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QUIZ ERT 317 BIOCHEMICAL ENGINEERING 1) Describe microbial growth and what are the results of growth to microorganisms or cells? [10 Marks] GROWTH - is the most essential response of microbes to their physiochemical environment Result – replication and change in cell size 2) Demonstrate or illustrate the microbial growth phases by a graph or diagram. [10 Marks] 3) Explain each phases of microbial growth from the diagram. [25 Marks] Lag phase (adaptation)– just after inoculation, they take some time to adjust to the new environment. Consequently, they start consuming the nutrients in the medium and multiply depending on composition of nutrient, new enzymes are synthesized, some other enzymes are repressed & the internal machinery of cells is adapted to the new environmental conditions these changes reflect the intracellular mechanisms for regulation of metabolic processes discussed in chapter 4 cell mass increase a little without an increase in cell number density Pseudolag phase- when the inoculum is small and has a low fraction of viable cells (poor condition of inoculum) Besides that low concentration of some nutrient and growth factors and the age of the inoculum culture may also cause a long lag phase. (the lag period increases with the age of inoculum.) Inoculum should be young and active. Inoculum size=5-10% by volume Diauxic growth – multiple lag phases is observed when the medium contains more than 1 carbon sources. This caused by a shift in metabolic pathways in the middle of a growth cycle. Log phase (logarithmic growth phase) – Cells have adjusted to their new environment once the growth process starts, the cell multiply in an exponential order (rapid). The growth is very fast. This is also known as exponential growth phase. This phase is very important and all the kinetic growth parameters are studied in this phase. Since the growth is exponential in nature, when the cell concentration is plotted versus time on a semi-log plot, the log phase results in straight line. This is a period of balanced growth – All component of cell grow at the same rate. The average composition of a single cell remain approximately constant during this phase. Deceleration growth phase: growth decelerates due to either depletion of one or more essential nutrients or the accumulation of toxic by-products of growth. These changes occurred over a very short period of time. The rapidly changing environment results in unbalanced growth During unbalanced growth, cell composition and size will change td will not be equal. In this phase, the stresses induced by nutrient depletion or waste accumulation cause a restructuring of the cell to increase the prospects of cellular survival in a hostile environment. Stationary phase – Once the cell numbers reach a maximum (point C), they stop growing any more (net growth rate is zero). Hence, the growth become stagnant and the growth continues to point D. It is probably in this period, that they start converting the substrate into metabolic products. More and more amount of substrate goes for biochemical reaction to yield some useful products. Even though the net growth rate is zero during stationary phase, cells are still metabolically active and produce secondary metabolites. Primary metabolites are growth related products Secondary metabolites are non-growth related. These phenomena may take place during stationary phase: 1. Total cell mass concentration may stay constant, but the number of viable cells may decrease 2. Cell lysis may occur and viable cell mass may drop. Cells may not be growing but may have active metabolism to produce secondary metabolites. • • • • • Stationary phase The cells catabolizes cellular reserves for new building blocks & for energy-producing monomers – endogenous metabolism The cell must always expend energy to maintain an energized membrane and transport of nutrients and for essential metabolic functions such as motility and repair of damage to cellular structures. This energy maintenance is called maintenance energy. The reason for termination of growth may be either exhaustion of an essential nutrient or accumulation of toxic products. If inhibitory product is produced and accumulates in the medium, the growth rate will slow down, depending on the inhibitor production, and at a certain level of inhibitor concentration, growth will stop. Death phase – the cells start decaying. After point D, probably cells face shortage of substrate for their consumption. Hence, they start decaying. The death phase (or decline phase) follows the stationary phase. Some cell death may start during the stationary phase. Often, dead cell lyse, and intercellular nutrients release into medium are used by the living organism during stationary phase. At the end of the stationary phase (because of nutrient depletion or toxic product accumulation, the death phase begins. The reestablisment of the culture may be possible in the early death phase if cells are transferred into a nutrient-rich medium. 4) List the three major categories of microbial products and give examples. [10 Marks]