Transport transmembrane

Fromm and Spanswick [79] found that electrical stimulation of a plant is followed by ion shifts which are most striking in the phloem cells. While their content of potassium and chloride was diminished after stimulation, the amount of cytoplasmic calcium increased slightly (Table 1). These displacements lead to the conclusion that Ca + influx as well as and CP efflux are involved in the propagation of action potentials. The main difference between propagation of action potentials in animals and plants is that in an axon there is the K /Na transmembrane transport but in phloem cells the K /Ca channels are involved in this process [Fig. 22(b)]. 

DM Matthews, JW Payne. Transmembrane transport of small peptides. Curr Topics Membrane Transport 14 331-425, 1980. 

Lipka, E., J. Crison, and G. L. Amidon. Transmembrane transport of peptide type compounds prospects for oral delivery. J. Control. Release 1996, 39, 121-129. 

For each description below, select the transmembranal transport mechanism it best defines ... 

The ideas of Overton are reflected in the classical solubility-diffusion model for transmembrane transport. In this model [125,126], the cell membrane and other membranes within the cell are considered as homogeneous phases with sharp boundaries. Transport phenomena are described by Fick s first law of diffusion, or, in the case of ion transport and a finite membrane potential, by the Nernst-Planck equation (see Chapter 3 of this volume). The driving force of the flux is the gradient of the (electro)chemical potential across the membrane. In the absence of electric fields, the chemical potential gradient is reduced to a concentration gradient. Since the membrane is assumed to be homogeneous, the... 

Many experimental variations are possible when performing uptake studies [246]. In a simple experiment for which the cells are initially free of internalised compound, the initial rates of transmembrane transport may be determined as a function of the bulk solution concentrations. In such an experiment, hydrophilic compounds, such as sugars, amino acids, nucleotides, organic bases and trace metals including Cd, Cu, Fe, Mn, and Zn [260-262] have been observed to follow a saturable uptake kinetics that is consistent with a transport process mediated by the formation and translocation of a membrane imbedded complex (cf. Pb uptake, Figure 6 Mn uptake, Figure 7a). Saturable kinetics is in contrast to what would be expected for a simple diffusion-mediated process (Section 6.1.1). Note, however, that although such observations are consistent... 

Deeley, R.G., Westlake, C. and Cole, S.P. (2006) Transmembrane transport of endo- and xenobiotics by mammalian ATP-binding cassette multidrug resistance proteins. Physiological Reviews, 86, 849-899. 

To solve this dilemma, a new method which allowed the separation of the transmembrane transporter not as the protein, but as the nucleic acid, followed by subsequent expression of the isolated nucleic acid and a screening of the functional activity of the encoded polypeptide was suggested. 

Today, there are a wide variety of laboratory protein expression systems available, ranging from cell-free systems over bacterial and yeast cultures to eukaryotic models including the Xenopus oocytes or insect and mammalian cell cultures, some of which even form polarised epithelial-like cells layers. In Table 24.1, an overview of the most important systems, as well as their particular strength and weaknesses in the expression of transmembrane transport proteins is provided. 

With the widespread availability of cell culture facilities, the reduced costs of media and reagents and above all, the commercialisation of a variety of transfection and expression kits, mammalian cells have now become probably the standard for functional studies of transmembrane transporters. Unsurpassed predictivity of the mammalian models may outweigh the higher costs and the lengthiness of the process, compared with bacterial cultures or Xenopus oocytes. Nevertheless, structural studies may require larger amounts than those easily produced in mammalian cells and the appeal of insect cell cultures for... 

The site of synthesis of numerous proteins is remote from their site of function. During transfer from one site to the other, proteins must, therefore, cross cellular membranes [43] [44], Proteins are usually synthesized as precursors containing an amino terminal extension, called the signal (leader) peptide, the sequence of which contains the necessary information to guide the protein to and across a specific membrane. After transmembrane transport (called translocation), the signal peptide is cleaved off by specific signal peptidases, which are found in the rough endoplasmic reticulum, and the... 

For designing effective topical drug therapy it is important to understand the principles of transmembrane transport and the unique structure of the stratum corneum. Some practical factors ... 

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