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lecture_16.__growth

lecture_16.__growth - 1 Lecture 16 Growth and nutrition...

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1 Lecture 16 Growth and nutrition Chapter 2 GROWTH AND NUTRITION OF CELLS As long as cells have all the nutrients they require, growth can continue in an uninterrupted manner. Under these conditions growth is UNRESTRICTED. MEASUREMENTS OF 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. Concept of VIABLE cells. 2. Plate assay, plate count. 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. Optical means. Estimate cell number by light scattering, in other word measure the turbidity of the cultureT urbidimetrically. This technique is based on the following: If parallel beams of light are passed through a suspension of bacteria, the light will hit some of the bacteria and be scattered, thus diminishing the amount of light passing through the culture. Because bacteria are virtually colorless they absorb very little of the light. Most of the loss in the transmission of light is due to light scattering. We therefore measure the fraction of incident light that is transmitted The amount of light scattered is proportional to the ratio of particle size to wavelength of incident light. The shorter the wavelength the more accurate the measurement. This is important for physiological measurement. We have a choice we can either measure the amount of scattered light, in which case the photodetector is placed at an angle from the incident light, usually at 90 ˚ angle. OR We can measure the amount of light that is transmitted, in which case we put the detector at 180 ˚ fro the source. The most common laboratory means is to measure light transmission in a devise known as a colorimeter or spectrophotometer . These two devices measure absorbance (A) , which is the log of the ratio of the incident
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