10 min readPathway sheet2 or 3-mark question on bioreactors or downstream processing, sometimes a 5-mark answer with a stirred-tank bioreactor diagram

Getting the product: bioreactors and downstream processing

A foreign gene's protein is made on a large scale in bioreactors under optimal conditions, then separated, purified, formulated and tested in downstream processing.

Recombinant proteinA protein made when its gene is expressed in a heterologous (foreign) host

Bioreactor volumeLarge volumes of culture, 100 to 1000 litres

Optimal conditionsTemperature, pH, substrate, salts, vitamins and oxygen

Downstream processingSeparation and purification of the product, then formulation with preservatives, clinical trials (for drugs) and quality control

Expressing the foreign gene

In almost all recombinant DNA work, the aim is to make a useful protein. So the cloned gene must be expressed (switched on to make protein) under suitable conditions. A protein made from a gene expressed in a heterologous (different) host is a recombinant protein. Small laboratory cultures can be used to extract and purify the protein, but they cannot give large amounts.

Small laboratory culture vs continuous culture

Small laboratory cultureA fixed volume of medium; the cells pass through all phases of growth; the yield is too small for industry.

Continuous culture systemSpent medium is drained out at one end while fresh medium flows in at the other. Cells stay in their most active log (exponential) phase, giving more biomass and a higher yield of protein.

Bioreactors

A bioreactor is a vessel in which raw materials are biologically converted into specific products, such as enzymes. The work is done by microbial, plant, animal or human cells. It can process 100 to 1000 litres of culture. It gives the optimum growth conditions: temperature, pH, substrate, salts, vitamins and oxygen. The most common type is the stirred-tank bioreactor, usually cylindrical or with a curved base so the contents mix well. In a simple stirred-tank, the stirrer mixes the culture and spreads oxygen. In a sparged stirred-tank, sterile air bubbles are also blown in from below.

Systems in a stirred-tank bioreactor

System or part	Role
Agitator (stirrer with flat-bladed impeller)	Mixes the culture evenly and spreads oxygen throughout
Oxygen delivery system (sparger)	Bubbles sterile air through the culture; small bubbles give more area for oxygen to dissolve
Foam control system (foam breaker)	Breaks the foam formed by stirring and aeration
Temperature control system	Keeps the best temperature for the cells
pH control system	Adds acid or base to keep the best pH
Sampling ports	Let small volumes of culture be withdrawn from time to time for testing

How to draw a stirred-tank bioreactor (labels as in your textbook figure)

Draw a tall cylinder with a curved base. Shade the lower part and label it culture broth.
Draw a motor on top with a shaft running down the middle. Add flat blades on the shaft inside the broth: flat bladed impeller.
On the shaft just above the broth, draw a small blade: foam breaker.
At the bottom, draw an inlet pipe that opens under the impeller: sterile air.
On the side, draw two short pipes: acid/base for pH control and steam for sterilisation. Title it Simple stirred-tank bioreactor.
For the sparged stirred-tank bioreactor, draw many small air bubbles rising through the broth. Label bubbles dramatically increase the oxygen transfer area, increased surface area for oxygen transfer at the broth surface, and gas entrainment.

Downstream processing

After the **biosynthetic stage** (the product is made in the bioreactor), it must pass through a series of processes before it can be sold. Separation and purification together are called **downstream processing**.

1
Separation of the product from the cells and culture broth.
2
Purification of the product.
3
Formulation with suitable preservatives.
4
Clinical trials, in the case of drugs.
5★
Strict quality control testing. The steps and tests differ from product to product.

★ key step

In real life

Indian biotech firms grow recombinant insulin, the hepatitis B vaccine and industrial enzymes in large stirred-tank bioreactors. Downstream processing and quality control are why a vial of insulin from the chemist is pure and safe.

Exam angle

3 marks: “What is a bioreactor? List the systems of a stirred-tank bioreactor that keep growth conditions optimal.” A vessel in which raw materials are biologically converted into specific products using cells, 100 to 1000 litres. Systems: agitator, oxygen delivery, foam control, temperature control, pH control, sampling ports.

2 marks: “What is downstream processing? Why is quality control essential?” Separation and purification of the product after biosynthesis, then formulation with preservatives and clinical trials for drugs. Quality control ensures every batch is pure, safe and effective; it varies with the product.

2 marks: “Why is a continuous culture system preferred for making a recombinant protein?” Used medium is drained and fresh medium added, so cells stay in the log phase. This gives more biomass and a higher yield.

Remember it

Upstream grows it, downstream cleans it, packs it and tests it.