Drawing On Science
If you placed a small, clear piece of onion skin under a microscope and examined it you would see what appears to be many small “bricks.” If any other bit of living matter were placed under a microscope you would observe similar shapes. The first person to see such shapes was the English scientist, Robert Hooke. Way back in the 1600’s he examined thin slices of cork and other plant material under a compound microscope (=a tube containing more than one magnifying lens). The units that Hooke saw reminded him of little rooms so he called them cells . At the same time that Hooke was examining his material a Dutch lens maker named Anton van Leeuwenhoek (LAY-ven-huke) was making very powerful simple microscopes (=single lens microscopes or, simply, magnifying glasses). He placed drops of pond water on his microscope and saw what he described as “little beasties.” He studied and drew these small living things, but did not know what they were.
Little was made of Hooke and Leeuwenhoek’s discoveries at the time. It was not until the 1800’s that scientists began to realize that cells were units found in all living things. It took a number of scientists to work out what is known as the Cell Theory . The Cell Theory states that living things, except viruses, are composed of either a single cell or many cells. This theory also states that cells carry on all life activities and all cells come from other living cells. Most cells are really very small and cannot be seen except under a microscope. A few cells can be seen without a microscope. The period at the end of this sentence is about the size of a human egg cell. The largest cell is the egg of an ostrich.
A cell is really a very complex thing—much more complex than a computer. It is composed of many organelles (=small organs) beginning with an outer, living cell membrane . Cell membranes allow for substances such as water to pass in and out of the cell. In plant cells the membrane is surrounded by a stiff cell wall . The cell wall is composed of cellulose (SEL-uh-lohs). It is the cellulose that gives plants their support. The membrane surrounds protoplasm (=a jelly-like, living material inside the cell). Protoplasm is composed mostly of water, but there are many organelles in the protoplasm. First there is the nucleus , which is sometimes referred to as the “brain” of the cell (no, it is not really a brain and it doesn’t think). The nucleus is composed of its own type of protoplasm known as nucleoplasm , which is surrounded by its own nuclear membrane . Like the cell membrane the nuclear membrane also allows materials to pass through it. Much of the nucleoplasm is composed of chromosomes ( KROH- muh-sohmz) on which are found genes (=the hereditary material that is passed on from parents to children). Near the nucleus in animal cells are found the centrioles (SEN-tree-oles), which play a role in cell division. The nucleus controls all of the life functions of the cell.
All of the protoplasm except for the nucleus is known as cytoplasm . There are many important substances dissolved in the cytoplasm. Cytoplasm also contains a variety of organelles needed for life functions. Let’s take a look at some of these organelles. Golgi (GOHL-jee) bodies are organelles that act as storage centers for products released by the cell. Mitochondria (myt-uh-KAHN-dree-uh; singular= mi-tochondrium) are referred to as the cell’s “furnace” or “powerhouse” because they contain the energy needed to keep the cell functioning. Near the nucleus are centrioles , which play a role in animal cell division. There are a number of vacuoles (VAK-yoo-wohls) in the cell. Water vacuoles store excess water that the cell has to get rid of. Water vacuoles in plant cells are very large. Food vacuoles are found in single-celled organisms and hold food for digestion. Plant cells have chloroplasts (KLOR-uh-plasts), which contain chlorophyll (KLOR-uh-fil) needed for photosynthesis.
Living things are either composed of a single cell or many cells. What Leeuwenhoek saw under his simple microscope were actually protozoans (pro-toh-ZOH-enz). Protozoans are microscopic, single-celled organisms like
a paramecium (pa-ruh-MEE-see-uhm). Bacteria, also, are single-celled organisms. You are composed of many cells—about 17 billion of them. All of your cells are acting together to keep you healthy. This is possible because of how your cells are arranged. There are different types of cells in your body. When similar cells perform the same function, they make up a tissue . Thus, we have muscle tissue, skeletal tissue, blood tissue, nerve tissue, etc. Now, if different tissues work together to perform a definite function, they form an organ . The largest organ in your body is your skin. It is made of nerve tissue, blood tissue, and fat tissue. Your heart is composed of muscle and blood tissues. Now, if you put all of your organs together they make up you, an organism . Plants do not have organs like animals, but they do have definite tissues that work together. So when you look at different living things try to imagine that each one is really a combination of many, complex cells. All of the cells are acting together to make up the plant/animal you are looking at.