Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
inner lines mark
edge of the
small square
small
square 1 2
3
count in
4 5
count in
count out
count out
Improved Neubauer counting chamber rulings
Figure P1.8 Counting cells in the haemocytometer.
252
The depth of
blood is 0.1 mm.
A small square is
0.05 by 0.05 mm.
The volume of blood
over one small square
is 0.00025 mm 3 .
Figure P1.9 Calculating the number of cells in a unit volume of the sample.
Now you can calculate the number of cells in a unit
volume of your sample. Look at Figures P1.8 and P1.9. Each
small square has sides of 0.05 mm. The depth of the sample
is 0.1 mm.
Therefore the volume of liquid over one small square is
0.05 × 0.05 × 0.1 = 0.00025 mm 3 .
You counted cells in 5 × 16 = 80 of these small squares.
This was a total volume of 80 × 0.00025 = 0.02 mm 3 .
Let’s say you counted N cells. If there are N cells in
0.02 mm 3 , then there are N × 10 6 ÷ 0.02 cells in 1 dm 3 .