How the Cell Lives 23 



simple living thing — the yeast cell — a unicellular organism which 

 causes fermentation of sugar. It is a jelly-like bag, containing a vis- 

 cous sticky fluid. It is in this fluid that the chemical changes take 

 place. Yeast will grow in a liquid containing a few simple compounds 

 like sugar, nitrogen compounds, phosphate, and a few other essential 

 nutrients. These enter the cell through its skin or membrane, and 

 when inside are transformed into many other compounds. The 

 nitrogen compounds are built up into the proteins required for 

 growth and reproduction, the sugars are partly elaborated into more 

 complex carbohydrates and partly burnt to provide the energy 

 required for the other transformations, yielding either carbon dioxide 

 or alcohol, according to the circumstances. 



This property of yeast was discovered in the early stages of pastoral 

 civilization. The carbon dioxide was used to leaven the dough in the 

 baking of bread; and the production of alcohol has been used for the 

 manufacture of alcoholic drinks from time immemorial. 



If you were to ask a chemist to take sugar and turn it into alcohol, 

 he could do it, but only with great difficulty and through a long and 

 complex series of changes. In fact most of the alcohol of commerce 

 is manufactured by the fermentation of sugars by the humble yeast 

 cell. How does it bring about this change with such surprising ease? 

 At one time it was thought that only the living cells could bring 

 about such changes, which were connected with the essential vital 

 activity of the cell. Then a German chemist, Biichner, showed that 

 it was possible to squeeze the juice out of the living cells, and this 

 juice, freed of cell fragments, was capable of fermenting sugar. It 

 contained substances, which he called enzymes^ which bring about 

 these changes. 



Before this, substances which assist chemical change had been 

 discovered and called catalysts; for example, platinum, which makes 

 hydrogen and oxygen combine. Enzymes were evidently vital cata- 

 lysts or bio-catalysts\ but for many years, indeed until quite recently, 

 their nature has been quite unknown. Many chemists tried unsuccess- 

 fully to purify and isolate them. They were found to be both varied 

 and ubiquitous. 



Before we examine their nature, let me gives a few examples of 

 enzymes. When food passes through the digestive tract of animals, 

 it encounters enzymes throughout its passage which break it down 

 and make it suitable for absorption. In the stomach there is pepsin, 

 which begins the break-down of proteins. Into the intestine, the 

 pancreas pours other enzymes, which continue the digestion of 

 protein and break down starch into sugars, digest the fats and so on. 

 A very large number of enzymes, which facilitate a multitude of 



