THE LOCALIZATION OF BIOCHEMICAL ACTIVITIES IN THE CELLS OF HIGHER PLANTS
Authors: HALLAWAY MARY 
Published: January 2008

Summary1. Since knowledge of the detailed morphology of the cell is essential for the study of intracellular localization of enzymic activities, a summary is presented of the results of electron microscopic examination of the fine structure of plant cells. Most of the data refer to meristematic cells, and relatively little is known of the structure of those fully differentiated tissues, such as roots, stems and fruits, which are commonly subjected to fractionation procedures.2. Cell fractionation involves disintegrating the selected tissue, separating the organelles, and purifying and characterizing them. A survey of the techniques which have been used for disintegration and separation reveals that they are all relatively crude compared with the complexity of the cell structures and that as a result each of the isolated fractions is likely to contain a number of types of organelles, with one predominating, and that damage to the structures is difficult to avoid. Characterization of the fractions has often been restricted to a definition of the conditions'of sedimentation, but electron microscopic and chemical examination are essential for definitive studies; in their absence any assignment of an enzyme to a particular organelle must be provisional.3. Chloroplasts and ribosomes can be isolated relatively intact and almost un‐contaminated. Methods have been developed for preparing biochemically active nuclei, mitochondria, cell walls, fragmented endoplasmic reticulum, starch grains and proteoplasts. Less satisfactory procedures are in use for isolating chromoplasts and proplastids, native fat, and vacuoles. So far dictyosomes and spherosomes cannot be separated from other subcellular particles.4. The histochemical determination of precise intracellular localization of enzymes requires the resolution of the electron microscope, and the recent adaptations of the methods of classical histochemistry for use with the electron microscope are briefly outlined. So far, none of these techniques is in routine use, but it is probable that the combination of improved fractionation procedures with electron microscopic histochemistry will provide definitive information on the intracellular sites of enzyme activity.5. Data on the intracellular distribution of enzymes and enzyme systems involved in nucleic acid, amino acid, protein, lipid and carbohydrate metabolism have been assembled. Ribonucleic acid synthesis occurs in the nuclei, and also in chloroplasts, deoxyribonucleic acid synthesis is apparently restricted to the nucleus. Amino acid incorporation into protein occurs in ribosomes, nuclei, chloroplasts and proteoplasts, and possibly in mitochondria; activating enzymes are usually detectable in the same subcellular fractions. Amino acid, lipid and carbohydrate metabolism involve enzymes present in a number of fractions, but in many cases the composition of the fractions is undefined and so the subcellular localization is uncertain.6. Photosynthetic fixation of carbon dioxide is restricted to the chloroplast, but the activity of isolated chloroplasts is less than 5 % of that of the intact tissue. This low activity is due to loss of enzymes and cofactors during isolation and to dilution of the remaining catalysts, but even slight damage to the chloroplast structure may also reduce the efficiency of photosynthesis by disorganizing the co‐ordinated electron transport and carbon fixation systems.7. It is technically difficult to determine the precise distribution of enzymes located partially in mitochondria. The procedures in use do not yield mitochondria free from contamination, and mitochondria or mitochondrial fragments may be sedimented with heavier or lighter particles. If mitochondria are ruptured, then the soluble intra‐mitochondrial enzymes will be found in the soluble fraction of the cell.