Plant Physiology - Musienko, M. M. 2001

Chemical and molecular composition, structure, and functions of the plant cell
Metabolic compartmentation of the cell

The Cell is not a homogeneous system. Lately, mounting evidence shows that THE PRINCIPLE OF heterogeneity, which underlies the structural-morphological and functional Organization of protoplasm, also extends to its localized metabolites and catalytic systems. The high degree of metabolic organization in both time and space, characteristic of living systems, is largely due to the presence of differentiated, specialized regions within the protoplasm. These regions differ in the activity levels of their constituent chemical compounds and the catalytic mechanisms regulating their transformations. Such regions are referred to as compartments, and this phenomenon of cellular space partitioning is known as compartmentalization. Compartments include specific Regions of the cell and its Organelles, distinct membrane domains, separate pools of metabolic Enzymes (substrates and products of enzymatic reactions), and all other morphological and Chemical Components of the protoplast.

Compartments are reaction spaces enclosed by membranes. There are cytoplasmic compartments with a high protein (enzyme) content, and non-cytoplasmic ones, such as vacuoles, the internal spaces of The Endoplasmic reticulum or dictyosomes of the Golgi apparatus, and the intermembrane space of Mitochondria and Plastids. The inner compartment of the latter two organelles—the matrix—is cytoplasmic, much like the Cytosol or karyoplasm.

Compartmentalization is evidence that the cell is a non-homogeneous system in which Different types of molecules are distributed in a specific manner. For instance, chlorophylls and certain Calvin cycle enzymes are found exclusively in METABOLISM/14.html">Chloroplasts, while cytochrome oxidase resides in mitochondria. Anthocyanins are synthesized in the Cytoplasm yet accumulate exclusively in the central vacuole. Most of the cellular DNA is located in The Nucleus, though a minor fraction is found in plastids and mitochondria. Practically all major pathways of Carbohydrate Metabolism are associated with specific regions of Plasma Membranes. Compartmentalization plays a fundamental role in regulating The Biosynthesis of enzymes and their isozymic components by influencing the repressor-derepressor system. Frequently, Isoenzymes with distinct catalytic properties are localized in different compartments. The compartmentalization of metabolic substrates and products ensures their storage by isolating them from enzymes. It should be noted that compartmentalization is not synonymous with "concentration", as the latter characterizes homogeneous systems. Thus, compartment Biomembranes act not merely as an insulating diffusion barrier, but also as a controlling, often selective factor in the mutual exchange of organic molecules and ions.