Compartmentalization in cells
Cells are not an amorphous mixture of proteins, lipids and other molecules. Instead, all cells are comprised of well-defined compartments, each specializing in a particular function. In many cases subcellular processes may be described based on whether they occur at the plasma membrane, within the cytosol or within membrane bound organelles such as the nucleus, Golgi apparatus or even vesicular components of the membrane trafficking system like lysosomes and endosomes.
Despite the morphological and functional variety of cells from different tissue types and different organisms, all cells share important similarities in their compartmental organization. These fundamental compartments, often referred to as organelles, are summarized in the drawing of the generic animal cell (central cell). Examples of specialized cell types, shown around the generic cell, include neuron, macrophage, intestine epithelial cell, adipocyte, muscle cell and osteoclast.
Compartmentalization increases the efficiency of many subcellular processes by concentrating the required components to a confined space within the cell. Where a specific condition is required to facilitate a given subcellular process, this may be locally contained so as not to disrupt the function of other subcellular compartments. For example, lysosomes require a lower pH in order to facilitate degradation of internalized material. Membrane bound proton pumps present on the lysosome maintain this condition. Similarly, a large membrane surface area is required by mitochondria to efficiently generate ATP from electron gradients across its lipid bilayer. This is achieved through the structural composition of this particular organelle.
Importantly, individual organelles may be transported throughout the cell, and this essentially localizes entire subcellular processes to regions where they are required. This has been observed in neurons, which have extremely long axonal processes and require mitochondria to generate ATP at various locations along the axon. It would be inefficient to rely on the passive diffusion of ATP down the length of the axon.
Compartmentalization can also have important physiological implications. For example, polarized epithelial cells, which possess distinct apical and basolateral membranes, can, for instance produce a secretory surface for various glands. Similarly, neuronal cells develop effective networks due to the production of dendrites and axonal processes from opposite ends of the cell body. Moreover, in the case of embryonic stem cells, cell polarization can result in distinct fates of the daughter cells.
With each organelle facilitating its own function, they may be considered as subcellular compartments in their own right. However, without a regulated supply of components to the compartment, the processes and mechanisms that produce their overall function will be impeded. With many proteins and molecular components participating in multiple subcellular processes, and therefore required throughout multiple subcellular compartments, effective transport of the protein and molecular components, either by passive diffusion or directed recruitment, is essential for the overall function of the cell.