Tok Essay

Only available on StudyMode
  • Topic: Microscope slide, Dilution, Volume
  • Pages : 8 (2443 words )
  • Download(s) : 21
  • Published : January 25, 2013
Open Document
Text Preview
Title: Counting the Number of Yeast Cells in a Suspension using Haemocytometer Objective: To estimate the number of cells of yeast per mm3 in five different dilutions of yeast suspension. Introduction: Biologists often need to count the density of cells in a liquid. “Density of cells” means “the number of cells per unit volume of liquid”. For example, they might want to find out the density of red blood cells in blood plasma, the density of bacteria in milk, or the population of Paramecium sp. (a protozoan) in water from a pond. The simplest, most convenient and cheapest means of accurately determining the number of cells in a sample is to use a haemocytometer and a microscope. A haemocytometer is a specialised slide that has a counting chamber with a known volume of liquid. The haemocytometer is a device originally designed for the counting of blood cells. It is now also used to count other types of cells as well as other microscopic particles. The haemocytometer was invented by Louis-Charles Malassez and consists of a thick glass microscope slide with a rectangular indentation that creates a chamber. This chamber is engraved with a laser-etched grid of perpendicular lines. The device is carefully crafted so that the area bounded by the lines is known, and the depth of the chamber is also known. It is therefore possible to count the number of cells or particles in a specific volume of fluid, and thereby calculate the concentration of cells in the fluid overall. The haemocytometer consists of a heavy glass slide with two counting chambers, which is 0.1mm deep, each of which is divided into nine large 1mm squares, on an etched and silvered surface separated by a trough. A cover slip sits on top of the raised supports of the ‘H’ shaped troughs enclosing both chambers. There is a ‘V’ notch at either end where the cell suspension is loaded into the haemocytometer. When loaded with the cell suspension it contains a defined volume of liquid. The engraved grid on the surface of the counting chamber ensures that the number of particles in a defined volume of liquid is counted. The haemocytometer is placed on a microscope stage and the cell suspension is counted. When a liquid sample containing immobilized cells is placed on the chamber, it is covered with a cover glass, and capillary action completely fills the chamber with the sample. Looking at the chamber through a microscope, the number of cells in the chamber can be determined by counting. Different kinds of cells can be counted separately as long as they are visually distinguishable. The number of cells in the chamber is used to calculate the concentration or density of the cells in the mixture from which the sample was taken: it is the number of cells in the chamber divided by the chamber's volume (the chamber's volume is known from the start), taking account of any dilutions and counting shortcuts. Research Question: How do the different dilutions of yeast suspension affect the density of yeast cells? Hypothesis: As the yeast suspension gets more diluted, the density of yeast cells will decrease. This is because as more distilled water is added to the yeast suspension, the number of yeast cells per mm3 of the yeast suspension decreases. The yeast cells are now more widely distributed because the volume of liquid (water) has increased. In diluted suspension, the number of yeast cells per unit volume ( in this case per mm3) of water is less than in a more concentrated solution, where the density of yeast cells is higher, due to the smaller ratio of yeast suspension to water. In a concentrated solution, the yeast cells are more tightly clustered together, because there is less water for the yeast cells to be distributed in, and the ratio of yeast suspension to water is greater. So for every mm3 of water, there are more yeast cell in a concentrated solution. Variables:

VariablesUnitRangePossible effect on resultsWays of manipulating/ measuring/
controlling variable...
tracking img