Macromolecules of Life



Biological systems are made up of four major macromolecules:

Carbohydrates, Lipids, Nucleic acids and Proteins



A figure showing structures of the 4 major types of macromolecules: carbohydrates, proteins, lipids and nucleic acids

Macromolecules are large molecules that are composed of smaller units. The four major macromolecules are carbohydrates, lipids, nucleic acids, and proteins.

Carbohydrates

Carbohydrates are the most abundant macromolecules on earth, and they are the source of immediate energy needs in living systems. Carbohydrates also participate in defining the structure of cells and living systems. Carbohydrates includes single sugars or multiple sugar molecules bonded together into polymers. Collectively, there are three types of carbohydrates. They are monosaccharides, disaccharides, and polysaccharides.

Monosaccharides, also referred to as simple sugars, are made up of a single sugar molecule. The major example of a monosaccharide is glucose. Sugar molecules, such as the glucose molecule, contain many OH functional groups. For example, the molecular formula for glucose is C6H12O6. Other examples of monosaccharides include isomers of glucose, such as fructose and galactose. Monosaccharides are transported in the blood of animals, broken down to produce chemical energy inside the cell, and can also be found in other macromolecules such as nucleic acids.
Some examples of simple sugars
Disaccharides are composed of two single monomers of sugar linked together. Examples of disaccharides are maltose (glucose + glucose) and sucrose (glucose + fructose). Disaccharides are broken down into their subunits for use inside living systems.

Polysaccharides are made up of chains of sugar monomers linked together, and they are stored inside the cell for future energy use. In plants, the major storage polysaccharide is starch, while in animals it is glycogen. Inside plants like the potato, starch is stored inside of granules throughout the winter until it is needed for growth in the springtime. In animals, glycogen is stored inside the liver and it released when the amount of glucose in the blood circulation is too low. At this time, glycogen is broken down chemically into monomers of glucose. Plants also contain cellulose, which is the most abundant of all carbohydrates. Cellulose is the found in the plant cell wall, where it provides structure and support to the plant cell.

Lipids

Lipids are macromolecules that are all insoluble in water. They include oils and fats, phospholipids, and steroids. Oils are found in plants, where they are used for long term energy storage. Fats are found in animals where they also provide long term energy storage, as well as insulation. Fats and oils are composed of glycerol and fatty acids. A fatty acid is a long chain of carbon-hydrogen (C-H) bonds, with a carboxyl group (-COOH) at one end. On average, a fatty acid contains sixteen to eighteen carbon atoms per molecule. Based on the number of carbons and the number of double bonds that are present, fatty acids can be classified as saturated or unsaturated. Fatty acids are saturated when they do not contain any double bonds between the carbons, and unsaturated when they contain double bonds. An example of a saturated fat is butter, while an example of an unsaturated fat is vegetable oil. Fats and oils are formed in a dehydration reaction in which, three fatty acids react with the -OH group in glycerol.
A figure showing some examples of different types of lipids. l. Triglycerides, which are the major component of vegetable oil as well stored fat in animals, are composed of three fatty acids and glycerol.



Phospholipids are found primarily in the cell membranes of living systems, of which they are the major component. Structurally, a phospholipid contains a hydrophilic head and a hydrophobic tail. The head of the phospholipid contains a phosphate group, while the tail is typically a diglyceride. The cell membrane is a double layer of phospholipids, in which the tails are turned inwards and the heads are exposed to the intracellular and extracellular environments. Phospholipids are also used inside biological systems for cell to cell signalling.

Steroids are also lipids, however they are the most unique category in the group. Steroids are typically made up of fused hydrocarbon rings. Each type of steroid is different in the type of chemically active functional groups that it contains. Examples of steroids include cholesterol, estrogen, and testosterone. Cholesterol in found in the cell membrane of animals, where it provides structural support. Cholesterol is also the precursor for other steroids, such as testosterone and estrogen. Some vitamins, such as Vitamin D, are also classified as steroids.

Proteins

Proteins are polymers that are made up of amino acids. Amino acids are small molecules that contain an amine (-NH2), a carboxyl acid (-COOH), and a side chain (R). There are twenty naturally occurring amino acids, and each amino acid has a unique side-chain or R-group. Amino acids are connected by peptide bonds to form protein polymers. This gives rise to different levels of structure for proteins. The primary (1�) structure of a protein is a made up of a string of connected amino acids. The secondary (2�) structure of a protein is formed by the coiling and folding of the 1� structure. At this level, structures called alpha-helices and beta-sheets are visible. The tertiary (3�) structure of a protein is formed by interactions between the components of the 2� structure. Some proteins have quaternary (4�) structure, which includes the assembly of multiple individual subunits to form the functional protein.

Proteins have numerous biological functions. The common types of proteins that are found in biological systems are enzymes, antibodies, transport proteins, regulatory proteins, and structural proteins. Structural proteins provide support, and examples included keratin that is found in hair and fingernails and collagen that is found in human skin. Proteins such as enzymes are actively involved in metabolism, in which they speed up or accelerate chemical reactions in the body. Enzymes are temperature sensitive, and most are optimal at the body temperature of the living system. Transport proteins are important for carrying compounds or chemicals that are important for survival throughout living systems. Examples of transport proteins include hemoglobin that carries oxygen through the blood, and pores in the cell's membrane that allow substances to enter and leave the cell. Proteins, such as antibodies, are a part of the immune system or defense mechanism in living systems. Antibodies bind to particles or cells that are foreign to the system and aid in their elimination. Regulatory proteins ensure that physiological processes are carried out in the correct manner inside living systems. Examples of regulatory proteins are hormones that control processes such as growth (growth hormone) and metabolism (insulin and glucagon).

Nucleic Acids

Nucleic acids are the molecules inside the cell that store and process genetic or hereditary information. The two types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). These macromolecules are called nucleic acids, because they were first found inside the nucleus of the cell. The monomers that make up nucleic acids are called nucleotides. Each nucleotide contains a sugar, a base, and a phosphate group.

DNA contains the genetic or hereditary information. It is made up four nucleotides: adenine (A), cytosine (C), guanine (G), and thymine (T). These nucleotides organize themselves to form a double helical structure that is often compared to a ladder. In the double helix, A pairs with T and C pairs with G forming the centre or rungs of the ladder. The nucleotide pairs are held together by hydrogen bonds. The backbone of the double helix is made of up the sugar and phosphate groups, which are held together by phosphodiester bonds. The base pairing in DNA is important, because it allows the DNA molecule to copy itself during DNA replication.
Chemical structure of the nitrogenous bases in DNA and RNA
Functionally DNA can be organized into three groups. Exons are the coding regions of DNA that contain the genes for proteins and other genes. Introns are the segments of the DNA that are not used in making genes. The third is the bulk of the DNA molecule, which is commonly referred to as Junk DNA. Although the function of Junk DNA is still relatively unknown, it is not truly junk because it is involved in several physiological processes as well as disease susceptibility.

Like DNA, RNA is made up of monomers of nucleotides. RNA is very similar in content to DNA. The major differences are:

(1.) the sugar used in RNA is ribose, which contains one less oxygen atom than the deoxyribose in DNA, and:

(2.) the base thymine is replaced by uracil (U) in RNA. As a result, in RNA A pairs with U and C still pairs with G. Structurally, RNA differs from DNA in that it does not form a double stranded molecule. Rather, it is a single stranded molecule that forms many different secondary structures.

There are three major types of RNA. The first, messenger RNA (mRNA) is involved in the interpretation of the genetic information stored in DNA. During the production of proteins for example, DNA is copied and translated into mRNA. The second type of RNA is ribosomal RNA (rRNA). rRNA is found in the ribosome, the site of protein synthesis in the cell. As a part of the ribosome, rRNA is responsible for the accurate production of proteins inside the cell. The third type of RNA is transfer RNA (tRNA). tRNA is also involved in protein synthesis. As the name implies, it transfers amino acids to the ribosome so that they can be assembled to form polymers of proteins during protein synthesis. Some RNA molecules also function as enzymes.