50 Practical Plant Biology. 



is active, the products of respiration are not observable because 

 all the carbon dioxide and water produced by respiration are built 

 up again into carbohydrates and never appear. So that although 

 this process of respiration is going on in the cell of Chlamydomo- 

 nas without ceasing, it only becomes apparent when the reduction 

 of light reduces photosynthesis sufficiently to allow it to pre- 

 ponderate. 



As we have already seen the combustion of carbohydrates sets 

 free energy. Thus i gramme molecule (or 162 g.) of starch in 

 complete combustion changing to carbon dioxide and water sets 

 free about 685 Calories, or i gramme of starch can generate more 

 than 4 Calories. This energy has been previously obtained from 

 the light absorbed by the chloroplast in photosynthesis. It is in 

 fact solar energy. Thus if a Chlamydomonas carries a starch 

 grain weighing i x 10 ~ 10 g. (i.e. about \ the size of a yeast cell) 

 it could do work equivalent to about o'2 gram-centimetre. This 

 energy is used in doing the work of life in the cell, much of which 

 eludes our observation. Some of it is visibly spent in propelling 

 Chlamydomonas through the water, or in causing and maintaining 

 the streaming motion of the protoplasm as in the cell of Elodea, 

 or again in raising the temperature of the cell. During illumina- 

 tion some of the substance actually being formed is used up, and 

 some of the energy being absorbed at the moment is set free 

 for this work. In this case it is probable that a soluble sugar 

 such as glucose is oxidised, and gives up its energy before it has 

 been built up into starch. Some provision, however, must be 

 made for times of darkness, during which there is still a demand 

 for energy for various processes. This provision is secured by 

 storing carbohydrates. There is an obvious difficulty in storing 

 large quantities of soluble carbohydrates like sugars, since as they 

 accumulate they necessitate a rise of osmotic pressure. In a cell 

 like Chlamydomonas this would ultimately lead to the expulsion 

 of part of the semi-permeable protoplasm through the anterior 

 opening in the cell-wall. It is probably to get over this difficulty 

 that for storage purposes the soluble carbohydrates are trans- 

 formed into the colloidal state, in which their osmotic pressure 

 practically disappears. As energy is required, the colloid starch 

 is gradually retransformed by the agency of the enzymes diastase 

 and maltase into glucose and is respired probably in that form. 

 It is this conversion of starch into glucose which may be observed 

 when Chlamydomonas is cut off from light. 



But whether the energy is used directly after it is absorbed, or 

 whether it is stored in the form of a soluble or colloidal carbo- 



