Use of Biological Resources

Using microorganisms in food production

Humans have used microorganisms to make food for thousands of years. Many of these processes rely on fermentation — the breakdown of nutrients by microorganisms, often in the absence of oxygen (anaerobic conditions).

Key terms Fermentation — the chemical breakdown of a substance by microorganisms, releasing energy and useful products.

Microorganism — a microscopic living thing such as a bacterium or a fungus.

Yoghurt production

Yoghurt is made from milk using bacteria of the genus Lactobacillus.

1. Equipment and milk are first pasteurised (heated) to kill unwanted microorganisms.
2. The milk is cooled to about $40\,°C$ — the ideal temperature for the bacteria.
3. A starter culture of Lactobacillus is added.
4. The bacteria ferment the milk sugar lactose into lactic acid.
5. The lactic acid lowers the pH, which makes the milk proteins clot (curdle), thickening the milk and giving yoghurt its sharp taste.
6. Flavourings and fruit may be added before packaging.

Bread making

Bread uses the fungus yeast (a single-celled organism).

When yeast is mixed with flour, sugar and water and kept warm, it respires and ferments the sugars. This releases carbon dioxide gas:

$$\text{glucose} \rightarrow \text{ethanol} + \text{carbon dioxide} \;(+\text{energy})$$

The bubbles of carbon dioxide become trapped in the stretchy dough, making it rise. During baking the ethanol evaporates and the yeast is killed.

Exam tip Be clear which gas does which job. In bread the useful product is the CO₂ (it makes dough rise) — the ethanol is driven off. In brewing the useful product is the ethanol — the CO₂ is the by-product.

Producing ethanol with yeast

The same fermentation reaction is used to make alcoholic drinks (brewing) and biofuel.

Brewing — yeast ferments sugars from barley (beer) or grapes (wine) into ethanol.
Biofuel — sugars from crops such as sugar cane are fermented to make ethanol, which can be burned as a renewable fuel.

The yeast works best in warm, anaerobic conditions. If too much ethanol builds up it eventually kills the yeast and fermentation stops.

The industrial fermenter

To make products like the antibiotic penicillin in large quantities, microorganisms are grown in a large stainless-steel vessel called a fermenter. Conditions inside are carefully controlled to keep the microorganisms growing well.

The conditions controlled inside a fermenter are:

Real world Penicillin was the first widely used antibiotic, made from the mould Penicillium. Growing it in giant fermenters during the 1940s allowed it to be mass-produced and saved millions of lives.

Growing crops: glasshouses and polythene tunnels

Growing plants inside glasshouses or polythene tunnels lets a farmer control the conditions that affect photosynthesis and growth, increasing the yield.

Temperature — heaters keep plants warm so enzymes and growth are fast; the structure traps heat.
Carbon dioxide — extra CO₂ can be added to increase the rate of photosynthesis.
Light — artificial lighting extends the growing day so plants photosynthesise for longer.

Fertilisers add mineral ions to the soil. Nitrate is needed to make amino acids and proteins for growth, while phosphate and other ions support healthy roots and reactions. Fertilisers replace minerals removed by previous crops and increase yield.

Watch out Too much fertiliser can wash into rivers and lakes and cause eutrophication — excess nutrients trigger algal growth, which blocks light and removes oxygen, killing other organisms.

Pest control

Pests reduce crop yields. There are two main approaches.

Chemical pesticides are chemicals sprayed to kill pests.

Advantages: fast acting, kill a wide range of pests, effective.
Disadvantages: may kill harmless or useful organisms; can bioaccumulate along food chains; pests may become resistant; must be re-applied.

Biological control uses a natural predator, parasite or disease of the pest.

Advantages: does not pollute, only targets the pest, does not need re-applying.
Disadvantages: slower; the control organism may itself become a pest or attack other species.

Fish farming

In fish farming (aquaculture) fish such as salmon are reared in enclosures to give a high yield. Factors are controlled to maximise growth and survival:

Water quality — kept clean and well-oxygenated.
Feeding — high-protein food given at the right amounts for fast growth.
Stocking density / intraspecific predation — controlling numbers so fish are not overcrowded and do not eat each other.
Interspecific predation — keeping out predators such as birds.
Disease and parasites — treated, since crowding spreads infection.
Selective breeding — choosing fast-growing fish to breed from.

Selective breeding (artificial selection)

In selective breeding humans choose which organisms reproduce, to develop desirable features. The process is repeated over many generations:

1. Choose parents with the desired characteristic (e.g. cows with high milk yield).
2. Breed them together.
3. From the offspring, select those that best show the characteristic.
4. Breed these together.
5. Repeat over many generations until the feature is strongly shown.

Worked example A farmer wants wheat with a high grain yield.

She plants many wheat plants and selects the few with the most grain. She crosses only these. From their seeds she again keeps only the highest-yielding plants and crosses them. After repeating this for several generations the whole crop reliably produces a high yield.

Genetic modification (genetic engineering)

Genetic modification transfers a gene from one organism into another, so the second organism makes a new, useful protein.

A famous example is making human insulin with bacteria. The human insulin gene is cut out using enzymes and inserted into a bacterial plasmid. The plasmid is put back into a bacterium, which is grown in a fermenter. As the bacteria multiply they make pure human insulin to treat diabetes.

GM crops have genes added to give useful traits, such as resistance to pests or herbicides, or extra vitamins.

Cloning and micropropagation

Cloning produces genetically identical offspring. In micropropagation (tissue culture), many small pieces of a plant are grown on a nutrient-rich, sterile medium. Each piece develops into a new plant identical to the parent.

This lets growers quickly produce large numbers of identical plants with a desired feature, and produce plants free of disease — useful for crops and endangered species.