The Molecules of Life PDF

The Molecules of Life ===================== - Within all cells, small organic molecules are joined together to form larger molecules. - All living things are made up of four main classes of **macromolecules**: carbohydrates, lipids, proteins, and nucleic acids. - These large mac...

The Molecules of Life ===================== - Within all cells, small organic molecules are joined together to form larger molecules. - All living things are made up of four main classes of **macromolecules**: carbohydrates, lipids, proteins, and nucleic acids. - These large macromolecules may consist of thousands of covalently bonded atoms, some with mass greater than 100,000 daltons. Most macromolecules are polymers, built from monomers. ====================================================== - Three of the four classes of macromolecules---carbohydrates, proteins, and nucleic acids---form chain-like molecules called polymers. - A **polymer** is a long molecule consisting of many similar or identical building blocks linked by covalent bonds. - The repeated units are small molecules called **monomers.** - Some of the molecules that serve as monomers have other functions of their own. - The chemical mechanisms that cells use to make and break polymers are similar for all classes of macromolecules. - Monomers are connected by covalent bonds that form through the loss of a water molecule. This reaction is called a **condensation reaction** or **dehydration reaction.** - When a bond forms between two monomers, each monomer contributes part of the water molecule that is lost. One monomer provides a hydroxyl group (---OH), while the other provides a hydrogen atom (---H). - Cells invest energy to carry out dehydration reactions. - Dehydration is facilitated by enzymes, specialized macromolecules that speed up chemical reactions in cells. - The covalent bonds that connect monomers in a polymer are disassembled by - In hydrolysis, bonds are broken by the addition of water molecules. A hydrogen atom attaches to one monomer, and a hydroxyl group attaches to the adjacent monomer. - We take in food as organic polymers that are too large for our cells to absorb. In the digestive tract, enzymes direct the hydrolysis of specific polymers. The resulting monomers are absorbed by the cells lining the gut and transported to the bloodstream for distribution to body cells. - The cells of our body then use dehydration reactions to assemble the monomers into new and different polymers that carry out functions specific to the particular cell type. An immense variety of polymers can be built from a small number of monomers. ---------------------------------------------------------------------------- - Each cell has thousands of different kinds of macromolecules. - Macromolecules vary among cells of the same individual. They vary more among unrelated individuals of a species, and even more between species. - This diversity comes from various combinations of the 40--50 common monomers and some others that occur rarely. - These monomers can be connected in a great many combinations, just as the 26 letters in the alphabet are used to create a whole dictionary of words. - Despite the great diversity in organic macromolecules, members of each of the four major classes of macromolecules are similar in structure and function. Carbohydrates serve as fuel and building material. ================================================== - **Carbohydrates** include sugars and their polymers. - The simplest carbohydrates are monosaccharides, or simple sugars. - Disaccharides, or double sugars, consist of two monosaccharides joined by a condensation reaction. - Polysaccharides are polymers of many monosaccharides. Sugars, the smallest carbohydrates, serve as fuel and a source of carbon. ------------------------------------------------------------------------- - **Monosaccharides** generally have molecular formulas that are some multiple of the unit CH~2~O. - For example, glucose has the formula C~6~H~12~O~6~. - Monosaccharides have a carbonyl group (\>C=O) and multiple hydroxyl groups (--- OH). - Depending on the location of the carbonyl group, the sugar is an aldose (aldehyde sugar) or a ketose (ketone sugar). - Most names for sugars end in -*ose*. - Glucose, an aldose, and fructose, a ketose, are structural isomers. - Monosaccharides are also classified by the number of carbon atoms in the carbon skeleton. - The carbon skeleton of a sugar ranges from three to seven carbons long. - Glucose and other six-carbon sugars are hexoses. - Five-carbon sugars are pentoses; three-carbon sugars are trioses. - Another source of diversity for simple sugars is the spatial arrangement of their parts around asymmetric carbon atoms. - For example, glucose and galactose, both six-carbon aldoses, differ only in the spatial arrangement of their parts around asymmetric carbons. - Although glucose is often drawn with a linear carbon skeleton, most sugars (including glucose) form rings in aqueous solution. - Monosaccharides, particularly glucose, are major nutrients for cellular work. - Cells extract energy from glucose molecules in the process of cellular respiration.