Nucleotides and the structure of DNA and RNA, and the bonds that join each kind of monomer. Then metabolism as anabolic and catabolic pathways, ATP, and the living state as a steady state that is not equilibrium.
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Created by StemPy LabNucleic acids are chains of nucleotides. A nucleotide is a nitrogenous base, a pentose sugar and a phosphate group; a base joined to a sugar is a nucleoside. DNA has deoxyribose and the bases A, G, C and T. RNA has ribose and uracil in place of thymine. In the Watson and Crick double helix, two antiparallel strands are held by hydrogen bonds: two for A with T and three for G with C. A turn is 3.4 nm and holds 10 base pairs. Chargaff found that A equals T and G equals C.
To join two monomers, a cell removes a molecule of water. Amino acids join by peptide bonds, sugars by glycosidic bonds, fatty acids to glycerol by ester bonds, and nucleotides by phosphodiester bonds. A chain of n monomers has n minus 1 bonds. Adding water breaks a bond: this is hydrolysis, as in digestion.
A cell holds thousands of small molecules that are made and broken down all the time. Chains of enzyme-catalysed steps are metabolic pathways; anabolic ones build molecules using energy, and catabolic ones break them down and release energy, stored in ATP. Material moves through a pathway at a flux. A closed system reaches equilibrium and can do no work. A living cell is an open system held in a steady state away from equilibrium.
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