The Game of Life is a serious one for all living organisms. An organism must be able to perform certain tasks to live, and survive competition from other individuals and species.
The Big Picture
While life forms are strikingly diverse on the outside, at the cellular level all life forms are strikingly similar.
History of the Cell
"Cells are the unit of structure, function, and reproduction in living things." If this sounds confusing, here's a simpler definition: "Anything that's alive is made up of cells."
Every organism, whether it's an octopus, bird, snail or ant, is made of cells. Humans are made of literally billions of cells. We can't see them 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 (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 organism unicellular. Unicellular organisms are very special in that they perform all of the functions necessary to live and yet are so tiny.
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:
Aything that is living is composed of cells.
The chemical reactions that occur in an organism occur in cells.
All cells come from preexisting cells.
Remember the definition of 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.
Types of Cells
There are many different types of unicellularorganisms, all of which fall into two general categories: prokaryotic organisms (also called bacteria) and eukaryotic organisms. In general, eukaryotic cells are more complex than prokaryotic organisms, because prokaryotes don't have membrane-bound organelles. Prokaryotes and eukaryotes also differ in the chemical composition of some of their structures as well as in the organization of their DNA. Below are the important differences between the two types of cells.
Prokaryote
Eukaryote
cell membrane
yes
yes
nucleus
no membranes
surrounded by two membranes
endoplasmic reticulum
no
yes
Golgi bodies
no
yes
mitochondria
no
yes
vacuoles
no
yes
lysosomes
no
yes
ribosomes
yes
yes
chromosomes
circular without histones
linear with histones
In addition to this classification, organisms may also be classified according to their method of nutrition. Heterotrophs are organisms which obtain food from the environment, whereas autotrophs have a method of producing their own food. Also, aerobes are organisms which require oxygen to survive, whereas anaerobes do not need oxygen.
