Lag phase
For a while, there is little or no change in the number of cells because they do not immediately reproduce in a new medium. This period of little of no cell division is called the lag phase, and it can last for an hour or several days. During this time there is intense metabolic activity, in particular enzyme synthesis. The population is analogous to a factory newly equipped to produce automobiles: there is considerable initial activity, but no immediate increase in the automobile population. Near the end of lag phase, some cells may enlarge, in preparation for division.

Log phase
Eventually , the cells being to divide and enter a period of growth, or logarithmic increase, called log phase or exponential growth phase. Cellular reproduction is most active during this period, and their generation time reaches a constant minimum. There is apparently a characteristic minimum generation time–maximum rate of doubling–genetically determined for different cells. During log phase, cells are more sensitive to adverse conditions than they usually are. Radiation and many chemicals exert their effects by interfering with some important step in the growth process, and are therefore most harmful to cells during this phase.

Stationary phase
If exponential growth continues unchecked, some startling numbers of cells can arise. For example, a cell dividing every 20 minutes for two days can theoretically produce 2144 cells (that’s a 44-digit number!) But this does not happen. Eventually, growth slows down, and sooner or later the number of deaths balances the number of new cells, and the population stabilizes. This period of equilibrium is called the stationary phase. The reason for stationary phase is not always clear. The accumulation of toxic waste products and the exhaustion of certain nutrients are usually involved. In animal-cell culture, cell growth ceases when the cells have formed a layer across the container such as a Petri plate. Cessation of growth by proximity to other cells is called contact inhibition.

Death phase
In a culture, the number of deaths soon exceeds the number of new cells formed, and the population enters the death phase, or logarithmic decline phase.

Sample protocol

1. Obtain a small culture tube. Using a Pasteur pipette fill the tube about two-thirds full with culture media (membrane-viltered creek water). Run the media down the side of the tube being filled to avoid making air bubbles.

2. Using a dissecting microscope, catch an alga or protozoan and place it in the tube.

3. Examine the tube under dissecting microscope or low-power of a compound microscope to locate the alga. Record this initial count (of 1) at time 0.

4. Examine the culture at regular intervals over the next week and record the number of cells. Try to count the cells at least once each day (Monday through Friday).

5. Graph your data.

Typical growth curve