Reserve materials, mobilization

For PHA-producing microbial cells, PHAs serve as reserve materials for carbon and energy. Under conditions of starvation, these reserve materials can be mobilized, thus providing the cell with an advantage for survival. Figure 1 shows electron-microscopic pictures of C. necator DSM 545 harbouring different intracellular amounts of PHA at different magnifications. 

Isolated axes of peas are not dependent on the reserve material of the cotyledons during the initial stages of radicle elongation [17]. Reserves of carbohydrate, protein and fat in the radicle itself must be sufficient for these early events, and sucrose, raffmose and stachyose probably serve as sources of respirable substrate. But after these early events have passed, the further development of the root and shoot systems depends upon the contributions from the cotyledons. Their stored carbohydrate (and the other reserves) are hydrolysed and transported into the axis. It is not yet clear if the initiation of mobilization is actually controlled by the embryonic axis this is discussed fully in Chapter 7. We can say, however, that the subcellular changes in the cotyledons that precede and accompany reserve mobilization do need the presence of axis, at least over the first 48 h after the start of imbibition. 

We will mention one more piece of evidence for the existence of mRNA, which will enable us, at the same time, to become acquainted with a new technique. This is the evidence for mRNA in the cotyledons of the ground nut (Arachis hypogaea). During germination of such cotyledons a considerable increase in the activity of a whole series of enzymes which are necessary for the mobilization of reserve materials, occurs. It seemed likely that this increase in activity was due, at least in a large part, to the de novo synthQSis of enzymes. However, such de novo synthesis is impossible without mRNA. The technique with which we are concerned here is the hybridization of nucleic adds. If a DNA double helix is slowly heated, a sharp increase in the extinction of 260 m t over a par-... 

The mobilization of the reserve materials in the cotyledons of the ground nut lead us to a new problem. Over a certain period of time, in this case several days, enzymes are needed, which always have the same function, namely, to mobilize certain reserve materials. It is known from bacteria that the mRNA is very short-lived. A given mRNA molecule can be degraded only a few seconds after it has been synthesized. As we shall learn later, this short life span of bacterial rhRNA enables bacteria to adjust rapidly to changing external conditions. Such a rapid adjustment, for... 

As already mentioned, starch is the most important reserve carbohydrate of plants. Such reserve materials must be mobilized when they are needed. This, in turn, requires that they be broken down into smaller units which can be transported to the sites where they are needed. Starch is degraded by means of phosphorylases and hydrolases. 

This supposition has been confirmed. In the mobilization of the reserve materials of barley the following events take place the embryo discharges gibberellic acid into the outermost layer of the endosperm, the aleuron. As a result genes for the synthesis of various hydrolytic enzymes are activated in the aleuron. The process culminates in the synthesis of... 

At germination, reserve materials localized in the storage tissues (cotyledons, endosperm, and, less often, the nucellus) must be mobilized. They are the only sources of organic substances available to the seedling until it has developed its own photosynthetic apparatus. As far as this... 

The major role of PHAs is of a beneficial reserve material, the ability to store large quantities of reduced carbon without significant effects on the osmotic pressure of the cell. As discussed earlier, the proteins attached to PHA granules include PHA polymerase/synthase, PHA depolymerase, structural proteins known as phasins and some other proteins with unknown functions. PHA depolymerases present inside the cells are known as intracellular PHA depolymerases (i-PHA depolymerases). i-PHA depolymerase is an enzyme responsible for degradation/mobilization/hydrolysis of PHA for the survival of the cell in absence of an exogenous carbon source.There are many studies on intracellular mobilization of PHB in PHB-accumulating organisms. 

The concept of a characteristic reaction temperature must, therefore, be accepted with considerable reservation and as being of doubtful value since the reactivity of a crystalline material cannot readily be related to other properties of the solid. Such behaviour may at best point towards the possible occurrence of common controlling factors in the reaction, perhaps related to the onset of mobility, e.g. melting of one component or eutectic formation, onset of surface migration or commencement of bulk migration in a barrier phase. These possibilities should be investigated in detail before a mechanism can be formulated for any particular chemical change. 

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Due to the expected demand in future applications, i.e., batteries for electric mobility, long-term bottlenecks in raw materials availability cannot be ruled out. This possibly applies for strategically important metals such as rare earth elements or others, as the regional distribution of the reserves and resources is very diverse [33]. 

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