Membrane diffusion simple

Another important vitamin is folate, which is required for purine and pyrimidine nucleotide synthesis. Since folate and its derivatives are generally lipo-phobic anions, they do not traverse biological membranes via simple diffusion but rather have to be taken up into the cells by specific transport processes... 

There is considerable evidence that the release of 5-HT occurs by exocytosis, i.e. by the discharge from the cell of the entire content of individual storage vesicles. First, 5-HT is sufficiently ionized at physiological pH so that it does not cross plasma membranes by simple diffusion. Second, most intraneuronal 5-HT is contained in storage vesicles and other contents of the vesicle including SPB are released together with serotonin. By contrast, cytosolic proteins do not accompany electrical stimulation-elicited release of 5-HT. Third, the depolarization-induced release of 5-HT occurs by a calcium-dependent process indeed, it appears that the influx of extracellular calcium ions with or without membrane depolarization can increase the release of 5-HT. Calcium stimulates the fusion of vesicular membranes with the plasma membrane (see Chs 9,10). 

Nonpolar compounds, including the majority of xenobiotic chemicals and some metal complexes [249-252], generally diffuse passively through the lipid portions of the membrane by simple diffusion [21,246,253,254]. In this case, internalisation rates are reflected by compound permeability in the bilipid membrane [254,255] and can be predicted by Fick s law [254,256] ... 

Substances can be transported across epithelial membranes by simple passive diffusion, carrier-mediated diffusion, and active transport, in addition to other specialized mechanisms, including endocytosis. 

A parameter (usually symbolized by P, and often containing a subscript to indicate the specific ion) that is a measure of the ease with which an ion can cross a unit area of membrane by simple (or passive) diffusion through a membrane experiencing a 1.0 M concentration gradient. For a particular biological membrane, the permeabilities are dependent on the concentration and activity of various channel or transporter proteins. In an electrically active cell (e.g., a neuron), increasing the permeability of K+ or CF will usually result in hyperpolarization of the membrane. Increasing will cause depolarization. 

SLM Simple flux equation based on film theory considering aqueous boundary layer resistance and membrane diffusion Optimal condition determined with respect to pH of the aqueous phase [57,58]... 

Small neutral molecules, such as water or ethanol, can penetrate membranes by simple diffusion.64 150 The rate is determined by the solubility of a substance in the membrane, by its diffusion coefficient (see Eq. 9-24) in the membrane, and by the difference in its concentration between the outside and the inside of the cell. This concentration difference is commonly referred to as the concentration gradient across the membrane. The ease of diffusion through a membrane is described quantitatively by a permeability... 

Most drugs and toxicant pass through membranes by simple diffusion down a concentration gradient. The driving force being the concentration gradient across the membrane. 

This mechanism is important for compounds that lack sufficient lipid solubility to move rapidly across the membrane by simple diffusion. A membrane-associated protein is usually involved, specificity, competitive inhibition, and the saturation phenomenon and their kinetics are best described by Michaelis-Menton enzyme kinetic models. Membrane penetration by this mechanism is more rapid than simple diffusion and, in the case of active transport, may proceed beyond the point where concentrations are equal on both... 

The movement of Na+ and glucose from the lumen of the intestine across the epithelial cell to the blood sets up a difference in osmotic pressure across the cell. As a result, water flows through the cell, across the apical and basolateral membranes by simple diffusion. Hence the uptake of water requires both Na+ and glucose (or amino acids) to be present in the lumen of the intestine. The presence of water alone in the lumen of the intestine is much less effective. This is the basis of glucose rehydration therapy as a remedy for dehydration a solution of glucose and salt (NaCl) is administered to the patient. This is a simple, inexpensive but extremely important treatment which has saved the lives of many infants in developing countries who would have otherwise died of the effects of dehydration, usually associated with diarrhea. 

Passive Transport. Transport by simple diffusion This mode of transport is available for apolar molecules. Permeation is predominantly governed by partitioning of the substrate between the lipid and water. The membrane simply acts as a permeability barrier small molecules pass more easily than large ones. The transport is explained in terms of a simple diffusion model involving three steps passage of the substrate from the exterior into the membrane, diffusion through the membrane, and passage out of the membrane. 

Which of the following molecules cannot move directly through the membrane by simple diffusion ... 

Various experimental techniques have demonstrated that cell membranes have a large lipid component, and most drugs cross such membranes by simple passive diffusion. In order to cross these lipid membranes, a drug should be in the lipid-soluble or un-ionized form and also be in solution. The various physicochemical characteristics of the drug are, therefore, of paramount importance as far as drug penetration across the oral mucosa is concerned. 

The previously described equations are characteristic of blood flow rate-limited models it is assumed that xenobiotics cross the cell membrane by simple diffusion and that equilibrium takes place instantaneously between blood and tissue compartments. This assumption is valid for a great number of chemicals. For certain xenobiotics, however, the kinetics of tissue uptake are not consistent with blood flow rate-limited processes since their distribution in a given tissue is limited by the resistance of the cell membrane to the passage of a xenobiotic. In these cases, the basic equation should account for such phenomena to describe adequately the time course of the xenobiotic disposition in the tissue. 

Nuclear receptors are intracellular proteins present in either the cytosol or the nucleus that bind hormones (steroids) which cross the ceU membrane by simple diffusion. The nuclear receptors undergo conformational changes that enable them to bind to DNA at specific sites. 

The three ways by which substances, including drugs, can cross cell membranes are simple diffusion, facilitated diffusion and active transport. 

Diffusion is the mechanism by which the vast majority of drugs pass across cell membranes. Both simple diffusion and facilitated diffusion are passive processes in that no energy is required other than the kinetic energy of the molecules themselves. 

What is the Henderson-Hasselbach equation and why is it an important consideration if a drug crossed membranes by simple passive diffusion Why is it far less important if a drug utilizes either facilitated transport or active transport carrier systems to cross membranes ... 

Vinca alkaloids are lipophilic molecules that can readily cross membranes by simple diffusion [186]. Experiments performed with several human cancer and tissue cell lines have shown in all cases rapid uptake of every one of Vinca alkaloids [23, 187-189]. Uptake is thought to occur by diffusion although energy dependence or independence of the uptake is seldom mentioned in reports. However, for instance in cultured human promyelocytic leukemia HL-60/C1 cells it has been shown that rates of uptake of vinblastine were unaffected by depletion of cellular adenosine triphosphate, reinforcing that uptake is not mediated by an energy-dependent system [189]. 

PASSIVE DIFFUSION Simple diffusion of a solute across the plasma membrane involves three processes partition from the aqueous to the lipid phase, diffusion across the Upid bilayer, and repartition into the aqueous phase on the opposite side. Diffusion of any solute (including drugs) occurs down an electrochemical potential gradient of the solute and is dependent on both its chemical and electrical potential. 

Figure 5.11 Facilitated transport proteins in cell membranes. Unlike simple diffusion through the membrane bilayer, facilitated transport systems become saturated as the solute concentration difference increases. In this hypothetical example, the permeability of the membrane in simple diffusion is 0.4 (units of flux/concentration).

Reflect and Apply Suggest a reason why inorganic ions, such as K, Na+, Ca +, and Mg +, do not cross biological membranes by simple diffusion. 

Biological membranes are highly nonpolar environments. Charged ions tend to be excluded from such environments rather than dissolving in them, as they would have to do to pass through the membrane by simple diffusion. Statements (a) and (c) are correct statement (b) is not correct because ions and larger molecules, especially polar ones, require channel proteins. Lipid-Soluble Vitamins and Their Functions... 

It is clear that the set of molecules that we will deal with, i.e. those for whom no specific transport system exists, will be defined by elimination. If there is no evidence for the existence of a specific mode of transport for the test molecule we will assume that this molecule crosses the cell membrane by simple diffusion, if it crosses the membrane at all. Such an assignment is, of course, temporary. As soon as evidence arises for the intervention of a specific system for the transport of our test molecule, this molecule will then be eliminated from the hst of those entering by simple diffusion. (Some molecules will of course cross the membrane both by simple diffusion and by a specific system, in parallel.)... 

Similar to the solid dosage forms containing chiral excipients, biological membranes may provide chiral environments (see Chapter 3). Most drugs cross the gastrointestinal membrane through simple passive diffusion thus, no stereoselectivity in the process is expected. It appears. 

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