<P> Cell membrane can form different types of "supramembrane" structures such as caveola, postsynaptic density, podosome, invadopodium, focal adhesion, and different types of cell junctions . These structures are usually responsible for cell adhesion, communication, endocytosis and exocytosis . They can be visualized by electron microscopy or fluorescence microscopy . They are composed of specific proteins, such as integrins and cadherins . </P> <P> The cytoskeleton is found underlying the cell membrane in the cytoplasm and provides a scaffolding for membrane proteins to anchor to, as well as forming organelles that extend from the cell . Indeed, cytoskeletal elements interact extensively and intimately with the cell membrane . Anchoring proteins restricts them to a particular cell surface--for example, the apical surface of epithelial cells that line the vertebrate gut--and limits how far they may diffuse within the bilayer . The cytoskeleton is able to form appendage - like organelles, such as cilia, which are microtubule - based extensions covered by the cell membrane, and filopodia, which are actin - based extensions . These extensions are ensheathed in membrane and project from the surface of the cell in order to sense the external environment and / or make contact with the substrate or other cells . The apical surfaces of epithelial cells are dense with actin - based finger - like projections known as microvilli, which increase cell surface area and thereby increase the absorption rate of nutrients . Localized decoupling of the cytoskeleton and cell membrane results in formation of a bleb . </P> <P> The content of the cell, inside the cell membrane, is composed of numerous membrane - bound organelles, which contribute to the overall function of the cell . The origin, structure, and function of each organelle leads to a large variation in the cell composition due to the individual uniqueness associated with each organelle . </P> <Ul> <Li> Mitochondria and chloroplasts are considered to have evolved from bacteria, known as the endosymbiotic theory . This theory arose from the idea that Paracoccus and Rhodopseaudomonas, types of bacteria, share similar functions to mitochondria and blue - green algae, or cyanobacteria, share similar functions to chloroplasts . The endosymbiotic theory proposes that through the course of evolution, a eukaryotic cell engulfed these 2 types of bacteria, leading to the formation of mitochondria and chloroplasts inside eukaryotic cells . This engulfment lead to the 2 membranes systems of these organelles in which the outer membrane originated from the host's plasma membrane and the inner membrane was the endosymbiont's plasma membrane . Considering that mitochondria and chloroplasts both contain their own DNA is further support that both of these organelles evolved from engulfed bacteria that thrived inside a eukaryotic cell . </Li> <Li> In eukaryotic cells, the nuclear membrane separates the contents of the nucleus from the cytoplasm of the cell . The nuclear membrane is formed by an inner and outer membrane, providing the strict regulation of materials in to and out of the nucleus . Materials move between the cytosol and the nucleus through nuclear pores in the nuclear membrane . If a cell's nucleus is more active in transcription, its membrane will have more pores . The protein composition of the nucleus can vary greatly from the cytosol as many proteins are unable to cross through pores via diffusion . Within the nuclear membrane, the inner and outer membranes vary in protein composition, and only the outer membrane is continuous with the endoplasmic reticulum (ER) membrane . Like the ER, the outer membrane also possesses ribosomes responsible for producing and transporting proteins into the space between the two membranes . The nuclear membrane disassembles during the early stages of mitosis and reassembles in later stages of mitosis . </Li> <Li> The ER, which is part of the endomembrane system, which makes up a very large portion of the cell's total membrane content . The ER is an enclosed network of tubules and sacs, and its main functions include protein synthesis, and lipid metabolism . There are 2 types of ER, smooth and rough . The rough ER has ribosomes attached to it used for protein synthesis, while the smooth ER is used more for the processing of toxins and calcium regulation in the cell . </Li> <Li> The Golgi apparatus has two interconnected round Golgi cisternae . Compartments of the apparatus forms multiple tubular - reticular networks responsible for organization, stack connection and cargo transport that display a continuous grape - like stringed vesicles ranging from 50 - 60 nm . The apparatus consists of three main compartments, a flat disc - shaped cisterna with tubular - reticular networks and vesicles . </Li> </Ul>

The lipid-based structure that surrounds some viruses is known as