Growing crops under cover

Farmers increase crop yield by growing plants in glasshouses and polythene tunnels. These control the environment so plants grow faster and for more of the year.

• Warmth is trapped, raising the temperature so enzymes work faster and photosynthesis speeds up.
• Carbon dioxide can be added to increase the rate of photosynthesis.
• Plants are protected from wind, frost and pests, and watering can be controlled.

Why the higher yield?

Fertilisers

Plants remove mineral ions from the soil as they grow. Fertilisers replace these ions so crops keep growing well:

• Nitrates — needed to make amino acids and proteins (for growth).
• Phosphates — needed for DNA and cell membranes (roots).
• Potassium — helps enzymes of respiration and photosynthesis.

Watch out: too much fertiliser washing into rivers causes eutrophication (that link belongs to Section 4, but it's why fertiliser use is controlled).

Controlling pests

Pests reduce yield, so farmers control them in two main ways:

• Chemical pesticides — sprayed to kill pests. Fast and effective, but can harm other organisms, may enter food chains (bioaccumulation), and pests can become resistant.
• Biological control — introducing a natural predator, parasite or disease of the pest. Slower and less complete, and the introduced organism may itself become a pest — but no chemicals, and the pest does not become resistant.

Example: using ladybirds to eat greenfly (aphids) is biological control — the ladybird is the pest's natural predator.

Which is biological control?

Why pump extra carbon dioxide into a glasshouse?

How do food makers keep the wrong microbes out?

Yoghurt and the role of bacteria

Micro-organisms are used to make food. In yoghurt making:

• Milk is first heated (pasteurised) to kill unwanted microbes, then cooled.
• A starter culture of bacteria (e.g. Lactobacillus) is added and the mixture kept warm.
• The bacteria ferment the lactose sugar into lactic acid. The acid lowers the pH, which thickens (clots) the milk and gives the sour taste.

Key idea: it is the lactic acid made by the bacteria that turns runny milk into thick, tangy yoghurt.

Yeast is the other food micro-organism on the spec: in bread dough it respires anaerobically, fermenting sugar into carbon dioxide (which makes the dough rise) and ethanol.

Your turn — the role of bacteria

The industrial fermenter

Large steel tanks called fermenters grow micro-organisms on a huge scale to make useful products, e.g. the antibiotic penicillin (from the Penicillium fungus) and the meat-substitute mycoprotein (from a fungus).

Conditions inside must be carefully controlled:

• Aseptic conditions — everything is sterilised to keep out other microbes.
• Nutrients — a food supply (sugars, etc.) is added.
• Optimum temperature — a water jacket / cooling coils stop overheating.
• Optimum pH — monitored and adjusted.
• Oxygen — air is bubbled in for aerobic respiration; a stirrer mixes it.

Why keep it aseptic?

Fish farming

Fish farms grow large numbers of fish (e.g. salmon) in cages or tanks to give a protein source. To get a high yield the farmer controls:

• Water quality — filtered and oxygenated; waste removed.
• Intraspecific predation (same species eating each other) — fish sorted by size/age.
• Interspecific predation (other species) — nets and cages keep predators out.
• Disease — treated with medicines/antibiotics.
• Feeding — the right food, in the right amount, at the right time.
• Selective breeding — breeding the fastest-growing fish.

Same species or different?

Food-production terms

Tap a term on the left, then tap its matching meaning on the right.

How do food makers keep the wrong microbes out?

How do breeding and genetic engineering differ?

Selective breeding

Selective breeding (artificial selection) means humans choosing which organisms breed, to develop useful features over many generations. The process is:

• Choose the parents with the desired characteristic (e.g. cows with high milk yield).
• Breed them together.
• From the offspring, again select the best and breed them.
• Repeat over many generations until the feature is strong.

Examples: cattle bred for high milk yield or beef; wheat bred for higher grain yield and disease resistance; dogs bred for temperament.

Your turn — improving milk yield

Don't confuse these two!

Misconception: "Selective breeding is a type of genetic engineering." No.

• Selective breeding uses natural reproduction — you just choose which organisms mate. Genes are never directly changed, and it takes many generations.
• Genetic engineering directly transfers a gene from one organism into another. It is much faster and can move genes between very different species.

Coming up next: how genetic engineering actually works.

Making human insulin with bacteria

Genetic engineering transfers a gene from one organism into another. The classic example is making human insulin in bacteria to treat diabetes:

• Restriction enzymes cut the human insulin gene out of human DNA, leaving "sticky ends".
• The same restriction enzyme cuts open a bacterial plasmid (a small ring of DNA used as a vector — viruses can also act as vectors), giving matching sticky ends.
• DNA ligase joins the insulin gene into the plasmid — this is now recombinant DNA.
• The plasmid is put into a bacterium, which is grown in a fermenter. As the bacteria multiply they make human insulin, which is collected and purified.

Put the steps in order

Tap the steps of insulin genetic engineering in the correct order, starting from the first.

Name that enzyme

How do breeding and genetic engineering differ?

GM crops & transgenic organisms

An organism with a gene from another species added to it is transgenic — for example a bacterium carrying the human insulin gene, or a crop given a useful gene.

GM crops can be engineered to:

• Resist insect pests or tolerate herbicides.
• Give a higher yield or extra nutrients (e.g. "golden rice" makes beta-carotene, which the body turns into vitamin A).

Benefits: higher yields, less pesticide, added nutrients. Risks/concerns: genes may spread to wild plants, effects on other organisms are uncertain, seeds may be costly, and some people have ethical objections.

Your turn — key term

Cloning plants: micropropagation

Micropropagation (tissue culture) makes many genetically identical clones of a plant from tiny pieces:

• Take small pieces of tissue (explants) from a good parent plant.
• Grow them on a sterile nutrient agar or gel containing plant hormones.
• They form a mass of cells that develops into many tiny plantlets, each a clone of the parent.

Uses: mass-producing identical, disease-free or genetically modified plants quickly and all year round.

Cloning mammals (Dolly the sheep): the diploid nucleus from a body cell of the adult being cloned is put into an enucleated egg cell (one with its own nucleus removed). The egg is stimulated to divide into an embryo, which is implanted into a surrogate mother. Cloned transgenic animals can be used to make human proteins, e.g. in their milk.

Clones and genes

Your turn — fermenter conditions

Section 5 in a nutshell

That's Edexcel 4BI1 Section 5 in a nutshell. Press Finish to see your score.