The aim of the experiment is to determine the relationship between Cell Number and Optical Density.


This experiment examines the number of cells per unit volume of a sample of yeast culture. A spectrophotometer, a device for measuring light intensity, is being used to determine the Optical Density, OD, of the cell. The optical density is associated with the absorbance of the cell culture. A microscope and a counting chamber, hemocytometer, are being used to obtain the number of cells per unit volume of the yeast culture. Spectrophotometer works on the principles of measuring light absorption of the sample tested. Due to the large quantity of cells present in a concentrated sample of the yeast culture, dilution is done to minimize the time spent and also counting errors. After the construction and calibration of a standard curve, the results show the direct relationship between the Optical Density and Cell Number.


In this experiment, yeast culture in YPUAD and fresh medium YPUAD are used to determine the quantity of yeast cells in a given sample. Under the light microscope, the yeast cells can be seen as ‘grey dots’. The yeast samples are diluted to an extent that the cells are widely and evenly distributed. This is to obtain a more accurate result and also reduce the time spent as counting a large number of cells can be quite time consuming.

A spectrophotometer is a photometer (a device for measuring light intensity) that can measure intensity as a function of the color (or more specifically the wavelength) of light. Important features of spectrophotometers are spectral bandwidth and linear range of absorption measurement.   This is a widely used method to obtain the cell density of any cell sample.

The given yeast sample is placed in a quartz cuvette and into a spectrophotometer. When light shines through, the Optical Density (OD) of the given sample will be determined by the absorbance of light that passed through...