Final answer:
Cell counts in primary squares of a counting chamber usually involve counting live cells. A minimum of five squares is commonly counted for statistical reliability. The purpose of a calibration curve in turbidity measurements is to relate cell concentration to turbidity levels.
Step-by-step explanation:
When performing cell counts in primary squares of a hemocytometer or a similar counting chamber, the cells that are typically counted are live cells to assess cell viability, or sometimes total cell count which includes both live and dead cells. The minimum number of squares that must be counted typically varies based on the chamber design and the concentration of cells, but a common practice is to count enough squares to ensure statistical reliability, often at least 5 squares. Direct counts, such as those performed using a hemocytometer, provide an estimate of the total number of cells, live or otherwise, in a sample.
Counts of live cells are particularly important for applications such as determining the extent of an infection, evaluating the effectiveness of antimicrobial compounds and medication, or assessing contamination in food and water. When estimating cell count from turbidity measurements, the purpose of a calibration curve is to establish a direct relationship between a known cell concentration and the level of turbidity caused by those cells. Newer indirect methods of counting live cells include flow cytometry and fluorescent staining, which are used to distinguish live from dead cells.