Difference between revisions of "Lipid" - New World Encyclopedia

From New World Encyclopedia
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'''Lipids''' are a class of [[hydrocarbon]]-containing [[organic compound]]s essential for the structure and function of living [[cell (biology)|cell]]s. Lipids are characterized by being [[water (molecule)|water]]-[[soluble|insoluble]] and soluble in nonpolar organic solvents. Although the term ''lipid'' is often used as a synonym for [[fat]], the latter is in fact a subgroup of lipids called [[triglyceride]]s.
 
'''Lipids''' are a class of [[hydrocarbon]]-containing [[organic compound]]s essential for the structure and function of living [[cell (biology)|cell]]s. Lipids are characterized by being [[water (molecule)|water]]-[[soluble|insoluble]] and soluble in nonpolar organic solvents. Although the term ''lipid'' is often used as a synonym for [[fat]], the latter is in fact a subgroup of lipids called [[triglyceride]]s.
  
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== Types of lipids ==
+
Along with [[proteins]], [[nucleic acids]] and [[carbohydrates]], lipids are one of the major classes of biologically important molecules or [[biomolecule]]s. However, unlike these other groups of molecules, lipids comprise a broader and more diverse range of structures.
 +
 
 +
functions
 +
* Cell membrane structure
 +
** Constitutes a barrier for the cell
 +
** Controls the flow of material in and out of the cell
 +
* Energy storage (for instance, fats stored in adipose tissue)
 +
* Lipid [[hormone]]s like [[steroid]]s and [[eicosanoid]]s - mediate communication between cells
 +
* [[Signal transduction]] - function in the transmission of information in cells
 +
* Lipid [[vitamin]]s - required for metabolism, usually as coenzymes
 +
 
 +
Lipids play diverse and important roles in [[nutrition]] and [[health]]. Many lipids are absolutely essential for life. However, there is also considerable awareness that abnormal levels of certain lipids, particularly cholesterol (in [[hypercholesterolemia]]) and, more recently, [[trans fatty acid]]s, are risk factors for [[heart disease]] and other diseases.
 +
 
 +
==The major classes of lipids and their properties==
 
[[Image:Basic_lipid_structure.png|frame|'''Figure 1:''' Structure of a Lipid. Many lipids consist of a polar head group (<font color="#AA0000">P</font>) and a nonpolar tail (<font color="#0000AA">U</font> for unpolar). The lipid shown is a phospholipid (two tails). The image on the left is a zoomed version of the more schematic image on the right, which will be used from now on to represent lipids with one, two, or three chains.]]
 
[[Image:Basic_lipid_structure.png|frame|'''Figure 1:''' Structure of a Lipid. Many lipids consist of a polar head group (<font color="#AA0000">P</font>) and a nonpolar tail (<font color="#0000AA">U</font> for unpolar). The lipid shown is a phospholipid (two tails). The image on the left is a zoomed version of the more schematic image on the right, which will be used from now on to represent lipids with one, two, or three chains.]]
  
−
Along with [[proteins]], [[nucleic acids]] and [[carbohydrates]], lipids are one of the major classes of biologically important molecules or [[biomolecule]]s. However, unlike these other groups of molecules, lipids comprise a broader and more diverse range of structures. The term lipid really is a catch-all phrase for a wide variety of [[hydrocarbon]]-based molecules of biological origin that are predominantly nonpolar or [[hydrophobic]] ("water-fearing"), meaning that they do not interact well with polar solvents like water, and are instead soluble in less polar organic solvents. Most lipids also have some [[polar molecule|polar]] or [[hydrophilic]] ("water-loving") character. This makes them [[amphipathic]] or [[amphiphilic]] molecules (having both hydrophobic and hydrophilic portions).  In the case of [[cholesterol]], the polar group is a mere -OH ([[hydroxyl]] or alcohol). In the case of [[phospholipids]], the polar groups are considerably larger and more polar. Lipids encompass a huge range of structures. They can be [[aliphatic]] or [[aromatic]]. They can be acyclic or cyclic, straight or branched, saturated or unsaturated. Lipids can be flexible or rigid. This diversity makes it impossible to define lipids on the basis of a single core structural feature or biosynthetic origin, as can be done with the other major groups of biomolecules. The basic classes of lipids are:
+
The term lipid really is a catch-all phrase for a wide variety of [[hydrocarbon]]-based molecules of biological origin that are predominantly nonpolar or [[hydrophobic]] ("water-fearing"), meaning that they do not interact well with polar solvents like water, and are instead soluble in less polar organic solvents. Most lipids also have some [[polar molecule|polar]] or [[hydrophilic]] ("water-loving") character. This makes them [[amphipathic]] or [[amphiphilic]] molecules (having both hydrophobic and hydrophilic portions).  In the case of [[cholesterol]], the polar group is a mere -OH ([[hydroxyl]] or alcohol). In the case of [[phospholipids]], the polar groups are considerably larger and more polar.  
 +
 
 +
[[Image:Lipid_bilayer_and_micelle.png|thumb|250px|'''Figure 2:''' Self-organization of lipids. A [[lipid bilayer]] is shown on the left and a [[micelle]] on the right.]]
 +
 
 +
Lipids encompass a huge range of structures. They can be [[aliphatic]] or [[aromatic]]. They can be acyclic or cyclic, straight or branched, saturated or unsaturated. Lipids can be flexible or rigid. This diversity makes it impossible to define lipids on the basis of a single core structural feature or biosynthetic origin, as can be done with the other major groups of biomolecules. The basic classes of lipids are:
  
 
* [[Fatty acid]]s
 
* [[Fatty acid]]s
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Note: There are different ways to classify lipids. A comprehensive new classification system has been proposed recently (J. Lipid Res. 46:839), which instead divides lipids into: (1) fatty acyls, (2) glycerolipids, (3) glycerophospholipids, (4) sphingolipids, (5) sterol lipids, (6) prenol lipids, (7) saccharolipids and (8) polyketides.
 
Note: There are different ways to classify lipids. A comprehensive new classification system has been proposed recently (J. Lipid Res. 46:839), which instead divides lipids into: (1) fatty acyls, (2) glycerolipids, (3) glycerophospholipids, (4) sphingolipids, (5) sterol lipids, (6) prenol lipids, (7) saccharolipids and (8) polyketides.
  
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== Lipid functions ==
+
== Fatty acids: the building blocks of fats==
−
 
+
[[Image:Myristic-acid-3D-vdW.png|thumb|left|500px|Three-dimensional representation of the saturated fatty acid [[myristic acid]].]]
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* Cell membrane structure
+
<br style="clear:both;"/>
−
** Constitutes a barrier for the cell
+
Chemically, fatty acids can be described as long-chain monocarboxylic acids and have a general structure of CH<sub>3</sub>(CH<sub>2</sub>)<sub>n</sub>COOH. The length of the chain usually ranges from 12 to 24, always with an even number of carbons. When the carbon chain contains no [[double bond]]s, it is called saturated. If it contains one or more such bonds, it is unsaturated. The presence of double bonds generally reduces the melting point of fatty acids. Furthermore, unsaturated fatty acids can occur either in [[cis]] or [[trans]] [[geometric isomerism|geometric isomers]]. In most naturally occurring fatty acids, the double bonds are in the cis configuration.
−
** Controls the flow of material in and out of the cell
 
−
* Energy storage (for instance, fats stored in adipose tissue)
 
−
* Lipid [[hormone]]s like [[steroid]]s and [[eicosanoid]]s - mediate communication between cells
 
−
* [[Signal transduction]] - function in the transmission of information in cells
 
−
* Lipid [[vitamin]]s - required for metabolism, usually as coenzymes
 
  
−
== Structure ==
+
A '''trans fatty acid''' (commonly shortened to '''trans fat''') is an unsaturated fatty acid molecule that contains a ''trans'' double bond between [[carbon]] atoms, which makes the molecule less 'kinked' in comparison to fatty acids with ''cis'' double bonds. These bonds are characteristically produced during industrial hydrogenation of plant oils. Research suggests that increasing amounts of trans fats are, for causal reasons not well understood, correlate with circulatory diseases such as [[atherosclerosis]] and [[coronary heart disease]], than the same amount of non-trans fats.
  
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=== Fatty acids and glycerides ===
+
Fatty acids can be bound or attached to other molecules, like triglycerides or [[phospholipid]]s.  When they are not attached to other molecules, they are known as "free" fatty acids. The '''uncombined fatty acids''' or '''free fatty acids''' may come from the breakdown of a triglyceride into its components (fatty acids and glycerol).
−
{{main|fatty acid}}
 
  
−
Chemically, fatty acids can be described as long-chain monocarboxylic acids and have a general structure of CH<sub>3</sub>(CH<sub>2</sub>)<sub>n</sub>COOH. The length of the chain usually ranges from 12 to 24, always with an even number of carbons. When the carbon chain contains no [[double bond]]s, it is called saturated. If it contains one or more such bonds, it is unsaturated. The presence of double bonds generally reduces the melting point of fatty acids. Furthermore, unsaturated fatty acids can occur either in [[cis]] or [[trans]] [[geometric isomerism|geometric isomers]]. In most naturally occurring fatty acids, the double bonds are in the cis configuration.
+
Free fatty acids are an important source of fuel for many tissues since they can yield relatively large quantities of [[Adenosine triphosphate|ATP]]. Many cell types can use either [[glucose]] or fatty acids for this purpose. However, heart and skeletal muscle prefer fatty acids. On the other hand, the brain cannot use fatty acids as a source of fuel, relying instead on glucose, or on [[ketone bodies]] produced by the liver from [[fatty acid metabolism]] during starvation, or periods of low carbohydrate intake.
  
−
[[Glyceride]]s are lipids possessing a [[glycerol]] core structure with one or more fatty acyl groups, which are fatty acid-derived chains attached to the glycerol backbone by [[ester]] linkages. Glycerides with three acyl groups ([[triglycerides]] or neutral fats) are the main storage form of fat in animals and plants.
+
==Glyderides: the energy storage lipids==
 +
[[Image:Fat triglyceride shorthand formula.PNG|thumb|Example of an unsaturated fat triglyceride. Left part: [[glycerol]], right part from top to bottom:  [[palmitic acid]], [[oleic acid]], [[alpha-linolenic acid]], chemical formula: C<sub>55</sub>H<sub>98</sub>O<sub>6</sub>]]
 +
[[Glyceride]]s are lipids possessing a [[glycerol]] core structure with one or more fatty acyl groups, which are fatty acid-derived chains attached to the glycerol backbone by [[ester]] linkages. Glycerides with three acyl groups ([[triglycerides]] or neutral fats) are the main storage form of fat in animals and plants.  
  
−
=== Lipids in membranes ===
+
'''Triglyceride''' (also known as '''triacylglycerol''' or '''triacylglyceride''') is [[glyceride]] in which the [[glycerol]] is [[ester]]ified with three [[fatty acid]]s. It is the main constituent of [[vegetable oil]] and [[animal fat]]s. Triglycerides play an important role in [[metabolism]] as energy sources. They contain more than twice as much energy (9 [[calorie|kcal]]/g) as [[carbohydrate]]s and [[protein]]s.
  
−
An important type of glyceride-based molecule found in [[biological membranes]], such as the cell's [[plasma membrane]] and the intracellular membranes of [[organelles]], are the phosphoglycerides or [[glycerophospholipids]]. These are [[phospholipids]] that contain a glycerol core linked to two fatty acid-derived "tails" by ester or, more rarely, [[ether]] linkages and to one "head" group by a [[phosphate]] ester linkage. The head groups of the phospholipids found in [[biological membrane]]s are phosphatidylcholine ([[lecithin]]), phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol. These phospholipids are subject to a variety of reactions in the cell: for instance, polar head groups or fatty acid tails can be released from specific phospholipids through enzyme-catalyze hydrolysis to generate [[second messengers]] involved in [[signal transduction]]. In the case of phosphatidylinositol, the head group can be enzymatically modified by the addition of one to three phosphate molecules, and this constitutes another mechanism of [[cell signaling]]. While phospholipids are the major component of biological membranes, other non-glyceride lipid components like [[sphingolipid]]s and [[sterol]]s (such as [[cholesterol]] in animal cell membranes) are also found in biological membranes.
+
==Phospholipids: membrane components==
 +
[[Image:Sphingomyelin.png|thumb|Sphingomyelin (Red:Phosphatidyl choline, Blue:Acyl CoA) Sphingomyelin is also present in all [[eukaryote|eukaryotic]] [[cell (biology)|cell]] [[membrane]]s, especially the plasma membrane, and is particularly concentrated in the nervous system because sphingomyelin is a major component of [[myelin]], the fatty insulation wrapped around nerve cells by [[Schwann cells]] or [[oligodendrocytes]]. Multiple Sclerosis is a disease characterised by deterioration of the myelin sheath, leading to impairment of nervous conductio<br>]] An important type of glyceride-based molecule found in [[biological membranes]], such as the cell's [[plasma membrane]] and the intracellular membranes of [[organelles]], are the phosphoglycerides or [[glycerophospholipids]]. These are [[phospholipids]] that contain a glycerol core linked to two fatty acid-derived "tails" by ester or, more rarely, [[ether]] linkages and to one "head" group by a [[phosphate]] ester linkage. The head groups of the phospholipids found in [[biological membrane]]s are phosphatidylcholine ([[lecithin]]), phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol. These phospholipids are subject to a variety of reactions in the cell: for instance, polar head groups or fatty acid tails can be released from specific phospholipids through enzyme-catalyze hydrolysis to generate [[second messengers]] involved in [[signal transduction]]. In the case of phosphatidylinositol, the head group can be enzymatically modified by the addition of one to three phosphate molecules, and this constitutes another mechanism of [[cell signaling]]. While phospholipids are the major component of biological membranes, other non-glyceride lipid components like [[sphingolipid]]s and [[sterol]]s (such as [[cholesterol]] in animal cell membranes) are also found in biological membranes.
  
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A biological membrane is a form of lipid bilayer, as is a [[liposome]]. Formation of lipid bilayers is a spontaneous process when the glycerophospholipids described above are placed in water. In an aqueous milieu, the polar heads of lipids tend to oriente toward the polar, aqueous environment, while the hydrophobic tails tend to minimize their contact with water. The nonpolar tails of lipids (<font color="#0000AA">U</font>) tend to cluster together, forming a [[lipid bilayer]] (1) or a [[micelle]] (2). The polar heads (<font color="#AA0000">P</font>) face the aqueous environment. Micelles form when single-tailed [[amphiphilic]] lipids are placed in a polar milieu, while lipid bilayers form when two-tailed phospholipids are placed in a polar environment (Fig. 2). Micelles are "monolayer" spheres and can only reach a certain size, whereas bilayers can be considerably larger. They can also form tubules. Bilayers that fold back upon themselves form a hollow sphere, enclosing a separate aqueous compartment, which is the basis of biological membranes.
+
==Sterol lipids: structure and signaling==
 +
add text
 +
===Cholesterol===
 +
[[Image:Cholesterol structure.png|249px|Cholesterol]]
 +
'''Cholesterol''' is a [[sterol]] (a combination [[steroid]] and [[alcohol]]) and a [[lipid]] found in the [[cell membrane]]s of all [[human body|body]] tissues, and transported in the [[blood]] plasma of all [[animal]]s. Lesser amounts of cholesterol are also found in [[plant]] membranes.  The name originates from the Greek ''chole-'' ([[bile]]) and  ''stereos'' (solid), and the chemical suffix ''-ol'' for an alcohol, as researchers first identified cholesterol (C<sub>27</sub>H<sub>45</sub>OH) in solid form in [[gallstone]]s in 1784.
  
−
Micelles and bilayers separate out from the polar milieu by a process known as the "hydrophobic effect." When dissolving a nonpolar substance in a polar environment, the polar molecules (i.e. water in an aqueous solution) become more ordered around the dissolved nonpolar substance, since the polar molecules cannot form [[hydrogen bond]]s to the nonpolar molecule. Therefore, in an aqueous environment, the polar water molecules form an ordered "[[clathrate]]" cage around the dissolved nonpolar molecule. However, when the nonpolar molecules separate out from the polar liquid, the [[entropy]] (state of disorder) of the polar molecules in the liquid increases. This is essentially a form of phase separation, similar to the spontaneous separation of oil and water into two separate phases when one puts them together.
+
Most cholesterol is not dietary in origin; it is synthesized internally. Cholesterol is present in higher concentrations in tissues which either produce more or have more densely-packed membranes, for example, the [[liver]], [[spinal cord]] and [[brain]], and also in [[atheroma|atheromata]]. Cholesterol plays a central role in many [[biochemistry|biochemical]] processes, but is best known for the association of [[cardiovascular disease]] with various [[lipoprotein]] cholesterol transport patterns and [[hypercholesterolemia|high levels of cholesterol]] in the blood.
  
−
[[Image:Lipid_bilayer_and_micelle.png|thumb|250px|'''Figure 2:''' Self-organization of lipids. A [[lipid bilayer]] is shown on the left and a [[micelle]] on the right.]]
+
When doctors talk to their patients about the health concerns of cholesterol, they are often referring to "bad cholesterol", or [[low-density lipoprotein]] (LDL). "Good cholesterol" is [[high-density lipoprotein]] (HDL); this denotes the way cholesterol is bound in [[lipoprotein]]s, the natural carrier molecules of the body.  
  
−
The self-organisation depends on the concentration of the lipid present in solution. Below the [[critical micelle concentration]], the lipids form a single layer on the liquid surface and are dispersed in solution. At the first critical micelle concentration (CMC-I), the lipids organise in spherical micelles, at the second critical micelle concentration (CMC-II) into elongated pipes, and at the lamellar point (LM or CMC-III) into stacked lamellae of pipes. The CMC depends on the chemical composition, mainly on the ratio of the head area and the tail length.
+
Cholesterol is required to build and maintain [[cell membrane]]s; it makes the membrane's fluidity - degree of [[viscosity]] -  stable over wider temperature intervals (the hydroxyl group on cholesterol interacts with the phosphate head of the membrane, and the bulky steroid and the hydrocarbon chain is embedded in the membrane). Some research indicates that cholesterol may act as an antioxidant.<ref name=Smith1991>Smith LL. Another cholesterol hypothesis: cholesterol as antioxidant. ''Free Radic Biol Med'' 1991;11:47-61. PMID 1937129.</ref> Cholesterol also aids in the manufacture of [[bile]] (which helps digest fats), and is also important for the metabolism of fat soluble vitamins, including vitamins [[vitamin A|A]], [[vitamin D|D]], [[vitamin E|E]] and [[vitamin K|K]].  It is the major precursor for the synthesis of [[vitamin D]], of the various [[steroid hormone]]s, including [[cortisol]] and [[aldosterone]] in the [[adrenal gland]]s, and of the sex hormones [[progesterone]], [[estrogen]], and [[testosterone]]. Further recent research{{citationneeded}} shows that cholesterol has an important role for the brain [[synapse]]s as well as in the [[immune system]], including protecting against cancer.
  
 +
===Steroids===
 +
A '''steroid''' is a [[lipid]] characterized by a [[carbon]] skeleton with four fused rings. All steroids are derived from the [[acetyl CoA]] biosynthetic pathway. Different steroids vary in the [[functional group]]s attached to these rings. Hundreds of distinct steroids have been identified in [[plant]]s, [[animal]]s, and [[Fungus|fungi]]. Their most important role in most living systems is as [[hormone]]s. Steroid hormones produce their physiological effects by binding to steroid hormone [[receptor (biochemistry)|receptor]] proteins. The binding of steroid hormones to their receptors causes changes in gene [[Transcription (genetics)|transcription]] and cell function.
  
 +
[[Image:Steroid-nomenclature.png|thumb|right|250px|Steroid skeleton. Carbons 18 and above can be absent.]]
  
−
==Nutrition and health==
+
In human physiology and medicine, the most important steroids are [[cholesterol]], the [[steroid hormone]]s, and their [[precursor]]s and [[metabolite]]s. In the bloodstream steroids are bound to carrier proteins.
  
−
Lipids play diverse and important roles in [[nutrition]] and [[health]]. Many lipids are absolutely essential for life. However, there is also considerable awareness that abnormal levels of certain lipids, particularly cholesterol (in [[hypercholesterolemia]]) and, more recently, [[trans fatty acid]]s, are risk factors for [[heart disease]] and other diseases.
+
[[Cholesterol]] is an important [[sterol|steroid alcohol]], being a common component of animal [[cell membrane]]s. However, a high level of it can cause various conditions and [[diseases]], such as [[atherosclerosis]]. Most other steroids are synthesized from cholesterol. Also, various hormones, including [[vertebrate]] [[sex hormone]]s, are steroids created from cholesterol.
  
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==See also==
+
Some of the common categories of steroids include:
−
 
+
*[[Anabolic steroid]]s are a class of steroids that interact with androgen receptors to increase muscle and bone synthesis. There are natural and synthetic anabolic steroids. These are the "steroids" used by [[athlete]]s to increase performance.
−
* [[Biochemistry]]
+
*[[Corticosteroid]]s include [[glucocorticoid]] and [[mineralocorticoid]]s:
−
* [[Fat]]
+
**[[Glucocorticoid]]s regulate many aspects of [[metabolism]] and [[immune system|immune function]], and often prescribed by doctors to reduce [[inflammation|inflammatory conditions]] like [[asthma]] and [[arthritis]].
−
* [[Saturation (chemistry)]]
+
**[[Mineralocorticoid]]s are corticosteroids that help maintain blood volume and control [[kidney|renal]] excretion of [[electrolyte]]s.
 +
*[[Sex steroid]]s are a subset of [[sex hormone]]s that produce [[sexual differentiation|sex differences]] or support [[reproduction]]. They include [[androgen]]s, [[estrogen]]s, and [[progestagen]]s.
 +
*[[Phytosterol]]s - steroids naturally occurring in plants.
  
 
==External links==
 
==External links==
−
 
 
* [http://www.apollolipids.org/ ApolloLipids - Provides dyslipidemia and cardiovascular disease prevention and treatment information as well as continuing medical education programs.]
 
* [http://www.apollolipids.org/ ApolloLipids - Provides dyslipidemia and cardiovascular disease prevention and treatment information as well as continuing medical education programs.]
 
* [http://www.biochemweb.org/lipids_membranes.shtml Lipids, Membranes and Vesicle Trafficking - The Virtual Library of Biochemistry and Cell Biology]
 
* [http://www.biochemweb.org/lipids_membranes.shtml Lipids, Membranes and Vesicle Trafficking - The Virtual Library of Biochemistry and Cell Biology]

Revision as of 19:57, 9 August 2006

Lipids are a class of hydrocarbon-containing organic compounds essential for the structure and function of living cells. Lipids are characterized by being water-insoluble and soluble in nonpolar organic solvents. Although the term lipid is often used as a synonym for fat, the latter is in fact a subgroup of lipids called triglycerides.

Along with proteins, nucleic acids and carbohydrates, lipids are one of the major classes of biologically important molecules or biomolecules. However, unlike these other groups of molecules, lipids comprise a broader and more diverse range of structures.

functions

  • Cell membrane structure
    • Constitutes a barrier for the cell
    • Controls the flow of material in and out of the cell
  • Energy storage (for instance, fats stored in adipose tissue)
  • Lipid hormones like steroids and eicosanoids - mediate communication between cells
  • Signal transduction - function in the transmission of information in cells
  • Lipid vitamins - required for metabolism, usually as coenzymes

Lipids play diverse and important roles in nutrition and health. Many lipids are absolutely essential for life. However, there is also considerable awareness that abnormal levels of certain lipids, particularly cholesterol (in hypercholesterolemia) and, more recently, trans fatty acids, are risk factors for heart disease and other diseases.

The major classes of lipids and their properties

Figure 1: Structure of a Lipid. Many lipids consist of a polar head group (P) and a nonpolar tail (U for unpolar). The lipid shown is a phospholipid (two tails). The image on the left is a zoomed version of the more schematic image on the right, which will be used from now on to represent lipids with one, two, or three chains.

The term lipid really is a catch-all phrase for a wide variety of hydrocarbon-based molecules of biological origin that are predominantly nonpolar or hydrophobic ("water-fearing"), meaning that they do not interact well with polar solvents like water, and are instead soluble in less polar organic solvents. Most lipids also have some polar or hydrophilic ("water-loving") character. This makes them amphipathic or amphiphilic molecules (having both hydrophobic and hydrophilic portions). In the case of cholesterol, the polar group is a mere -OH (hydroxyl or alcohol). In the case of phospholipids, the polar groups are considerably larger and more polar.

Figure 2: Self-organization of lipids. A lipid bilayer is shown on the left and a micelle on the right.

Lipids encompass a huge range of structures. They can be aliphatic or aromatic. They can be acyclic or cyclic, straight or branched, saturated or unsaturated. Lipids can be flexible or rigid. This diversity makes it impossible to define lipids on the basis of a single core structural feature or biosynthetic origin, as can be done with the other major groups of biomolecules. The basic classes of lipids are:

  • Fatty acids
    • Saturated
    • Unsaturated
  • Glycerides or glycerolipids
      • Monoglycerides
      • Diglycerides
      • Triglycerides (neutral fats)
    • Phosphoglycerides or glycerophospholipids
  • Nonglycerides
    • Sphingolipids
    • Sterol lipids (includes cholesterol and steroid hormones)
    • Prenol lipids (includes terpenoids)
    • Waxes
    • Polyketides
  • More complex lipid derivatives
    • Sugar-linked lipids: glycolipids
    • Protein-linked lipids

Note: There are different ways to classify lipids. A comprehensive new classification system has been proposed recently (J. Lipid Res. 46:839), which instead divides lipids into: (1) fatty acyls, (2) glycerolipids, (3) glycerophospholipids, (4) sphingolipids, (5) sterol lipids, (6) prenol lipids, (7) saccharolipids and (8) polyketides.

Fatty acids: the building blocks of fats

Three-dimensional representation of the saturated fatty acid myristic acid.


Chemically, fatty acids can be described as long-chain monocarboxylic acids and have a general structure of CH3(CH2)nCOOH. The length of the chain usually ranges from 12 to 24, always with an even number of carbons. When the carbon chain contains no double bonds, it is called saturated. If it contains one or more such bonds, it is unsaturated. The presence of double bonds generally reduces the melting point of fatty acids. Furthermore, unsaturated fatty acids can occur either in cis or trans geometric isomers. In most naturally occurring fatty acids, the double bonds are in the cis configuration.

A trans fatty acid (commonly shortened to trans fat) is an unsaturated fatty acid molecule that contains a trans double bond between carbon atoms, which makes the molecule less 'kinked' in comparison to fatty acids with cis double bonds. These bonds are characteristically produced during industrial hydrogenation of plant oils. Research suggests that increasing amounts of trans fats are, for causal reasons not well understood, correlate with circulatory diseases such as atherosclerosis and coronary heart disease, than the same amount of non-trans fats.

Fatty acids can be bound or attached to other molecules, like triglycerides or phospholipids. When they are not attached to other molecules, they are known as "free" fatty acids. The uncombined fatty acids or free fatty acids may come from the breakdown of a triglyceride into its components (fatty acids and glycerol).

Free fatty acids are an important source of fuel for many tissues since they can yield relatively large quantities of ATP. Many cell types can use either glucose or fatty acids for this purpose. However, heart and skeletal muscle prefer fatty acids. On the other hand, the brain cannot use fatty acids as a source of fuel, relying instead on glucose, or on ketone bodies produced by the liver from fatty acid metabolism during starvation, or periods of low carbohydrate intake.

Glyderides: the energy storage lipids

Example of an unsaturated fat triglyceride. Left part: glycerol, right part from top to bottom: palmitic acid, oleic acid, alpha-linolenic acid, chemical formula: C55H98O6

Glycerides are lipids possessing a glycerol core structure with one or more fatty acyl groups, which are fatty acid-derived chains attached to the glycerol backbone by ester linkages. Glycerides with three acyl groups (triglycerides or neutral fats) are the main storage form of fat in animals and plants.

Triglyceride (also known as triacylglycerol or triacylglyceride) is glyceride in which the glycerol is esterified with three fatty acids. It is the main constituent of vegetable oil and animal fats. Triglycerides play an important role in metabolism as energy sources. They contain more than twice as much energy (9 kcal/g) as carbohydrates and proteins.

Phospholipids: membrane components

Sphingomyelin (Red:Phosphatidyl choline, Blue:Acyl CoA) Sphingomyelin is also present in all eukaryotic cell membranes, especially the plasma membrane, and is particularly concentrated in the nervous system because sphingomyelin is a major component of myelin, the fatty insulation wrapped around nerve cells by Schwann cells or oligodendrocytes. Multiple Sclerosis is a disease characterised by deterioration of the myelin sheath, leading to impairment of nervous conductio

An important type of glyceride-based molecule found in biological membranes, such as the cell's plasma membrane and the intracellular membranes of organelles, are the phosphoglycerides or glycerophospholipids. These are phospholipids that contain a glycerol core linked to two fatty acid-derived "tails" by ester or, more rarely, ether linkages and to one "head" group by a phosphate ester linkage. The head groups of the phospholipids found in biological membranes are phosphatidylcholine (lecithin), phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol. These phospholipids are subject to a variety of reactions in the cell: for instance, polar head groups or fatty acid tails can be released from specific phospholipids through enzyme-catalyze hydrolysis to generate second messengers involved in signal transduction. In the case of phosphatidylinositol, the head group can be enzymatically modified by the addition of one to three phosphate molecules, and this constitutes another mechanism of cell signaling. While phospholipids are the major component of biological membranes, other non-glyceride lipid components like sphingolipids and sterols (such as cholesterol in animal cell membranes) are also found in biological membranes.

Sterol lipids: structure and signaling

add text

Cholesterol

Cholesterol Cholesterol is a sterol (a combination steroid and alcohol) and a lipid found in the cell membranes of all body tissues, and transported in the blood plasma of all animals. Lesser amounts of cholesterol are also found in plant membranes. The name originates from the Greek chole- (bile) and stereos (solid), and the chemical suffix -ol for an alcohol, as researchers first identified cholesterol (C27H45OH) in solid form in gallstones in 1784.

Most cholesterol is not dietary in origin; it is synthesized internally. Cholesterol is present in higher concentrations in tissues which either produce more or have more densely-packed membranes, for example, the liver, spinal cord and brain, and also in atheromata. Cholesterol plays a central role in many biochemical processes, but is best known for the association of cardiovascular disease with various lipoprotein cholesterol transport patterns and high levels of cholesterol in the blood.

When doctors talk to their patients about the health concerns of cholesterol, they are often referring to "bad cholesterol", or low-density lipoprotein (LDL). "Good cholesterol" is high-density lipoprotein (HDL); this denotes the way cholesterol is bound in lipoproteins, the natural carrier molecules of the body.

Cholesterol is required to build and maintain cell membranes; it makes the membrane's fluidity - degree of viscosity - stable over wider temperature intervals (the hydroxyl group on cholesterol interacts with the phosphate head of the membrane, and the bulky steroid and the hydrocarbon chain is embedded in the membrane). Some research indicates that cholesterol may act as an antioxidant.[1] Cholesterol also aids in the manufacture of bile (which helps digest fats), and is also important for the metabolism of fat soluble vitamins, including vitamins A, D, E and K. It is the major precursor for the synthesis of vitamin D, of the various steroid hormones, including cortisol and aldosterone in the adrenal glands, and of the sex hormones progesterone, estrogen, and testosterone. Further recent researchTemplate:Citationneeded shows that cholesterol has an important role for the brain synapses as well as in the immune system, including protecting against cancer.

Steroids

A steroid is a lipid characterized by a carbon skeleton with four fused rings. All steroids are derived from the acetyl CoA biosynthetic pathway. Different steroids vary in the functional groups attached to these rings. Hundreds of distinct steroids have been identified in plants, animals, and fungi. Their most important role in most living systems is as hormones. Steroid hormones produce their physiological effects by binding to steroid hormone receptor proteins. The binding of steroid hormones to their receptors causes changes in gene transcription and cell function.

Steroid skeleton. Carbons 18 and above can be absent.

In human physiology and medicine, the most important steroids are cholesterol, the steroid hormones, and their precursors and metabolites. In the bloodstream steroids are bound to carrier proteins.

Cholesterol is an important steroid alcohol, being a common component of animal cell membranes. However, a high level of it can cause various conditions and diseases, such as atherosclerosis. Most other steroids are synthesized from cholesterol. Also, various hormones, including vertebrate sex hormones, are steroids created from cholesterol.

Some of the common categories of steroids include:

  • Anabolic steroids are a class of steroids that interact with androgen receptors to increase muscle and bone synthesis. There are natural and synthetic anabolic steroids. These are the "steroids" used by athletes to increase performance.
  • Corticosteroids include glucocorticoid and mineralocorticoids:
  • Sex steroids are a subset of sex hormones that produce sex differences or support reproduction. They include androgens, estrogens, and progestagens.
  • Phytosterols - steroids naturally occurring in plants.

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  1. ↑ Smith LL. Another cholesterol hypothesis: cholesterol as antioxidant. Free Radic Biol Med 1991;11:47-61. PMID 1937129.
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