Onions can cause interest as well as tears in the classroom. Easy and cheap to source, they make a great resource for looking at cell structure, particularly stomata, and for extracting DNA, much more reliably than kiwi fruits.

Practicals

• Stomata: Stomatal Peel
• DNA: Extracting DNA (a resource from the NCBE)
• Enzymes: Catalase is a widespread enzyme, and its presence can be identified in onions and radishes with this deceptively simple protocol. Resource: Microscale investigations with Catalase
• Cell structure: Observing plant cells under microscope
• Cell membranes: Student investigation starter - Finding the concentration of lead which affects the permeability of onion cell membranes.
• Cell membranes: Student investigation starter - Variation of solute potential in onion epidermal cells

Teaching Topics

• Cell structure
• Stomata
• DNA
• Enzymes

Description

Onion, Allium ceta

Onions have been cultivated for many centuries. Along with chives, leeks, garlic and shallots they belong to the genus Allium with their characteristic smell caused by alkyl sulphides.

Growing and sourcing

Obtaining: Easily obtained from greengrocers, markets and supermarkets at reasonable cost. A small number can provide the material for several investigations. Although some awarding bodies suggest the use of kiwi for extracting DNA, there is a problem with pectin levels. We suggest onion as a better option.

Curriculum Links   

GCSE

• EDEXCEL GCSE additional science 2.13 stomata for gas exchange
• AQA Biology B3.1.3 observing guard cells and stomata
• EDEXCEl GCSE additional science 1.8 extracting DNA
• AQA Biology extracting DNA B2.7.1 extracting DNA
• AQA Biology B2.1.1 observation of plant cell under microscope
• AQA biology AS/A2 3.2.4 use of optical microscope to examine plant cells
• Scottish Highers Unit 1 The structiure and replication of DNA