The term macromolecules refer to molecules with a high molecular mass (greater than 1000 Uma). The dimensions of the macromolecules are extremely varied and depend on the number and nature of the component atoms; you can get to the size of the order of 100 Å (Angstrom), that is, the size of the colloidal particles. Macromolecules can be of both artificial and natural origin. Examples of artificial macromolecules are some types of polymers such as nylon 6.6, polystyrene, and polypropylene. Examples of natural macromolecules are proteins, carbohydrates, starch, lipids, cellulose, DNA and RNA, and, in general, all nucleic acids.
Macromolecules are only single large molecules. Therefore, polymers notoriously formed from smaller molecules known by the term monomers should not be included among them. Polymers (artificial macromolecules) are formed by the repetition of numerous structural units (constitutional repeating unit); the starting units, with which the polymer is formed, are called monomers, and the reaction that joins the monomers together to make up the polymer is called polymerization. An example of a polymerization reaction is that which leads to the formation of polyethylene.
As can be seen from the above case, the polymers are usually represented by indicating the CRU, without specifying the type of end groups of the chains. The monomer of polyethylene is ethylene (H2C = CH2).
An example of a natural macromolecule is cellulose. It has a chemical formula (C6H10O5) n.
The most important chemical bond of cellulose is the 1, 4-ß-glucosidic bonds used to join the glucose units together.
Cellulose is insoluble in water and has an important structural function in plants. The wood contains about 50% of cellulose, and the cotton fibers are practically made of pure cellulose.
Most synthetic or natural macromolecules can be obtained by addition or condensation reactions. In the first case, the rearrangement of the chemical bonds in the monomer molecule leaves two values free, which allow the linkage with other monomers without the elimination of atoms. In the condensation reaction, the monomer, which has two functional groups, can react with another monomer with the elimination of a small molecule, often of water. The building blocks of some macromolecules, both synthetic and natural, are described below; the examination has only an example purpose, to allow the reader to more easily follow the exposition of the structural aspects.
Carbohydrates are mostly ternary compounds because their molecules are made up of three types of atoms, i.e., carbon C, hydrogen H, and oxygen O. The main carbohydrates are divided into monosaccharides, disaccharides, and polysaccharides. Small molecules constitute the monosaccharides or simple sugars. They are divided into pentose monosaccharides and hexose monosaccharides. Pentose monosaccharides are made up of 5 carbon atoms, and the most important is deoxyribose C5H1005 (which forms DNA) and ribose C5H1004 (which forms RNA). The hexose monosaccharides are glucose, fructose, and galactose, all having brute formula C6H1206. However, these compounds differ in the structural formula. The three hexoses are, therefore, isomers. The disaccharidethey are formed by two monosaccharide molecules joined together by a condensation reaction:
2C6H1206 ---> C12H22011 + H20
The most common disaccharides have the formula C12H22011 and are sucrose (glucose and fructose), maltose (two glucose molecules), and lactose (glucose and galactose). The polysaccharides consist of several monosaccharides linked together by means of a polycondensation reaction. The most important with reserve functions are starch and glycogen. Starch is a reserve molecule characteristic of plants, while glycogen is the reserve polysaccharide of animal cells. In cellulose, monomers are bound together with a different bond than that found in starch and glycogen. It is a polymer with a structural function, characteristic of the plant cell wall.
Lipids are divided into triglycerides, phospholipids, and steroids. They are ternary compounds because their molecules are made up of three types of atoms: carbon C, hydrogen H, and oxygen O. They are insoluble in water and soluble instead in organic compounds such as benzene, ether, and chloroform. Lipids are made up of glycerol, and fatty acids joined together through an esterification reaction. The glycerol (or glycerine) is a molecule is an alcohol that has three -OH groups. The fatty acids are molecules consisting of a linear hydrocarbon chain (R), which carries a carboxylic -COOH group at one end. That is, they are long-chain organic acids. Fatty acids can be saturated if only simple bonds are present between the carbon atoms in the chain, monounsaturated if there is a double bond, and polyunsaturated if there are two or more double bonds. The triglycerides are lipids formed by the union of three molecules of fatty acids with glycerol according to an esterification reaction, which gives the triglyceride and three water molecules. Triglycerides are the main form of energy storage in animal cells; they are a source of metabolic water and allow thermal insulation through subcutaneous deposits. The phospholipids are made up of a glycerol molecule combined with a phosphate group and two fatty acids. The phospholipid part of the molecule is called the polar head, while the two hydrocarbon chains of fatty acids are called apolar tails.
Proteins are the most common macromolecules in living cells. Quaternary compounds are formed from carbon C, hydrogen H, oxygen O, and nitrogen N. Proteins are polymers made up of monomers, and amino acids.
Amino acids are organic compounds made up of a carboxylic group and an amino group. These two groups are linked together by alpha carbon and differ in group R. The amino acids present in nature are 20, of which 12 that man can synthesize and 8 essential, that is, the cells cannot synthesize them and are inserted through the diet.
In most proteins, different levels of the organization are recognized, coexisting in the same protein molecule. They are generally determined by each other. Thus, the primary structure determines the secondary structure and the latter, the tertiary. Functional specialization depends on the final form taken by the protein. Each protein has a precise number and sequence of amino acids. This sequence is called the primary structure. The secondary structure is found as an alpha helix, tight spiral winding of the polypeptide chain, or beta lamina, plus traits of parallel polypeptide chains. The tertiary structure assumes a compact and globular shape and is the result of folding the secondary structure on itself (folding). Sometimes two or more globular proteins associate, giving rise to the quaternary structure.
DNA is the genetic material of cells and is located in the nucleus of eukaryotic cells and in the cytoplasm of prokaryotic cells. DNA is made up of two spiral wound strands that form the “double helix.” Each filament is a polymer made up of monomers called nucleotides. Each nucleotide is made up of three basic molecules:
- a phosphate group
- a pentose sugar deoxyribose
- a nitrogenous base (adenine, thymine, cytosine, guanine)
Each strand of DNA is a polynucleotide and is joined to the other strand by hydrogen bonds. These bonds are established between the complementary nitrogen base pairs (adenine and thymine; cytosine and guanine) AT, CG, TA, and GC. The nitrogen bases are divided into purines (Adenine and Guanine) and pyrimidines (Thymine and Cytosine), and a purine is always bound with a pyrimidine.
RNA is a macromolecule whose monomers are the ribonucleotides consisting of a phosphate group.
- One pentose sugar ribose a nitrogenous base (adenine, uracil, cytosine, guanine).
RNA is made up of a single strand, i.e., a single nucleotide chain held together by phosphoester bonds. RNA is found in the cytoplasm of eukaryotic and prokaryotic cells. RNA plays a fundamental role in the process that determines the union of the various amino acids to form proteins. There are three types of RNA in cells, with different structures and functions, which interact with each other and collaborate in protein synthesis.