Exercise XI 



FERMENTATION AND RESPIRATION 55 



The main business of fermentation and 

 respiration is to supply cells with ~P; and the 

 yield of such groups is a measure of the efficiency 

 of these processes. 



Yeast, a unicellular organism, can live nor- 

 mally either by fermentation, when no oxygen 

 is available, or by respiration, when oxygen is 

 present: 



Yeast fermentation: 



CnHiL'Oo -^ 2C2H5OH + 2CO2 + 2~P. 



elhylalcohol 



Yeast respiration: 



CgHi20g + 6O2 -^ 



6CO2 + 6H2O + (approx.) 38~P. 



Various other microorganisms ferment sugar 

 to different products, namely butyric acid, ace- 

 tone, etc., but the principle is always the same. 

 Animal cells also ferment sugar. Muscle cells, 

 for example, are often required to work more 

 rapidly than they can be supplied with oxygen, 

 and do so by fermenting sugar to lactic acid: 



Muscle fermentation : 



CgHioOb -> 2C3H6O3 + 2~P. 



lactic acid 



It will be noted that the chemical changes of 

 respiration just reverse those of photosynthesis; 

 similarly the energy of sunlight stored in sugars 

 by photosynthesis is released in respiration to 

 make high-energy phosphate bonds. Green 

 plants carry out both processes, photosynthesis 

 in the light, and respiration at ail times. 



You have already measured photosynthesis in 

 Elodea by the rate of oxygen evolution in the 

 light. Now we will measure respiration in a 

 higher plant by the rate of oxygen consumption. 

 As you see by the above equation for respira- 

 tion of sugars, one molecule of CO2 is produced 

 for each molecule of O2 consumed, so that, 

 according to Avogadro's law, one would expect 

 no change in gas volume. We shall absorb CO2 

 as fast as it is formed, however, with soda lime 

 (a mixture of solid sodium hydroxide and cal- 

 cium hydroxide). Write the equation for this 

 process. 



The rate of respiration varies greatly over the 

 life span of many organisms, being most rapid 

 during growth and development and slowing 

 down with maturity. Pea seedlings that are 3 

 to 4 days old have very rapid rates of respira- 

 tion, and thus were chosen for this experiment. 

 Since the products of respiration are also the 

 reactants of photosynthesis, it is advisable to 

 hold the latter process to a minimum during 

 your measurements. For that reason, the pea 

 seedlings were germinated in the dark, and so 

 lack chlorophyll. They green rapidly, however, 

 when exposed to light, so keep them shaded. 



EXPERIMENTS 

 Respiration 



We shall be working again with the volumeter 

 described originally in Exercise VI (pp. 35-36). 

 Fill one of the test tubes to within 2 inches of 

 the top with pea seedlings, tapping the test tube 

 against your hand to pack the seedlings. Insert a 

 cotton plug over the seedlings, and layer about 1 

 inch of soda lime over the cotton plug, to absorb 

 all carbon dioxide. Be sure that no soda 

 lime touches the seedlings. The second test tube, 

 which again will act as thermobarometer, should 

 be filled to 2 to 3 inches from the top with water, 

 to approximate the volume occupied by solid 

 material in the experimental tube. 



Insert the rubber stoppers and adjust indicator 

 drops in the side-arms, this time placing the drop 

 in the experimental tube near the distal end of 

 the scale. Clamp the escape tubes and wait 

 about 5 minutes for equilibration. Take readings 

 in both side-arms at 3-minute intervals, each 

 time subtracting the reading in the thermo- 

 barometer from that in the experimental tube, 

 until the rate of change in the experimental tube 

 becomes constant. This measures the rate of 

 oxygen consumption. 



If you now knew the rate of oxygen consump- 

 tion minus carbon dioxide production, you could 

 calculate the rate of evolution of carbon dioxide. 

 Figure out how to do this yourself; then do the 

 experiment. 



