Introduction:
The cells in a culture reproduce by splitting in half forming two new daughter cells in a process called fission. As the population of cells grows more cells will be available to divide. Provided there are no barriers to cell division such as limited nutrients or space the rate of growth of cells will increase in directly proportion to the number of cells present.
 

Procedure:
In this lab we will model cell growth using tacks to represent cells. Each lab pair should stat with about 80 tacks of the same type.

We will test two different rules for cell growth.

Growth Rule 1 -- Nutrient Rich
Take 20 tacks and place them in the petri dish -- these are the cells. Count out another 60 tacks and keep them at the side — these are the nutrient supply for the cells in the petri dish. Firmly shake  the dish in the horizontally direction then count the number of tacks which point up. These are the tacks which are ready to divide and will do so provided sufficient nutrients are available to them. If sufficient nutrients are available add one tack for each tack which is pointing up in the petri dish.  This process represents one growth cycle. Repeat this process until the nutrients have run out then stop. Record the total number of tacks in the petri dish as a function of the number of cycles. Due to the statistical variation that will occure repeat the entire experiment several times and use the average number of tacks for each cycle as your results.

Growth Rule 2 -- Nutrient Poor
Follow the same procdure as above except now there is a restricted food supply. The food supply starts with only 6 tacks. At the end of each cycle add 6 more tacks to the food supply. In a given cycle if more cells are ready to divide then there are nutirents in the food supply for them then only those cells that get nutrients divide. Those that cannot divide die and are removed from the dish and placed into the food supply. Repeat this process until the population reaches 75 tacks. Repeat the whole experiment several time and find the average number of tacks in each cycle.

Analysis
For each growth rule use Excel to find the rate of change in population per cycle for each cycle. Also find the percentage change per cycle in each cycle. Decide on this evidence whether a linear or an exponential growth is a better model for the growth rules. Due to statistical variation the evidence may not be conclusive.

Next plot average number of tacks vs number of cycles. Use the Excel trendline to fit an exponential or a linear fit to the graph. Which one gives the best fit. If the fit is an exponential one find the percentage change per cycle. If the fit is linear write down the growth rate per cycle.

The time it takes for the population to double is called the generation time g or the doubling time. Use the graph to estimate the generation time for each growth rule.

In your lab notebook write a short summary of your conclusions from this experiment. Refer to your results and remark on their significance. Can you suggest modifications to this experiment which might improve the reality of the model.