<P> 3 . Endocytosis: Endocytosis is the process in which cells absorb molecules by engulfing them . The plasma membrane creates a small deformation inward, called an invagination, in which the substance to be transported is captured. This invagination is caused by proteins on the outside on the cell membrane, acting as receptors and clustering into depressions that eventually promote accumulation of more proteins and lipids on the cytosolic side of the membrane . The deformation then pinches off from the membrane on the inside of the cell, creating a vesicle containing the captured substance . Endocytosis is a pathway for internalizing solid particles ("cell eating" or phagocytosis), small molecules and ions ("cell drinking" or pinocytosis), and macromolecules . Endocytosis requires energy and is thus a form of active transport . </P> <P> 4 . Exocytosis: Just as material can be brought into the cell by invagination and formation of a vesicle, the membrane of a vesicle can be fused with the plasma membrane, extruding its contents to the surrounding medium . This is the process of exocytosis . Exocytosis occurs in various cells to remove undigested residues of substances brought in by endocytosis, to secrete substances such as hormones and enzymes, and to transport a substance completely across a cellular barrier . In the process of exocytosis, the undigested waste - containing food vacuole or the secretory vesicle budded from Golgi apparatus, is first moved by cytoskeleton from the interior of the cell to the surface . The vesicle membrane comes in contact with the plasma membrane . The lipid molecules of the two bilayers rearrange themselves and the two membranes are, thus, fused . A passage is formed in the fused membrane and the vesicles discharges its contents outside the cell </P> <P> Prokaryotes are divided into two different groups, Archaea and Bacteria, with bacteria dividing further into gram - positive and gram - negative . Gram - negative bacteria have both a plasma membrane and an outer membrane separated by periplasm, however, other prokaryotes have only a plasma membrane . These two membranes differ in many aspects . The outer membrane of the gram - negative bacteria differ from other prokaryotes due to phospholipids forming the exterior of the bilayer, and lipoproteins and phospholipids forming the interior . The outer membrane typically has a porous quality due to its presence of membrane proteins, such as gram - negative porins, which are pore - forming proteins . The inner, plasma membrane is also generally symmetric whereas the outer membrane is asymmetric because of proteins such as the aforementioned . Also, for the prokaryotic membranes, there are multiple things that can affect the fluidity . One of the major factors that can affect the fluidity is fatty acid composition . For example, when the bacteria Staphylococcus aureus was grown in 37 C for 24h, the membrane exhibited a more fluid state instead of a gel - like state . This supports the concept that in higher temperatures, the membrane is more fluid than in colder temperatures . When the membrane is becoming more fluid and needs to become more stabilized, it will make longer fatty acid chains or saturated fatty acid chains in order to help stabilize the membrane . Bacteria are also surrounded by a cell wall composed of peptidoglycan (amino acids and sugars). Some eukaryotic cells also have cell walls, but none that are made of peptidoglycan . The outer membrane of gram negative bacteria is rich in lipopolysaccharides, which are combined poly - or oligosaccharide and carbohydrate lipid regions that stimulate the cell's natural immunity . The outer membrane can bleb out into periplasmic protrusions under stress conditions or upon virulence requirements while encountering a host target cell, and thus such blebs may work as virulence organelles . Bacterial cells provide numerous examples of the diverse ways in which prokaryotic cell membranes are adapted with structures that suit the organism's niche . For example, proteins on the surface of certain bacterial cells aid in their gliding motion . Many gram - negative bacteria have cell membranes which contain ATP - driven protein exporting systems . </P> <P> According to the fluid mosaic model of S.J. Singer and G.L. Nicolson (1972), which replaced the earlier model of Davson and Danielli, biological membranes can be considered as a two - dimensional liquid in which lipid and protein molecules diffuse more or less easily . Although the lipid bilayers that form the basis of the membranes do indeed form two - dimensional liquids by themselves, the plasma membrane also contains a large quantity of proteins, which provide more structure . Examples of such structures are protein - protein complexes, pickets and fences formed by the actin - based cytoskeleton, and potentially lipid rafts . </P>

What are the four major functions of the plasma membrane