D-glucose
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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.
The common name for this group of compounds is sugars.
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, glucose, is shown here :
D-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 (as shown above) 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.
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 + galactose = lactose
Glucose + fructose = sucrose
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.
All of the above are polymers of glucose only.
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.
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.
The basic structure of an amino acid is :
The symbol R in the above structure designates a variable 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, as shown below.
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 R-groups and other reactive molecules determines the structure and properties of the protein molecule.
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.
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.
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 as in stearic acid and palmitic acid below.
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.
triglycerides - the major fuel store in the body
This molecule is composed of three molecules of fatty acid (which may all be the same or all different) attached to a glycerol molecule.
phospholipids - a major component of all cell membranes
Phospholipids differ from the above structure in having only two molecules of fatty acid attached to glycerol with the third hydroxyl group on glycerol bonded 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 is a precursor for other steroids, including a number of hormones.
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 :
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.
The structure of a nucleotide is shown below.
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.
Note that 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.
| DNA | RNA |
| contains A, G, C and T | contains A, G, C and U |
| contains deoxyribose | contains ribose |
| is double stranded | is single stranded |
| is the STORE of genetic information | is used in the EXPRESSION of genetic information |
DNA is described as a double stranded molecule. It consists of two non-covalently linked nucleotide polymer strands which are wound about each other to form a double helix.
The two strands are held together by weak interactions between the bases in each strand.
Because the bases stick out from the polymeric nucleotide chain, the bases in each strand of the DNA molecule are opposite each other and form hydrogen bonds.
Even though each of the hydrogen bonds is very weak (compared to a covalent bond) the total strength of the many thousands of hydrogen bonds between apposed bases holds the two strands firmly together.
There is a high degree of organisation in the arrangement of the two nucleotide strands.
The particular bases which occur opposite each other in the double helix are restricted :
Adenine (A) always occurs opposite Thymine (T) - forming an A -T base pair
Guanine (G) always occurs opposite Cytosine (C) - forming a G-C base pair
DNA as a double helix is shown in the diagram below.
This completes the section on classes of biological molecules.