LIPIDS AND CARBOHYDRATES

If you are puzzled by the chemistry in this section you should read Bare Bones Chemistry

This file contains information on Lipids and Carbohydrates

Monomers and Polymers
Plants are made up of organic compounds of diverse size and structure. The large molecules are usually polymers made up of chains of smaller, simpler molecules which are monomers. Thus starch is a polymer based on glucose monomers. Three classes of polymer are important in plant function: polysaccharides, proteins and nucleic acids. Together with lipids (most of which are not polymers) these molecules make up most of plant structure.

Lipids
Lipids are fatty substances with long hydrocarbon chains and often ester linkages somewhere in the molecule. There are three classes of lipids in plants. The simplest are the triglycerides or fats in which three fatty acids are attached to a glycerol molecule by ester bonds. These are the most energy-rich form of food reserve (for plants and for us). Plants tend to accumulate fats only when it is important to pack a lot of energy into a small space, such as a seed.

Membrane lipids are similar to triglycerides except that one of the fatty acids is replaced by a polar group such as a sugar in a glycolipid or a phosphate compound in a phospholipid. The combination of a polar head group and a non-polar tail is important in membrane structure and function

(The two structures above show the five main fatty acids found in plant lipids. Generally speaking the unsaturated acids (oleic, linoleic and linolenic acids) are more fluid (and nutritionally desirable) than the saturated acids (palmitic and stearic). Margarines are often made from plant oils by chemically hydrogenating the fatty acids; this makes them more saturated and more solid.

Cuticular lipids are a complex mix of hydrocarbons and esters of long chain aliphatic acids and alcohols embedded in a lipid polymer called cutin. Waxes and cutin make up the cuticle which is a water-retaining barrier around the above ground parts of plants.

Carbohydrates

As their name implies carbohydrates are composed of the elements of water and carbon so their formula approximates to a multiple of CH2O. Most of the dry weight of plants is carbohydrate of one kind or another. All carbohydrates are polar and the low-molecular forms are what we commonly know as sugars. Sugars are freely soluble in water. The simplest is the three carbon sugar, glyceraldehyde but most of the carbohydrates in plants are based on glucose or other six carbon sugars.

Like other sugars glucose can exist in two basic forms, an open chain or a ring structure. When the ring closes the OH group on the last carbon can be up (beta) or down (alpha). This then influences how the sugars can be joined together in larger molecules.

When sugars move around the plant they are usually in the form of sucrose in which glucose and fructose are joined by a glycosidic bond. This bond is made by removing the elements of water from two sugars. It can be broken by re-introducing those elements - the process of hydrolysis ("water splitting") can often be accomplished by heating in acidic solution. For example corn syrup is made by heating starch from corn in dilute acid. Organisms use enzymes to carry out this kind of reaction without heating and in neutral solution.

Polysaccharides are made up by joining hundreds of monosaccharides by glycosidic bonds. The most common food reserve in plants is starch which you probably know can be dissolved in boiling water:

Joining glucose in the beta configuration produces a very different polysaccharide,cellulose. This is completely insoluble in water and is the most important structural component of plant cell walls:

Another important cell wall component, particularly in herbaceous tissue is pectin. This is made up of a modified sugar, galacturonic acid and in the plant the carboxyl groups are esterified with methyl (CH3) groups which are not shown in the picture:

Fungal cell walls are made of chitin a polymer of N-acetyl glucosamine. The kind of cell wall that an organism makes is a fundamental property that cannot easily be changed. So the fact that true fungi have chitin cell walls makes them more like animals than plants. Conversely the water molds have cellulose cell walls that make them more plant-like, so they have been separated from the fungi in modern classification.

Continue to proteins and nucleic acids

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The Ohio State University
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