Cell Metabolism - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 509

10.6. HO W A CELL SENSES ITS EXTRACELLULAR ENVIR ONMENT

When cells are placed into a new environment, whether cells respond to the change in
the environment depends on whether cells are “aware” of the environment. For the cell
to be aware of the environment, substances in the environment must first be moved into
the cell.

10.6.1. Mechanisms to Transport Small Molecules across Cellular Membranes
A cell must take nutrients from its extracellular environment if it is to grow or retain meta-
bolic activity. As we discussed in Example 10-1, the rate at which nutrients enter the cell can
be important in regulating metabolic activity.
Molecules enter the cell through either passive diffusion (i.e. via concentration or Gibbs
potential gradient of the nutrients being transported) or facilitated diffusion (coordinated reac-
tion and diffusion). While passive diffusion is spontaneous, the facilitated diffusion can be
either spontaneous or energy dependant. The energy-dependent uptake mechanisms include
active transport and group translocation.
In passive diffusion, molecules move down a concentration gradient (from high to low
concentration) that is thermodynamically favorable. Consequently,
J A ¼ D d S A ðC AE C AI Þ¼k p ðC AE C AI Þ (10.27)

where J A is the flux of species A across the membrane, mol/(m 2 $ s); D A is the diffusivity of

species A through the membrane, m 2 /s; d is the effective thickness of the membrane, m;
S A is the solubility or “partition coefficient” of species A in the membrane, k p is the perme-
ability, m/s, C AE is the extracellular concentration of species A, mol/m 3 ; and C AI is the intra-
cellular concentration, mol/m 3 . The cytoplasmic membrane consists of a lipid core with
perhaps very small pores, thus the permeability and/or diffusivity is inversely related to
the molecular size of species A. A collection of permeability coefficients for a few compounds
in the cytoplasmic membrane of the plant cell Chara ceratophylla is given in Table 10.4 . Pres-
ence of a polar group can significantly reduce the permeability: an extra hydroxyl group on
the molecule decreases the permeability by 100- or 1000-fold. A carboxyl group has an even
larger effect. An extra amide group is more or less equivalent to two extra hydroxyl groups.
Conversely, an extra methyl group in the compound is likely to increase the permeability
5-fold, while a doubling of molecular volume decreases the permeability 30-fold. Therefore,
for charged or large molecules, the value of k p is very low and the flow of material across the
membrane is negligible. The cellular uptake of water and oxygen appears to be due to
passive diffusion. Furthermore, lipids or other highly hydrophobic compounds have rela-
tively high diffusivities (10 12 m 2 /s) in cellular membranes, and passive diffusion can be
a mechanism of quantitative importance in their transport.
With facilitated transport, a carrier molecule (protein) can combine specifically and revers-
ibly with the molecule of interest. The carrier protein is considered embedded in the
membrane. The carrier protein, after binding the target molecule, undergoes conformational
changes, which result in release of the molecule on the intracellular side of the membrane.
The carrier can bind to the target molecule on the intracellular side of the membrane,