The cells of living organisms are of two basic types; prokaryotic and eukaryotic. The prokaryotic cell, the simpler type, is characteristic of organisms like bacteria and blue-green algae. The eukaryotic cell, the more complex type with internal structures called organelles, is typical of the vast majority of familiar living organisms.
The cell itself is an organized structure and represents the highest level of organization at the cellular level. Multicellular organisms, of course, feature more than one cell and in this case cells can become specialized. Though the organism may have cells of the eukaryotic basic type, in a multicellular organism, individual cells can differentiate while retaining their eukaryotic features, becoming specialized for different functions. An example of this in animals would be the bone cell versus the muscle cell.
There are numerous so-called organelles, specialized structures within the eukaryotic cell that are essential in maintaining the living cell. These organelles have interrelated functions and together they represent the next level of organization down from the whole cell. Examples of organelles are the nucleus containing DNA (the cell's genetic material) and the mitochondrion (singular of mitochondria) where cellular respiration occurs. The word organelle and the word organize stem from the same root word, meaning tool or instrument.
Just as the overall cell is composed of organelles, the organelles themselves are made up of complex molecular structures. Many of these are not simply physical building blocks that compose the cell but are physiologically integral to the cell's organization and structure. Good examples of these are the proteins that are associated with, or even embedded in cellular membranes. Membranes surround the cell itself and organelles are within the cell. These membrane proteins play important roles in transport of materials into and out of the cell and into and out of organelles within the cell.
Perhaps the most prominent complex molecule in the cells is the deoxyribonucleic acid (DNA) molecule. It is at the heart or the nerve center of the cell, orchestrating through all of its coded instructions, many complex cellular functions.
Simple doesn't mean unimportant. The water molecule is relatively simple compared to the complex proteins and DNA but no less important. The cytosol of the cell, the mixture of molecules and ions that makes up the cell cytoplasm in which the organelles are contained, is largely water. Other relatively small or simple molecules in addition to water are part of the cytosol and together they represent a level of cellular organization below the complex molecules.
Atoms and Ions
At the smallest level of organization is the atom and its electrons. Many cellular components exist and function in the form of single atoms or ions. An ion is an atom or molecule that as a positive or negative charge. One critical metabolic process in the cell that fits this description, the electron transport system, functions by the movement of electrons and hydrogen ions. Although facilitated and mediated by more complex levels of organization, this "simple" ion and electron movement is essential to cell structure and function.