Cells are in general divided into two types. These are: eukaryotes and prokaryotes. Simply speaking, eukaryotic cells differ than prokaryotes by the absence of nuclei in the latter with all accompanied characteristics to this such as inability to divide and the lack of the ability to synthesize their proteins.
In this article, I will discuss very briefly the function of the organelles of the cell. This includes the cell membrane and the motochondria in addition to the nucleus. In addition to the afore mentioned organelles the cell possess other organelles as well but will not be discussed here such as the endoplasmic reticulum and golgi apparatus. Here I will discuss the main features that distinguish these organelles.
The cell whether eukaryote or prokaryote is surrounded by a membrane which keeps the integrity of the cell by preventing leakage of cellular contents to the outside. Few molecules are able to penetrate this cellular barrier such as the water molecule which is able to pass through this barrier by diffusion.
This barrier that forms the membrane aroun the cell is composed of phospholipid bilayer. Due to this structure this membrane is freely permeable to lipidlike material such as vitamins which are soluble in fats. In contrast water soluble compounds are not freely permeable across the cellular membrane.
Another mechanism can occur for the passage of molecules across the phospholipid bilayer other than passive diffusion and which is called active transport. Before discussing this it is important to note that along the phospholipid bilayer there is proteins embedded within this layer. These are called integral proteins. They play a specific role in ion pumps in particular.
By changing conformation as a result of a stimulus they can open ion channels that in turn permits the flow of ions from one side of the cell to the other side. This active transport uses energy to accomplish the transport in the form of ATP molecules. Example of such a transport that uses ATPase enzyme is the transport of calcium across the cellular membrane. In addition another example is the sodium / potassium ions pump in nerves and muscle cells.
Hormones are another class of compounds that can be either lipid soluble or water soluble. Thus some of them can penetrate the phospholipid bilayer. The water soluble hormones such as proteins make their effect by binding to receptors that are embedded in the cell membrane. Lipid soluble hormones succeed in penetrating the phospholipid bilayer due to their solubility in lipids. They exert their effect by binding to nuclear receptors. Thus they can change the rate of RNA transcription or change the rate of proteins synthesis.
The other important strucuture of the cell is called the mitochondria. This is the machinery for energy production for the body and the cell. They produce ATP molecules which have phosphoanhydride bonds rich in energy. This molcule is produced in several processes which occur in the motochondria.
Some of these processes do not require oxygen and which is called glycolysis. Other processes consume oxygen such as the process which is called oxidative phosphorylation. This process couples the oxidation of certain molecules with phosphorylation. Important in this process are compounds which are called cytochromes which uses Fe-S clusters that can be oxidized and reduced in this process.
The other important structure in the cell of eukaryotes is the nucleus. This structure is not present in prokaryotes. This fact has important consequences. One of them is that prokaryotes do not divide. Therefore they cannot proliferate by division. Another consequence is that proteins cannot be synthesized in prokaryotes and must be given from sources outside the cell.