Extracellular
Encyclopedia
In cell biology
, molecular biology
and related fields, the word extracellular (or sometimes extracellular space) means "outside the cell
". This space is usually taken to be outside the plasma membranes, and occupied by fluid. The term is used in contrast to intracellular
(inside the cell).
According to the Gene Ontology
database the Extracellular Space is a Cellular Component defined as: "That part of a multicellular organism outside the cells proper, usually taken to be outside the plasma membranes, and occupied by fluid. Note that for multicellular organisms, the extracellular space refers to everything outside a cell, but still within the organism (excluding the extracellular matrix). Gene products from a multi-cellular organism are secreted from a cell into the interstitial fluid or blood can therefore be annotated to this term".
The composition of the extracellular space includes metabolites, ion
s, various protein
s and non-protein substances (i.e. DNA, RNA, lipids, microbial products etc) that might affect cellular function. For example, hormone
s, growth factor
s, cytokine
s and chemokine
s act by travelling the extracellular space towards biochemical receptors on cells. Other proteins that are active outside the cell are various enzymes, including digestive enzymes (Trypsin
, Pepsin
), extracellular proteinases (Matrix metalloproteinase
s, ADAMTS
s, Cathepsin
s) and antioxidant enzymes (extracellular superoxide dismutase
). Often, proteins present in the extracellular space are stored outside the cells by attaching to various Extracellular matrix
components (Collagen
s, Proteoglycan
s, etc). In addition, Extracellular matrix
proteolytic products are also present in the extracellular space, especially in tissues undergoing remodelling [2].
The term 'extracellular' is often used in reference to the extracellular fluid
(ECF) compartment which composes about 15 litre
s of an average adult 70 kg human body which is assumed to contain a total of about 50 litre
s of water (thus, about 30% of the body's water is in the ECF compartment).
The cell membrane
(and, in plants and fungi, the cell wall
) is the barrier between the two, and chemical composition of intra- and extracellular milieu can be radically different. In most organisms, for example, a Na+/K+-ATPase
pump maintains a high concentration
of sodium
ion
s outside cells while keeping that of potassium
low, leading to chemical excitability. Many cold-tolerant plants force water into the extracellular space when the temperature drops below 0 degrees Celsius, so that when it freezes, it does not lyse
the plants' cells.
Two compartments comprise the extracellular space: the vascular space and the interstitial space.
Cell biology
Cell biology is a scientific discipline that studies cells – their physiological properties, their structure, the organelles they contain, interactions with their environment, their life cycle, division and death. This is done both on a microscopic and molecular level...
, molecular biology
Molecular biology
Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...
and related fields, the word extracellular (or sometimes extracellular space) means "outside the cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
". This space is usually taken to be outside the plasma membranes, and occupied by fluid. The term is used in contrast to intracellular
Intracellular
Not to be confused with intercellular, meaning "between cells".In cell biology, molecular biology and related fields, the word intracellular means "inside the cell".It is used in contrast to extracellular...
(inside the cell).
According to the Gene Ontology
Gene Ontology
The Gene Ontology, or GO, is a major bioinformatics initiative to unify the representation of gene and gene product attributes across all species...
database the Extracellular Space is a Cellular Component defined as: "That part of a multicellular organism outside the cells proper, usually taken to be outside the plasma membranes, and occupied by fluid. Note that for multicellular organisms, the extracellular space refers to everything outside a cell, but still within the organism (excluding the extracellular matrix). Gene products from a multi-cellular organism are secreted from a cell into the interstitial fluid or blood can therefore be annotated to this term".
The composition of the extracellular space includes metabolites, ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s, various protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s and non-protein substances (i.e. DNA, RNA, lipids, microbial products etc) that might affect cellular function. For example, hormone
Hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...
s, growth factor
Growth factor
A growth factor is a naturally occurring substance capable of stimulating cellular growth, proliferation and cellular differentiation. Usually it is a protein or a steroid hormone. Growth factors are important for regulating a variety of cellular processes....
s, cytokine
Cytokine
Cytokines are small cell-signaling protein molecules that are secreted by the glial cells of the nervous system and by numerous cells of the immune system and are a category of signaling molecules used extensively in intercellular communication...
s and chemokine
Chemokine
Chemokines are a family of small cytokines, or proteins secreted by cells. Their name is derived from their ability to induce directed chemotaxis in nearby responsive cells; they are chemotactic cytokines...
s act by travelling the extracellular space towards biochemical receptors on cells. Other proteins that are active outside the cell are various enzymes, including digestive enzymes (Trypsin
Trypsin
Trypsin is a serine protease found in the digestive system of many vertebrates, where it hydrolyses proteins. Trypsin is produced in the pancreas as the inactive proenzyme trypsinogen. Trypsin cleaves peptide chains mainly at the carboxyl side of the amino acids lysine or arginine, except when...
, Pepsin
Pepsin
Pepsin is an enzyme whose precursor form is released by the chief cells in the stomach and that degrades food proteins into peptides. It was discovered in 1836 by Theodor Schwann who also coined its name from the Greek word pepsis, meaning digestion...
), extracellular proteinases (Matrix metalloproteinase
Matrix metalloproteinase
Matrix metalloproteinases are zinc-dependent endopeptidases; other family members are adamalysins, serralysins, and astacins. The MMPs belong to a larger family of proteases known as the metzincin superfamily....
s, ADAMTS
ADAMTS
ADAMTS is a family of peptidases. 19 members of this family have been identified in humans. Known functions of the ADAMTS proteases include processing of procollagens and von Willebrand factor as well as cleavage of aggrecan, versican, brevican and neurocan...
s, Cathepsin
Cathepsin
Cathepsins are proteases: proteins that break apart other proteins, found in many types of cells including those in all animals. There are approximately a dozen members of this family, which are distinguished by their structure, catalytic mechanism, and which proteins they cleave...
s) and antioxidant enzymes (extracellular superoxide dismutase
Superoxide dismutase
Superoxide dismutases are a class of enzymes that catalyze the dismutation of superoxide into oxygen and hydrogen peroxide. As such, they are an important antioxidant defense in nearly all cells exposed to oxygen...
). Often, proteins present in the extracellular space are stored outside the cells by attaching to various Extracellular matrix
Extracellular matrix
In biology, the extracellular matrix is the extracellular part of animal tissue that usually provides structural support to the animal cells in addition to performing various other important functions. The extracellular matrix is the defining feature of connective tissue in animals.Extracellular...
components (Collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
s, Proteoglycan
Proteoglycan
Proteoglycans are proteins that are heavily glycosylated. The basic proteoglycan unit consists of a "core protein" with one or more covalently attached glycosaminoglycan chain. The point of attachment is a Ser residue to which the glycosaminoglycan is joined through a tetrasaccharide bridge...
s, etc). In addition, Extracellular matrix
Extracellular matrix
In biology, the extracellular matrix is the extracellular part of animal tissue that usually provides structural support to the animal cells in addition to performing various other important functions. The extracellular matrix is the defining feature of connective tissue in animals.Extracellular...
proteolytic products are also present in the extracellular space, especially in tissues undergoing remodelling [2].
The term 'extracellular' is often used in reference to the extracellular fluid
Extracellular fluid
Extracellular fluid usually denotes all body fluid outside of cells. The remainder is called intracellular fluid.In some animals, including mammals, the extracellular fluid can be divided into two major subcompartments, interstitial fluid and blood plasma...
(ECF) compartment which composes about 15 litre
Litre
pic|200px|right|thumb|One litre is equivalent to this cubeEach side is 10 cm1 litre water = 1 kilogram water The litre is a metric system unit of volume equal to 1 cubic decimetre , to 1,000 cubic centimetres , and to 1/1,000 cubic metre...
s of an average adult 70 kg human body which is assumed to contain a total of about 50 litre
Litre
pic|200px|right|thumb|One litre is equivalent to this cubeEach side is 10 cm1 litre water = 1 kilogram water The litre is a metric system unit of volume equal to 1 cubic decimetre , to 1,000 cubic centimetres , and to 1/1,000 cubic metre...
s of water (thus, about 30% of the body's water is in the ECF compartment).
The cell membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
(and, in plants and fungi, the cell wall
Cell wall
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
) is the barrier between the two, and chemical composition of intra- and extracellular milieu can be radically different. In most organisms, for example, a Na+/K+-ATPase
Na+/K+-ATPase
Na+/K+-ATPase is an enzyme located in the plasma membrane in all animals.- Sodium-potassium pumps :Active transport is responsible for cells containing relatively high...
pump maintains a high concentration
Concentration
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
of sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s outside cells while keeping that of potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
low, leading to chemical excitability. Many cold-tolerant plants force water into the extracellular space when the temperature drops below 0 degrees Celsius, so that when it freezes, it does not lyse
Lysis
Lysis refers to the breaking down of a cell, often by viral, enzymic, or osmotic mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a "lysate"....
the plants' cells.
Two compartments comprise the extracellular space: the vascular space and the interstitial space.