In a green plant, cell metabolism is highly compartmentalised. Photosynthesis and photorespiration involve the participation of three different organelles: chloroplasts, mitochondria and peroxisomes. Mitochondria and chloroplasts are surrounded by two membranes; the outer membrane is freely permeable to small molecules like metabolites (Pfaff et al., 1968; Heldt & Sauer, 1971), owing to the presence of pores formed by porins. In chloroplasts these pores allow the passage of substances up to a molecular weight of 10000 (Flügge & Benz, 1984), whereas an exclusion limit of 4000–6000 was found in mitochondria from animal tissues (Zalman et al., 1980). Thus in mitochondria and chloroplasts the inner boundary membranes represent the border between metabolic compartments and are the site of metabolite translocators. Peroxisomes are surrounded by a single membrane. Recent studies have suggested that the boundary membrane of animal peroxisomes also contains porins, allowing the passage of metabolites of molecular mass up to 800 Da (van Veldhoven et al., 1987).

This raises the question, to what extent and by what means peroxisomal metabolism is compartmentalised. This chapter presents a summary of our current knowledge of the processes by which metabolites are transferred between different subcellular compartments and between cells in the course of photosynthetic metabolism.