Chapter 22: Chemistry of Living Things

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The basic message of this chapter is to show how life is rooted in the fundamental wave-particle duality of nature. Wave-particle duality led us to the wave model of the atom. The wave model of the atom helps us to understand how the Octet Rule arises and leads to covalent bonding of atoms to form molecules. Here we look at some of the molecules, mostly covalently bonded, that characterize life. The fundamental molecules of life arise from the tendency of atoms to satisfy the Octet Rule. When we look for conditions in the solar system where life might originate, we look for planets that have: sufficient to hold an atmosphere; proximity to a star as a source of ordered, but not too close, not too far away; atmospheres containing simple molecules such as H2O, CO2, H2, CH4, NH3, O2, N2, etc. One wonders if the molecules that characterize life can be built from these simple building blocks. The Urey-Miller Experiment demonstrated that (what kind of molecules?) form when ultraviolet energy is supplied to an atmosphere made of the simple molecules thought to have made up the earth's early atmosphere. The single element most closely associated with life is . The reason is that covalent bonds with this atom allow it to form chains and rings and other structures of great strength. Methane, ethane, , and butane are examples of carbon chains of growing length. Ethene, propene, , ..., polyethylene are also examples of chains of growing length. "Isomers" are molecules with the same chemical , but different structures. "Functional groups" are simple substructures that attach to carbon chains and give certain generic (common) characteristics to the chain that are somewhat independent of the length of the chain. We concentrate on four functional groups: Hydroxyl Group: (Draw one on a piece of paper.) Example molecule: A simple carbon chain with hydrogens attached to the carbon to satisfy the Octet Rule but with a hydroxyl group attached to one end of the chain is an . The (OH)- group satisfies the Octet Rule. If a free hydroxyl ion, (OH)-, and a free hydrogen ion, H+, "find" each other, say in a water solution, they combine to form a water molecule, H2O. Any molecule that is a H+ acceptor (like (OH)-) is called a(n) . Any molecule that is a H+ donor is a(n) . Carbonyl Group: (Draw one on a piece of paper.) Example molecule: A carbon chain; each carbon has a single hydrogen and a single hydroxyl group attached to it, but at the end of the chain is a carbonyl group. The molecule is a . Two sugars of particular interest are five-carbon sugars that typically form rings. These two are and deoxyribose. Carboxyl Group: (Draw one on a piece of paper.) Example molecule: Fatty acid. Amino Group: (Draw one on a piece of paper.) Example molecule: Amino acid. Sugars are examples of carbohydrates. Fatty acids are the stuff of which fat is made and amino acids are the stuff of which proteins are made. You will probably recognize carbohydrates, fats, and proteins as the elements of a balanced diet. Amino acids (which Urey-Miller found in their experiment) are particularly important because they chain together to form . When amino acids link together, a molecule of (sugar, water, urine?) is also formed. Protein is the building material for hair, wool, hemoglobin, brain cells, etc. It is the building stuff of life. We can demonstrate the chaining together of molecules that have an amino group on both ends with molecules that have carboxyl groups on both ends. When we mix the molecules, they form a kind of nylon. Sheep insulin is a long protein chain of considerable complexity. How can a cell in the pancreas of a sheep possibly know now to make such complicated molecules? We turn to this question in the following chapter.

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