Organic molecules are the chemicals of life, compounds composed of more than one type of element, that are found in, and produced by, living organisms. The feature that distinguishes an organic from inorganic molecule is that organic contain carbon-hydrogen bonds, whereas inorganic molecules do not. The four major classes of organic molecules include carbohydrates, proteins, lipids and nucleic acids.
* Carbohydrates *
The term carbohydrates is actually a descriptor of what these molecules are composed of; carbon hydrates, in a ratio of one carbon molecule to one water molecule (CH2O)n. The word saccharide is a handy synonym for carbohydrate, because it can be preceded with a prefix indicating the size of the molecule (mono-, di-, tri- poly-):
1. Monosaccharides: The simplest, single sugars. Examples: Glucose and fructose are monosaccharides.
2. Disaccharides: Double sugars that are a combination of two monosaccharides. Example: Sucrose (table sugar) is made of glucose and fructose together.
3. Polysaccharides: These are polymers composed of several sugars. They can be one type of monomer (many of same monosaccharide) or mixture of monomers. Example: Starch is a polysaccharide composed of many glucose molecules.
* Proteins *
Proteins are large organic molecules that have many roles within living cells. They are part of cell membranes, act as enzymes in cellular reactions, and are vital molecules of our immune system, protecting us from invading microbes. Proteins are composed of monomers called amino acids. Each amino acid contains contain a…
1. base amino group ( -NH2)
2. acidic carboxyl group ( -COOH)
3. hydrogen atom
…all three attached to same carbon atom (the alpha-carbon). A fourth bond attaches the alpha-carbon to a side group that varies among different amino acids. These side groups are important, as they affect the way a protein’s amino acids interact with one another, and how the protein interacts with other molecules.
Peptide chemical bonds link amino acids together into chains, like the beads on a necklace. A dipeptide has two amino acids linked together, a polypeptide, more than two. Although there are hundreds of different amino acids, most organisms use only 21 to build proteins.
* Lipids *
Lipids are molecules that are generally hydrophobic (not attracted to water) because the non-polar covalent bonds linking carbon and hydrogen aren’t attracted to the polar bonds of water. The four major groups of lipids include fats, phospholipids, waxes and steroids.
1. Fats: These lipids are the fats and oils that we are familiar with as part of our diet. They are made from a combination of two kinds of molecules:
* glycerol (a type of alcohol)
* three fatty acids (so known as triglycerides)
2. Phospholipids: These lipids have hydrophobic (water hating) hydrocarbon tails at one end and a hydrophilic (water loving) phosphate group at the other end. This means that phospholipids are soluble in both water and oil. Our cell membranes are made mostly of phospholipids arranged in a double layer, with the tails from both layers facing inward and the heads facing outward; an arrangement known as a lipid bilayer.
3. Waxes: Wax lipids are esters of alcohol, which are insoluble in water and difficult to break down. Wax forms protective and waterproof layers on some plants, bacteria, animal fur and integuments of insects.
4. Steroids: Cholesterol is one example of a steroid. The central core of a cholesterol molecule consists of four fused rings, a chemical arrangement that is shared by all steroids. Cholesterol is precursor to our sex hormones and Vitamin D. Our cell membranes also contain cholesterol, which helps to keep the membrane flexible and fluid even when our cells are exposed to cooler temperatures.
* Nucleic Acids *
Your genetic material is made of DNA (deoxyribonucleic acid), the molecule which contains the code for making an operating your cells and your entire body. Nucleic acids are polymers made up of nucleotide monomers. Each monomer of nucleic acid is a nucleotide, and consists of 3 portions:
1. a pentose sugar
2. one or more phosphate groups
3. one of five cyclic nitrogenous bases
Nucleotides are linked by covalent bonds between the phosphate of one nucleotide and the sugar of next, forming a phosphate-sugar backbone. Nitrogenous bases extend from the backbone like teeth of a comb. Ribonucleic acid (RNA) is a single stranded molecule involved in the production of cellular proteins. In contrast, DNA molecules are double stranded, with hydrogen bonds attracting the complimentary bases that hold the two phosphate-sugar strands together. The hydrogen bonds also twist the phosphate-deoxyribose backbones into a double helix.
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Park Talaro, K. (2008) Foundations in Microbiology.