lecture_16__growth

lecture_16__growth - ModelingGrowth Reading:Chapter2...

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Making Two Cells From One Modeling Growth Reading: Chapter 2
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Unrestricted Growth As long as cells have all the nutrients they require, growth can continue in an uninterrupted manner. Under these conditions growth is UNRESTRICTED. However, Cells will eventually use up a limiting nutrient
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Measurements of cell growth 1. Count cells directly: using microscope and grid of a known size and volume you can count cells and get a number of cells/unit volume. Pro you see all cells, it is a real number. Con you see all cells, dead cells as well as alive cells which in some instances could be a large fraction of the cells. 2. Plate assay, plate count. Concept of VIABLE cells. Dilute the cells plate them on a medium where all viable cells will live and count the colonies. Pro get a real number of viable cells, Con takes a long time, what if your cells have doubling time of 3 days!!! Also, variability in sampling, if cells stick together, etc. Finally sampling error, which is rather significant under low dilutions. 3. Determine the amount of some constituent of the organism , such as amount of protein or dry Weight. This means that you collect cells, heat them to get rid of the water and then weigh them. Pro once obtained you can relate the dry weight to cell number by keeping conditions the same. Con time consuming. 4. Indirect means by Optical density. Estimate cell number by light scattering, in other word measure the turbidity of the culture . Pro easy and fast, very reliable. Con Indirect, measures both alive and dead cells. Cells must be evenly dispersed in solution. So, which approach would you use?
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Growth: 1 cell to 2, to 4, to 8, 8 to 16, 16 to ………. In unrestricted growth cells multiply exponentially, 2 n where n is number of generations. 2 1, 2 2 , 2 3 , 2 4 , ……. Exponential Phase Once a limiting nutrient runs out, cell growth becomes unbalanced and there is no net growth. Stationary Phase
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Cells can continue in Stationary Phase for quite some time. Eventually, cell death exceeds cell growth Death Phase
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Lets look at growth graphically If you take a fresh colony from a plate and put it into liquid medium, and incubate it under favorable growth conditions for that bacterium, and you take measurements of the growth of the culture you plot the numbers on log scale you observe the following pattern: 1. First nothing appears to happen for a while: Lag phase 2. Cell mass increases exponentially: exponential growth or balanced growth