Sugars, transport through membranes

The various forms of the sugars must also be considered in such processes as transport through membranes. If one form is exclusively (or even preferentially) transferred, the concentrations on each side of the membrane will not be expressed by the total concentrations (as usually measured), especially if the rates of mutarotation are different on both sides of the membrane. Keston has explained active transport of sugars across the membranes of the kidney and intestine on this basis, but assumed that an active form (the aldehydo form) is present in only a small proportion.305 320 The concept seems theoretically sound under non-equilibrium, steady-state conditions, but it cannot be applied to the reversal of the concentration gradient of a particular anomer. 

As might be expected, many membrane activities reflect the physical state of the membrane bilayer. An example of this is seen in Figure 4. Here the kinetics of protein mediated sugar transport through a lipid bilayer is seen to follow the melting of the bilayer. For these studies the human red blood cell hexose transport protein was removed from its native membrane and reconstituted into artificial membranes with a predetermined thermal profile. Sugar transport was monitored by a turbidometric method to obtain the parameters (maximum rate... 

In conclusion, some of the most important observations made on insulin s mechanism of action in muscle can be summarized as follows the intracellular concentration of glucose is increased after insulin administration insulin stimulates the transport of metabolizable and nonmetabolizable sugars with an identical configuration in carbons 1, 2, and 3 and different sugars compete for the site of action of insulin. On the basis of such observations, it has been suggested that insulin promotes sugar transport through the muscle cell membrane by activation of a specific transfer system rather than by an increase in membrane permeability. 

Sugar Transport Through Lipid Bilayer Membranes. 

Special carrier molecules exist for certain substances that are important for cell function and too large or too insoluble in lipid to diffuse passively through membranes, eg, peptides, amino acids, glucose. These carriers bring about movement by active transport or facilitated diffusion and, unlike passive diffusion, are saturable and inhibitable. Because many drugs are or resemble such naturally occurring peptides, amino acids, or sugars, they can use these carriers to cross membranes. 

Boron s role in plant nutrition is an essential one, but the specific role that it plays is still in question. The most persistent hypothesis is that B facilitates the transport of sugars through membranes (Gauch and Dugger, 1954). 

A rather widespread family of proteins, found in the periplasmic space of gramnegative bacteria, complexes certain small molecules and allows them to be transported through the cell wall or activate chemotaxis. Each of these functions involves a consecutive interaction with specific membrane proteins. The molecules transported are amino acids, sulfate, mono- and oligosaccchrides. In this way ABP complexes L-arabinose (K 0.98 x 10 M), and MBP (maltodextrin-binding protein) complexes maltose (ATj 35 x 10 M) and maltodextrins. It is in this series that are found the strongest possible bonds between sugars and proteins. The dissociation rate ( i 1.5 s ) is indicative of the upper limit of the ionic transport rate. Hydrogen bonds... 

Hydrolysis The organic polymers (carbohydrates, proteins, and fats) are hydrolyzed to their respective monomers (sugars, amino acids, lipids) by extracellular enzymes to facUitate the nutrient transport through the cell membrane. In order to improve the biomass hydrolysis, the biomass is generally pretreated. Pretreatments make the complex substrate matrix more amenable to microorganisms and enzymatic attacks. 

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