Cell membranes facilitated transport

Fatty acids enter cells both by a saturable transport process and by diffusion through the lipid plasma membrane. A fatty acid binding protein in the plasma membrane facilitates transport. An additional fatty acid binding protein binds the fatty acid intracellularly and may facilitate its transport to the mitochondrion. The free fatty acid concentration in cells is, therefore, extremely low. 

Damage to the blood-brain barrier (BBB) as a result of trauma or disease permits hormones (molecules), such as epinephrine, or toxic substances to reach the brain. Normally, the BBB keeps the contents within blood vessels from reaching the brain. Closely packed endothelial cells line blood vessels. The endothelial cell membrane has transport systems that facilitate the movement of desired molecules and nutrients into the brain, while keeping undesirable molecules out. Damage to the BBB from trauma may be involved in posttraumatic stress disorder. 

A permease (carrier [16]) is assumed to have the characteristics of an enzyme and thus exhibits stereospecificity for the transported solute. It can traverse the cell membrane, thus transporting the solute from the outside to the inside of the cell, and both influx and efflux of solute occur. The transport will usually exhibit saturation (Michaelis-Menten) kinetics. Enzymes involved in facilitated diffusion or active transport (i.e. permeases and molecules responsible for the intracellular modification of the transported solute) may be either constitutive or inducible. This means that the system may be present at all times (constitutive) or developed by the cell only in the presence of the transported solute (inducible). Clearly for induction to occur some of the solute must enter the cell either by way of a small (constitutive) amount of the transport system or by another means (e.g. diffusion). When the inducer concentration falls to zero or below a critical level, the transport system fails to operate thus a degree of control on the entry of the inducer is exercised. The presence of glucose in the medium may prevent the synthesis of the transport system (catabolite repression) thereby enabling glucose to be... 

Steroid and thyroid hormones are minimally soluble in the blood. Binding to plasma proteins renders them water soluble and facilitates their transport. Protein binding also prolongs the circulating half-life of these hormones. Because they are lipid soluble, they cross cell membranes easily. As the blood flows through the kidney, these hormones would enter cells or be... 

Figure 4.4 Comparison of oxidase-dependent iron transport in mammals and yeast. In mammals, the plasma glycoprotein cerulpolasmin mediates iron oxidation, facilitating iron export from the cells and delivery to other tissues throughout the body. In yeast, Fet3p, an integral membrane protein mediates iron oxidation, resulting in plasma membrane iron transport through the permease Ftrlp. Reprinted from Askwith and Kaplan, 1998. Copyright (1998), with permission from Elsevier Science.

Adenosine and inosine can be transported across cell membranes in either direction, facilitated by a membrane-associated nucleoside transport protein. Concentrative transporters have also been identified. Messenger RNA for a pyrimidine-selective Na+-nucleoside cotransporter (rCNTl) and a purine-selective Na+-nucleoside cotransporter (rCNT2) are found throughout the rat brain. Most degradation of adenosine is intracellular, as evidenced by the fact that inhibitors of adenosine transport, such as dipyridamole, increase interstitial levels of adenosine. Dipyridamole is used clinically to elevate adenosine in coronary arteries and produce coronary vasodilation. In high doses, dipyridamole can accentuate adenosine-receptor-mediated actions in the CNS, resulting in sedation and sleep, anticonvulsant effects, decreased locomotor activity and decreased neuronal activity. 

Most of the naturally occurring chelating agents are substituted hydroxamates which are produced by a variety of protista so that iron(III) subsequently becomes available for biochemical processes. Neilands (73) has suggested that the hydroxamates facilitate the transport of iron across cell membranes. The distribution of hydroxamates in the biosphere appears limited. However, if there was a wider distribution of hydroxamates in the environment then the management of actinide wastes could become a problem of horrifying dimensions if these chelators facilitated the transport of actinides across cell membranes. 

The apparent failure of trivalent and tetravalent cations to enter plants could result from the interaction of the cations with the phospholipids of the cell membranes. Evidence for such interactions is provided by the use of lanthanum nitrate as a stain for cell membranes (143) while thorium (IV) has been shown to form stable complexes with phospholipid micelles (144). However, it is possible that some plant species may possess ionophores specific to trivalent cations. Thomas (145) has shown that trees such as mockernut hickory can accumulate lanthanides. The proof of the existence of such ionophores in these trees may facilitate the development of safeguards to ensure that the actinides are not readily transported from soil to plants. 

Ambient concentrations of COj are very low and usually biolimiting. Hence phytoplankton generally rely on bicarbonate as their carbon source. Phytoplankton must convert this bicarbonate to COj to enable production of organic matter. This conversion is facilitated by the Zn-containing enzyme, carbonic anhydrase (Table 11.4). Some phytoplankton release carbonic anhydrase into seawater with the resulting COj then transported across their cell membrane. 

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