The cell is a fundamental component of our modern definition of life and living things. Cells are regarded as the basic building blocks of life and are used in the elusive definition of what it means to be 'alive'.
Let's take a look at one definition of life:
"Living things are chemical organizations composed of cells and capable of reproducing themselves. (Keeton 1986, 85)"
This definition is rooted in two theories, cell theory and biogenesis theory. Cell theory, first proposed in the late 1830s by two German scientists Matthias Jakob Schleiden and Theodor Schwann, states that all living things are composed of cells. Biogenesis theory, proposed in 1858 by Rudolf Virchow states that all living cells arise from existing (living) cells and no cells are created spontaneously from non-living matter (Keeton 1986, 84).
Cells organize things. They keep chemical processes tidy and compartmentalized so individual cell processes do not interfere with others and the cell can go about its business of metabolizing, reproducing, etc. To organize things, cell components are enclosed in a membrane which serves as a barrier between the outside world and the cell's internal chemistry. The cell membrane is a selective barrier, meaning that it lets some chemicals in and others out and in doing so maintains the balance necessary for the cell to live.
Two Fundamental Cell Types
All living organisms can be sorted into one of two groups depending on the fundamental structure of their cells. These two groups are the prokaryotes and the eukaryotes. Prokaryotes are organisms made up of cells that lack a cell nucleus or any membrane-encased organelles. Eukaryotes are organisms made up of cells that possess a membrane-bound nucleus (that holds genetic material) as well as membrane-bound organelles.
Anatomy of a Prokaryotic Cell
A typical prokaryotic cell might contain the following parts:
- cell wall
- plasma membrane
- flagella and pili
Anatomy of a Eukaryotic Cell
A typical eukaryotic cell might contain the following parts:
- plasma membrane
- endoplasmic reticulum
- Golgi apparatus
The Cell Membrane
The cell membrane regulates the crossing of chemicals in and out of the cell in several ways: by diffusion (the tendency of solute molecules to minimize concentration and thus move from an area of higher concentration towards an area of lower concentration until concentrations equalize), osmosis (the movement of solvent across a selective boundary in order to equalize the concentration of a solute that is unable to move across the boundary), and selective transport (via membrane channels and membrane pumps).