<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 things that can affect the fluidity is fatty acid composition . For example, when the bacteria S. 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> <P> Lipid bilayers form through the process of self - assembly . The cell membrane consists primarily of a thin layer of amphipathic phospholipids that spontaneously arrange so that the hydrophobic "tail" regions are isolated from the surrounding water while the hydrophilic "head" regions interact with the intracellular (cytosolic) and extracellular faces of the resulting bilayer . This forms a continuous, spherical lipid bilayer . Hydrophobic interactions (also known as the hydrophobic effect) are the major driving forces in the formation of lipid bilayers . An increase in interactions between hydrophobic molecules (causing clustering of hydrophobic regions) allows water molecules to bond more freely with each other, increasing the entropy of the system . This complex interaction can include noncovalent interactions such as van der Waals, electrostatic and hydrogen bonds . </P>

All transport through the cell membrane can be classified as