The cellular biology is a wide and versatile area of research. Your mind cannot imagine the huge number of chemical reactions that take place inside the cell. Each cell is comprised of organelles which each has a special function related to its unique structure. Each cell is surrounded by a membrane which makes sure the cell sustain life.
Without the cell membrane the constituents of the cell would leak outside the cell. Thus predisposing to its death. The eukaryotic cell or human cells are composed of a membrane that is composed of phospholipid bilayer that separates the cell from its exterior environment.
The communication between the cell and its outer world is done via two important mechanisms. These are the mechanism of diffusion and the mechanism of active transport. The cell is in a continuous state of nonequilibrium that is maintained by active transport across the cellular membrane.
Molecules and ions are actively transported in and out of the cell using mechanisms which are called pumps. These are molecular pumps that function using special proteins in the cellular membrane and which they allow for the passage of specific molcules and ions by changing its conformation in response to an external stimulus such as the neurotransmitter acetylcholine.
These pumps require energy for their proper functioning. This energy is supplied by the molecule ATP. It is used by an enzyme which is called ATPase which catalyzes or drive this uphill reaction that is energetically unfavourable to completion. An example of a molecular pump that is abundant in nerve and muscle cells is the sodium/potassium molecular pump, which guarantees the existence of electric potential across the cell membrane of these tissues.
The other way that the cell communicates with the exterior world is by diffusion. Some molecules such as the polar compoud water can enter or exit the cell using the process of diffusion. The process of diffusion in and out of the cell will be determined by the osmolality of the blood plasma.
Receptors on the surface of the cellular membrane in the form of integral proteins are also another way of communication between the cell and its exterior world. Hormones are a major and important way that the cells use to communicate with each other and with the external environment.
The type of hormone will dictate the method of communication between the the cells. Steriod hormones such as aldosterone are lipid soluble. Therefore, they can penetrate the cell phospholipid bilayer and exert their effect inside the cell by binding to nuclear receptors for example in which case it accelerates the synthesis of proteins by affecting the rate of genetic translation.
Other hormones such as the protein hormones which are the majority of of the types of hormones cannot penetrate the cell membrane due to its polar structure or lipophobic character. Therefore they bind instead to receptors on the cellular membrane surface. Thus activating this way a second messenger which as called cAMP.
This messenger in turn exerts its effect on other parts of the cell which depends on the speciality of the hormone, thus making its physiological effect indirectly by stimulating other secondary messengers. An example of such a hormone is the polypeptides antidiuretic hormone and the polypeptide oxytocin.
Other compounds such as glucose molecules enter the cell via the assitance of a hormone which is called insulin. This is also a polypeptide hormone which is important in the homeostasis of sugar in the body. Glucose undergoes the important process for generating energy in the cell which is called glycolysis.
Other organelles inside the cell are also important such as the endoplasmic reticulum which is divided into smooth and rough parts in addition to the geolgi appararus and the peroxisomes. These structures are also important but their discussion is beyond this small article which discusses the methods of signaling in cells.