"Cells are the unit of structure, function, and reproduction in living things." That's probably the definition you'd find if you looked up the word "cells" in a biology textbook. To a lot of people, that definition is pretty confusing.
After reading through the Guide, it will make a lot more sense to you, but for now, here's a simpler definition: "Anything that's alive is made up of cells." Every organism that you can think of, whether it be an octopus or a bird, a snail or an ant, is made of cells. Humans are made of literally billions of cells. We can't see them though, because cells are so small that they can only be seen under a microscope. Most organisms, like us, are made up of many different types of cells. Some carry oxygen in our blood, some produce sweat, and some allow us to feel whether it's hot or cold.
Although most organisms (an organism is just a living thing) are made of many different types of cells, all of which work together, some organisms are just one, single cell. Biologists call this type of organismunicellular.
Think about it. A unicycle has one wheel, a unicorn has one horn, so an organism that's unicellular has just one cell. Unicellularorganisms are very special in that they perform all of the functions necessary to live and yet are so tiny. Webcytology focuses on how these extraordinary organisms live and function.
The study of cells is known as cytology. Cytology began in 1665, when Robert Hooke, an English scientist, first glimpsed into the microscopic world of cells by examining dead cork cells under a primitive microscope which he constructed. However, all Hooke was able to observe were the thick walls that surrounded each cell. The thought that cells might be the basis for life was not to come for nearly two centuries.
During the next 170 years, other scientists used microscopes to further advance their knowledge of cells. The most important discovery during that period came in 1838, when a German botanist named Matthias Schleiden suggested that all plant tissues are made of cells. Just one year later, zoologist Theodore Schwann made a similar proposal for animals. In 1858, Rudolf Virchow suggested that all cells come from preexisting cells. The ideas of these three scientists led to the creation of what is now called the cell theory. The three main aspects of the theory are:
Perhaps we should return to the first definition of what a cell is to see if it makes more sense now. That definition called a cell the "unit of structure, function, and reproduction in living things." The cell theory says that all living things are composed of cells, so it makes sense that cells are called the unit of structure for an organism. An organism requires chemical reactions to function, and since the second part of the cell theory says that these reactions occur in cells, we can say that cells are the unit of function for the organism. The final component of the theory states that cells reproduce to form new cells. Since all living things reproduce and are also made of cells, the cells themselves must reproduce to form the new organism. That's why the definition also calls cells the unit of reproduction in living things.