Exercise III 



CHoOH 



C- 

 H/H 

 C 



-O 



CHEMICAL COMPONENTS OF CELLS (1] 

 CH2OH 



-0 



13 



C- 

 H/H 

 C 



\H H/H \H 



c c c 



|\OH H/l |\OH H/l 



-0 — I C C I — o — I c — c I — o- 



H OH T H OH 



HO— H 



HO— H 



HO— H 



from a cell, and learn something of their proper- 

 ties and the units of which they are composed. 

 For convenience we shall work with yeast cells, 

 but the results would be much the same if we 

 used any others. 



The simplest of the macromolecules are the 

 polysaccharides. They include the starches and 

 glycogens, forms in which the cell stores sugar 

 for future use; and such inert structural poly- 

 saccharides as cellulose, the principal component 

 of plant cell walls. Each of these molecules, 

 though very large, is made of a single repeating 

 unit, glucose. 



Glucose contains 6 carbon, 12 hydrogen, and 

 6 oxygen atoms, so that its empirical formula is 

 C6H12O6. What is much more important is 

 the arrangement of these atoms in the molecule, 

 the so-called structural formula: 



H— C=0 



I CH2OH 



H— C— OH \ 



I C O 



HO— C— H H/l \ H 



I 1/ H \| 



H— C— OH ^ C C 



I |\ OH H /I 



H— C— OH OH\| 1/ OH 



I c — c 



H— C— OH I I 



I H OH 



H 



These are two forms in which glucose exists at 

 all times, in equilibrium with each other. The 

 straight-chain form at the left is present in 

 minor amounts, but exposes the aldehyde 

 (HC=0) reducing group upon which the test 



you will perform next week depends. The ring 

 structure at the right is by far the more prevalent. 



A starch or glycogen is formed by stringing 

 hundreds or thousands of glucose molecules 

 together by eliminating a molecule of water 

 between each pair, as shown above. That is, « 



(C6Hi206)^[(C6H,o05)„-H20] + (/7-l)H20, 



in which n is the number of glucose units in- 

 volved. [Why is the number of water mole- 

 cules eliminated {n — 1)?] 



Such a chain, several hundred glucose units 

 in length but unbranched, is the component of 

 starch that yields a blue color when treated 

 with iodine. A second component of starch is 

 formed of similar chains, but highly branched; 

 and glycogen, the characteristic storage poly- 

 saccharide of animal tissues and yeast, consists 

 entirely of such highly branched chains. The 

 highly branched polysaccharides yield brownish 

 or reddish colors when treated with iodine. 



The nucleic acids are composed of units 

 called nucleotides, tied together to form long 

 unbranched chains, thousands of nucleotide 

 units long. Each nucleic acid contains four 

 different nucleotides; and since so many of 

 these units are involved, and they can be ar- 

 ranged in any sequence along the chain, it is 

 possible to construct in this way an enormous 

 number of different nucleic acids. Such variety 

 is needed, for among other things nucleic acids 

 form the functional components of the genes, 

 and it takes a lot of genes to account for the 

 heredity of all living things. 



Any cell, even one that looks as simple as 

 yeast, contains a large number of different 

 nucleic acids, representatives of the two great 

 families, ribonucleic (RNA) and deoxyribonu- 

 cleic acid (DNA). Each, as already said, is 

 made of four different nucleotides; and each 



