Cytoplasm diffusion

One immediate impression of plant cells is the great prevalence of membranes. In addition to surrounding the cytoplasm, membranes also separate various compartments in the cytoplasm. Diffusion of substances across these membranes is much more difficult than is diffusion within the compartments. Thus, organelle and vacuolar membranes can control the contents and consequently the reactions occurring in the particular compartments that they surround. Diffusion can also impose limitations on the overall size of a cell because the time for diffusion can increase with the square of the distance, as we will quantitatively consider in the next section. 

Ruenraroengsak P, Al-Jamal K, Hartell N, et al. Cell uptake, cytoplasmic diffusion and nuclear access of a 6.5mn diameter dendrimer. Int J Pharm 2007 331 215-219. 

The role of ammonia is in buffering acid and it makes a contribution which is particularly important when the acid load is high. Ammonia produced in the tubular cell cytoplasm diffuses readily across the tubular cell membrane into the tubular fluid. Cell membranes consist largely of lipid material and so are in general permeable to non-polar particles such as ammonia The solubility of ionized particles such as ammonium ions in lipid is very low so that ions permeate cell membranes very slowly. 

Figure 16 Polymers with ATDs can be generated by ROMP. The ATD promotes cellular internalization. HeLa cells were incubated with polymer and visualized by using confocal fluorescence microscopy. The polymeric ATD is seen in endocytic vesicles (punctatefluorescence) as well as in the cytoplasm (diffuse fluorescence). Scale = 25 pm.

Figure 27 Block copolymers In which one block serves as an AID can be internalized by cells. Fluorescence microscopic images of live HeLa cells incubated with the rhodamine-labeled polymer for varying time points at 37 °C are shown. The block copolymer localizes in both endocytic vesicles (punctate staining) and the cytoplasm (diffuse fluorescence). Scale = 25 pm.

Considerable work has been done to try to explain why quats are antimicrobial. The following sequence of steps is beheved to occur in the attack by the quat on the microbial cell (/) adsorption of the compound on the bacterial cell surface (2) diffusion through the cell wall (J) binding to the cytoplasmic membrane (4) dismption of the cytoplasmic membrane (5) release of cations and other cytoplasmic cell constituents (6) precipitation of cell contents and death of the cell. 

The nuclear pore complex, located in the nuclear envelope, contains more than 50 proteins. It allows diffusion of small proteins between cytoplasm and nucleoplasm. Larger molecules (>50kD) are selectively transported by an energy-dependent mechanism. 

Gershon, N.D., Porter, K.R., Trus, B.L. (1985). The cytoplasmic matrix Its volume and surface area and the diffusion of molecules through it. Proc. Natl. Acad. Sci. USA 82, 5030-5034. 

Acetylcholine is synthesised in nerve terminals from its precursor choline, which is not formed in the CNS but transported there in free form in the blood. It is found in many foods such as egg yolk, liver and vegetables although it is also produced in the liver and its brain concentration rises after meals. Choline is taken up into the cytoplasm by a high-affinity (Am = 1-5 pM), saturable, uptake which is Na+ and ATP dependent and while it does not appear to occur during the depolarisation produced by high concentrations of potassium it is increased by neuronal activity and is specific to cholinergic nerves. A separate low-affinity uptake, or diffusion (Am = 50 pM), which is linearly related to choline concentration and not saturable, is of less interest since it is not specific to cholinergic neurons. 

Unphosphorylated functioning according to Fig. 5 catalyzes facilitated diffusion of mannitol across the membrane. The same process has been reported for purified II reconstituted in proteoliposomes [70]. The relevance of this activity in terms of transport of mannitol into the bacterial cell is probably low, but it may have important implications for the mechanism by which E-IIs catalyze vectorial phosphorylation. It would indicate that the transmembrane C domain of Il is a mannitol translocating unit which is somehow coupled to the kinase activity of the cytoplasmic domains. We propose that the inwardly oriented binding site which is in contact with the internal water phase (Ecyt Mtl, see Fig. 5) is the site from where mannitol is phosphorylated when transport is coupled to phosphorylation. Meehan-... 

The phosphorylation of cytoplasmic sugar and the facilitated diffusion from the cytoplasm to the periplasm are catalyzed by the E-IIs under conditions where they are also active in the vectorial phosphorylation reaction. Therefore, the former two activities should be integral parts of any kinetic scheme representing the mechanism of E-IIs. Such a scheme should explain how vectorial phosphorylation, transport coupled to phosphorylation, is still achieved while the uncoupled pathways are integral parts of the scheme. 

Demonstration of catalytic activities of E-IIs other than vectorial phosphorylation, like phosphorylation of free substrate at the cytoplasmic side of the membrane and facilitated diffusion catalyzed by E-IIs and phosphorylated E-IIs. 

Another situation is found for the Na+ ions. When the membrane is permeable to these ions, even if only to a minor extent, they will be driven from the external to the internal solution, not only by diffusion but when the membrane potential is negative, also under the effect of the potential gradient. In the end, the unidirectional flux of these ions should lead to a concentration inside that is substantially higher than that outside. The theoretical value calculated from Eq. (5.15) for the membrane potential of the Na ions is -1-66 mV. Therefore, permeabihty for Na ions should lead to a less negative value of the membrane potential, and this in turn should lead to a larger flux of potassium ions out of the cytoplasm and to a lower concentration difference of these ions. All these conclusions are at variance with experience. 

K. Luby-Phelps, P. E. Castle, D. L. Taylor, and F. Lanni, Hindered diffusion of inert tracer particles in the cytoplasm of mouse 3T3 cells, Proc. Natl. Acad. Sci. USA 84, 4910 (1987). 

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