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.^[1] The endoplasmic reticulum (ER) is a synthesis and transport organelle that branches into the cytoplasm in plant and animal cells.^[2] 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.^[3] Vacuoles, which are found in both plant and animal cells (though much bigger in plant cells), are responsible for maintaining the shape and structure of the cell as well as storing waste products.^[4] A vesicle is a relatively small, membrane-enclosed sac that stores or transports substances.^[5] The plasma membrane, also referred to as the cell membrane, is a protective barrier that regulates what enters and leaves the cell. ^[6]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.^[7]

Components

Nuclear envelope

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 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. Histones, gene regulatory proteins, DNA and RNA polymerases, 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.^[10]

Endoplasmic reticulum

1 Nucleus     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 ER has a central role in producing, processing, 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 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 chaperon 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.^[11]

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.^[12]

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 vital to the synthesis of lipids, including oils, phospholipids, and steroids. Sex hormones of vertebrates and the steroid hormones secreted by the adrenal glands are among the steroids produced by the smooth ER in animal cells. The cells that synthesis these hormones are rich in smooth ER.

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, glycogen hydrolysis requires glucose phosphate, an ionic form of the sugar that can not exit the cell. An enzyme of the liver cell's smooth ER removes the phosphate from the glucose, so that it can then leave the cell.

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 water-insoluble drugs or metabolites that would otherwise accumulate to toxic levels in cell membrane.

Muscle cells have another specialized function of smooth ER. The ER membrane pumps calcium ions from the cytosol into the cisternal space. When a muscle cell becomes stimulated by a nerve impulse, calcium goes back across the ER membrane into the cytosol and generates the 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 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.

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 transport vesicles.

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.^[13]

Golgi apparatus

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

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 removes and substitutes sugar monomers, producing a large variety of oligosaccharides. 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.

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.^[15]

Vacuoles

Vacuoles, like vesicles, are membrane-bounded sacs within the cell. They are larger than vesicles and their specific function varies. The operations 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. ^[16] 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+ surging into the cytosol can be transferred to a vacuole in order to keep the cytosol's pH constant. ^[17]

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.^[18]

Vesicles

Vesicles are small membrane-enclosed transport units that can transfer molecules between different compartments. Some vesicles transfer the membranes assembled in the endoplasmic reticulum to the golgi apparatus, and then from the golgi apparatus to various locations. There are three types of vesicles known each with a different protein configuration. ^[4]

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 recyle worn out cell parts. The enzymes inside of lysosomes are acid hydrolases. That means that for peak performance the enzymes require an acidic environment, lysosomes provide such an environment by maintaing a pH of 5.0 inside of the organelle.^[19] If a lysosome were to burst 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 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 ther hydrolytic enzymes to recycle the cell's obsolete organelles in a process caleed autophagy. The lysosme engulfs another organelle and uses it's 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.^[20]

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.^[21]

Plasma Membrane

Illustration of an Eukaryotic cell membrane

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. In addition to these universal functions, the plasma membrane has a role in multicellular organisms. Glycoprotiens on the membrane assist the cell in recognizing other cells, in order to exchange metabolities 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 shape that matches with a chemical messenger, resulting in various cellular responses.^[22]

Notes

References

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See also

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