46 ORGANIC SUBSTANCES : CHEMICAL CHARACTERS 



tive concentration of the soluble end products. Thus, 

 if the sugar concentration is low, starch will be converted 

 into sugar, but when it rises above a certain point the 

 sugar will be converted into starch. We shall see in 

 the sequel that these reversible reactions are very 

 important in the living organism. 



In nature organic compounds always occur as 

 constituents or products of living organisms, but 

 chemists can now make many of them out of simpler 

 substances in the laboratory, for instance the sugars, 

 and even some of the simpler proteins. 



PRACTICAL WORK. 



Carbohydrates, Fats and Proteins. 



(i) Compare the samples of solid glucose and of solid sucrose 

 provided. Note that the first is a crystalline powder, while the 

 second forms large well-defined crystals. Compare their sweet- 

 ness by tasting. Dissolve the samples in two separate test 

 tubes each containing a little water, and note that each dissolves 

 completely. Test a little of each with very dilute solution of 

 iodine in potassium iodide ' and note that there is no coloration. 

 Add a few drops of Fehling's solution (copper sulphate made 

 strongly alkaline) to each test tube, and heat. Note that one 

 solution turns red owing to the formation of red cuprous oxide, 

 while the other does not. 



(2) Examine a thin slice of potato under the low and high powers 

 of the microscope. Draw one or two of the starch grains under 

 the high power. Measure their long and short diameters with 

 the micrometer eyepiece and express the values in /j, (see Practical 

 Work I (4) p. 33). Treat the section with very dilute iodine 

 solution and examine again. 



(3) The tube provided contains very dilute starch paste. 

 Pour a little into a watchglass, and add one drop of very dilute 

 iodine solution. Now add a little taka-diastase (a commercial 

 preparation of diastase also containing maltase, see p. 45) to the 

 contents of the tube, and shake the tube vigorously at intervals. 

 After a minute pour a little of the mixture into another watch- 



' This solution will be alluded to in future simply as " iodine 

 solution." 



