Carbohydrates and Lipids

Carbohydrates are chemical compounds that contain oxygen, hydrogen, and carbon atoms. They consist of monosaccharide sugars of varying chain lengths and that have the general chemical formula C_n(H₂O)_n or are derivatives of such.

Certain carbohydrates are an important storage and transport form of energy in most organisms, including plants and animals. Carbohydrates are classified by the number of sugar units into monosacchharides (such as glucose), disaccharides (such as saccharose), oligosaccharides, and polysaccharides (such as starch, glycogen, and cellulose).

Structure

Glucose as a straight-chain carbohydrate (Fischer projection)

Fructose (Fischer projection)

Pure carbohydrates contain carbon, hydrogen, and oxygen atoms, in a 1:2:1 molar ratio, giving the general formula C_n(H₂O)_n. (This applies only to monosaccharides, see below, although all carbohydrates have the more general formula C_n(H₂O)_m.) However, many important "carbohydrates" deviate from this, such as deoxyribose and glycerol, although they are not, in the strict sense, carbohydrates. Sometimes compounds containing other elements are also counted as carbohydrates (e.g. chitin, which contains nitrogen).

The simplest carbohydrates are monosaccharides, which are small straight-chain aldehydes and ketones with many hydroxyl groups added, usually one on each carbon except the functional group. Other carbohydrates are composed of monosaccharide units and break down under hydrolysis. These may be classified as disaccharides, oligosaccharides, or polysaccharides, depending on whether they have two, several, or many monosaccharide units..

Monosaccharides

Monosaccharides may be divided into aldoses, which have an aldehyde group on the first carbon atom, and ketoses, which typically have a ketone group on the second. They may also be divided into trioses, tetroses, pentoses, hexoses, and so forth, depending on how many carbon atoms they contain. For instance, glucose is an aldohexose, fructose a ketohexose, and ribose an aldopentose.

Further, each carbon atom that supports a hydroxyl group (except for the first and last) is optically active, allowing a number of different carbohydrates with the same basic structure. For instance, galactose is an aldohexose but has different properties from glucose because the atoms are arranged differently.

A heterocyclic form of ribose (Haworth projection)

The straight-chain structure described here is only one of the forms a monosaccharide may take. The aldehyde or ketone group may react with a hydroxyl group on a different carbon atom to form a hemiacetal or hemiketal, in which case there is an oxygen bridge between the two carbon atoms, forming a heterocyclic ring. Rings with five and six atoms are called furanose and pyranose forms and exist in equilibrium with the straight-chain form.

It should be noted that the ring form has one more optically active carbon than the straight-chain form, and so has both an alpha and a beta form, which interconvert in equilibrium. However, the carbohydrate may further react with an alcohol to form an acetal or ketal, in which case the two forms become distinct. This is the basic type of link between the monosaccharide units of larger carbohydrates.

Disaccharides

Disaccharides are composed of two monosaccharide units bound together by a covalent glycosidic bond. The binding between the two sugars results in the loss of a hydrogen atom (H) from one molecule and a hydroxyl group (OH) from the other.

The most common disaccharides are sucrose (cane or beet sugar - made from one glucose and one fructose), lactose (milk sugar - made from one glucose and one galactose) and maltose (made of two glucoses). The formula of these disaccharides is C₁₂H₂₂O₁₁.

Oligosaccharides and polysaccharides

Oligosaccharides and polysaccharides are composed of longer chains of monosaccharide units bound together by glycosidic bonds. The distinction between the two is based upon the number of monosaccharide units present in the chain. Oligosaccharides typically contain between three and nine monosaccharide units, and polysaccharides contain greater than ten monosaccharide units. Definitions of how large a carbohydrate must be to fall into each category vary however.

Oligosaccharides are found as a common form of protein posttranslational modification. Polysaccharides represent an important class of biological polymer. Examples include starch, cellulose and chitin. Table and powdered sugar are some of the foods you find disaccharides in.

Nutrition

Refined grain products are rich sources of complex carbohydrates

Strictly speaking, carbohydrates are not necessary for human nutrition because proteins can be converted to carbohydrates. The traditional diet of some cultures consists of very little carbohydrate, whose people remaining relatively healthy. However, carbohydrates require less water to digest than proteins or fats and are the most abundant source of energy. Proteins and fat are vital building components for body tissue and cells, and thus it could be considered advisable not to deplete such resources.

Very low carbohydrate diets can slow down brain and neural function because the nervous system especially relies on glucose.

Some problems have been cited for the long term effects of a no-carbohydrate diet for some individuals. Athletes, for instance, or those that participate in high intensity activities, will have a considerable reduction in performance, due to having little to no glycogen supplies stored in muscle tissue. Additionally, nephrotoxicity may occur, particularly in persons that are not very well hydrated.

Some examples of different carbohydrate rich foods are beans, bread and pasta.

Catabolism

There are two major metabolic pathways of carbohydrate catabolism:

1. Glycolysis
2. Citric acid cycle

See also

• Biochemistry
• Macromolecules
• Glycolysis
• Gluconeogenesis
• Pentose phosphate pathway

External links

• IUPAC-IUBMB Joint Commission on Biochemical Nomenclature (JCBN): Carbohydrate Nomenclature
• Carbohydrates detailed
• Carbohydrates Overview
• Carbohydrates and Glycosylation - The Virtual Library of Biochemistry and Cell Biology
• Consortium for Functional Glycomics

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 comprises a diverse range of molecules and some extent is a catch-all for relatively water-insoluble or nonpolar compounds of biological origin, including waxes, fatty acids, fatty-acid derived phospholipids, sphingolipids, glycolipids, and terpenoids such as retinoids and steroids. Some lipids are linear aliphatic molecules, while others have ring structures. Some are aromatic, while others are not. Some are flexible, while others are rigid.

Most lipids have some polar character in addition to being largely nonpolar. Generally, the bulk of their structure is nonpolar or hydrophobic ("water-fearing"), meaning that it does not interact well with polar solvents like water. Another part of their structure is polar or hydrophilic ("water-loving") and will tend to associate with polar solvents like water. This makes them 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, as described below.

Phospholipids or, more precisely, glycerophospholipids, are built on a glycerol core to which are linked two fatty acid-derived "tails" by ester linkages and one "head" group by a phosphate ester linkage. Fatty acids are unbranched hydrocarbon chains, connected by single bonds alone (saturated fatty acids) or by both single and double bonds (unsaturated fatty acids). The chains are usually 10-24 carbon groups long. The head groups of the phospholipids found in biological membranes are phosphatidylcholine (lecithin), phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol, whose head group can be modified by the addition of one to three more phosphate groups. While phospholipids are the major component of biological membranes, other lipid components like sphingolipids and sterols (such as cholesterol in animal cell membranes) are also found in biological membranes.

In an aqueous milieu, the heads of lipids tend to face the polar, aqueous environment, while the hydrophobic tails tend to minimize their contact with water. The nonpolar tails of lipids (U) tend to cluster together, forming a lipid bilayer (1) or a micelle (2). The polar heads (P) 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 an a separate aqueous compartment, which is essentially the basis of cellular membranes.

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 bonds 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.

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

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.

Lipid bilayers form the foundation of all biological membranes and of liposomes.

See also

• Biochemistry
• Fat

External links

• ApolloLipids - Provides dyslipidemia and cardiovascular disease prevention and treatment information as well as continuing medical education programs.
• Lipids, Membranes and Vesicle Trafficking - The Virtual Library of Biochemistry and Cell Biology
• IUPAC glossary entry for the lipid class of molecules what is IUPAC?