(h) (i) the standard growth curve of a microorganism in a closed culture

The idea

A closed culture (batch culture) contains a fixed volume of nutrient medium with no additions or removals, so nutrients fall and waste accumulates. The standard growth curve plots population size, often as log₁₀ of cell number, against time and has four phases. In the lag phase cells are metabolically active but not yet dividing, synthesising enzymes and ribosomes. In the exponential (log) phase, provided nutrients are plentiful and conditions suitable, the population doubles at a constant rate, giving a straight line on a logarithmic plot. Growth then slows in the stationary phase as nutrients become limiting and toxic waste builds up, so reproduction balances death. In the decline (death) phase, death exceeds reproduction and the population falls. Interpreting which phase a point lies in, and why the curve changes shape, is the core skill.

What earns the marks

• Identifies the lag phase as a period of metabolic activity and enzyme synthesis without an increase in cell number.
• Explains the exponential phase as rapid, constant-rate doubling while nutrients are plentiful and waste is low.
• States that a logarithmic (log₁₀) scale makes exponential growth appear as a straight line.
• Explains the stationary phase as reproduction balancing death when nutrients become limiting and waste accumulates.
• Describes the decline phase as death exceeding reproduction as conditions become increasingly unfavourable.
• Recognises that a closed culture receives no fresh nutrients and has no removal of waste products.

Common mistakes and how to put them right

• Thinking cells are inactive during the lag phase.

  Put it right: They are metabolically active, making enzymes and ribosomes, but not yet dividing.

• Assuming the population grows at a constant rate throughout.

  Put it right: Constant-rate doubling occurs only in the exponential phase.

• Believing the stationary phase means growth has stopped entirely.

  Put it right: Reproduction and death are balanced, so numbers stay roughly constant.

• Confusing a closed (batch) culture with a continuous culture that is continually supplied with fresh medium.

Exam tips

• Label all four phases on a sketch curve before answering questions about them.
• Link each phase to the underlying cause, such as nutrient supply or waste accumulation, rather than just naming it.
• Remember that a log scale turns exponential growth into a straight line, which is why the exponential phase looks linear.
• Check whether the question asks about cell number or log cell number before interpreting the graph.