Biochemical Engineering Fundamentals: Part 1 - Bailey, J., & Ollis, D. 1989

Stoichiometry and Energetics of Metabolic Pathways
Transport Across Cell Membranes
Active Transport

As we demonstrated in Fig. 5.17, Active Transport is characterized, first, by the movement of a substance against its chemical (or electrochemical) gradient—that is, from a region of low concentration to one of high concentration—and second, as equation (5.41) shows, by the requirement for metabolic Energy Expenditure. For instance, if an uncharged compound is transported from a medium where its concentration is 0.001 M into a Cell where its concentration is 0.1 M, then

Class="center">∆G°=1,98 cal/(mol∙deg) (298 deg) (ln 100) =

= 2,72 kcal/mol

Consequently, this process requires at least the specified amount of energy to take place. Active transport is also mediated by specific permeases.

While active transport is fundamental to Nerve Impulse transmission, Cells utilize this mechanism on a much broader scale. Indeed, virtually all cells possess active transport systems (ion pumps) that maintain the necessary intracellular concentrations of K⁺, Na⁺, and Water. These systems pump Na⁺ ions out of The Cell and K⁺ ions inward, thereby counteracting the passive diffusion of these ions. These transport systems are interconnected and coupled to ATP Hydrolysis (Fig. 5.19). In red Blood Cells, the sodium-potassium pump function is performed by an oligomeric protein known as Na⁺—K⁺-ATPase. As shown in Fig. 5.19, this protein is embedded within The Plasma Membrane and spans its entire thickness; such Proteins are referred to as transmembrane proteins.

A second common group of active transport systems supplies the cell with nutrient molecules, such as glucose and Amino Acids, at rates vastly exceeding those achievable by passive diffusion alone. In higher animal cells, The active transport of glucose is coupled to the inward transport of Na⁺, while the extrusion of Na⁺ from the cell occurs concurrently with ATP hydrolysis driven by Na⁺—K⁺-ATPase. The overall process is illustrated schematically in Fig. 5.20. In Bacteria, active transport of glucose relies on a different mechanism called group translocation. In this process, glucose is released into the intracellular space as high-energy glucose-6-phosphate, which poorly penetrates membranes (Fig. 5.21). This process is believed to be the rate-limiting step for growth in A number of cell types.

FIG. 5.19. The Na⁺—K⁺ pump (Na⁺—K⁺-ATPase) transports Na⁺ out of the cell and K⁺ into the cell; the driving force for this process is ATP hydrolysis.

The driving force for many bacterial transport systems is a proton flux across the plasma membrane. The primary role of proton flux in bacterial Bioenergetics is described by the chemiosmotic theory formulated by Mitchell in 1961. Figure 5.22 depicts the cotransport of protons and lactose during Respiration in E. coli. Protons and lactose are cotransported into the cell by lactose permease (the METABOLISM/31.html">Transcription product of the lac Operon Gene). Simultaneously, protons are extruded from the cell along with an electron flow in the Respiratory Chain. Overall, the intracellular pH is higher than that of the immediate extracellular environment.

FIG. 5.20. Na⁺ ions and glucose are transported from the medium into the cell by a shared passive carrier. The low intracellular concentration of Na⁺ is maintained by active transport. (Reprinted with permission from: Lehninger A., Bioenergetics, 2d ed., p. 205, W. A. Benjamin, Inc., Palo Alto, CA, 1974.)

FIG. 5.21. Phosphorylation of glucose within The cell membrane provides a continuous driving force for glucose Transport Across the membrane and traps the glucose (as a derivative) inside the cell. Here G denotes glucose; E_H, membrane enzyme; E₁, cytoplasmic enzyme; HPr, cytoplasmic protein; PEP, phosphoenolpyruvate. (Reprinted with permission from: Lehninger A. L., Biochemistry, 2d ed., p. 799, Worth Publishers, New York, 1975.)

Eukaryotic cells must possess a complex array of transport systems because the concentrations of numerous substances within Organelles differ from those in the Cytoplasm. This fact further underscores that any cell, and particularly a eukaryotic one, is not a homogeneous mixture of countless components, but a highly organized system down to THE MOLECULAR LEVEL.

Although numerous Mathematical models of active transport have been developed, we will not discuss them here. Readers seeking more detailed information can consult the References and exercises provided at the end of the chapter.

FIG. 5.22. Coupling of lactose active transport with proton transport driven by the electron flow gradient of the respiratory chain.

Before examining fluxes and regulatory systems in metabolic pathways, it is useful to review the Structure and Organization OF THE cell membrane more thoroughly and in greater detail. Figure 5.23 illustrates the Cytology/cytology/92.html">SCHEMATIC STRUCTURE OF the bacterial cell envelope; it is readily apparent that its architecture is considerably more complex than the simplified model described in Chapter 2, which comprised a lipid bilayer and a protein coat. The figure shows that numerous protein molecules are embedded within the membrane; some reside near the outer or inner membrane surfaces, while others span the entire thickness of the membrane. These proteins (permeases and receptor proteins) recognize specific compounds in the environment and interface with the cell's regulatory systems, enabling a cellular response to environmental changes. In bacteria, these membrane protein components allow cells to rapidly adapt to fluctuating environmental conditions. Surface receptors of higher eukaryotic cells participate in Cell Recognition processes, which play a major role in both tissue organization and immune responses.

Membranes are generally asymmetric; their outer and inner surfaces harbor different components and perform distinct Functions. Lipids and various Membrane Proteins diffuse very rapidly within the plane of the membrane; by some estimates, the lipid diffusion rate in certain membranes is 10^-8 cm²/s.

FIG. 5.23. Structural details of the E. coli cell envelope. Here, white globular entities On the surface and within the cell membranes represent individual proteins (OM, outer membrane; PG, peptidoglycan; PP, periplasmic space; CM, cytoplasmic membrane). (Reprinted with permission from: Inouye M., Bacterial Outer Membranes, John Wiley, New York, 1979.)