Simultaneous synthesis and degradation of starch in spinach chloroplasts in the light

The rates of starch breakdown in isolated starch-loaded spinach chloroplasts have been measured in the dark and light at a range of phosphate concentrations, and compared with the rates of starch synthesis occurring simultaneously in the light. (1) Illumination has little inhibitory effect on starch breakdown when chloroplasts are suspended in medium containing a high phosphate level. At low phosphate levels, up to 67% inhibition is observed. (2) CO2 evolution is prevented by illumination at all phosphate levels, showing that the oxidative pentose phosphate pathway has been inhibited. (3) The rates of starch degradation in the light are significant compared with the rates of starch synthesis, even at phosphate levels which are optimal for photosynthesis or for starch synthesis. (4) It is concluded that net starch accumulation in the light represents a balance between synthesis and breakdown. This balance is very sensitive to the rate at which carbon is withdrawn from the chloroplast in exchange for Pi.