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Physical Structure
Main Concepts
Lipids and Polysaccharides


The basic 'monomer' from which general hydrocarbons are constructed is a -[CH2]- unit. These are joined together in long, straight chains to form molecules such as octane.

Hydrocarbons contain and store a lot of energy in their bonds, and are thus good fuel molecules (gasoline, for example contains a lot of hydrocarbons). However, they are strongly hydrophobic (they 'hate' water), so it is very difficult for living cells and organisms to manipulate and use pure hydrocarbons.

About the only use for nearly pure hydrocarbons is wax, which is so strongly hydrophobic that it is used as a waterproofing material.

Fatty Acids

Fatty acids consist of long, unbranched hydrocarbons with a carboxylic acid group at one end. The number of carbon atoms in a fatty acid molecule is usually even (6, 8, 12, 32, 36, etc.), although it is not impossible to find a fatty acid with an odd number of carbon atoms in its structure.

While the long, hydrocarbon chain of the fatty acid continues to be strongly hydrophobic, the presence of the carboxylic acid group at one end of the molecule adds some hydrophilic properties. Small fatty acids such as propionic acid (with 3 carbon atoms) mixes with water readily, caproic acid (with 6 carbon atoms) is only 0.4 percent soluble in water.

Saturated and Unsaturated

Typical animal fatty acids are palmitic (C16), and stearic (C18), which have hydrocarbon chains in which each carbon atom is also linked with two hydrogen atoms (-CH2-CH2-CH2-CH2-CH2-). These are called saturated fatty acids.

Animals also contain fatty acids in which there are less hydrogen atoms joined to some of the carbon atoms, and a double bond between two carbon atoms takes their place. These are the unsaturated fatty acids, such as oleic acid (CH3-[CH2]7-CH==CH-[CH2]7-COOH), which is the most common fatty acid found in nature.

Unsaturated fatty acids generally melt at lower temperatures than saturated fatty acids, and the common ones are liquids at room temperatures. There are some fatty acids in which there are more than one double bond, such as linolenic acid.

Neutral Lipids

Neutral lipids are very abundant in nature. These molecules consist of one, two or three fatty acid molecules joined to one molecule of glycerol, hence forming mono-, di-, or triglycerides.

Fats are insoluble in water, and most animal fats contain mainly palmitic, stearic, palmitoleic, oleic and linoleic fatty acids in their structure.

Fatty acids in Animals

Acid Human Cow Pig
Palmitic 23 29 27
Stearicic 6 22 10
Palmitoleic 6 - -
Oleic 50 40 59
Linoleic 10 2 4

approximate composition in molar percentage


Glycerol, in its concentrated form, is a very thick, sticky, sweet tasting liquid that dissolves easily and readily in water. It can form more complex molecules by reacting with molecules such as fatty acids, or with inorganic reactive groups, such as phosphate. These compound molecules are called ethers, and use the general name glycerides. Hence one fatty acid linked to a glycerol molecule is called a monoglyceride.


These are a second class of glycerol based lipids in which (usually) two fatty acid molecules and one phosphate reactive group are all joined to one glycerol molecule.

These phospholipids play many roles in cells, but one of their most important is in the cell membrane.


The carbohydrates are a large, very widely distributed class of compounds found in almost all animals and plants. They are so named because of their basic chemical composition, which is usually some variation on the general formula CH2O. The smallest molecule to be generally considered a carbohydrate is glyceraldehyde, with only three carbon atoms in a short chain. Larger, single, molecules can have up to seven carbon atoms in a chain, but the most common members of this class have 5, or 6 carbon atoms in their structure. The largest molecules are huge polymers of smaller carbohydrate units.

The carbohydrate class can be subdivided into three smaller groups, monosaccharides ('single sugars'), Oligosaccharides (two and three sugars joined together), and Polysaccharides (polymers of many sugars in long chains).

Glucose, a monosaccharide

Glucose is a hexose sugar (meaning it has 6 carbon atoms in it's structure). All the carbon atoms are joined to one another in a chain. Each of the carbon atoms is also joined to at least one hydrogen atom and to one oxygen atom. The presence of all this oxygen in the structure of the glucose molecule ensures that it is strongly hydrophilic ('loves' water). Most monosaccharides, like glucose, contain a lot of energy in their bonds, but, unlike the hydrocarbons, they dissolve readily in water.

Cyclic structure of glucose

When scientists shone polarized light through a freshly prepared solutions of glucose, they often got very different results from one solution to the next. Sometimes the polarized light would be rotated +112.2 degrees, and sometimes only +18.7 degrees. What was going on?

The solution to this puzzle turned out to be the three dimensional structure of the glucose molecule itself. Two different forms of the molecule could exist (called isomers), both of which had the same chemical structure, but different arrangements of the molecular shape.

This, seemingly trivial difference, in molecular structure turned out to be very important when these different types of the glucose molecule participated in 'joining' reactions to form larger structures.

Joining sugars together

Individual sugar molecules, the monosaccharides, can be used as monomers joined together to form larger structures. For example, two glucose molecules can be joined to form the disaccharide called maltose,.


Or two different sugars (fructose and glucose) can be joined together to form the disaccharide sucrose.


The vast majority of carbohydrates in nature are found in the form of very large polymers, made up by joining together various monosaccharide sugars. Glucose is the most abundant sugar used this way, but mannose, galactose, xylose, and arabinose are also used as monomers. Polysaccharides vary in their monosaccharide composition, in the number of monomers in a chain (its molecular weight) and structural features such as branching.

Almost all polysaccharides are polydisperse,, meaning that, even when in a pure form, any given sample of the substance could vary in its size or number of monomer units in its structure. The very common polysaccharide starch is a mixture of branched chains of glucose that can have as little as a 100 sugars per chain, all the way up to chains as long as 10,000 glucose monomers.


Made by plants as a way of storing chemical energy, starch comes in two common forms. Amylose is believed to be a long, unbranched chain of alpha-glucose molecules, in which the fourth carbon atom of one sugar is joined to the first carbon atom of the next sugar.

Amylopectin is a branched series of glucose chains. Glucose molecules are joined to each other by links between their first and fourth carbon atoms (as above), but then branches occur when other glucose molecules are also joined to the sixth carbon atom of a sugar in the chain. Such a branch occurs about every 24 to 30 units along the chain.


These giant molecules are probably the most prevalent and abundant substance in nature. It has been calculated that, of all the organic carbon on the planet, a full 50 percent is in the form of cellulose. This molecule is most commonly found in plants (although a small amount has been found in tunicates) and in its purest form in cotton (about 90 percent cellulose).

It is formed when beta-glucose molecules are joined together using their first and fourth carbon atoms. There are no branches in these polysaccharides which can reach lengths of between 300 and over 2,000 units.


This polysaccharide molecule is the animal equivalent of starch. It is found stored in the liver and muscles, where it can be seen under the microscope as small particles. Glycogen is a branched molecule, with a new branch occurring about every 10 or so units along the chain, but there is a lot of variety in this molecule both with regards to its size and structure.

© 2003, Professor John Blamire