280 PEACTICAL PHYSIOLOGY 



adding some NaOH to the tube by means of a pipette and shaking : 

 the gas disappears. As a control, a tube filled with water and yeast 

 should also be incubated. This should yield no gas. 



EXPERIMENT. Repeat the above experiment with similar solutions of 

 maltose, lactose and cane sugar, and note that, after 24 hours, lactose has not 

 undergone any fermentation, whereas it is marked in the case of maltose ; 

 cane sugar also shows a certain amount of fermentation. Yeast contains an 

 invertase (maltase) which readily hydrolyses maltose into dextrose, on which 

 the zymase of the yeast then acts, forming alcohol and carbon dioxide. Another 

 invertase in the yeast acts on cane sugar. These invertases have no action on 

 lactose. 



///. Rotation of Polarised Light. All simple carbohydrates rotate 

 the plane of polarisation of polarised light to the right except laevulose, 

 which rotates to the left. 



This effect is due to the presence in the molecule of asymmetrical carbon 

 atoms. 



6 carbon aldose (hexose). 6 carbon ketose. 



CH 2 OH CH 2 OH 



*CHOH *CHOH 



*CHOH *CHOH 



*CHOH *CHOH 



*CHOH CO 



CHO CH 2 OH 



* Denotes an asymmetrical carbon atom. 



Examination of the above formulae shows that the aldoses contain four 

 asymmetrical carbon atoms, whilst the corresponding ketoses contain only 

 three. The different arrangements in space of the hexose carbon atoms allow 

 of the existence of sixteen different hexoses, of which twelve have been 

 identified. Only two, however, are of physiological importance, dextrose and 

 galactose. The different spacial arrangement of the atoms in the molecule 

 accounts for the difference in rotatory powers shown by these aldoses and also 

 for slight differences in chemical properties, such as crystalline form and 

 melting point of the osazones. 



Polarisation Of Light, When two slices of tourmaline, a semi-transparent 

 mineral, are cut parallel to the axis of the crystal and laid over one another, it 

 will be noticed that the amount of light which passes through the combination 

 varies according to the relative positions of the two slices to one another. If 

 the slices be at right angles to one another no light passes through, and in 

 intermediate positions only a certain amount, so that an opaque combination is 

 obtained. A ray of ordinary light contains vibrations in all planes passing 

 through the ray ; but when the light passes through a tourmaline plate it 

 vibrates in one plane only. Ordinary light may, therefore, be likened to a 

 wheel, the axle representing the ray of light and the spokes the planes along 

 which it vibrates. On passing through the tourmaline plate, however, the 



