Organic Compounds
Carbohydrates

Monosaccharides (dextrose)
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Disaccharides (sucrose)
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Polysaccharides (starches)
Proteins

Amino acids
Lipids
 fatty acids
 glycerol
Nucleic acids
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DNA

RNA
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Introduction
A living cell is extremely complex; however the basic principles of chemistry and chemical reactions apply. The types of compounds used by cells are generally more complex than those occurring in the non-biological world. All types of different cells, in spite of markedly different functions, use the same fundamental biological molecules.
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Carbohydrates
The common names for this group of compounds are sugars and starches. The simplest sugar is called a monosaccharide. They have roles as both biological fuels (to supply energy as
ATP) and as structural units within cells. There is a wide variety of different sugars but only a few are very abundant. Their structure is described as a hydrocarbon chain with attached hydroxyl groups. The structure of the most abundant sugar is glucose. The nomenclature of different sugars is usually based on the number of carbon atoms they contain. For example, a monosaccharide with six carbons (such as glucose above) is referred to as a hexose. Each monosaccharide will have either a ketone or an aldehyde reactive group. Monosaccharides can occur as a straight chain form or a ring structure formed by creating a covalent bond between the oxygen of the ketone or aldehyde group and the last carbon in the chain.

1) Disaccharides
Sugars do not exist only as monosaccharides. They can be covalently linked to form units of two or more sugars. The simplest is the linkage of two monosaccharides to form a disaccharide.
The common disaccharides are :
Glucose + glucose = maltose
Glucose + galactose = lactose
Glucose + fructose = sucrose
Polysaccharides
Much of the carbohydrate in nature is in the form of many units linked together to form polysaccharides. There are a vast number of different polysaccharides which occur with the three most abundant being :
1) Glycogen
Glycogen functions as a fuel store in animals. Large amounts are stored in liver and muscle and maintain circulating blood glucose levels between meals.
2) Starch
Starch is the fuel store in plants - it provides their energy needs. It also has the very important role of being humans' major source of dietary carbohydrate. It has a similar structure to glycogen, but with less branch points.
3) Cellulose
Cellulose is a structural carbohydrate in plants. It is one of the molecules which gives plant material rigidity and thus provides some of the useful properties of materials that are used by humans, such as wood and paper.
Because cellulose has a different bonding structure linking adjacent glucose molecules in the polymer it is indigestible by humans. The human gut lacks the enzymes necessary to digest cellulose, whereas the gut of herbivores contains the appropriate enzymes. (The enzymes are actually produced by bacteria resident in the herbivore gut.) Thus cellulose is an important dietary source of carbohydrate to herbivores.

Non-repetitive polymers i.e. those containing a mixture of different monosaccharides can exist, giving quite complex molecules. These occur in nature often as cell surface recognition sites such as antigens.
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Proteins
Proteins are an extremely diverse group of biological molecules. A vast number of different proteins exist in nature; the actual number of different proteins is unknown. Certainly it must be thousands of millions. They have a wide range of different functions in nature. In spite of this diversity, all proteins have the same basic structure - they are all chains of subunits called amino acids. There are only twenty different amino acids which, when arranged in different combinations, make up all the different proteins. There is a variable group ( R group) attached to the central carbon atom. Different R-groups attached to alpha-carbon determine the chemical characteristics of the different amino acids. In proteins the amino acids are linked by amide linkages (called peptide bonds). These are formed by linking the carboxyl group of one amino acid to the amino group of the adjacent amino acid.
The only reactive parts left exposed are the R-groups. These determine the properties of the protein and regions within it. Interaction between R-groups within the protein and between Rgroups and other reactive molecules determines the structure and properties of the protein molecule.
Why do proteins differ from each other?
The huge diversity of proteins results simply from different sequences of amino acids in the different proteins. Proteins are large structures consisting of from 50 to many thousand amino acids. The different possible arrangements of twenty amino acids within polymers of this length is enormous.
Protein structure
The polymer of amino acids does not remain linear but folds into a three-dimensional shape which is the most stable for that sequence of amino acids. It is the interaction between the component R-groups which determines the shape. This shape is called the native conformation for that particular protein and is essential for that protein's biological activity. The shape may be altered by various factors e.g. heat or large pH changes. Once the three-dimensional shape is altered, biological activity is lost. This change in shape (and loss of activity) is termed denaturation. A denatured protein is no longer functional.

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Lipids
The first group of lipids found in nature are commonly called fats and contain structures called fatty acids. Fatty acids consist of a long hydrocarbon chain with a terminal carboxyl group.
The hydrocarbon chain commonly contains 15-20 carbons. They may contain all single covalent bonds between carbon atoms along the length of the chain. These are termed saturated fatty acids.
Unsaturated fatty acids are represented by structures such as oleic acid and contain one or more double covalent bonds between carbon atoms along the length of the chain. This type of structure is highly non-polar and hence water insoluble.
Fatty acids have roles as biological fuels and structural molecules (when combined into more complex structures). Fatty acids rarely occur on their own. They are usually combined into more complex molecules. There are two major classes of molecule which contain fatty acids. Both consist of fatty acids attached to a backbone molecule of glycerol. The molecule is composed of three molecules of fatty acid (which may all be the same or all different) attached to a glycerol molecule.
Phospholipids differ from the above structure in having only two molecules of fatty acid attached to glycerol with the third hydroxyl group on glycerol b õonded to a phosphate and then to a polar alcohol group. This gives the molecule the structure shown below with a polar
"head" (the phosphate and alcohol) and a long non-polar "tail", the latter contributed by the fatty acid molecules.
The second major group of lipids is called steroids. These DO NOT contain fatty acids.
They contain a mostly hydrocarbon structure, but instead of being linear like fatty acids, they are composed of ring structures. Cholesterol is the most abundant of the steroids. Cholesterol is important as it has a role in membrane structure, and it is a precursor for other steroids, including a number of hormones.
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Nucleic acids
Each organism has a strictly defined genetic makeup. This is inherited from its predecessors and used by the organism itself to direct its cellular functions. It then passes on its own genetic information to its progeny. Each cell of a single organism contains the same genetic information.
This genetic information must be stored in some way. It is stored in the chromosomes of the cell as nucleic acid.
Nucleic acids are subdivided into two types. They are called deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The two types are both polymers built from the same basic type of subunit. These subunits are called nucleotides. Each nucleic acid is a polymer usually many thousands of nucleotides long, but each polymer is composed of a variable sequence of only four different nucleotide structures.

The degree of similarity in the structure of all nucleic acids is even greater when it is observed that three of the nucleotides are common to both DNA and RNA. Each consists of a base composed of a carbon and nitrogen ring structure, a pentose (5 carbon) sugar which is either deoxyribose or ribose, and one, two or three phosphates attached to the sugar. This type of structure has been seen earlier in this module in a different context. ATP, the "energy currency molecule", is a nucleotide. The nucleotides are covalently linked to form a polymer by covalent bonds between the phosphate group of one to the sugar group of the next. The structure is shown below.
The bases do not participate in the covalent linkages along the length of the chain but stick out from the chain. There are four different bases in each class of nucleic acid. Three are common to both and one is unique to each. The bases are described as either a purine type or a pyrimidine type.
The five bases are :
adenine (A) - purine type guanine (G) - purine type cytosine (C) - pyrimidine type thymine (T) - pyrimidine type uracil (U) - pyrimidine type
There are subtle but important differences between the subunits used to synthesise DNA and those which are incorporated into RNA. In addition, there are differences in the structure of the two nucleic acids. from: http://www.prm.unisa.edu.au/h&p1map.htm