Cell metabolism 55 



Atoms are neither destroyed nor created in the course oi a chemical 

 reaction; thus the sum ot each kind ot atom on one side ot the arrow 

 must equal the sum ot that kind of atom on the other side. This is an 

 expression ol one of the basic laws of physics, the Law of the Conserva- 

 tion of Matter. The direction of a reversible reaction is determined by 

 the energy relations of the several chemicals involved, their relative 

 concentrations, and their solubility. 



One of the factors determining the rate of a chemical reaction is the 

 temperature; the reaction rate approximately doubles with each increase 

 of 10° C. This is true of the chemical reactions occurring in living cells 

 as well as those in a test tube, and is another bit of evidence that the 

 chemical reactions of living things are fundamentally similar to those of 

 nonliving ones. 



The over-all formula for the metabolism of glucose in the presence 

 of oxygen is: 



CuHi.Ou + 6 O. ^ 6 H.O + 6 CO. + energy 



A census of the carbon, hydrogen and oxygen atoms will reveal that 

 there are equal numbers of each kind on the two sides of the arrow. 

 Energy is released as the glucose molecule is broken down. To reverse 

 the reaction, and thus synthesize glucose, an equivalent amount of 

 energy must be supplied. In photosynthesis the radiant energy of sun- 

 light is absorbed by the green pigment chlorophyll and used to split 

 water to yield oxygen and an unstable hydrogen compound, which in 

 turn reacts with carbon dioxide to begin the synthesis of carbohydrates. 



There are a number of units of energy, including the erg, the joule 

 and the foot-pound, but the one most widely used in the biological 

 sciences is the Calorie. The kilocalorie, or Calorie written with a capital 

 C, is the amount of heat required to raise one kilogram of water one 

 degree Centigrade (strictly, from 14.5° C. to 15.5° C). Other forms of 

 energy, such as light, electricity or the energy of motion or position, can 

 be converted to heat and measured by the resulting increase in tempera- 

 ture of a known amount of water. Each gram of glucose, when metab- 

 olized to carbon dioxide, yields 3.74 Calories. An easy figure to remember 

 is that a gram of carbohydrate yields about 4 Calor.es. 



Catalysis. Many of the substances that are rapidly metabolized by 

 living cells are remarkably inert outside the body. A glucose solution, 

 for example, will keep indefinitely in a bottle if it is kept free of bacteria 

 and molds. It must be subjected to high temperature or to the action of 

 strong acids or bases before it will decompose. Protoplasm cannot fur- 

 nish conditions as extreme as these, for the protoplasm itself would be 

 destroyed long before the glucose, yet glucose is rapidly decomposed 

 within cytoplasm at ordinary temperatures and pressures and in a solu- 

 tion which is neither acidic nor basic. The reactions within the cell are 

 brought about by special agents known as enzymes, which belong to the 

 class of substances known as catalysts. A catalyst is an agent which affects 

 the velocity of a chemical reaction without altering its end point and 

 without being used up in the course of the reaction. The list of sub- 

 stances which may serve as a catalyst in one or more reactions is long 



