis,
and are free-floating.
Volvox (30x) Living balls made of many volvox
live together as a colony. Each has its own
flagellum and makes its own food by
photosynthesizing.
Observing Cells
21
The Advantages of Being Multicellular
Imagine you are a microscopic, unicellular organism. Your whole body is one cell.
This one cell must carry out all the functions needed to keep you alive. I t must be
able*to move, obtain food, reproduce, and respond to the environment. There are
many living organisms that consist of only one cell. What disadvantages do you
think they have, compared with multicellular organisms?
In the last chapter, you learned of one disadvantage. Unicellular organisms cannot grow very large. Also, because they must take in all the materials they need
through their cell membranes, most unicellular organisms can only live in watery,
food-rich surroundings.
Multicellular organisms have several advantages compared to unicellular living
things. They can live in a wide variety of environments. They are able to grow
very large — as large as a whale or a Douglas fir. Multicellular animals can obtain
their energy from a wide variety of foods. Their bodies are more complex. By
specializing in particular functions, each cell in a multicellular organism can work
much more efficiently than the cell of a unicellular organism, which must do every
job itself.
In multicellular organisms, specialized cells of a similar kind work closely
together, and are usually found grouped closely together in the body. Groups of
specialized cells, in turn, work in harmony with other groups. You will investigate
this organization of cells in the next chapter.
Check Your Understanding
1. W h y do cells in your body need to be specialized?
2. W h y do nerve cells have long fibres, whereas red blood cells are thin and
disklike?
3. W h y do unicellular organisms live mainly in a watery environment?
4. Apply Most people think of the skin as just a body covering. How do you
think skin cells are important to other body cells?