EXCRETION OF GLYCOLIC ACID 229 



equilibrium exists across the cell membrane, a change in bicarbonate 

 ion concentration should result in an opposite movement of glyco- 

 late ion to help restore the following equality: 



(glycolate-)ceii (HC03~)ceii 



(glycolate ) medium (HCO3 ) medium 



The enormous rate of glycolate-C^^ excretion during active bicar- 

 bonate uptake of photosynthesis indicates that this organic anion is 

 uniquely able to respond quickly to the upset of equihbrium when bi- 

 carbonate ion in the cell suddenly diminishes in concentration and 

 more of this anion moves in from the medium. 



That glycolate should be the anion functioning in this manner may 

 be the result of several factors. It is a small organic acid, and yet it is 

 one of the strongest acids associated with the cell. It is somewhat 

 unique in being readily available from the photosynthetic carbon 

 cycle. In this respect, it is believed to be formed from a side oxidation 

 pathway from this cycle, starting from a C2 complex at the oxidation 

 level of glycolaldehyde. 



The excreted glycolate does not accumulate in the Chlorella culture 

 medium in large amounts, but rather it is rapidly reabsorbed by the 

 algae when one or more of the conditions for its excretion are not met. 

 This accumulation inside the cell, in relatively large amounts, is about 

 equal to the total normally found in the medium, but it should not 

 persist for a long time, since glycolate is metabolized to glycine and 

 serine (1). This factor could contribute to induction effects in photo- 

 synthesis after periods of darkness of about 1 hour or longer. Little 

 glycolate would then be left in the cell to exchange rapidly for bi- 

 carbonate ion. When the algae are placed in the hght, a photosyn- 

 thetic induction period would ensue while the glycolate pool was re- 

 plenished by a slow rate of photosynthesis or from reduction of gly- 

 oxylate arising from glycine (2) or from isocitrate (3,4). 



Other consequences of glycolate excretion during photosynthesis, 

 and its reabsorption and accumulation inside Chlorella during dark 

 respiration, have not yet been evaluated. This bicarbonate-glycolate 

 shift may be involved in acid bursts, pH shifts, CO2 bursts, and 

 quantum efficiency as calculated from short-time or flashing-light 

 experiments. 



