Growth Curves
A growth curve plots size or number of cells against time and typically produces an S-shaped (sigmoid) curve with a lag, exponential, stationary and death phase. Animal growth generally stops at maturity, while many plants keep growing throughout life.
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The four phases of a sigmoid growth curve
- Lag phase: growth is slow as the organism adjusts, and cells are still preparing to divide.
- Exponential (log) phase: growth is rapid as cells divide repeatedly under favourable conditions.
- Stationary phase: growth rate slows and levels off as resources become limited or growth-inhibiting factors increase.
- Death phase: the population or growth rate declines as conditions become unfavourable or resources run out.
Plant growth versus animal growth
Most animals show determinate growth: they grow rapidly while young and stop growing once they reach adult size, matching the stationary phase of the sigmoid curve. Many plants show indeterminate growth: because of continuously active meristems, they keep producing new organs and can keep growing in height or girth for as long as they live, so a single sigmoid curve better describes one growth period, such as one leaf or one growing season, rather than the whole lifespan.
Factors that affect the shape of the curve
The exact shape and length of each phase depends on factors such as nutrient supply, temperature, light and space. Limiting one of these factors shortens the exponential phase and brings on the stationary phase earlier, since resources become insufficient to support further growth sooner.
A population growing in a closed system, such as bacteria in a fixed volume of broth, is more likely to show a clear death phase than a population in a more open natural environment, where new resources may occasionally become available.
How it is examined
Common questions give a growth curve graph and ask you to label the phases, identify which phase corresponds to the fastest rate of growth, or explain why the curve levels off in the stationary phase. You may also be asked to compare the general shape of plant versus animal growth curves.
Worked exam-style question
Question. A student cultures a population of bacteria in a fixed volume of nutrient broth and counts the number of bacteria every hour for 10 hours. The population increases very slowly for the first hour, then increases rapidly between hour 1 and hour 6, before the increase slows and the population size remains roughly constant from hour 7 onwards.
(a) Name the phase of growth occurring between hour 1 and hour 6. (b) Explain why the population growth slows and becomes constant after hour 6.
(c) Predict and explain what would happen to the growth curve if fresh nutrient broth were added to the culture at hour 8.
Model answer. (a) This is the exponential (log) phase, when the bacteria are dividing repeatedly under favourable conditions. (b) The population enters the stationary phase because nutrients in the fixed volume of broth are becoming limited, and waste products may also be accumulating, so the rate of new cells being produced becomes roughly equal to the rate of cells dying.
(c) Adding fresh nutrient broth would restart rapid growth, since the limiting factor (lack of nutrients) has been removed; the curve would show the population rising again in a new exponential phase before eventually levelling off once the new supply also becomes limited.
Practice question
Try this. A pond's algae population grows very rapidly for several weeks after a nearby farm's fertiliser washes into the water, then the population suddenly crashes soon after. Suggest which phase of the sigmoid growth curve corresponds to the sudden crash, and suggest one reason for it.
Exam tip
Key terms
These terms help you describe what drives (or limits) the shape of a growth curve:
- Mitosis, the type of cell division that produces the new cells responsible for growth during the exponential phase.
- Meristem, the region of actively dividing cells in a plant that allows many plants to keep growing throughout their life.
- Xylem, the plant tissue that transports water and minerals needed to support continued growth.
- Phloem, the plant tissue that transports the food produced by photosynthesis to growing regions of a plant.
Source:SRC-DSKP-EN
Frequently asked questions
Why is a growth curve S-shaped?
How does plant growth differ from animal growth on a graph?
What causes the death phase of a growth curve?
More for Organisation of Plant Tissues and Growth
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