GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

11. MAIN MECHANISMS OF METABOLISM AND ENERGY CONVERSION IN MICROORGANISMS

11.7. SUBSTRATE UPTAKE MECHANISMS

For an exogenous substrate to be utilized by a Cell, it must enter The Cell, which means crossing its boundary layers. The Cell wall is generally not a significant barrier to small molecules and ions, but it retains macromolecules with a molecular weight exceeding 600 Da. The boundary layer responsible for The transport of nutrients is The Plasma Membrane.

The transport of nutrients across the plasma membrane is specific: only those substances for which a corresponding transport system exists can be taken up. With few exceptions, transport depends on the presence of specific permeases or translocases. These are Membrane Proteins whose very name indicates that they possess enzyme-like properties—meaning they can be induced by the substrate, are substrate-specific, and are synthesized under conditions that permit Protein Synthesis.

There are several Mechanisms of nutrient transport into the cell. Some of these are capable of providing only transport, but not the accumulation of the substance within the cell. In contrast, Active Transport processes result in the accumulation of substances inside the cell.

When comparing the ways in which environmental substances cross the plasma membrane into the Cytoplasm, four distinct mechanisms can be identified: passive diffusion, Facilitated Diffusion, active transport, and group translocation.