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6.1.3Unit 6: Practical Skills in Biology II

The Bacterial Growth Curve — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 6 parts Lesson 4 of 48 Unit quiz

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Section 1 of 6Binary Fission

6.1.3 The Bacterial Growth Curve
1
The single circular DNA molecule of the bacterium is replicated.
2
Any plasmids in the cell are also replicated.
3
The cell divides in two, with the cytoplasm shared roughly equally.
4
Each daughter cell receives one copy of the chromosome and a variable number of plasmids.
5
The two daughter cells are genetically identical, so the population doubles each time they all divide.

Section 2 of 6Phases of the Growth Curve

Lag phase
  • The population grows slowly while cells adjust to their new environment and begin to divide.
Exponential phase
  • With plenty of nutrients and space, the population doubles at regular intervals,
    • giving a steep curve; this is also called the log phase.
Stationary phase
  • Resources run low and waste accumulates, so the number of cells dividing equals the number dying and the curve levels off.
Death phase
  • Death rate exceeds the rate of reproduction because of nutrient shortage and toxic waste, so the population declines.

Section 3 of 6Logarithmic Scales

Logarithmic Scales
Why use them
  • Bacterial numbers can range from a few to millions, which is hard to fit onto an ordinary linear axis.
Log₁₀ scale
  • Each step on the axis represents a ten-fold increase, so the value 3 means 10³, which is 1000.
Recognising one
  • The intervals between numbers on the axis are not equal when the scale is logarithmic.
Exam skill
  • You need to interpret graphs with logarithmic axes and explain their advantage.

Section 4 of 6The Exponential Growth Equation

Equation
  • N = N₀ × 2^kt, where N is the number at time t, and N₀ is the number at time zero.
Rate constant
  • The constant k is the number of times the population doubles in one unit of time.
Finding k
  • k = (log₁₀N − log₁₀N₀) ÷ (log₁₀2 × t).
Use
  • Once k is known, the equation predicts the population at any time during exponential growth.

Section 5 of 6Worked Example

Question

  • A culture contains 1000 bacteria.
  • After 2 hours there are 32 000 bacteria.
  • Calculate k, and then the number of bacteria after 3 hours at the same rate.

  1. log₁₀(32 000) = 4.505 and log₁₀(1000) = 3, so the difference is 1.505.
  2. The denominator is log₁₀2 × t = 0.301 × 2 = 0.602.
  3. k = 1.505 ÷ 0.602 = 2.5 doublings per hour.
  4. After 3 hours: N = 1000 × 22.5 × 3) = 1000 × 27.5 ≈ 181 000.

Answer k = 2.5 per hour, and N ≈ 1.8 × 10⁵ bacteria after 3 hours.

Section 6 of 6Exam Checklist & Tips

6.1.3 – Key Points for the Exam

  • Binary fission Asexual division of a bacterial cell into two identical daughter cells.
  • Exponential phase The stage of growth in which the population doubles at regular intervals.
  • Stationary phase The stage in which growth and death are balanced and population size is constant.
  • Growth rate constant (k) The number of population doublings per unit time.

Common Mistakes

  • Saying that bacteria stop dividing in the stationary phase; some still divide, but the same number die.
  • Describing the lag phase as having no activity, when cells are making enzymes and growing.
  • Reading a log scale as a linear one, which makes equal gaps look like equal increases.
  • Using the natural logarithm instead of log₁₀ in the formula for k.

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