Membranes

The definition of a membrane is a thin, film-like structure that acts as a selective barrier or a separating structure that allows only certain particles or chemicals to pass through. A cell has an outer plasma membrane and an internal plasma membrane. The outer plasma membrane forms a barrier between the cell and its surroundings, controlling the traffic of molecules in and out of the cell, exhibiting the characteristics of a selective permeable membrane. The internal plasma membrane provides a structural order for the metabolism. A membrane’s structure is like a complex fluid mosaic. It is made up of proteins and molecules that are embedded in a phospholipid bilayer. The phospholipids are the main structural component of the membrane, the organization of the phospholipid bilayer allows the hydrophobic tails to be protected by the hydrophilic heads, due to the polarization of the heads and the non-polarity of the tails.

The proteins that are embedded in the membrane are either integral proteins or peripheral proteins. Integral proteins span the entire membrane but peripheral proteins are either on one side of the membrane or the other.

Though the structure of the membrane is made of phospholipids, most of the membranes functions are accomplished by the embedded proteins. There are many kinds of proteins and each have a specific function. There are cell-cell recognition  proteins that identify the type of cell and indicate what is the cell’s and what is “foreign.” Most of the cell-cell recognition proteins are  glycoproteins, which are short chain carbohydrates that attach onto the extracellular side of the membrane, all carbohydrate chains are very specific and vary between species, individuals and cell types, which is why they are used by the cell to make sure that nothing foreign enters that specific cell. Another type of protein that is embedded in the membrane are integrins. Integrins are the main integral protein that strengthens and provides structure to the membrane. There are also intercellular junction proteins, that help corresponding cells stick together to form tissues; enzymes, which activate the biochemical reactions that occur in the cells; and receptor proteins, which bind hormones and other substances to the outside of the cell allowing a change inside the cell to be triggered. The other protein that should be mentioned are the transport proteins, which have two different forms. There are passive transport proteins which do not require energy and allow soluble substances to pass through the membrane and there are active transport proteins that allows the cell to expend energy in order to transport water soluble substances against their concentration gradient.