Metabolism need for

Contrast the situation in an adult. Little growth takes place, but the metabolism must vary with time and physiological state. The body must make drastic readjustments from normal feeding to a starvation situation and from resting to heavy exercise. The metabolism needed for rapid exertion is different from that needed for sustained work. A fatty diet requires different metabolism than a high-carbohydrate diet. The necessary control mechanisms must be rapid and sensitive. 

The correct answer = A. Resistant cells may have elevated levels of dihydrofolate reductase. Methotrexate inhibits dihydrofoiate reductase and, therefore, leads to lower than normal levels of the reduced tetrahydrofolate derivatives. Methotrexate does undergo metabolism by the host, but this is not a factor in resistance. The metabolic need for folate is high in all rapidly dividing cells. A decrease in influx may be associated with resistance. 

Based on a lengthy metabolic study with men, Sandstead et al. (8 ) have reported that the dietary requirement for zinc increased as the level of protein Intake increased. All of these investigations suggest that a higher amount of protein in the diet results in poorer utilization and/or increased metabolic need for zinc. 

Animals store energy in the form of triacylglycerols, kept in a layer of fat cells below the surface of the skin. This fat serves to insulate the organism, as well as provide energy for its metabolic needs for long periods. The first step in the metabolism of a triacylglycerol is hydrolysis of the ester bonds to form glycerol and three fatty acids. This reaction is simply ester hydrolysis. In cells, this reaction is carried out with enzymes called lipases. 

MEMBRANE TRANSPORT Membrane transport mechanisms are vital to living organisms. Ions and molecules constantly move across cell plasma membranes and across the membranes of organelles. This flux must be carefully regulated to meet each cell s metabolic needs. For example, a cell s plasma membrane regulates the entrance of nutrient molecules and the exit of waste products. Additionally, it regulates intracellular ion concentrations. Because lipid bilayers are generally impenetrable to ions and polar substances, specific transport components must be inserted into cellular membranes. Several examples of these structures, referred to as transport proteins or permeases, are discussed. 

As is also shown in Figure 6.1, an interleukin-1-mediated [29] acute-phase response to many diseases involves a release of copper-thioneine-stored copper from the liver as ceruloplasmin, copper amino-acid complexes, and a copper albumin complex to meet increased metabolic needs for copper, which exceed normal needs, and plasma copper concentrations increase 200-300% above normal, as illustrated in Figure 6.3. The appropriate increase coupled... 

Animals have a subcutaneous layer of fat cells that serves as both an energy source and an insulator. The fat content of the average man is about 21%, whereas the fat content of the average woman is about 25%. Humans can store sufficient fat to provide for the body s metabolic needs for two to three months, but can store only enough carbohydrate to provide for its metabolic needs for less than 24 hours. Carbohydrates, therefore, are used primarily as a quick, short-term energy source. 

If substances spare the B vitamins by increasing vitamin synthesis, or by reducing metabolic requirements, they should act equally well when given parenterally. Ekman and Strcimbeck (1949) reported that rats survived a very short while longer on riboflavin-deficient diets when they were injected with small doses of ascorbic acid, though there was no difference in the appearance of the signs of deficiency. If these results are confirmed, they would imply that ascorbic acid can indeed reduce the metabolic need for riboflavin, but only to a very limited extent. 

Water soluble vitamins are generally not stored in the body, or are stored only for a limited time and the excess is excreted in the urine. Lipophilic vitamins are stored mainly in the Hver. The reserve capacity, defined as the time during which the need for the vitamin is covered by the organism reserves, is the longest for corrinoids (3-5 years) and vitamin A (1-2 years). The reserve capacity for folacin is 3-4 months, for vitamins C, D, E and K, riboflavin, pyridoxine and niacin it is 2-6 weeks, and for thiamine, pantothenic acid and biotin it is only 4-10 days. Reserve capacity is affected by the history of vitamin intake, the metabolic need for the vitamin and the health status of the individual. 

Thus, the metabolic needs for choline can be supplied in either of two ways by dietary choline as such, or by choline synthesis in the body which makes use of labile methyl groups. But the synthesis in the body cannot take place fast enough to meet the choline needs for rapid growth hence, the symptoms of deficiency may result. 

Accurate, precise isotope ratio measurements are used in a variety of applications including dating of artifacts or rocks, studies on drug metabolism, and investigations of environmental issues. Special mass spectrometers are needed for such accuracy and precision. 

Mineral oil and paraffins should not be used, because these are not metabolized and may irritate tissue. Various other additives are needed for stabiUty, stefihty, and isotonicity antimicrobial preservatives, antioxidants (qv), chelating agents (qv), and buffers. No parenteral container material is completely inert to parenteral solvent systems. 

Microorganisms exhibit nutritional preferences. The enzymes for common substrates such as glucose are usually constitutive, as are the enzymes for common or essential metabohc pathways. Furthermore, the synthesis of enzymes for attack on less common substrates such as lactose is repressed by the presence of appreciable amounts of common substrates or metabolites. This is logical for cells to consei ve their resources for enzyme synthesis as long as their usual substrates are readily available. If presented with mixed substrates, those that are in the main metabolic pathways are consumed first, while the other substrates are consumed later after the common substrates are depleted. This results in diauxic behavior. A diauxic growth cui ve exhibits an intermediate growth plateau while the enzymes needed for the uncommon substrates are synthesized (see Fig. 24-2). There may also be preferences for the less common substrates such that a mixture shows a sequence of each being exhausted before the start of metabolism of the next. 

Aquatic organisms, such as fish and invertebrates, can excrete compounds via passive diffusion across membranes into the surrounding medium and so have a much reduced need for specialised pathways for steroid excretion. It may be that this lack of selective pressure, together with prey-predator co-evolution, has resulted in restricted biotransformation ability within these animals and their associated predators. The resultant limitations in metabolic and excretory competence makes it more likely that they will bioacciimiilate EDs, and hence they may be at greater risk of adverse effects following exposure to such chemicals. 

Neurons have three parts the cell body and dendrites, the axon, and axon terminals. The cell body contains the nucleus and the organelles needed for metabolism, growth, and repair. The dendrites are branched extensions of the cell body membrane. The axon is a long, thin structure which transfers electrical impulses down to the terminals. The axon divides into numerous axon terminals and it is in this specialized region that neurotransmitters are released to transmit information from one neuron to its neighbors. The synapse has been defined as the space between two subsequent interrelated neurons. ... 

Figure 1.20). All of these reactions, many of which are at apparent crosspurposes in the cell, must be fine-tuned and integrated so that metabolism and life proceed harmoniously. The need for metabolic regulation is obvious. This metabolic regulation is achieved through controls on enzyme activity so that the rates of cellular reactions are appropriate to cellular requirements. 

To this point, the pathway has generated a pool of pentose phosphates. The AG° for each of the last two reactions is small, and the three pentose-5-phosphates coexist at equilibrium. The pathway has also produced two molecules of N/ DPH for each glucose-6-P converted to pentose-5-phosphate. The next three steps rearrange the five-carbon skeletons of the pentoses to produce three-, four-, six-, and seven-carbon units, which can be used for various metabolic purposes. Why should the cell do this Very often, the cellular need for... 

The natural polymers known as proteins make up about 15% by mass of our bodies. They serve many functions. Fibrous proteins are the main components of hair, muscle, and skin. Other proteins found in body fluids transport oxygen, fats, and other substances needed for metabolism. Still others, such as insulin and vasopressin, are hormones. Enzymes, which catalyze reactions in the body, are chiefly protein. 

Usually fairly high concentrations of such a drug are needed for effective control of an infection because the inhibitor (the false substrate) should occupy as many active centers as possible, and also because the natural substrate will probably have a greater affinity for the enzyme. Thus the equilibrium must be influenced and, by using a high concentration of the false substrate, the false substrate-enzyme complex can be made to predominate. The bacteria, deprived of a normal metabolic process, cannot grow and multiply. Now the body s defense mechanisms can take over and destroy them. 

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