Why are microorganisms so useful for studying and exploiting metabolism?
The use of microorganisms in research and industry, their diverse metabolism and ability to use a range of substrates, growth in culture media and the phases of a growth curve, the production of primary and secondary metabolites, and the control of growth conditions in fermenters.
An SQA Higher Biology answer on the environmental control of metabolism in microorganisms, covering their use in research and industry, growth requirements and the phases of a growth curve, primary and secondary metabolites, and the control of conditions in fermenters.
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What this key area is asking
The SQA wants you to explain why microorganisms are useful in research and industry, describe their growth requirements and the phases of a growth curve, distinguish primary from secondary metabolites, and describe how growth conditions are controlled in a fermenter.
Why microorganisms are useful
They are grown in a culture medium that supplies all the raw materials and energy sources needed for growth, such as a carbon source, a nitrogen source and other essential elements. Some microorganisms can make all the complex molecules they need from simple raw materials, while others must be supplied with specific additional substances.
The growth curve
In a closed culture (one with no fresh medium added and no waste removed), a population passes through four phases:
- Lag phase - cells adjust to the new medium, making enzymes and other molecules; little division occurs.
- Log (exponential) phase - rapid division when conditions and resources are favourable, so the number of cells doubles at a steady interval.
- Stationary phase - resources run low and toxic waste builds up, so the rate of new cells produced matches the rate of cell death and the population stays roughly constant; many secondary metabolites are produced here.
- Death (decline) phase - cells die faster than they are produced as nutrients are exhausted and waste becomes harmful.
Primary and secondary metabolites
Secondary metabolites are not needed for growth itself, but they give the microorganism an advantage when resources run short. Antibiotics, for instance, kill competing microorganisms, which is why penicillin from the mould Penicillium is made mainly once the culture stops growing rapidly.
Controlling growth in a fermenter
In industry, microorganisms are grown in fermenters where conditions are carefully controlled to maximise the yield of the desired product:
- Temperature, pH and oxygen are kept at optimum levels for the microorganism and product.
- Nutrients are supplied and waste removed so growth is not limited.
- Sterility is maintained to prevent contamination by unwanted microorganisms, which would compete with the culture or spoil the product.
Recombinant DNA technology can also be used so that microorganisms produce a chosen, valuable product, such as human insulin made by genetically modified bacteria.
Examples in context
Example 1. Penicillin production. Penicillin is a secondary metabolite made by the mould Penicillium. In industry it is grown in large stirred fermenters where temperature, pH, oxygen and nutrient supply are tightly controlled and sterility is maintained. The antibiotic is harvested mainly during the stationary phase, after rapid growth has slowed, which matches the timing of secondary metabolite production seen on the growth curve.
Example 2. Genetically modified bacteria making human insulin. Diabetics rely on insulin made by bacteria such as E. coli that have had the human insulin gene inserted using recombinant DNA technology. The bacteria are grown in fermenters under carefully controlled and sterile conditions, then the insulin is extracted and purified. This shows how the diverse and easily managed metabolism of microorganisms, combined with genetic technology, can produce a valuable human medicine.
Try this
Q1. Name the four phases of a microbial growth curve in order. [2 marks]
- Cue. Lag, log (exponential), stationary, death (decline).
Q2. State the difference between a primary and a secondary metabolite. [2 marks]
- Cue. Primary metabolites are made during the log phase as part of normal metabolism; secondary metabolites are made in the stationary phase and often aid survival.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher 20184 marksDescribe the four phases of a microbial growth curve in a closed culture, and explain what happens to the cells in the stationary phase.Show worked answer →
A 4-mark answer needs the four named phases in order plus the stationary phase explanation.
The four phases in order are the lag phase, the log (exponential) phase, the stationary phase and the death (decline) phase.
In the lag phase, cells adjust and make enzymes and other molecules, with little division. In the log phase, cells divide rapidly because resources are plentiful.
In the stationary phase, resources run low and toxic waste builds up, so the rate at which new cells are produced equals the rate at which cells die, and the population stays roughly constant. Many secondary metabolites, such as antibiotics, are produced during this phase.
Markers reward the four phases in the correct order and a correct account of the stationary phase.
SQA Higher 20214 marksA culture of bacteria starts with 200 cells and divides every 30 minutes under ideal conditions. Calculate the number of cells after 3 hours, and explain why this exponential growth does not continue indefinitely.Show worked answer →
This is an exponential growth calculation.
Step 1. Work out how many divisions occur: 3 hours is 180 minutes, and the cells divide every 30 minutes, so there are divisions.
Step 2. Each division doubles the number of cells, so multiply the start by . Calculate .
Step 3. Multiply by the starting number: cells.
Step 4. This growth cannot continue indefinitely because, in a closed culture, nutrients run out and toxic waste builds up, so the population enters the stationary and then death phases.
Markers reward the working (), the value 12800, and a reason linked to limited resources or waste.
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Sources & how we know this
- SQA Higher Biology Course Specification — SQA (2018)