Exercise 



CHEMICAL COMPONENTS OF CELLS (] 



15 



A WORD ON MOLECULAR 

 STRUCTURE 



Molecular structure is anatomy carried to the 

 level of small dimensions. We hope that by 

 now you would have no difficulty recognizing 

 an ameba or a Paramecium when seeing one 

 under the microscope. In exactly the same 

 sense you should learn to know a sugar, fat, or 

 a section of a protein or nucleic acid molecule 

 from its molecular appearance. Molecules are 

 three-dimensional structures, with characteristic 

 anatomies upon which many of their properties 

 depend. Some violence is done by the habit of 

 portraying them on the plane surfaces of paper 

 and blackboards; yet even such two-dimensional 

 representations are useful and recognizable. 

 After all, this is no greater violence than is in- 

 volved in pictures of animals and plants. 



Fortunately, however, we can do something 

 much better, and we hope you will take full 

 advantage of it. You will find in the laboratory 

 sets of molecular models, from which you can 

 construct sugars, fats, representative sections of 

 proteins and nucleic acids, and many other types 

 of molecule that we encounter in this course. 

 With these models you can also inquire into 

 such interesting and important matters as optical 

 activity, associated with the right- or left-handed- 

 ness characteristic of many of the organic mole- 

 cules found in cells. 



It would be altogether wrong to deal with 

 these molecules simply as words, the names of 

 abstractions. Use this opportunity in the labora- 

 tory to handle them and look at them as things, 

 which is what they are. Make yourselves models 

 of glucose, and join them together by taking 

 out molecules of water between them, as in 

 polysaccharide formation; then split them apart 

 again by inserting water molecules, as in hy- 

 drolysis. Similarly construct a polypeptide 

 chain from a few generalized amino acids, and 

 see what it means to hydrolyze such a chain, 

 the process catalyzed by such protein-hydrolyz- 

 ing enzymes as are found in pancreatic extracts. 



From now on whenever you have a little free 

 time in the laboratory, one good thing to do 



with it is to construct molecular models, and 

 carry out reactions with them. This is fun to 

 do, it will help you greatly, and it is as close to 

 synthetic organic chemistry as many of you will 

 ever come. 



One last word about these models. They are 

 probably of a relatively inexpensive type, that 

 represents fairly correctly interatomic distances 

 and bond angles. The little balls that represent 

 the atoms, however, show only the relative 

 positions of the centers of those atoms, not the 

 space they occupy. In a more correct and much 

 more expensive type of molecular model, which 

 tries in addition to represent the space-filling 

 properties of the atoms, one sees that molecules 

 are much more solid structures. In such a more 

 correct model, for example, the six-membered 

 ring of glucose is seen to have almost no hole 

 in the middle. 



EXPERIMENTAL PROCEDURE 



Yeasts are a unicellular type of fungus which 

 reproduces by budding. The species of yeast we 

 shall use, Saccharomyces cereviseae, serves many 

 human uses. Different strains of it have been 

 developed as baker's yeast, for raising dough; 

 brewer's yeast, for fermenting malt to make 

 beer; and various types of wine yeast. We shall 

 be working with baker's yeast, which ordinarily 

 comes in cakes with starch as a binding mate- 

 rial. We have carefully washed the starch away, 

 leaving a clean suspension of yeast cells with 

 which to work. 



Stir a pinch of yeast into 1 ml of glucose 

 medium, and set it aside. Toward the end of 

 the laboratory period, when you have time, 

 make a slide of a drop of this, and examine 

 the budding cells under the high power of the 

 microscope. During this interval the yeast will 

 have begun to ferment the glucose, and you 

 will see the bubbles of carbon dioxide which is 

 one of the products. 



Our work in the laboratory will involve a 

 number of processes that are new to many of 

 you: centrifuging, neutralization of acids with 



