Biology · Biotechnology: Principles and Processes · NEET
A stirred-tank bioreactor uses a stirrer (agitator) to mix the culture and spread oxygen through it. A sparged (bubble-column) bioreactor has no stirrer; it passes air upward as fine bubbles to both mix the liquid and supply oxygen. Memory hook: stirred = "spoon stirs the tea," sparged = "straw blows bubbles in the drink."
Left: stirred-tank uses a rotating stirrer to mix and spread oxygen (NCERT's common type, may also bubble air). Right: sparged (bubble-column) has no stirrer; rising air bubbles do both mixing and oxygen supply.
Your doubts, answered
What is the main difference between a stirred-tank and a sparged bioreactor?
In a stirred-tank bioreactor a moving stirrer (agitator) mixes the culture and helps oxygen reach every part. In a sparged (bubble-column) bioreactor there is no stirrer at all; air is pushed in from the bottom as fine bubbles, and these rising bubbles do both jobs, mixing the liquid and adding oxygen. So the difference is simple: mechanical stirrer vs air bubbles.
How does the stirrer supply oxygen if it only spins?
The spinning stirrer breaks up the air that enters the tank into small bubbles and pushes the liquid around. This keeps oxygen mixed evenly through the whole vessel instead of only at the top. NCERT says the stirrer does two jobs: even mixing AND oxygen availability throughout the bioreactor. Air can also be bubbled in at the same time.
Does a stirred-tank bioreactor also use air bubbles?
Yes. NCERT clearly states that in a stirred-tank reactor, 'alternatively air can be bubbled through the reactor.' So a stirred-tank can use both the stirrer and some air bubbling. The sparged type is the version that removes the stirrer and relies on bubbles alone.
Which bioreactor is more commonly used according to NCERT?
NCERT says the most commonly used bioreactors are of the stirring type (stirred-tank). This is a common one-line NEET fact. The stirred-tank shape is usually cylindrical or has a curved base to help the contents mix well.
What parts does a stirred-tank bioreactor have?
NCERT lists: an agitator (stirrer) system, an oxygen delivery system, a foam control system, a temperature control system, a pH control system, and sampling ports. The sampling ports let you take out small amounts of culture from time to time to check it without stopping the process.
Why does a sparged bioreactor remove the stirrer?
Some cultures, like fragile animal cells, can be damaged by a fast-moving stirrer. Bubbling air gently (sparging) mixes and oxygenates without the shear force of a blade. The curved or cylindrical base in NCERT's design is meant to help mixing, which becomes even more important when there is no stirrer.
⚠️ The NEET trap ✗ A stirred-tank bioreactor is designed only to mix the culture, and it keeps the vessel anaerobic (no oxygen). ✓ A stirred-tank bioreactor is designed for availability of oxygen throughout the process (the stirrer mixes AND spreads oxygen). It is aerobic, never anaerobic. Purification and preservatives belong to downstream processing, not the bioreactor. 🧠 Read what the stirrer is FOR, not just that it mixes.
Real NEET questions
NEET 2016
Stirred-tank bioreactors have been designed for
A · Purification of product
B · Addition of preservatives to the product
C · Availability of oxygen throughout the process ✓
D · Ensuring anaerobic conditions in the culture vessel
Solution: In a stirred-tank bioreactor the stirrer does two jobs: it mixes the contents evenly and it makes oxygen available throughout the vessel; air may also be bubbled (sparged) in. Cultures need a continuous oxygen supply, so the design is aerobic, not anaerobic. Purification and preservative addition are part of downstream processing, not the bioreactor.
NEET 2019
Which one of the following equipments is essentially required for growing microbes on a large scale, for industrial production of enzymes?
A · BOD incubator
B · Sludge digester
C · Industrial oven
D · Bioreactor ✓
Solution: Large-scale (100-1000 litres) culturing of microbes for industrial products like enzymes and vaccines needs a bioreactor, a vessel that provides optimum growth conditions (temperature, pH, substrate, oxygen). Small-volume cultures cannot yield enough product, so bioreactors were developed for large-scale production.
NEET 2019
Which of the following statements is incorrect?
A · Most commonly used bio-reactors are of stirring type.
B · Bio-reactors are used to produce small scale bacterial cultures. ✓
C · Bio-reactors have an agitator system, an oxygen delivery system and foam control system.
D · A bio-reactor provides optimal growth conditions for achieving the desired product.
Solution: The incorrect statement is B. Bioreactors are built for LARGE-scale processing (100-1000 litres), not small-scale, because small cultures cannot give enough product. The other statements are all correct NCERT facts: the commonest type is the stirring type, they have agitator/oxygen-delivery/foam-control systems, and they provide optimal growth conditions.
Solved Biotechnology: Principles and Processes NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Is the sparged bioreactor the same as a bubble-column bioreactor?
Yes. A sparged bioreactor is commonly called a bubble-column (or airlift) bioreactor. Air is 'sparged', meaning pushed in as fine bubbles from the bottom, and there is no mechanical stirrer.
Which type does NCERT Figure 9.7 mainly show?
NCERT Figure 9.7 shows the stirred-tank bioreactor, the most commonly used type, with its agitator, oxygen delivery, foam control, temperature and pH control systems, and sampling ports.
Do both types provide the same growth conditions?
Yes. Both provide the same optimum growth conditions (temperature, pH, substrate, salts, vitamins, oxygen). The only real difference is HOW they mix and oxygenate: stirrer versus air bubbles.
Is a bioreactor aerobic or anaerobic?
For most industrial cultures it is aerobic. This is why both types focus so much on oxygen delivery. Saying a bioreactor keeps the vessel anaerobic is a classic NEET trap answer and is wrong.
What is a common one-line NEET fact about bioreactors?
The most commonly used bioreactors are of the stirring type, and they can process 100-1000 litres of culture for large-scale production. Remember that stirred-tanks may also bubble air through them.
