Endomembrane system

The endomembrane system is composed of the different membranes that are suspended in the cytoplasm within a eukaryotic cell. These membranes divide the cell into functional and structural compartments, or organelles. The organelles of the endomembrane system include: the nuclear envelope, the endoplasmic reticulum, the golgi apparatus, vacuoles, vesicles, and the cell membrane. The nuclear envelope is a membrane containing two layers, that encompasses the contents of the nucleus.Harvnb|Davidson|2005|loc= [http://micro.magnet.fsu.edu/cells/nucleus/nuclearenvelope.html The Nuclear Envelope] .] The endoplasmic reticulum (ER) is a synthesis and transport organelle that branches into the cytoplasm in plant and animal cells. [Harvnb|Davidson|2005|loc= [http://micro.magnet.fsu.edu/cells/endoplasmicreticulum/endoplasmicreticulum.html The Endoplasmic Reticulum] .] The golgi apparatus is a series of multiple compartments where molecules are packaged for delivery to other cell components or for secretion from the cell. [Harvnb|Graham|2000.] Vacuoles, which are only found in plant cells, are responsible for maintaining the shape and structure of the cell.Harvnb|Lodish|Berk|Zipursky|Matsudaira|2000|loc= [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=Vacuoles&rid=mcb.section.1182#1187 Section 5.4 Organelles of the Eukaryotic Cell] .] A vesicle is a relatively small, membrane-enclosed sac that stores or transports substances. [Harvnb|Lodish|Berk|Zipursky|Matsudaira|2000|loc= [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=Vesicles&rid=mcb.section.4883#4913 Section 17.10. Molecular Mechanisms of Vesicular Traffic] ] The cell membrane, also referred to as the plasma membrane, is a protective barrier that regulates what enters and leaves the cell. [Harvnb|Lodish|Berk|Zipursky|Matsudaira|2000|loc= [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=Cell%20Membrane&rid=mcb.section.1146#1168 Section 5.3. Biomembranes: Structural Organization and Basic Functions] ] The organelles of the endomembrane system are related through direct contact or by the transfer of membrane segments as vesicles. Despite these relationships, the various membranes are not alike in structure and function. The thickness, molecular composition, and metabolic behavior of a membrane are not fixed, they may be modified several times during the membrane’s life. One identical charactaristic the membranes have is a lipid bilayer, with proteins attached to either side or traversing them. [Harvnb|Campbell|Reece|2002|p=117–123 .]

History

Components

The Nuclear Envelope

The nuclear envelope encloses the nucleus, separating its contents from the cytoplasm. It has two membranes, each a lipid bilayer with associated proteins. [Harvnb|Childs|2003.] The outer nuclear membrane is constant with the rough endoplasmic reticulum membrane, and like that structure, features ribosomes attached to the surface. The outer membrane is also constant with the inner nuclear membrane since the two layers are fused together at numerous tiny holes called nuclear pores that perforate the nuclear envelope. These pores are about 120 nm in diameter and regulate the passage of molecules between the nucleus and cytoplasm, permitting some to pass through the membrane, but not others. [Harvnb|Cooper|2000|loc= [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=Nuclear%20Envelope&rid=cooper.section.1323#1324 The Nuclear Envelope and Traffic between the Nucleus and Cytoplasm] ] Since the nuclear pores are located in an area of high traffic, they play an important role in the physiology of cells. The space between the outer and inner membranes is termed the perinuclear space and is connected with the lumen of the rough ER.

The nuclear envelopes structure is determined by a network of intermediate filaments (protein filaments). This network is organized into a special 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 have a role in directing materials inside the nucleus toward the nuclear pores for export and in the disintegration of the nuclear envelope during cell mitosis and its reformation at the end of the process.

The nuclear pores are highly efficeint 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 constantly transferred from the nucleus, their orgin, to the cytoplasm. Histones, gene regulatory proteins, DNA and RNA polymerases, and other substances required 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. [Harvnb|Alberts|Walter|Roberts|Raff|2002|loc= [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=Nuclear%20Envelope&rid=mboc4.section.2154#2156 Nuclear Pore Complexes Perforate the Nuclear Envelope] .]

The Endoplasmic Reticulum

2 Nuclear pore 3 Rough endoplasmic reticulum (RER) 4 Smooth endoplasmic reticulum (SER) 5 Ribosome on the rough ER 6 Proteins that are transported 7 Transport vesicle ] 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 convulted space is called the ER lumen and is also referred to as the ER cisternal 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 double-layered nuclear envelope, it further provides a pipeline between the nucleus and the cytoplasm.

The ER has a central role in producing, proccessing, and transporting 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 most of the cell's organelles, including the ER itself, the Golgi apparatus, lysosomes, endosomes, Mitochondrion, Peroxisome, 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 orginally delivered to the ER lumen. Consequently, many of the proteins found in the cisternal space of the endoplasmic reticulum lumen are there only temporarly as they pass on their way to other locations. Other proteins, however, are targeted to constantly remain in the lumen and are known as endoplasmic reticulum resident proteins. These special proteins, which are necessary for the endoplasmic reticulum to carry out its normal functions, contain a specialized retention signal consisting 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 known as BiP which identifies other proteins that have been improperly built or processed and keeps them from being sent to their final destinations. [Harvnb|Bertolotti|Zhang|Hendershot|Harding|2000]

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. [Harvnb|Alberts|Walter|Roberts|Raff|2002|loc= [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=endoplasmic%20reticulum&rid=mboc4.section.2202#2204 Membrane-bound Ribosomes Define the Rough ER] .]

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 theses specialized cells function in diverse metabolic processes, including synthesis of lipids, metabolism of carbohydrates, and detoxification of drugs and poisons.

Enzymes of the smooth ER are important to the synthesis of lipids, including oils, phospholipids, and steroids. Among the steroids prodeced by the smooth ER in animal cells are the sex hormones of vertebrates and the various steroid hormones secreted by the adrenal glands. Cells that synthesis these hormones are rich in smooth ER which is the structural feature that fits the function of these cells.

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 hydrolysis of glycogen leads to the release of glucose from the liver cells, which is important in the regulation of sugar concentration in the blood. However, the first product od glycogen hydrolysis is glucose phosphate, an ionic form of the sugar that can not exit the cell. An enzyme embedded in the membrane of the liver cell's smooth ER removes the phosphate from the glucose, which can then leave the cell.

Enzymes of the smooth ER can also help detoxify drugs and poisons. Detoxification usually involves adding hydroxly groups to drugs, making them more soluble and eaier to flush from the body. One extensively studied detoxification reaction is carried out by the cytochrome P450 family of enzymes, which catalyze water-insoluble drugs or metabolites that would otherwise accumulate to toxic levels in cell membrane.

Muscle cells exhibit another specialized function of smooth ER. The ER membrane pumps calcium ions from the cytosol into the cisternal space. When a muscle cell is stimulated by a nerve impulse, calcium rushes back across the ER membrane into the cytosol and triggers contraction of the muscle cell.

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 transported into the rough endoplasmic reticulum for further processing. 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 completely 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.

Once secretory proteins are formed, the ER membrane keeps them separate from the proteins, produced by free ribosomes, that will remain in the cytosol. Secretory proteins depart from the ER wrapped in the membranes of vesicles that bud like bubbles from the transitional ER. Such vesicles in transit from one part of the cell to another are called transport vesicles.

In addition to making secretory proteins, the rough ER is a membrane factory that grows in place by adding proteins and phospholipids. As polypeptides destined to be membrane proteins grow from the ribosomes, they are inserted into the ER membrane itself and are anchored there by their hydrophobic portions. The rough ER also makes its own membrane phospholipids; enzymes built into the ER membrane assemble phospholipids from precursors in the cytosol. The ER membrane expands and can be transferred in the form of transport vesicles to other components of the endomembrane system. [Harvnb|Kimball|2007.]

The Golgi Apparatus

The golgi apparatus (also know as the golgi body and the golgi complex) is composed of interconnected sacs called cisternae. Its shape can be related to that of a stack of pancakes. The number of these stacks varies with the specific function of the cell. The golgi apparatus is known as the packing and shipping department for the cell. The section of the golgi apparatus that recevies 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 compounds modified leave. The trans face is usually facing the plasma membrane, which is where most of the substances the golgi apparatus modifies are sent. [Harvnb|Rothman|1981.]

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 protiens 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 removes and substitutes sugar monomers, producing a large variety of oligosaccharides. In addition to modifing proteins, the golgi also manufactures macromolecules itself. In plant cells, the golgi produces pectins and other polysaccharides needed by the plant structure.

Once the modification proccess is completed the golgi apparatus sorts the products of its proccesing 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. [Harvnb|Cooper|2000|loc= [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=Golgi%20apparatus&rid=cooper.section.1498 The Golgi Apparatus] .]

Vacuoles

Vacuoles, like vesicles, are membrane-bounded sacs withing the cell. They are larger than vesicles and their specific function varies. The opeartions of vacuoles can be broken down into plant and animal vacuoles.

In plant cells, vacuoles cover anywhere from 30% to 90% of the total cell volume. [Harvnb|Alberts|Walter|Roberts|Raff|2002|loc= [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=Vacuoles&rid=mboc4.section.2363#2369 Plant and Fungal Vacuoles Are Remarkably Versatile Lysosomes] .] 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 enivornment drops, the H+ surging into the cytosol can be transferred to a vacuole in order to keep the cytosol's ph constant. [Harvnb|Lodish|Berk|Zipursky|Matsudaira|2000|loc= [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=Vacuoles&rid=mcb.section.1182#1187 Plant Vacuoles Store Small Molecules and Enable the Cell to Elongate Rapidly] Section 5.4. Organelles of the Eukaryotic Cell]

In animals, vacuoles serve in exocytosis and endocytosis processes. Endocytosis refers to when particles are taken into the cell. The 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. [Harvnb|Cooper|2000|loc= [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=phagocytosis&rid=cooper.section.2017#2018 Endocytosis] ]

Vesicles

Vesicles are small membrane-enclosed transport units that can transfer molecules between different compartments.

Lysosomes

The Spitzenkörper

The spitzenkörper is a component of the endomembrane system found only in fungi, and is associated with hyphal tip growth. It is a 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. [Harvnb|Steinberg|2007.]

The Cell Membrane

The cell 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.

Notes

References

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