Cell Specialization and Organization
Students explore the structure and function of multicellular cells.
Multicellular organisms, even though they are made up of individual cells, have different specialized cells that perform specialized functions. For example, your muscle cells are shaped to move parts of your body, and your skin cells are built to protect your body from the drying rays of the Sun. Humans have about a hundred different types of cells, each with its own unique structure and function.
Nerve cells have long, branched fibres running from the main part of the cell, shaped to carry nerve signals from one part of the body to another. Red blood cells, which carry oxygen in the bloodstream, have a thin, disklike shape. This gives them a large surface area to pick up large amounts of oxygen. The water-conducting cells of a plant are tubelike, with thick walls and a network of holes that lets water pass easily through them. Onion skin cells are flat and brick-shaped, so they can fit closely together to form a continuous protective layer.
Now look at the photographs of different cells below and think about the structure of each and what function it might perform. Can you match the cells that come from the following part of your body — blood in your heart, nerve in your toe, muscle in your arm, bone in your leg, and skin on your head?
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. It 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?
You have already learned 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.
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.
Many animals and plants are made of trillions of cells. To learn how these cells are organized, compare the organization of cells with the way students are organized in a school district. First, students in the same grade are grouped together in classes. Then, different classes of students together make up a school. Finally, a number of schools are organized into a single school district.
Similarly, as you saw at the beginning of Topic 1, cells with the same structure and function are grouped into tissues. Groups of different tissues form organs. Organs work together in systems. Systems work together to form an organism. This arrangement of cells, tissues, organs, and systems forms several different levels of organization in living things. Each level can be studied on its own, as you have done with cells. Or they can be studied in relation to the levels above or below it, as you have done with plant cells and tissues.
Tissues are groups of similar cells. Onion skin is a tissue made of sheets of similar, thin, tightly packed cells. These specialized skin cells form a layer that covers and protects the onion. Look below for the different types of tissues.
Suppose you feel hungry, see a juicy apple, and eat it. This simple action would not be possible without the next level of organization in the body — the organs. Organs are distinct structures in the body that perform particular functions. You used your eyes (to sense the apple), your brain (to plan and coordinate your actions), and your mouth and stomach (to start digesting the apple). Each organ is made of several tissues working together. For example, your stomach is made of four main types of tissues. Other examples of organs in your body are the lungs, the heart, and the kidneys. Plants have organs, as well. Plant organs include roots, stems, and leaves.
As you have seen, organs work together just as cells and tissues do. Organs form systems to perform activities that help plants and animals function as a whole. Because of differences in how plants and animals survive, plants have fewer systems than animals have. Plants have only two main systems: a root system below ground and a shoot system (the stems and leaves) above ground. The functions of the root system are to obtain water and minerals from the soil and to anchor the plant in the ground. The function of the shoot system is to make food for the plant. At certain times, flowering plants produce a third system for reproduction. The reproductive system can include flowers, fruits, and seeds.
The information on this page and it associated figures are from the Science Focus 8.Science Focus 8, pp. 130-137