Using Beetroot in the Lab

Beetroot make a useful and easily available resource for the science lab. They’re also a fascinating example of how many years of selective breeding by farmers can turn one original plant species into a whole range of different varieties, providing us with resources from sugar to salad vegetables.

Practicals

• Osmosis: Place slices of fresh beetroot in different concentrations of liquid to observe effect then take thin slices to observe the cells
• Plant cells and tissues: Cells can be looked at under optical microsope
• Student investigation idea: Investigating plasmolysis in beetroot cells
• Student investigation idea: The effect of lead nitrate on the cell membrane of a beetroot cell

Teaching Topics

• Cell Structure
• Plasmolysis
• Membrane permeability

Description

Beetroot, Beta vulgaris

Beta vulgaris is an edible crop with many varieties created by hundreds of years of selective breeding, which have very different uses. In most of the varieties, such as our common edible beetroot, the stem tuber (the swollen base of the stem) is eaten. One particular variety, sugar beet, contains around 17% sugar, and is an important crop in the UK, providing over half the sugar we use. The sugar beet leaves are used for feed for cattle and sheep. Beetroot and their relatives are members of the Amaranthaceae family.

A standard A-level biology practical is to examine the effect of temperature on membrane stability in beetroot, assessed by the degree of pigment leakage. But what is this pigment, and what function does it serve?

The pigments are actually betalain pigments, named after the Red beet (Beta vulgaris). They replace anthocyanins in plants of the order Caryophyllales (cacti, beets & co., bougainvillaea, phytolacca, large-flowered purslane etc. and also in some fungi such as fly agaric). Their function is not really known, but it is guessed that when present in the flowers they serve to attract pollinating insects and when present in seeds they may attract birds for the dispersal of the seeds. In cultivated species man may have arbitrarily selected for more coloured lines, because they are attractive to look at (red beet!), or the colour may have co-segregated with another useful trait. (For example, the colour may be genetically linked with a characteristic that improves flavour – by selecting varieties for flavour, man may have also selected for colour by chance.) There is no indication that betalains protect plants against pathogens (fungal, bacterial or viral) or herbivores, and they do not absorb UV light. Unlike anthocyanins, betalains are poor pH indicators.

Betalains are in the vacuole. They serve as markers for people who want to isolate intact vacuoles; the red-violet vesicles you can get from protoplasts are intact beet vacuoles. So betalain pigments have to cross 2 membranes. In living tissue (beet slices) you can get leakage by a heat shock, by acid treatment or of course by incubating them in acidified 80 % methanol. Thinner slices have a larger surface, thus speeding up pigment leakage. Putting beet slices in the deep-freeze of course kills them, and afterwards the pigments will leak out. A pH of about 3 – 4 stabilises the pigments and protects against oxidation. Pigment extracts must be protected from direct sun light and should be kept in a cool and dark place.

Beet pigments are unstable at higher temperature, but the chemistry depends on the pH and composition of the solution, oxygen concentration, how long the solution is boiled for etc etc. Generally, plant extracts are complex, i.e. they contain phenolics and lots of reactive substances in addition to what you are looking at, and they just turn brown (eg at 80ºC). (Ursula Hinz, Lausanne University)

Growing and sourcing

Obtaining: Easily obtained from greengrocer, market or supermarket at reasonable cost. Use fresh uncooked beetroot.