Rubisco: its role in photorespiration

The release of CO 2 during photosynthesis that is due to the production and metabolism of glycollic acid is usually regarded as outward evidence for the wasteful process of photorespiration in plants. In the light, glycollic acid is produced almost entirely as a result of the oxygenase activity of ribulose bisphosphate carboxylaseoxygenase (Rubisco). Metabolism of the glycollic acid not only releases recently assimilated carbon back into the atmosphere but also uses a considerable amount of energy to recycle remaining carbon from the glycollate to intermediates of the photosynthetic carbon reduction cycle. Furthermore, nitrogen from amino acids is released as ammonia during the metabolism of glycollate; some further energy is needed for this ammonia to be reassimilated. The oxygenation of ribulose bisphosphate is competitive with carboxylation and it appears to be the relative concentrations of oxygen and carbon dioxide present in cells containing the enzyme that mainly determine the relative rates of the two reactions in leaves. Systems which concentrate carbon dioxide in photosynthetic cells decrease the extent of photorespiration in C 4 species, certain algae and cyanobacteria. However, carboxylases from different species also vary considerably in their relative capacities to catalyse carboxylation and oxygenation of ribulose bisphosphate under standard conditions. This variation allows some hope that photorespiration might be decreased without recourse to energydependent systems for increasing cellular CO 2 concentrations.

The significance of dissolved organic compounds in the nutrition of Siboglinum ekmani and other small species of Pogonophora

Siboglinum ekmani can accumulate labelled alanine, glycine, phenylalanine, glutamic acid, galactose, glucose, mannose, acetic acid, palmitic acid, lactic acid and glycollic acid from sea water at concentrations between 1 and 100 μM/1. Uptake of fructose and mannitol was not detected. The neutral amino acids were absorbed against a concentration gradient, probably by a carrier-mediated transport mechanism, and similar mechanisms may exist for acidic amino acids and glucose. Most of the compounds entered into the metabolism, but 90% of the glycine accumulated was retained as free amino acid. The presence of glucose appeared to depress uptake of alanine, but uptake of glucose was stimulated by alanine.