Colours in plants and food

Section 3 of 4

Plants are full of coloured molecules, and each one has a long run of alternating double bonds.

Green leaves

Chlorophyll is the green of leaves. Its core is a big, flat ring of carbon and nitrogen atoms with a magnesium atom at the centre, and a run of alternating double bonds goes round the ring. Chlorophyll a, the most common kind, is C₅₅H₇₂MgN₄O₅.

Chlorophyll absorbs violet and blue light strongly, and red light too. A plant uses the energy of that light for photosynthesis, which turns carbon dioxide and water into sugar, as you saw earlier. Green light, in the middle, is absorbed least, so leaves send it back and look green.

Chlorophyll takes in red and blue light and sends back green The bar of visible colours from violet at 400 nanometres to red at 700 nanometres. The violet and blue part, 400 to 480 nanometres, and the red part, 630 to 690 nanometres, are faded and marked taken in. The green part in the middle, around 500 to 570 nanometres, is marked sent back. 400 nm 500 nm 600 nm 700 nm taken in taken in sent back: green
Chlorophyll absorbs violet and blue light, and red light. The green in between is sent back, so leaves look green.

Berries and red cabbage

Blueberries, blackberries, cherries, red grapes and red cabbage get their reds, purples and blues from anthocyanins. The name comes from the Greek words for flower and blue. An anthocyanin is a coloured core with one or more sugar molecules attached. In red cabbage and blackberries, the core is cyanidin.

Cyanidin as a skeletal formula Three rings in a row. The left ring is a benzene ring with two -OH groups, the middle ring holds an oxygen atom with a positive charge and one -OH group, and the right ring is a benzene ring with two -OH groups. Double and single bonds take turns through all three rings. O⁺ OH OH HO OH OH
Cyanidin, C₁₅H₁₁O₆⁺, the coloured core of the anthocyanins in red cabbage and blackberries. Double and single bonds take turns through all three rings.

Cyanidin has three rings in a row. The outer two are flat benzene rings, and the alternating double bonds run through all three, so the run is long enough to absorb visible light. The ⁺ on the oxygen in the middle ring means the molecule carries a positive charge: it has one more proton than it has electrons. That is the form it takes in acid, and its colour changes as the pH goes up. The next section shows how.

Colours in plants and food
MoleculeColour, and where you find it
chlorophyllgreen, in leaves, spinach and peas
beta-caroteneorange, in carrots, sweet potatoes and mangoes
lycopenered, in tomatoes and watermelon
anthocyaninsred, purple or blue, in blueberries, blackberries and red cabbage

Quick check

Why do leaves look green?