Diffusion within cells

The cytoskeleton is an organized, but dynamic, assembly of proteins that is responsible for cell shape and behavior. The cytoskeleton provides mechanical 

Relative volumes of cell compartments Percentage Number 

The total percentage volume and the total number of organelles is presented for a hepatocyte. ER = endoplasmic reticulum. Adapted from [122], p. 553. 

The axons of neurons appear to have special transport processes for moving molecules into and out of the cell body (soma). The presence of special transport mechanisms in these cells is essential for their function, since neurons vary greatly in size, with some extending over 1 m, while protein synthesis occurs predominantly in the soma. Transport rates for different solutes occur in two categories slow (1-lOmm/day) and fast (100-400mm/day), and occur in the anterograde (away from the cell body) and retrograde (toward the cell body) directions. The speed of the slow processes is consistent with rates of diffusion, but the fast processes require additional mechanisms [9, 128]. 

The characteristic time for diffusion within cells or cell compartments of various sizes can be estimated from the diffusion coefficient (Table 4.8). Diffusion is an efficient method for distributing molecules throughout a small cell ( 1 /xm) even when the diffusion coefficient is low, diffusion times are less than 10 s. For larger cell compartments, diffusion is very slow, so that large cells ( 100 q,m) cannot rely on diffusion to distribute substrates or newly produced proteins. 

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Diffusion within cells is even more complex than within simple gels first the cytoplasm is a molecularly crowded zone, a complex gel with structural obstacles such as actin and myosin fibers and strands. There is the additional tortuosity that occurs in gels as the moving particle avoids the regions of the macromolecular chains and the obstruction effects from the impenetrable regions of the cytoplasm. If we designate as the effective... 

One problem with enzyme labeling is that the dense reaction product can diffuse within cells, masking the precise location of the antigen. Diffusion of the reaction products might limit resolution and obscure the underlying cellular structure. For restricted distribution of the reaction product use NiDAB (Angelov et al., 1998). 

The importance of the simple kinetics of molecular diffusion within cells and the factors which might alter these kinetics are often overlooked in research. However, FRAP experiments have illuminated the significance of kinetics of molecules as they relate to changes in cell shape, developmental stage, cell cycle progression,... 

In contrast to the cell experiments of Gibilaro et al., it is now seen from equation (10.45) that measurement of the delay time gives no information about diffusion within the pellets this can be obtained only through equation (10.46) from measurements of the second moment. As in the case of the cell experiment, the results can also be Interpreted in terms of an "effective diffusion coefficient" associated with a Fick equation for the... 

W. R. Lieb and W. D. Stem, Non-stochesian nature of the transverse diffusion within hmnan red cell membranes. J. Membr. Biol. 1986, 92, 111-110. 

Figure 1 General pathways through which molecules can actively or passively cross a monolayer of cells. (A) Endocytosis of solutes and fusion of the membrane vesicle with the opposite plasma membrane in an active process called transcytosis. (B) Similar to A, but the solute associates with the membrane via specific (e.g., receptor) or nonspecific (e.g., charge) interactions. (C) Passive diffusion between the cells through the paracellular space. (C, C") Passive diffusion (C ) through the cell membranes and cytoplasm or (C") via partitioning into and lateral diffusion within the cell membrane. (D) Active or carrier-mediated transport of an otherwise poorly membrane permeable solute into and/or out of a cellular barrier.

Cation and anion flux across cultured cell monolayers by molecular restricted diffusion within an electrostatic field of force across aqueous pores has been described with a model derived by Adson et al. (1994). The ion fluxes per cross-sectional area of the cell monolayer are defined as... 

Of the large number of protein interactions that take place in cells, perhaps the vast majority may be described as transient. Most proteins that modify other molecules do so very rapidly and so interact only briefly with their substrates or binding partners (i.e., enzymes). In addition, since proteins within cells are highly compartmentalized, the affinity of most interactions doesn t have to be very great, because each potential binding partner is within short diffusion distances and the relative concentration of molecules within these small volumes is high. 

These selection and evaluation criteria were applied systematically to four technological fields, three of which contribute to new energy-efficient solutions. Passive houses, for example, with their major components of insulation solutions, window systems, ventilation and control techniques are close to market diffusion within the next ten years. Fuel cells for mobile uses in vehicles, however, are still a long way from market introduction, for instance, because of unresolved problems regarding the deactivation of the membrane electrode assembly (MEA) and the need for cost reductions by about one order of magnitude. Other types of fuel cells for stationary uses may be closer to market introduction, owing to less severe technical bottlenecks and better economic competitiveness. 

Lieb WR, Stein WD (1986) Non-stokesian nature of transverse diffusion within human red blood cells. J Membrane Biol 92 111-119. 

The prototype of a small pore-forming toxin is the S. aureus a-toxin, also called ct-hemolysin, that has been extensively investigated hy Bhakdi and coworkers. Monomers of ct-hemolysin (33 kDa) hind to the surface of erythrocytes, and after lateral diffusion within the lipid hilayer, seven monomers oligomerize to form pores in the cell membrane. The ct-hemolysin forms mushroom-shaped pores with an outer diameter of lOnm and an inner diameter of approximately 2.5 nm. Small molecules can pass through the pore and diffuse into/out of the cytosol, along with water. As a consequence of such movement, cell homeostasis is greatly disturbed and pushed into an unhealthy state. In animals, the a-hemolysin represents a major virulence factor of S. aureus which causes hemolysis as well as tissue destruction. ... 

Facilitated diffusion within organisms takes place when carriers or proteins residing within membranes—ion channels, for instance—organize the movement of ions from one location to another. This diffusion type is a kinetic, not thermodynamic, effect in which a for the transfer is lowered and the rate of diffusion is accelerated. Facilitated diffusion channels organize ion movements in both directions, and the process can be inhibited both competitively and noncompetitively. It is known that most cells maintain open channels for K+ most of the time and closed channels for other ions. Potassium-ion-dependent enzymes include NaVK+ ATPases (to be discussed in Section 5.4.1), pyruvate kinases, and dioldehydratases (not to be discussed further). 

Compounds can cross biological membranes by two passive processes, transcellu-lar and paracellular mechanisms. For transcellular diffusion two potential mechanisms exist. The compound can distribute into the lipid core of the membrane and diffuse within the membrane to the basolateral side. Alternatively, the solute may diffuse across the apical cell membrane and enter the cytoplasm before exiting across the basolateral membrane. Because both processes involve diffusion through the lipid core of the membrane the physicochemistry of the compound is important. Paracellular absorption involves the passage of the compound through the aqueous-filled pores. Clearly in principle many compounds can be absorbed by this route but the process is invariably slower than the transcellular route (surface area of pores versus surface area of the membrane) and is very dependent on molecular size due to the finite dimensions of the aqueous pores. 

Another technique, single particle tracking, allows one to follow the movement of a single lipid molecule in the plasma membrane on a much shorter time scale. Results from these studies confirm the rapid lateral diffusion within small, discrete regions of the cell sur-... 

Organelles within cells have their own ion-concentrating mechanisms. Thus, mitochondria can concentrate K+, Ca2+, Mg2+, and other divalent metal ions as well as dicarboxylic acids (Chapter 18). The entrance and exit of many substances from mitochondria appear to occur by exchange diffusion, i.e., by secondary active transport. Such ion exchange processes may also occur in other membranes. 

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