Endomembrane system

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Detail of the endomembrane system and its components Endomembrane system diagram en.svgnucleussecretory vesicle
Detail of the endomembrane system and its components

The endomembrane system is composed of the different membranes (endomembranes) that are suspended in the cytoplasm within a eukaryotic cell. These membranes divide the cell into functional and structural compartments, or organelles. In eukaryotes the organelles of the endomembrane system include: the nuclear membrane, the endoplasmic reticulum, the Golgi apparatus, lysosomes, vesicles, endosomes, and plasma (cell) membrane among others. The system is defined more accurately as the set of membranes that forms a single functional and developmental unit, either being connected directly, or exchanging material through vesicle transport. [1] Importantly, the endomembrane system does not include the membranes of plastids or mitochondria, but might have evolved partially from the actions of the latter (see below).

Contents

The nuclear membrane contains a lipid bilayer that encompasses the contents of the nucleus. [2] The endoplasmic reticulum (ER) is a synthesis and transport organelle that branches into the cytoplasm in plant and animal cells. [3] 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. [4] 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. [5] A vesicle is a relatively small, membrane-enclosed sac that stores or transports substances. [6] The cell membrane is a protective barrier that regulates what enters and leaves the cell. [7] There is also an organelle known as the Spitzenkörper that is only found in fungi, and is connected with hyphal tip growth. [8]

In prokaryotes endomembranes are rare, although in many photosynthetic bacteria the plasma membrane is highly folded and most of the cell cytoplasm is filled with layers of light-gathering membrane. [9] These light-gathering membranes may even form enclosed structures called chlorosomes in green sulfur bacteria. [10] Another example is the complex "pepin" system of Thiomargarita species, especially T. magnifica . [11]

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 identical 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 unifying characteristic the membranes share is a lipid bilayer, with proteins attached to either side or traversing them. [12]

History of the concept

Most lipids are synthesized in yeast either in the endoplasmic reticulum, lipid particles, or the mitochondrion, with little or no lipid synthesis occurring in the plasma membrane or nuclear membrane. [13] [14] Sphingolipid biosynthesis begins in the endoplasmic reticulum, but is completed in the Golgi apparatus. [15] The situation is similar in mammals, with the exception of the first few steps in ether lipid biosynthesis, which occur in peroxisomes. [16] The various membranes that enclose the other subcellular organelles must therefore be constructed by transfer of lipids from these sites of synthesis. [17] However, although it is clear that lipid transport is a central process in organelle biogenesis, the mechanisms by which lipids are transported through cells remain poorly understood. [18]

The first proposal that the membranes within cells form a single system that exchanges material between its components was by Morré and Mollenhauer in 1974. [19] This proposal was made as a way of explaining how the various lipid membranes are assembled in the cell, with these membranes being assembled through lipid flow from the sites of lipid synthesis. [20] The idea of lipid flow through a continuous system of membranes and vesicles was an alternative to the various membranes being independent entities that are formed from transport of free lipid components, such as fatty acids and sterols, through the cytosol. Importantly, the transport of lipids through the cytosol and lipid flow through a continuous endomembrane system are not mutually exclusive processes and both may occur in cells. [17]

Components of the system

Nuclear envelope

Diagram of the nucleus with the nuclear envelope shown as the orange portion Diagram human cell nucleus.svg
Diagram of the nucleus with the nuclear envelope shown as the orange portion

The nuclear envelope surrounds the nucleus, separating its contents from the cytoplasm. It has two membranes, each a lipid bilayer with associated proteins. [21] The outer nuclear membrane is continuous with the rough endoplasmic reticulum membrane, and like that structure, features ribosomes 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 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. [22] 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. [2]

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 to synthesize proteins. [23]

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 8 Golgi apparatus 9 Cis face of the Golgi apparatus 10 Trans face of the Golgi apparatus 11 Cisternae of the Golgi apparatus Nucleus ER golgi.svg
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  8  Golgi apparatus  9 Cis face of the Golgi apparatus 10 Trans face of the Golgi apparatus 11 Cisternae of the Golgi apparatus

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 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 nuclear envelope, it provides a channel between the nucleus and the cytoplasm. [24]

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 many of the cell's organelles, including the ER itself, the Golgi apparatus, lysosomes, endosomes, 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 known as BiP which identifies other proteins that have been improperly built or processed and keeps them from being sent to their final destinations. [25]

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. [26]

By using electron microscope, ribosomes ("particles") on the rough endoplasmic reticulum can be observed 0313 Endoplasmic Reticulum b en.png
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. [27]

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, metabolism of carbohydrates, and detoxification of drugs and poisons. [24] [27]

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 synthesize these hormones are rich in smooth ER. [24] [27]

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 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. [24] [27]

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. [24] [27]

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. [24] [27]

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. [24] [27]

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. [24] [27] An alternative mechanism for transport of lipids and proteins out of the ER are through lipid transfer proteins at regions called membrane contact sites where the ER becomes closely and stably associated with the membranes of other organelles, such as the plasma membrane, Golgi or lysosomes. [28]

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. [24] [27]

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. Human leukocyte, showing golgi - TEM.jpg
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.

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. [29]

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 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. [30]

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. [31]

Vacuoles

Vacuoles, 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. [32] 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+ ions surging into the cytosol can be transferred to a vacuole in order to keep the cytosol's pH constant. [33]

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. [34]

Vesicles

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. [35]

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. [36]

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. [36]

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. [37] 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 phagein, to eat and 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. [38]

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. [8]

Detailed illustration of the plasma membrane. Including the structure of a phospholipid. Cell membrane detailed diagram 4.svg
Detailed illustration of the plasma membrane. Including the structure of a phospholipid.

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. [39]

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. [39]

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. [40]

Evolution

The origin of the endomembrane system is linked to the origin of eukaryotes themselves and the origin of eukaryoties to the endosymbiotic origin of mitochondria. Many models have been put forward to explain the origin of the endomembrane system (reviewed in [41] ). The most recent concept suggests that the endomembrane system evolved from outer membrane vesicles the endosymbiotic mitochondrion secreted, and got enclosed within infoldings of the host prokaryote (in turn, a result of the ingestion of the endosymbiont). [42] This OMV (outer membrane vesicles)-based model for the origin of the endomembrane system is currently the one that requires the fewest novel inventions at eukaryote origin and explains the many connections of mitochondria with other compartments of the cell. [43] Currently, this "inside-out" hypothesis (which states that the alphaproteobacteria, the ancestral mitochondria, were engulfed by the blebs of an asgardarchaeon, and later the blebs fused leaving infoldings which would eventually become the endomembrane system) is favored more than the outside-in one (which suggested that the endomembrane system arose due to infoldings within the archaeal membrane).

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<span class="mw-page-title-main">Biological membrane</span> Enclosing or separating membrane in organisms acting as selective semi-permeable barrier

A biological membrane, biomembrane or cell membrane is a selectively permeable membrane that separates the interior of a cell from the external environment or creates intracellular compartments by serving as a boundary between one part of the cell and another. Biological membranes, in the form of eukaryotic cell membranes, consist of a phospholipid bilayer with embedded, integral and peripheral proteins used in communication and transportation of chemicals and ions. The bulk of lipids in a cell membrane provides a fluid matrix for proteins to rotate and laterally diffuse for physiological functioning. Proteins are adapted to high membrane fluidity environment of the lipid bilayer with the presence of an annular lipid shell, consisting of lipid molecules bound tightly to the surface of integral membrane proteins. The cell membranes are different from the isolating tissues formed by layers of cells, such as mucous membranes, basement membranes, and serous membranes.

Cell biology is a branch of biology that studies the structure, function, and behavior of cells. All living organisms are made of cells. A cell is the basic unit of life that is responsible for the living and functioning of organisms. Cell biology is the study of the structural and functional units of cells. Cell biology encompasses both prokaryotic and eukaryotic cells and has many subtopics which may include the study of cell metabolism, cell communication, cell cycle, biochemistry, and cell composition. The study of cells is performed using several microscopy techniques, cell culture, and cell fractionation. These have allowed for and are currently being used for discoveries and research pertaining to how cells function, ultimately giving insight into understanding larger organisms. Knowing the components of cells and how cells work is fundamental to all biological sciences while also being essential for research in biomedical fields such as cancer, and other diseases. Research in cell biology is interconnected to other fields such as genetics, molecular genetics, molecular biology, medical microbiology, immunology, and cytochemistry.

<span class="mw-page-title-main">Endoplasmic reticulum</span> Cell organelle that synthesizes, folds and processes proteins

The endoplasmic reticulum (ER) is a part of a transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum (RER), and smooth endoplasmic reticulum (SER). The endoplasmic reticulum is found in most eukaryotic cells and forms an interconnected network of flattened, membrane-enclosed sacs known as cisternae, and tubular structures in the SER. The membranes of the ER are continuous with the outer nuclear membrane. The endoplasmic reticulum is not found in red blood cells, or spermatozoa.

<span class="mw-page-title-main">Endocytosis</span> Cellular process

Endocytosis is a cellular process in which substances are brought into the cell. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a vesicle containing the ingested materials. Endocytosis includes pinocytosis and phagocytosis. It is a form of active transport.

<span class="mw-page-title-main">Golgi apparatus</span> Cell organelle that packages proteins for export

The Golgi apparatus, also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles inside the cell before the vesicles are sent to their destination. It resides at the intersection of the secretory, lysosomal, and endocytic pathways. It is of particular importance in processing proteins for secretion, containing a set of glycosylation enzymes that attach various sugar monomers to proteins as the proteins move through the apparatus.

<span class="mw-page-title-main">Lysosome</span> Cell membrane organelle

A lysosome is a single membrane-bound organelle found in many animal cells. They are spherical vesicles that contain hydrolytic enzymes that digest many kinds of biomolecules. A lysosome has a specific composition, of both its membrane proteins and its lumenal proteins. The lumen's pH (~4.5–5.0) is optimal for the enzymes involved in hydrolysis, analogous to the activity of the stomach. Besides degradation of polymers, the lysosome is involved in cell processes of secretion, plasma membrane repair, apoptosis, cell signaling, and energy metabolism.

Protein targeting or protein sorting is the biological mechanism by which proteins are transported to their appropriate destinations within or outside the cell. Proteins can be targeted to the inner space of an organelle, different intracellular membranes, the plasma membrane, or to the exterior of the cell via secretion. Information contained in the protein itself directs this delivery process. Correct sorting is crucial for the cell; errors or dysfunction in sorting have been linked to multiple diseases.

<span class="mw-page-title-main">Vesicle (biology and chemistry)</span> Any small, fluid-filled, spherical organelle enclosed by a membrane

In cell biology, a vesicle is a structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer. Vesicles form naturally during the processes of secretion (exocytosis), uptake (endocytosis), and the transport of materials within the plasma membrane. Alternatively, they may be prepared artificially, in which case they are called liposomes. If there is only one phospholipid bilayer, the vesicles are called unilamellar liposomes; otherwise they are called multilamellar liposomes. The membrane enclosing the vesicle is also a lamellar phase, similar to that of the plasma membrane, and intracellular vesicles can fuse with the plasma membrane to release their contents outside the cell. Vesicles can also fuse with other organelles within the cell. A vesicle released from the cell is known as an extracellular vesicle.

<span class="mw-page-title-main">Nucleoplasm</span> Protoplasm that permeates a cells nucleus

The nucleoplasm, also known as karyoplasm, is the type of protoplasm that makes up the cell nucleus, the most prominent organelle of the eukaryotic cell. It is enclosed by the nuclear envelope, also known as the nuclear membrane. The nucleoplasm resembles the cytoplasm of a eukaryotic cell in that it is a gel-like substance found within a membrane, although the nucleoplasm only fills out the space in the nucleus and has its own unique functions. The nucleoplasm suspends structures within the nucleus that are not membrane-bound and is responsible for maintaining the shape of the nucleus. The structures suspended in the nucleoplasm include chromosomes, various proteins, nuclear bodies, the nucleolus, nucleoporins, nucleotides, and nuclear speckles.

<span class="mw-page-title-main">Endosome</span> Vacuole to which materials ingested by endocytosis are delivered

Endosomes are a collection of intracellular sorting organelles in eukaryotic cells. They are parts of endocytic membrane transport pathway originating from the trans Golgi network. Molecules or ligands internalized from the plasma membrane can follow this pathway all the way to lysosomes for degradation or can be recycled back to the cell membrane in the endocytic cycle. Molecules are also transported to endosomes from the trans Golgi network and either continue to lysosomes or recycle back to the Golgi apparatus.

<span class="mw-page-title-main">Secretion</span> Controlled release of substances by cells or tissues

Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. The classical mechanism of cell secretion is via secretory portals at the plasma membrane called porosomes. Porosomes are permanent cup-shaped lipoprotein structures embedded in the cell membrane, where secretory vesicles transiently dock and fuse to release intra-vesicular contents from the cell.

<span class="mw-page-title-main">Endoplasm</span> Also known as entoplasm

Endoplasm generally refers to the inner, dense part of a cell's cytoplasm. This is opposed to the ectoplasm which is the outer (non-granulated) layer of the cytoplasm, which is typically watery and immediately adjacent to the plasma membrane. The nucleus is separated from the endoplasm by the nuclear envelope. The different makeups/viscosities of the endoplasm and ectoplasm contribute to the amoeba's locomotion through the formation of a pseudopod. However, other types of cells have cytoplasm divided into endo- and ectoplasm. The endoplasm, along with its granules, contains water, nucleic acids, amino acids, carbohydrates, inorganic ions, lipids, enzymes, and other molecular compounds. It is the site of most cellular processes as it houses the organelles that make up the endomembrane system, as well as those that stand alone. The endoplasm is necessary for most metabolic activities, including cell division.

A secretory protein is any protein, whether it be endocrine or exocrine, which is secreted by a cell. Secretory proteins include many hormones, enzymes, toxins, and antimicrobial peptides. Secretory proteins are synthesized in the endoplasmic reticulum.

<span class="mw-page-title-main">Cellular compartment</span> Closed part in cytosol

Cellular compartments in cell biology comprise all of the closed parts within the cytosol of a eukaryotic cell, usually surrounded by a single or double lipid layer membrane. These compartments are often, but not always, defined as membrane-bound organelles. The formation of cellular compartments is called compartmentalization.

Cell physiology is the biological study of the activities that take place in a cell to keep it alive. The term physiology refers to normal functions in a living organism. Animal cells, plant cells and microorganism cells show similarities in their functions even though they vary in structure.

<span class="mw-page-title-main">Outline of cell biology</span> Overview of and topical guide to cell biology

The following outline is provided as an overview of and topical guide to cell biology:

Berkeley bodies are organelles unique to the yeast cell Saccharomyces cerevisiae, with a secretory mutation in the genes sec7 and sec14. The function of the organelle lies in the CTV pathway, which is a transport pathway for certain vacuolar hydrolases to be degenerated. The Berkeley body acts as the transport medium from the cytoplasm to the vacuole within this pathway. Studies have shown that Berkeley bodies share structural similarities with autophagosomes, which are involved in autophagy.

Reticulons are a group of evolutionary conservative proteins residing predominantly in endoplasmic reticulum, primarily playing a role in promoting membrane curvature. In addition, reticulons may play a role in nuclear pore complex formation, vesicle formation, and other processes yet to be defined. They have also been linked to oligodendrocyte roles in inhibition of neurite outgrowth. Some studies link RTNs with Alzheimer's disease and amyotrophic lateral sclerosis.

<span class="mw-page-title-main">Cell membrane</span> Biological membrane that separates the interior of a cell from its outside environment

The cell membrane is a biological membrane that separates and protects the interior of a cell from the outside environment. The cell membrane consists of a lipid bilayer, made up of two layers of phospholipids with cholesterols interspersed between them, maintaining appropriate membrane fluidity at various temperatures. The membrane also contains membrane proteins, including integral proteins that span the membrane and serve as membrane transporters, and peripheral proteins that loosely attach to the outer (peripheral) side of the cell membrane, acting as enzymes to facilitate interaction with the cell's environment. Glycolipids embedded in the outer lipid layer serve a similar purpose. The cell membrane controls the movement of substances in and out of a cell, being selectively permeable to ions and organic molecules. In addition, cell membranes are involved in a variety of cellular processes such as cell adhesion, ion conductivity, and cell signalling and serve as the attachment surface for several extracellular structures, including the cell wall and the carbohydrate layer called the glycocalyx, as well as the intracellular network of protein fibers called the cytoskeleton. In the field of synthetic biology, cell membranes can be artificially reassembled.

<span class="mw-page-title-main">Intracellular transport</span> Directed movement of vesicles and substances within a cell

Intracellular transport is the movement of vesicles and substances within a cell. Intracellular transport is required for maintaining homeostasis within the cell by responding to physiological signals. Proteins synthesized in the cytosol are distributed to their respective organelles, according to their specific amino acid’s sorting sequence. Eukaryotic cells transport packets of components to particular intracellular locations by attaching them to molecular motors that haul them along microtubules and actin filaments. Since intracellular transport heavily relies on microtubules for movement, the components of the cytoskeleton play a vital role in trafficking vesicles between organelles and the plasma membrane by providing mechanical support. Through this pathway, it is possible to facilitate the movement of essential molecules such as membrane‐bounded vesicles and organelles, mRNA, and chromosomes.

References

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  4. ↑ Graham TR (2000). Eurekah Bioscience Collection Cell Biology. University of New South Wales and Landes Bioscience. ISBN   978-0-7334-2108-2.
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