Plant chemical defences

Biology · Plant defences

Which chemical, which enemy?

Four plants, four chemical defences. Pick a plant, then pick where it belongs. Sort by what the chemical does first, then switch the rule and sort by who it's aimed at.

What does this plant's chemical do?

4 of 4 still to sort.

Plants have physical, chemical and mechanical defences. Here we're sorting four chemical ones.

Predict, then check

Think about the energy bill.

Making defensive chemicals takes a plant energy. So what do you think some plants do about it?

What you need to know

• Pathogens such as viruses, bacteria and fungi can infect plants. Pests, including insects, damage plants by feeding and can spread pathogens.
• Against both, plants have physical, chemical and mechanical defences. This lesson is about the chemical ones.
• An antimicrobial substance kills microorganisms or stops their growth, and some plants make antimicrobial chemicals that kill pathogens.
• Mint, for example, produces antimicrobial substances that kill bacteria.
• Have a goMint's antimicrobial substances are aimed at one of these enemies: a bacterium getting in, or an insect chewing the plant. Which one?

  A bacterium, which is a pathogen.

  Antimicrobial substances act on microorganisms (mint's kill bacteria). An insect that feeds on the plant is the other kind of enemy, a pest.

• Some plants make poisons that deter herbivores from eating them, and foxglove and stinging nettles are two examples.
• Animals, including humans, avoid those plants and leave them alone to grow, and a stinging nettle irritates the skin.
• Have a goSam says: "Nettles are poisonous, so every plant chemical defence must be a poison." Mint has just made Sam's claim wobble. How?

  Mint's chemicals aren't poisons that put herbivores off. They are antimicrobial and kill bacteria.

  Poisons are one chemical defence, the kind that deters herbivores. Antimicrobial substances are a different kind, aimed at pathogens.

• Making defensive chemicals takes energy, so some plants make them only in response to damage, to kill or deter herbivores.
• Maize has a different trick: damaged by a herbivore such as a caterpillar, it releases compounds into the air that attract wasps.
• Those wasps feed on the caterpillar, which protects the maize from further damage.
• Have a goThe maize's chemical doesn't come out until something happens to the plant. Name that trigger, then say what the chemical does next.

  Damage by a herbivore such as a caterpillar; the maize then releases compounds that attract wasps.

  The release is a response to damage, and the wasps turn up because of the compounds. It is the wasps that feed on the caterpillar.

The big picture

Plants face two kinds of enemy: pathogens and pests. Some defend themselves chemically: antimicrobial substances (mint) kill microorganisms or stop their growth, and poisons (foxglove, stinging nettles) deter herbivores. Making these chemicals takes energy, so some plants make them only in response to damage. Maize does something different: damaged maize releases compounds that attract wasps, which feed on the caterpillar.

⚠ Watch out

Thinking every plant chemical defence is a poison. Poisons (foxglove, stinging nettles) deter herbivores, but antimicrobial substances (mint) kill microorganisms or stop their growth, and maize's compounds work by attracting wasps.

Memory hook

Kill, deter, call for backup: mint kills bacteria, foxglove and nettles deter herbivores, maize calls in the wasps.

Check yourself

Cover the page. For mint, foxglove, stinging nettle and maize, say which enemy each chemical is aimed at. Then say why some plants make their chemicals only after damage.