Editing Endomembrane system (section)

== Components of the system ==

=== Nuclear envelope ===
[[Image:Diagram human cell nucleus.svg|thumb|350px|Diagram of the nucleus with the nuclear envelope shown as the orange portion]]
{{main|Nuclear envelope}}

The [[nuclear envelope]] surrounds the [[cell nucleus|nucleus]], separating its contents from the cytoplasm. It has two membranes, each a [[lipid bilayer]] with associated proteins.<ref>{{cite web| last = Childs| first = Gwen V. | name-list-style = vanc | title = Nuclear Envelope| publisher = UTMB| year = 2003| url = http://cellbio.utmb.edu/CELLBIO/nuclear_envelope.htm| access-date = 2008-09-28|archive-url = https://web.archive.org/web/20060620093536/http://cellbio.utmb.edu/CELLBIO/nuclear_envelope.htm |archive-date = June 20, 2006|url-status=dead}}</ref> The outer nuclear membrane is continuous with the rough endoplasmic reticulum membrane, and like that structure, features [[ribosome]]s attached to the surface. The outer membrane is also continuous with the inner nuclear membrane since the two layers are fused together at numerous tiny holes called [[nuclear pore]]s that perforate the nuclear envelope. These pores are about 120 [[Nanometre|nm]] in diameter and regulate the passage of molecules between the nucleus and cytoplasm, permitting some to pass through the membrane, but not others.<ref name=Cooper>{{cite web|url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=Nuclear%20Envelope&rid=cooper.section.1323#1324|title=The Nuclear Envelope and Traffic between the Nucleus and Cytoplasm|access-date=2008-12-09|last=Cooper|first=Geoffrey| name-list-style = vanc |year=2000|work= The Cell: A Molecular Approach|publisher=Sinauer Associates, Inc}}</ref> Since the nuclear pores are located in an area of high traffic, they play an important role in [[cell physiology]]. The space between the outer and inner membranes is called the [[perinuclear space]] and is joined with the lumen of the rough ER.

The nuclear envelope's structure is determined by a network of intermediate filaments (protein filaments). This network is organized into a mesh-like lining called the [[nuclear lamina]], which binds to [[chromatin]], integral membrane proteins, and other nuclear components along the inner surface of the nucleus. The nuclear lamina is thought to help materials inside the nucleus reach the nuclear pores and in the disintegration of the nuclear envelope during [[mitosis]] and its reassembly at the end of the process.<ref name=Davidson />

The nuclear pores are highly efficient at selectively allowing the passage of materials to and from the nucleus, because the nuclear envelope has a considerable amount of traffic. [[RNA]] and ribosomal subunits must be continually transferred from the nucleus to the cytoplasm. [[Histone]]s, gene regulatory proteins, [[DNA polymerase|DNA]] and [[RNA polymerase]]s, and other substances essential for nuclear activities must be imported from the cytoplasm. The nuclear envelope of a typical mammalian cell contains 3000–4000 pore complexes. If the cell is synthesizing DNA each pore complex needs to transport about 100 histone molecules per minute. If the cell is growing rapidly, each complex also needs to transport about 6 newly assembled large and small ribosomal subunits per minute from the nucleus to the cytosol, where they are used to synthesize proteins.<ref name=Alberts2>{{cite web|url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=Nuclear%20Envelope&rid=mboc4.section.2154#2156|title=Nuclear Pore Complexes Perforate the Nuclear Envelope|access-date=2008-12-09|last=Alberts|first=Walter | name-list-style = vanc |year=2002|work= Molecular Biology of the Cell 4th edition|publisher=Garland Science|display-authors=etal}}</ref>

=== Endoplasmic reticulum ===
[[Image:nucleus ER golgi.svg|thumb|350px|'''1'''&nbsp;[[Cell nucleus|Nucleus]]{{nbsp}}
'''2'''&nbsp;[[Nuclear pore]]{{nbsp}}
'''3'''&nbsp;Rough endoplasmic reticulum (RER){{nbsp}}
'''4'''&nbsp;Smooth endoplasmic reticulum (SER){{nbsp}}
'''5'''&nbsp;[[Ribosome]] on the rough ER{{nbsp}}
'''6'''&nbsp;[[Protein]]s that are transported{{nbsp}}
'''7'''&nbsp;Transport [[Vesicle (biology)|vesicle]]{{nbsp}}
'''8'''&nbsp;[[Golgi apparatus]]{{nbsp}}
'''9'''&nbsp;Cis face of the Golgi apparatus{{nbsp}}
'''10'''&nbsp;Trans face of the Golgi apparatus{{nbsp}}
'''11'''&nbsp;Cisternae of the Golgi apparatus]]

{{main|Endoplasmic reticulum}}

The [[endoplasmic reticulum]] (ER) is a membranous synthesis and transport organelle that is an extension of the nuclear envelope. More than half the total membrane in eukaryotic cells is accounted for by the ER. The ER is made up of flattened sacs and branching tubules that are thought to interconnect, so that the ER membrane forms a continuous sheet enclosing a single internal space. This highly convoluted space is called the ER lumen and is also referred to as the ER [[cisterna]]l space. The lumen takes up about ten percent of the entire cell volume. The endoplasmic reticulum membrane allows molecules to be selectively transferred between the lumen and the cytoplasm, and since it is connected to the nuclear envelope, it provides a channel between the nucleus and the cytoplasm.<ref name=Cooper3>{{cite web|url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=endoplasmic%20reticulum&rid=cooper.section.1466|title=The Endoplasmic Reticulum|access-date=2008-12-09|last=Cooper|first=Geoffrey| name-list-style = vanc |year=2000|work= The Cell: A Molecular Approach|publisher=Sinauer Associates, Inc}}</ref>

The ER has a central role in producing, processing, and transporting [[Biochemistry|biochemical compounds]] for use inside and outside of the cell. Its membrane is the site of production of all the transmembrane proteins and lipids for many of the cell's organelles, including the ER itself, the Golgi apparatus, lysosomes, [[endosome]]s, secretory vesicles, and the plasma membrane. Furthermore, almost all of the proteins that will exit the cell, plus those destined for the lumen of the ER, Golgi apparatus, or lysosomes, are originally delivered to the ER lumen. Consequently, many of the proteins found in the cisternal space of the endoplasmic reticulum lumen are there only temporarily as they pass on their way to other locations. Other proteins, however, constantly remain in the lumen and are known as endoplasmic reticulum resident proteins. These special proteins contain a specialized retention signal made up of a specific sequence of [[amino acids]] that enables them to be retained by the organelle. An example of an important endoplasmic reticulum resident protein is the [[Chaperone (protein)#Human chaperone proteins|chaperone protein]] known as [[Binding immunoglobulin protein|BiP]] which identifies other proteins that have been improperly built or processed and keeps them from being sent to their final destinations.<ref name="Bertolotti et al.">{{cite journal | vauthors = Bertolotti A, Zhang Y, Hendershot LM, Harding HP, Ron D | title = Dynamic interaction of BiP and ER stress transducers in the unfolded-protein response | journal = Nature Cell Biology | volume = 2 | issue = 6 | pages = 326–32 | date = June 2000 | pmid = 10854322 | doi = 10.1038/35014014 | s2cid = 22684712 }}</ref>

The ER is involved in cotranslational sorting of proteins. A polypeptide which contains an ER signal sequence is recognised by the [[signal recognition particle]] which halts the production of the protein. The SRP transports the nascent protein to the ER membrane where it is released through a membrane channel and translation resumes.<ref>{{Cite book|title=Biology |year=2011 |url=https://archive.org/details/biologythedition00maso |url-access=limited |publisher=McGraw Hill education|pages=[https://archive.org/details/biologythedition00maso/page/n117 89]|isbn=9780078936494 }}</ref>
[[File:0313 Endoplasmic Reticulum b en.png|thumb|243x243px|By using [[electron microscope]], ribosomes ("particles") on the [[rough endoplasmic reticulum]] can be observed ]]
There are two distinct, though connected, regions of ER that differ in structure and function: smooth ER and rough ER. The rough endoplasmic reticulum is so named because the cytoplasmic surface is covered with ribosomes, giving it a bumpy appearance when viewed through an [[electron microscope]]. The smooth ER appears smooth since its cytoplasmic surface lacks ribosomes.<ref name=Alberts3>{{cite web|url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=endoplasmic%20reticulum&rid=mboc4.section.2202#2204|title=Membrane-bound Ribosomes Define the Rough ER|access-date=2008-12-09|last=Alberts|first=Walter | name-list-style = vanc |year=2002|work= Molecular Biology of the Cell 4th edition|publisher=Garland Science|display-authors=etal}}</ref>

==== Functions of the smooth ER ====
In the great majority of cells, smooth ER regions are scarce and are often partly smooth and partly rough. They are sometimes called transitional ER because they contain ER exit sites from which transport vesicles carrying newly synthesized proteins and lipids bud off for transport to the Golgi apparatus. In certain specialized cells, however, the smooth ER is abundant and has additional functions. The smooth ER of these specialized cells functions in diverse metabolic processes, including synthesis of lipids, [[Carbohydrate metabolism|metabolism of carbohydrates]], and detoxification of drugs and poisons.<ref name=Cooper3 /><ref name=Alberts3 />

Enzymes of the smooth ER are vital to the synthesis of lipids, including [[oil]]s, [[phospholipid]]s, and [[steroid]]s. Sex hormones of vertebrates and the steroid hormones secreted by the [[adrenal gland]]s are among the steroids produced by the smooth ER in animal cells. The cells that synthesize these hormones are rich in smooth ER.<ref name=Cooper3 /><ref name=Alberts3 />

[[Liver]] cells are another example of specialized cells that contain an abundance of smooth ER. These cells provide an example of the role of smooth ER in carbohydrate metabolism. Liver cells store carbohydrates in the form of [[glycogen]]. The [[Glycogenolysis|breakdown of glycogen]] eventually leads to the release of [[glucose]] from the liver cells, which is important in the regulation of sugar concentration in the blood. However, the primary product of glycogen breakdown is glucose-1-phosphate. This is converted to glucose-6-phosphate and then an enzyme of the liver cell's smooth ER removes the phosphate from the glucose, so that it can then leave the cell.<ref name=Cooper3 /><ref name=Alberts3 />

Enzymes of the smooth ER can also help detoxify drugs and poisons. Detoxification usually involves the addition of a hydroxyl group to a drug, making the drug more soluble and thus easier to purge from the body. One extensively studied detoxification reaction is carried out by the [[cytochrome P450]] family of enzymes, which catalyze oxidation reactions on water-insoluble drugs or metabolites that would otherwise accumulate to toxic levels in cell membrane.<ref name=Cooper3 /><ref name=Alberts3 />

In muscle cells, a specialized smooth ER ([[sarcoplasmic reticulum]]) forms a membranous compartment (cisternal space) into which [[calcium]] ions are pumped. When a muscle cell becomes stimulated by a nerve impulse, calcium goes back across this membrane into the cytosol and generates the contraction of the muscle cell.<ref name=Cooper3 /><ref name=Alberts3 />

==== Functions of the rough ER ====
Many types of cells export proteins produced by ribosomes attached to the rough ER. The ribosomes assemble [[amino acids]] into protein units, which are carried into the rough ER for further adjustments. These proteins may be either [[transmembrane proteins]], which become embedded in the membrane of the endoplasmic reticulum, or water-soluble proteins, which are able to pass through the membrane into the lumen. Those that reach the inside of the endoplasmic reticulum are folded into the correct three-dimensional conformation. Chemicals, such as carbohydrates or sugars, are added, then the endoplasmic reticulum either transports the completed proteins, called secretory proteins, to areas of the cell where they are needed, or they are sent to the Golgi apparatus for further processing and modification.<ref name=Cooper3 /><ref name=Alberts3 />

Once secretory proteins are formed, the ER membrane separates them from the proteins that will remain in the cytosol. Secretory proteins depart from the ER enfolded in the membranes of vesicles that bud like bubbles from the transitional ER. These vesicles in transit to another part of the cell are called [[Vesicular transport protein|transport vesicles]].<ref name=Cooper3 /><ref name=Alberts3 /> An alternative mechanism for transport of lipids and proteins out of the ER are through lipid transfer proteins at regions called [[membrane contact site]]s where the ER becomes closely and stably associated with the membranes of other organelles, such as the plasma membrane, Golgi or lysosomes.<ref>{{cite journal | vauthors = Levine T, Loewen C | title = Inter-organelle membrane contact sites: through a glass, darkly | journal = Current Opinion in Cell Biology | volume = 18 | issue = 4 | pages = 371–8 | date = August 2006 | pmid = 16806880 | doi = 10.1016/j.ceb.2006.06.011 }}</ref>

In addition to making secretory proteins, the rough ER makes membranes that grows in place from the addition of proteins and phospholipids. As [[polypeptides]] intended to be membrane proteins grow from the ribosomes, they are inserted into the ER membrane itself and are kept there by their [[hydrophobic]] portions. The rough ER also produces its own membrane phospholipids; enzymes built into the ER membrane assemble phospholipids. The ER membrane expands and can be transferred by transport vesicles to other components of the endomembrane system.<ref name=Cooper3 /><ref name=Alberts3 />

=== Golgi apparatus ===
[[Image:Human leukocyte, showing golgi - TEM.jpg|thumb|350px|Micrograph of Golgi apparatus, visible as a stack of semicircular black rings near the bottom. Numerous circular vesicles can be seen in proximity to the organelle.]]

{{main|Golgi apparatus}}

The [[Golgi apparatus]] (also known as the Golgi body and the Golgi complex) is composed of separate sacs called [[cisternae]]. Its shape is similar to a stack of pancakes. The number of these stacks varies with the specific function of the cell. The Golgi apparatus is used by the cell for further protein modification. The section of the Golgi apparatus that receives the vesicles from the ER is known as the cis face, and is usually near the ER. The opposite end of the Golgi apparatus is called the trans face, this is where the modified compounds leave. The trans face is usually facing the plasma membrane, which is where most of the substances the Golgi apparatus modifies are sent.<ref name=Rothman>{{cite journal | vauthors = Rothman JE | title = The golgi apparatus: two organelles in tandem | journal = Science | volume = 213 | issue = 4513 | pages = 1212–9 | date = September 1981 | pmid = 7268428 | doi = 10.1126/science.7268428 | url = https://www.science.org/doi/10.1126/science.7268428 | bibcode = 1981Sci...213.1212R }}</ref>

Vesicles sent off by the ER containing proteins are further altered at the Golgi apparatus and then prepared for secretion from the cell or transport to other parts of the cell. Various things can happen to the proteins on their journey through the enzyme covered space of the Golgi apparatus. The modification and synthesis of the carbohydrate portions of glycoproteins is common in protein processing. The Golgi apparatus removes and substitutes sugar monomers, producing a large variety of [[oligosaccharide]]s. In addition to modifying proteins, the Golgi also manufactures macromolecules itself. In plant cells, the Golgi produces [[pectins]] and other polysaccharides needed by the plant structure.<ref name=Alberts4>{{cite web|url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=golgi%20apparatus&rid=mboc4.section.2333|title=Transport from the ER through the Golgi Apparatus|access-date=2008-12-09|last=Alberts|first=Walter| name-list-style = vanc |year=2002|work= Molecular Biology of the Cell 4th edition|publisher=Garland Science|display-authors=etal}}</ref>

Once the modification process is completed, the Golgi apparatus sorts the products of its processing and sends them to various parts of the cell. Molecular identification labels or tags are added by the Golgi enzymes to help with this. After everything is organized, the Golgi apparatus sends off its products by budding vesicles from its trans face.<ref name=Cooper4>{{cite web|url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=Golgi%20apparatus&rid=cooper.section.1498|title=The Golgi Apparatus|access-date=2008-12-09|last=Cooper|first=Geoffrey| name-list-style = vanc |year=2000|work= The Cell: A Molecular Approach|publisher=Sinauer Associates, Inc}}</ref>

=== Vacuoles ===
{{main|Vacuole}}

[[Vacuole]]s, like vesicles, are membrane-bound sacs within the cell. They are larger than vesicles and their specific function varies. The operations of vacuoles are different for plant and animal vacuoles.

In plant cells, vacuoles cover anywhere from 30% to 90% of the total cell volume.<ref name=Alberts5>{{cite web|url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=Vacuoles&rid=mboc4.section.2363#2369|title=Plant and Fungal Vacuoles Are Remarkably Versatile Lysosomes|access-date=2008-12-09|last=Alberts|first=Walter| name-list-style = vanc |year=2002|work= Molecular Biology of the Cell 4th edition|publisher=Garland Science|display-authors=etal}}</ref> Most mature plant cells contain one large central vacuole encompassed by a membrane called the tonoplast. Vacuoles of plant cells act as storage compartments for the nutrients and waste of a cell. The solution that these molecules are stored in is called the [[cell sap]]. [[Pigments]] that color the cell are sometime located in the cell sap. Vacuoles can also increase the size of the cell, which elongates as water is added, and they control the [[turgor pressure]] (the osmotic pressure that keeps the cell wall from caving in). Like lysosomes of animal cells, vacuoles have an acidic pH and contain hydrolytic enzymes. The pH of vacuoles enables them to perform homeostatic procedures in the cell. For example, when the pH in the cells environment drops, the H<sup>+</sup> ions surging into the cytosol can be transferred to a vacuole in order to keep the cytosol's pH constant.<ref name=Lodish1>{{cite web|url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=Vacuoles&rid=mcb.section.1182#1187|title=Plant Vacuoles Store Small Molecules and Enable the Cell to Elongate Rapidly|access-date=2008-12-09|last=Lodish|first=Harvey| name-list-style = vanc |year=2000|work= Molecular Cell Biology|publisher=W. H. Freeman and Company|display-authors=etal}}</ref>

In animals, vacuoles serve in [[exocytosis]] and [[endocytosis]] processes. Endocytosis refers to when substances are taken into the cell, whereas for exocytosis substances are moved from the cell into the extracellular space. Material to be taken-in is surrounded by the plasma membrane, and then transferred to a vacuole. There are two types of endocytosis, [[phagocytosis]] (cell eating) and [[pinocytosis]] (cell drinking). In phagocytosis, cells engulf large particles such as bacteria. Pinocytosis is the same process, except the substances being ingested are in the fluid form.<ref name=Cooper5>{{cite web|url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=phagocytosis&rid=cooper.section.2017#2018|title=Endocytosis|access-date=2008-12-09|last=Cooper|first=Geoffrey| name-list-style = vanc |year=2000|work= The Cell: A Molecular Approach|publisher=Sinauer Associates, Inc}}</ref>

=== Vesicles ===
{{main|Vesicle (biology)}}

[[Vesicle (biology)|Vesicles]] are small membrane-enclosed transport units that can transfer molecules between different compartments. Most vesicles transfer the membranes assembled in the endoplasmic reticulum to the Golgi apparatus, and then from the Golgi apparatus to various locations.<ref name=Lodish2>{{cite web|url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=Vesicles&rid=mcb.section.4883#4887|title=Section 17.10 Molecular Mechanisms of Vesicular Traffic|access-date=2008-12-09|last=Lodish|first=Harvey| name-list-style = vanc |year=2000|work= Molecular Cell Biology|publisher=W. H. Freeman and Company|display-authors=etal}}</ref>

There are various types of vesicles each with a different protein configuration. Most are formed from specific regions of membranes. When a vesicle buds off from a membrane it contains specific proteins on its cytosolic surface. Each membrane a vesicle travels to contains a marker on its cytosolic surface. This marker corresponds with the proteins on the vesicle traveling to the membrane. Once the vesicle finds the membrane, they fuse.<ref name=Alberts6>{{cite web|url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=vesicles&rid=mboc4.section.2306#2308|title=The Molecular Mechanisms of Membrane Transport and the Maintenance of Compartmental Diversity|access-date=2008-12-09|last=Alberts|first=Walter| name-list-style = vanc |year=2002|work= Molecular Biology of the Cell 4th edition|publisher=Garland Science|display-authors=etal}}</ref>

There are three well known types of vesicles. They are [[clathrin]]-coated, [[COPI]]-coated, and [[COPII]]-coated vesicles. Each performs different functions in the cell. For example, clathrin-coated vesicles transport substances between the Golgi apparatus and the plasma membrane. COPI- and COPII-coated vesicles are frequently used for transportation between the ER and the Golgi apparatus.<ref name=Alberts6 />

=== Lysosomes ===
{{main|Lysosomes}}

[[Lysosomes]] are organelles that contain hydrolytic enzymes that are used for intracellular digestion. The main functions of a lysosome are to process molecules taken in by the cell and to recycle worn out cell parts. The enzymes inside of lysosomes are [[acid hydrolases]] which require an acidic environment for optimal performance. Lysosomes provide such an environment by maintaining a pH of 5.0 inside of the organelle.<ref name=Alberts1>{{cite web|url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=Lysosomes&rid=mboc4.section.2363#2365|title=Transport from the Trans Golgi Network to Lysosomes|access-date=2008-12-09|last=Alberts|first=Walter| name-list-style = vanc |year=2002|work= Molecular Biology of the Cell 4th edition|publisher=Garland Science|display-authors=etal}}</ref> If a lysosome were to rupture, the enzymes released would not be very active because of the cytosol's neutral pH. However, if numerous lysosomes leaked the cell could be destroyed from autodigestion.

Lysosomes carry out intracellular digestion, in a process called phagocytosis (from the Greek {{lang|grc-Latn|phagein}}, to eat and {{lang|grc-Latn|kytos}}, vessel, referring here to the cell), by fusing with a vacuole and releasing their enzymes into the vacuole. Through this process, sugars, amino acids, and other monomers pass into the cytosol and become nutrients for the cell. Lysosomes also use their hydrolytic enzymes to recycle the cell's obsolete organelles in a process called [[autophagy]]. The lysosome engulfs another organelle and uses its enzymes to take apart the ingested material. The resulting organic monomers are then returned to the cytosol for reuse. The last function of a lysosome is to digest the cell itself through [[Autolysis (biology)|autolysis]].<ref name=Cooper6>{{cite web|url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=lysosomes&rid=cooper.section.1519|title=Lysosomes|access-date=2008-12-09|last=Cooper|first=Geoffrey| name-list-style = vanc |year=2000|work= The Cell: A Molecular Approach|publisher=Sinauer Associates, Inc}}</ref>

=== Spitzenkörper ===
{{main|Spitzenkörper}}

The spitzenkörper is a component of the endomembrane system found only in [[fungi]], and is associated with [[Hypha|hyphal tip growth]]. It is a [[phase contrast|phase]]-dark body that is composed of an aggregation of membrane-bound vesicles containing cell wall components, serving as a point of assemblage and release of such components intermediate between the Golgi and the cell membrane. The spitzenkörper is motile and generates new hyphal tip growth as it moves forward.<ref name=Steinberg />

[[Image:Cell membrane detailed diagram 4.svg|thumb|350px|Detailed illustration of the plasma membrane. Including the structure of a [[phospholipid]].]]

=== Plasma membrane ===
{{main|Plasma membrane}}

The [[plasma membrane]] is a phospholipid bilayer membrane that separates the cell from its environment and regulates the transport of molecules and signals into and out of the cell. Embedded in the membrane are proteins that perform the functions of the plasma membrane. The plasma membrane is not a fixed or rigid structure, the molecules that compose the membrane are capable of lateral movement. This movement and the multiple components of the membrane are why it is referred to as a fluid mosaic. Smaller molecules such as carbon dioxide, water, and oxygen can pass through the plasma membrane freely by [[diffusion]] or [[osmosis]]. Larger molecules needed by the cell are assisted by proteins through [[active transport]].<ref name=Cooper7>{{cite web|url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=plasma%20membrane&rid=cooper.section.1967|title=Structure of the Plasma Membrane|access-date=2008-12-09|last=Cooper|first=Geoffrey| name-list-style = vanc |year=2000|work= The Cell: A Molecular Approach|publisher=Sinauer Associates, Inc}}</ref>

The plasma membrane of a cell has multiple functions. These include transporting nutrients into the cell, allowing waste to leave, preventing materials from entering the cell, averting needed materials from leaving the cell, maintaining the pH of the cytosol, and preserving the [[osmotic pressure]] of the cytosol. Transport proteins which allow some materials to pass through but not others are used for these functions. These proteins use ATP hydrolysis to pump materials against their concentration gradients.<ref name="Cooper7"/>

In addition to these universal functions, the plasma membrane has a more specific role in multicellular organisms. Glycoproteins on the membrane assist the cell in recognizing other cells, in order to exchange metabolites and form tissues. Other proteins on the plasma membrane allow attachment to the [[cytoskeleton]] and [[extracellular matrix]]; a function that maintains cell shape and fixes the location of membrane proteins. Enzymes that catalyze reactions are also found on the plasma membrane. Receptor proteins on the membrane have a shape that matches with a chemical messenger, resulting in various cellular responses.<ref name=Lodish3>{{cite web|url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=Cell%20Membrane&rid=mcb.section.1146#1168|title=Section 5.3. Biomembranes: Structural Organization and Basic Functions|access-date=2008-12-09|last=Lodish|first=Harvey| name-list-style = vanc |year=2000|work= Molecular Cell Biology|publisher=W. H. Freeman and Company|display-authors=etal}}</ref>