The interaction of cells with their surrounding environment is necessary for their survival, this is done by way of the plasmatic membrane, which is their outermost structure. It is made up of lipids and proteins perforated by tiny needles, through which substances pass into the cell (oxygen for example) and waste products out of it (carbon dioxide for example). Molecules that are too big to pass through the cell membrane are dissolved by the lipids of the membrane and then pass into the cell (see the inset of Transport Mechanisms). Other nutrients, like glucose, fatty acids and the amino acids that form proteins, pass to the interior of the cell thanks to the presence of substance known as membrane transporters. Cells are capable of discriminating molecules, letting in some and denying access to others.
Cytosol and Cytoskeleton
Cytosol is a watery based gel which contains a grand quantity of large and small molecules. In majority of cells, it is what uses up most of the cell volume. Also, it produces many of the most important metabolic functions of the great cellular molecules. Just as the skeleton gives shape to our body, each cell has a framework of fine tubes and filaments known as cytoskeleton.
Transportation of large molecules
The macromolecules enter or are expelled from the cell by way of two methods.
Exocytosis: The movement of large particles out of the cell, by means of packing the material in a membrane sack towards the cell surface. It fuses with the cell membrane which opens outwards and allows the contents to flow out.
Endocytosis: It is the ingestion of macromolecules with the formation within the cell of vesicles originating from the membrane. There are different types of endocytosis: pinocytosis (the incorporation of intercellular fluid and dissolved solids) and phagocytosis (the incorporation of large solid particles like microorganisms)
Vital Program
Tiny chromosomes, which contain all the information necessary for the cell to work, are found in the nucleus (in humans there are 46: 44 autosomes and two sexual chromosomes). The chromosomes are formed by a large molecule known as DNA (deoxyribonucleic acid), which is a blueprint for life. DNA contains about 100 thousand genes.
