MICROBIOLOGY Study Guide - 2012

CHAPTER 9. MICROBIAL PHYSIOLOGY

9.2. NUTRITION OF MICROORGANISMS

9.2.2. Mechanisms of Nutrient Uptake into the Cell

The rate of nutrient uptake into The Cell is influenced by molecular size and solubility in Water or Lipids, medium pH, solute concentration, and other factors. The Cell wall permits only small molecules and ions to pass, acting as a barrier that prevents The entry of high molecular weight substances (greater than 600 Da) into the cell. The selective uptake of nutrients into the microbial cell is regulated by the cytoplasmic membrane. Currently, the following MECHANISMS OF NUTRIENT transport into the cell have been identified: passive diffusion, Facilitated Diffusion, Active Transport, and group translocation.

Passive diffusion is the process by which water and certain dissolved low-molecular-weight substances (e.g., toxins) enter the cell across the cytoplasmic membrane along a concentration gradient. The rate of passive diffusion is low, and the process occurs without Energy Expenditure.

Facilitated diffusion also proceeds along a concentration gradient without energy expenditure; however, unlike passive diffusion, The transport of the nutrient molecule is mediated by a substrate-specific transport protein known as a permease. The permease catalyzes the binding of the substance molecule to an Active Site on its surface and transports it across the cytoplasmic membrane into the Cytoplasm. There, the permease releases the substance and returns to the outer surface of the membrane for another molecule. Permeases are synthesized and localized in the cytoplasmic membrane.

Active transport differs from facilitated diffusion in that the translocation of a substance molecule is mediated by a permease but occurs against a concentration gradient, which requires the expenditure of energy in the form of ATP.

Group translocation. In this mechanism, the transport of a substance molecule is carried out by specific carrier Proteins called translocases against a concentration gradient with the expenditure of energy. Unlike active transport, the substance molecule undergoes chemical modification while passing through the cytoplasmic membrane. Specifically, The process of Glucose Transport into bacterial Cells is accompanied by its phosphorylation.