# Biology Exams 4 U

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### If a bacterium divides every 20 minutes-Formula

1. If a bacterial cell doubles every 20 minutes. How many bacteria will be formed in 2 hours?

a) 4
b) 8
c) 16
d) 64

2. If you start out with a population density of 300 CFU/ml of a bacterium that divides every 30 minutes, what will the population density be at the end of three hours, assuming the cells are in the log phase of growth?
a) 26 CFU/ml
b) 3006 CFU/ml
c) 19200 CFU/ml
d) 9000 CFU/ml

3. A bacterial cell divides once in every minute and takes one hour to fill a cup. How much time will it take to fill half the cup?
a) 30 min
b) 29 min
c) 59 min
d) 60 min

First find out the number of generation, let it be =n (no. of generation)
n=total time of division / time taken for one division
i.e. 2 hours=120 min/20 min=6
You can answer any question like this using the given formula
f=ix2n where    f=final number of bacteria
i=initial number of bacteria
n=6
f=1X26=64

f=ix2n
n=180/30=6
F=300x2=19200 CFU/ml
Q3 Answer: 59 min as the growth is exponential.
Suppose 10000 bacteria is required to fill the cup and it takes 1 hour (60 min) to form 10000 bacteria. Then half the cup is 5000 bacteria and that is just before the final division, which is it takes 59 min to fill half the cup and 60 min to fill the cup as the growth is exponential.

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