ORGANIC COMPOUNDS
Organic compounds are compounds that contain carbon. In addition to carbon, nearly all organic compounds contain hydrogen, and most contain oxygen as well. The atoms of these elements are arranged in various forms to make up most of the dry weight of living organisms.
The four main types of organic compounds are carbohydrates, proteins, lipids and nucleic acids.
Carbohydrates
Carbohydrates are compounds that contain carbon combined with hydrogen and oxygen. They are one of the most significant groups of organic compounds that are made (ie. synthesised) by living systems (ie. within the tissues of plants or animals). Carbohydrates are significant both in terms of both quantity made and the importance of their use in living organisms.
Carbohydrates are compounds which, when analysed, give empirical formulae which are multiples of the simple formula CH2O. Chemically, carbohydrates are defined as "polyhydroxy aldehydes" or "ketones". (Don't worry too much if you don't yet understand these terms.) Examples of carbohydrates include sugars, starch, glycogen, cellulose and chitin.
Carbohydrates are made or synthesised in plants by the process of photosynthesis.
(This will be studied in detail later in the course.)
TYPES OF CARBOHYDRATE
There are three main types of carbohydrates:
1. Monosaccharides
These are the simplest carbohydrates. They are made up of a chain of carbon atoms to which Hydrogen and Oxygen atoms are attached in the proportion of 1 carbon atom to 2 hydrogen atoms to 1 oxygen atom (CH20). These simple sugars, under reasonably mild conditions, can be hydrolysed into smaller compounds. (Hydrolysis is the process of splitting one molecule into two by the addition of H+ and OH- ions of water.)
Glucose is a monosaccharide and is the form of sugar which is most often transported through animal systems.
A combination of glucose and fructose forms sucrose, which is the form that plants use.
2. Oligosaccharides
These are compound sugars which, when hydrolysed, will yeild two to six molecules. Disaccharides are therefore oligosaccharides which yield two monosaccharide molecules when they hydrolyse.
3. Polysaccharides
These are made up of monosaccharides linked together in long chains. They yield a large number of monosaccharides when they hydrolyse.
Starch, which is made of many glucose molecules, is the main storage form of sugar in plants. (Glycogen is the common storage form of sugar in animals.) They must be hydrolysed before they can be used as energy sources for living systems.
Proteins
These are complex compounds made up of a number of amino acids. A single protein molecule can contain hundreds of thousands of amino acids joined by peptide links into one or more very long chains. There are 20 different types of amino acids which can be found in these chains. All types are found in animals; but there are fewer types found in plants.
Amino Acids
These are organic compounds which contain both the amino group (ie. NH2) and the carboxyl group
(ie. COOH). Note: the carboxyl group is acidic.
Different species of organisms have the need for "specific" essential amino acids. Humans require the following 8 specific types of amino acids: valine, leucine, phenylalanine, tryptophane, lysine, isoleucine, methionine and threonine.
Lipids
Lipids are fats and oils. They should not be confused with petroleum or mineral oils.
They are insoluble in water, but are very soluble in organic substances such as ether or hot alcohol. The term lipid doesn't refer to a structural characteristic of these compounds - it refers to behavioural characteristics. The structural characteristics of lipids are extremely variable.
Fats are solid or semi solid at room temperature, while oils are liquid.
Lipids occur in both plants and animals, and are among other things, used to store chemical energy. In plants, the seeds and fruits in particular are used to store fats (oils).
Lipids are divided into the following types:
a. Neutral Lipids
b. Phosphatides and Sphingolipids
c. Glycolipids
d. Terpenoids eg. Carotenoids and steroids.
Nucleic Acids
Nucleic acids are the chemicals which make up genetic material in a living cell. There are two types of nucleic acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).
DNA molecules store the genetic information. RNA molecules translate and transmit this genetic information.
DNA molecules (like proteins) are very long (ie. polymers). The molecular weight of a DNA molecule can be about 100 million. Different DNA molecules are similar, but still different to each other. A DNA molecule can self duplicate itself: this characteristic being the basis for reproduction of an offspring which shares characteristics with its parent.
