Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Metabolism: An Overview
Various metabolic pathways may be localized in different compartments of the cell

In Ch. 1, we already discussed that Cells fall into two major classes: prokaryotic and eukaryotic. Representatives of these two classes differ significantly in size, internal Structure, and genetic and metabolic Organization. Prokaryotic Cells, which include Bacteria and blue-green Algae (cyanobacteria), are very small, relatively simple in structure, with a single membrane system—namely, Cell/30.html">The Plasma Membrane surrounding The Cell.

Prokaryotic cells lack compartments separated by internal membranes. However, bacteria do exhibit a certain degree of compartmentalization of certain enzyme systems (Fig. 13-22). For example, most Enzymes involved in METABOLISM/35.html">Protein Biosynthesis are localized in their Ribosomes, whereas some enzymes responsible for phospholipid biosynthesis are concentrated in The cell membrane.

Eukaryotic cells, which include those of higher animals and plants, Fungi, higher algae, and Protozoa, are considerably larger and much more complex in organization than prokaryotic cells (Fig. 13-23). In addition to the plasma membrane, they contain a membrane-enclosed Nucleus that houses the Chromosomes (the number of which varies among organisms and can be quite large in some species). They also contain other membrane-enclosed Organelles, namely Mitochondria, The Endoplasmic reticulum, the Golgi apparatus, and the Chloroplasts of green plant cells. In eukaryotic cells, enzymes of specific Metabolic pathways are often localized within distinct organelles or compartments. How was this discovered? Cellular organelles can be isolated from Cells and Tissues by centrifugation (Fig. 13-24). To achieve this, animal or plant tissues are first subjected to gentle homogenization in an isotonic sucrose solution. This Treatment disrupts the plasma membrane while leaving most cellular organelles intact. Sucrose is advantageous because it permeates membranes relatively poorly and therefore does not cause osmotic Swelling of intracellular structures such as mitochondria or chloroplasts. Different types of organelles—such as nuclei and mitochondria—differ in size and specific gravity, and consequently sediment at different rates in a centrifugal field. They can subsequently be isolated from the homogenate by differential centrifugation (Fig. 13-24). These isolated fractions of nuclei, mitochondria, and other components are then tested for their ability to catalyze specific metabolic processes. It was this approach that made it possible to establish that various metabolic pathways are compartmentalized within eukaryotic cells (Fig. 13-23). For instance, it was found that in certain cells, all the enzymes involved in The conversion of glucose to lactic acid reside in the Cytosol (the soluble portion of the Cytoplasm), whereas the enzymes of The Citric Acid Cycle, electron transport, and Oxidative Phosphorylation are localized within the mitochondria.

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Fig. 13-22. Localization of certain types of metabolic activity in a bacterial cell.