Chapter 2
Table 2.2. Summary of transport processes for different compounds.
Amino acids
Active transport
Active transport
Active transport (PTS* and permease)
Facilitated diffusion and active transport
Active transport (PTS and permease)
Facilitated diffusion and active transport
Free and facilitated diffusion
Free and facilitated diffusion
Free diffusion
Free diffusion
Lactic acid
Active transport and free diffusion
Free diffusion
Acetic acid
Free diffusion
Free diffusion
Carbon dioxide
Free diffusion
Free diffusion
Free diffusion
Free diffusion
Free diffusion
Free diffusion
PTS - phosphotransferase system
There are three different transport mechanisms across the cytoplasmic membrane:
free diffusion
facilitated diffusion
active transport
By the first two mechanisms species are transported down a concentration gradient, i.e. they are
passive processes that in principle require no extraneous energy supply to run properly, whereas
sp ecies transported against a concentration gradient b y an a ctiv e p ro cess require
con siderable
input of Gibbs free energy. Table 2.2 summarizes the type of transport process for a few substrates
and metabolic products in bacteria and fungi, respectively. It is observed that most of the substances
are transported by the same type of process in the two groups of microorganisms, an important
exception being sugars, which are transported actively in bacteria, whereas they may also be
transported passively in fungi.
2.I.2.I. Free Diffusion
Transport of a substance across a lipid membrane by free diffusion involves three steps: (1) transfer
of the substance from the extracellular medium to the membrane phase, (
) molecular diffusion
through the lipid membrane, and (3) transfer from the lipid phase to the cytoplasm. Normally the
physical and chemical properties of the cytoplasm are similar to the properties of the extracellular
medium, and steps 1 and 3 are therefore similar. Furthermore, the interphase processes can be
assumed to be in equilibrium; i.e., the characteristic time for these processes to equilibrate is much
smaller than the characteristic time for the molecular diffusion through the lipid layer. The
concentration in the lipid layer at the interface can therefore be described as the product of the
concentration in the water phase and the so-called partitioning coefficient
the ratio of the
solubility of the compound in the lipid layer to its solubility in water. Mass flux due to molecular
diffusion follows Fick's first law, and the rate of mass transport of a compound into the cell
mass per membrane area per time, e.g., g m
s l)
through a lipid membrane of thickness
therefore be described by
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