Proteins barrier model approximation

It has been claimed that reactions in proteins can, as an approximation, be formulated within the Kramers reaction theory of barrier crossing [106]. The highly nonexponential relaxation pattern can now be explained by our model,... 

The outermost layer of the skin, the cornified layer or stratum corneum, has been identified as the principal diffusion barrier for substances, including water [2,3]. It is approximately 10 to 20 pm thick when dry but swells to several times this thickness when fully hydrated [17], It contains 10 to 25 layers lying parallel to the skin surface of nonviable cells, the corneocytes, which are surrounded by a cell envelope and imbedded in a lipid matrix. This architecture is often modeled as a wall-like structure, with the corneocytes as protein bricks embedded in a lipid mortar [18]. Similarly to the viable epidermis, desmosomes (corneodesmosomes) contribute to the cell cohesion. 

The cell membranes are predominantly a lipid matrix or can be considered a lipid barrier with an average width of a membrane being approximately 75 A. The membrane is described as the fluid mosaic model (Figure 6.2) which consist of (1) a bilayer of phospholipids with hydrocarbons oriented inward (hydrophobic phase), (2) hydrophilic heads oriented outward (hydrophilic phase), and (3) associated intra- and extracellular proteins and transverse the membrane. The ratio of lipid to protein varies from 5 1 for the myelin membrane to 1 5 for the inner structure of the mitochondria. However, 100% of the myelin membrane surface is lipid bilayer, whereas the inner membrane of the mitochondria may have only 40% lipid bilayer surface. In this example the proportion of membrane surface that is lipid will clearly influence distribution of toxicants of varying lipophilicity. 

Most organelles have a characteristic architecture that is important for function, but may serve as an additional impediment to diffusion within the cell compartment. Organelle membranes within the cell can serve as local barriers (Table 4.9), which affect the rate of diffusion of molecules locally. This effect has been estimated for diffusion in the mitochondrial matrix and the endoplasmic reticulum (Figure 4.28a and b). The mitochondrial matrix was modeled as a closed cylinder with multiple barriers occluding the lumen to simulate the mitochondrial cristae the presence of multiple barriers produced a modest decrease in the rate of diffusion down the axis of the cyUnder (Figure 4.28a) this calculation is consistent with recent measurements of green fluorescent protein (GFP) diffusion in the mitochondrial matrix, which was approximately three-fold slower than diffusion in saline. The endoplasmic reticulum was modeled as an array of interconnected cylinders with a continu-... 

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