3-The Importance of Weak and Strong Chemical bonds.docVIP

幻灯片1 The Importance of Weak and Strong Chemical Bonds Chapter 3 幻灯片2 A. Characteristics of Chemical Bonds B. Free Energy C. Weak bonds in biological systems D. High Energy Bonds E. Molecules that Donate Energy Are Thermodynamically Unstable F. Enzymes Lower Activation Energies in Biochemical Reactions 幻灯片3 G. Free Energy in Biomolecules H. High Energy Bonds Drive Biosynthesis I. Activation of Precursors in Group Transfer Reactions 幻灯片4 Based on what they could see with microscopes, scientists in the 1900s thought cells were very simple. They thought cells were bags filled with a living substance named they named protoplasm. They thought protoplasm was made of simple chemicals like ions and amino acids. 幻灯片5 At that time most scientists thought the molecules inside cells were all very small and were held together by weak associations such as hydrogen bonds. 幻灯片6 In the 1900s scientists there was no method for measuring the sizes of molecules. Based on what they knew, most scientists had no idea of the existence of macromolecules (高分子)—large polymer molecules that make up cells. 幻灯片7 Organic chemists had synthesized molecules that were about 4,000 Daltons (道尔顿). Scientists thought that molecules larger than this did not exist. 幻灯片8 In 1901 Emil Fischer discovered that amino acids are linked by peptide bonds to form proteins. He thought the longest polypeptide could be no longer than 30 to 40 amino acids. Anything longer was considered “very improbable” by all the leading chemists. It was too difficult to imagine how amino acids could be joined together in the correct order to produce larger proteins. Emil won the Nobel Prize for his work in 1902. 幻灯片9 Other scientists began to consider the existence of larger molecules based on observing the behavior of large molecules in solution. For example, some molecules such as cellulose (纤维素) would not diffuse through a fine filter (滤纸). But others thought this could be explained by the weak bondin