Variety of fruits and vegetables (leonori, iStockphoto)
Variety of fruits and vegetables (leonori, iStockphoto)
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BC11Chemistry 11 (June 2018)Big Idea: Organic chemistry and its applications have significant implications for human health, society, and the environment.
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Why are there so many colours of plants? Learn about plant pigments and their role in plants.
Have you ever wondered where the wonderful colours of plants come from? What makes some leaves green and some tomatoes red? Pigments are behind the many colours we see in plants.
What is a pigment?
In biology, a pigment is any coloured material found in a plant or animal cell. Pigments are what give colour to our skin, hair and eyes. They are also what colour plants. Pigments make things appear to be certain colours because theyanddifferentof light. There are three major pigments found in plants. These are chlorophylls, carotenoids, and flavonoids.
Types of Pigments
Chlorophylls
Molecules of chlorophyll cause the green colours in plants.
These molecules are critical for the process of. Chlorophyll absorbs energy from light. Plants use this energy to convert carbon dioxide and water intoand oxygen. Chlorophyll is produced byknown as the chloroplasts. For more details, check out thisbackgrounder about light and plants.
Image - Text Version
Shown is a colour photograph of a tray piled with green fruits, vegetables and a smoothie.
In the foreground is a glass mug filled with a bright green smoothie. A silver metal straw is in the drink. The mug is surrounded by piles of food in various shades of green. Spinach, kale, parsley and green apples are visible.
There are two types of chlorophylls in green plants. These arechlorophyll aandchlorophyll b. The structure of these molecules is very similar. The only difference is in one side chain.
Image - Text Version
The diagram on the left is labelled Chlorophyll a. The diagram on the right is labelled Chlorophyll b. Both diagrams have clusters near the top, and long tails reaching down and to the left. At the top of the Chlorophyll A structure, an area labelled CH3(3 in subscript) is highlighted. At the top of the Chlorophyll bstructure, the same area has two additionalcovalent chemical bonds. The one on the left is la double bond labelled O. The one on the right is a single bond labelled H.
Both Chlorophyll a and b absorb light in the blue and red regions of the.
They are different in that they absorb more light in different parts of the spectrum. As you can see in this graph, Chlorophyll b has its largest peak in the blue part of the spectrum. Chlorophyll a has its largest peak in the red part of the spectrum.
Neither absorb much light in the green part of the spectrum (490 to 550 nanometers (nm)). They reflect green light instead.
This is why plants containing chlorophyll a and b appear green to us. Chlorophyll a produces dark green colours and chlorophyll b produces yellowish green colours.
Image - Text Version
Shown is a colour line graph showing where Chlorophyll A and B absorb light along the electromagnetic spectrum.
The x axis is labelled Wavelength [nm] and is marked from 400 on the left, to 700 on the right. Below, a ruler illustrates the colours of these wavelengths. Purple at 400, blueish green at 500, orange at 600 and red at 700.The y axis is labelled Absorbance.
The absorbance of Chlorophyll A is illustrated with a dark green line that starts about halfway up the Absorbance scale at 400 nm. It then peaks at about 425 nm and dips very low till it peaks again at about 675 nm.
The absorbance of Chlorophyll B is illustratedwith a bright green line that starts about 1/4 of the way up the Absorbance scale at 400 nm. It then peaks at about 475 nm and dips low till it peaks again at about 625 nm.
Plants have more chlorophyll a than b. About three quarters of the pigment in plants is chlorophyll a. Chlorophyll a absorbs light better than chlorophyll b, so it is no wonder there is more of it in plants. Plants do not actually need chlorophyll b to photosynthesize. This is why scientists often call it anaccessory pigment. Chlorophyll b’s purpose is to absorb light in a wider range of the visible light spectrum. Plants that live in low-light conditions tend to have more chlorophyll b than plants which get lots of sunlight.
Did you know?
There are also other chlorophylls? Chlorophyll c, d and f are found in some types of red algae and in some. They absorb light in the red part of the spectrum.
Carotenoids
Green is not the only colour plants can be. Fruits, vegetables and flowers come in a rainbow of colours!
Yellow, orange, and red colours usually come from a group of pigments calledcarotenoids.
A common carotenoid, beta-carotene, is produced in theof sunflower petals. This produces the bright yellow and orange colours we see in these flowers.
Beta-carotene is also responsible for the orange colour in carrots and sweet potatoes.
Autumn leaves appear red and yellow because carotenoids are revealed. This happens when chlorophyll breaks down in response to less sunlight.
Image - Text Version
Shown is a colour photograph of bright red and yellow flowers with green stems and leaves.
The frame is filled with flowering plants. They have tall, thin green stems and long pointed leaves. The flowers of each plant are in full bloom. Each one appears either red or yellow.
Carotenoids absorb light at wavelengths from 400 to 600 nm. This part of the spectrum is primarily blue and green.
Carotenoids reflect light in the yellow, orange and red parts of the spectrum. That is why they appear yellow, orange, and red to us.
Image - Text Version
Shown is a colour line graph showing where lutein, beta-carotene, and lycopene absorb light along the electromagnetic spectrum.
The x axis is labelled Wavelength [nm] and is marked from 400 on the left, to 700 on the right. Below, a ruler illustrates the colours of these wavelengths. Purple at 400, blueish green at 500, orange at 600 and red at 700. The y axis is labelled Absorbance.
The absorbance of lutein is illustrated with a yellow line that starts low on the Absorbance scale at 400 nm. It then peaks at about 490 nm, with a smaller peak at about 510 nm. The line dips to the bottom and disappears at about 590 nm.
The absorbance of beta carotene is illustrated with an orange line that starts low on the Absorbance scale at 400 nm. It then peaks just below lutein at about 500 nm and dips to a plateau at about 525 nm. The line dips to the bottom and disappears at about 590 nm.
The absorbance of lycopene is illustratedwith a red line that starts low on the absorbance scale at about410 nm. Then it reaches a small plateau at about 490 nm, followed by a peak at about 510 nm. This peak is below both lutein and beta-carotene, about halfway up the absorbance scale. The line has another, smaller peak at about 550 nm before dipping low and disappearing at 600 nm.
There are two main types of carotenoids.These are carotenes and xanthophylls.
Carotenesinclude pigments like 𝛃-carotene (beta-carotene) and lycopene. They all have the chemical formula C40H56. All their names also end in -ene.
Image - Text Version
Shown are black and white molecular diagrams for beta-carotene and lycopene.
The top diagram is labelled Beta-carotene in orange. The bottom is labelled Lycopene in red. Both are long, thin, horizontal structures. beta-carotene has additional bonds, forming hexagons with three additional bonds on each end. All bonds are labelled CH3 (3 in subscript).
Did you know?
The red colour of the tomato fruit is due mainly to the carotenoid lycopene. There have been many studies linking lycopene with potential health benefits. These includereducing the risk of certain cancers.
Xanthophyllsinclude pigments likeluteinandzeaxanthin. They have a chemical structure similar to carotenes. But they have additional oxygen molecules. All their names end in -in.
Xanthophylls are found in many green vegetables, and in some flowers. Some xanthophylls are also used as food colouring.
Image - Text Version
Shown are black and white molecular diagrams labelled Lutein and Zeaxanthin, each with small areas highlighted in yellow.
Both diagrams are long and narrow with hexagons on each end. The top one is labelled Lutein in yellow. The bottom one is labelledZeaxanthin in yellow. One double bond labelled CH3 (3 in subscript) is circled and highlighted in yellow on the left end of each diagram. This is on the bottom right side of the hexagon in Lutein, and the right side of the hexagon inZeaxanthin.
Like chlorophylls, carotenoids absorb energy from sunlight. Then they transmit the energy to chlorophyll molecules to boost photosynthesis.
In all living things, carotenoids act asantioxidants. Antioxidants are molecules that can slow down theeactions that can damage cells.
Flavonoids
Flavonoids are a family of compounds found in plants. They produce red, yellow, blue and purple colours.
The most common type of flavonoid is anthocyanin which is found in cell.
The red colour in roses, apples, cherries, red cabbage and autumn maple leaves, is due to anthocyanins.
Image - Text Version
Shown is a colour photograph of red and purple fruits and vegetables on a white surface.
The camera looks straightdown at the display from above. The foods are grouped together neatly by type. From the top left they include: strawberries, red grapes, red peppers, tomatoes, red onions, beets, red cabbage, blueberries, cherries, peppers, pomegranates and apples.
Many flavonoid pigments absorb light in wavelengths between 250 to 550 nm. They absorb the most light in the ultraviolet and blue green parts of the spectrum.
They reflect light in the blue and violet part of the spectrum. This is why they look purplish to us.
Image - Text Version
Shown is a colour line graph showing where Oenin, Chlorophyll A and B absorb light along the electromagnetic spectrum.
The x axis is labelled Wavelength [nm] and is marked from 200 on the left, to 700 on the right. Below, a ruler illustrates the colours of these wavelengths. The ruler starts with dark purple just before 400 nm. The ruler then goes from purple at 400, blueish green at 500, orange at 600, to red at 700. The y axis is labelled Absorbance.
The absorbance of oenin is illustrated by a purple line. It starts at about 2/3 up the absorbance scale at about 280 nm. It quickly peaks at about 290 nm, then dips quite low before peaking again at about 510 nm. It drops quickly and disappears at about 590 nm.
The absorbance of Chlorophyll A is illustrated with a dark green line that starts about halfway up the Absorbance scale at 400 nm. It then peaks at about 425 nm and dips very low till it peaks again at about 675 nm.
The absorbance of Chlorophyll B is illustrated with a bright green line that starts about 1/4 of the way up the Absorbance scale at 400 nm. It then peaks at about 475 nm and dips low till it peaks again at about 625 nm.
Flavonoids have many different functions.
For example, they are responsible for some of the colour and aroma in flowers and fruit. Because they reflect light in the ultraviolet part of the spectrum, insects who see in this range can spot them easily. This can mean flowers with flavonoids are more visible to.
Flavonoids also help protect plants from stresses like ultraviolet light, frost, heat, and dry conditions.
Scientists have studied flavonoids because of their potential health benefits to humans. These compounds have anti-inflammatory properties, and they may help prevent cancer.
Image - Text Version
Shown are 15 colour photographs of flowers that appear yellow under visible light, and bright blue under ultraviolet light.
Under visible light, the flowers are bright yellow on black backgrounds. In ultraviolet light, different parts of the flowers are more visible.
For example, in the first photograph, the flower appears entirely one shade of yellow. In the second photograph, the flower's outside petals appearbright blue, while a large part of the centre is very dark blue.
Each photograph is labelled in the top left or bottom left corner, from A - O.
Plant Pigments(2020)
This video (4:50 min.) by Professor Dave Explains what plant pigments are and their role.
Plant Functions
This backgrounder by Let’s Talk Science presents how photosynthesis and respiration function in plants.
Why Do Leaves Change Color in the Fall?(2020)
This article by How Stuff Works explains why leaves of trees change colours with the seasons.
Leaf Pigments and Light(2014)
This video (3:11 min.) by Teacher’s Pet explains how light and leaf pigments interact in plants.
Carotenoids Found in Tomatoes
This article by the Healthy Eating SF Gate explains the nutritional value and benefits of carotenoids found in tomatoes.
References
Gronquist,M., Bezzerides, A., Attygalle, A., Meinwald, J., Eisner, M., & Eisner, T. (2001). Attractive and defensive functions of the ultraviolet pigments of a flower (Hypericum calycinum).PNAS, 98 (24) 13745-13750.https://www.pnas.org/doi/10.1073/pnas.231471698.
Harvard Forest. (n. d.)Leaf Pigments.
Helmenstine, A. M. (2021)Pigment Definition and Chemistry. ThoughtCo.
Jenkins, J. A. & MacKinney G. (1951)Color in Tomatoes.California Agriculture, February 1951, 13-14.
May, P. (n. d.)Chlorophyll. School of Chemistry, University of Bristol.
Mierziak, J., Kostyn, K., & Kulma, A. (2014). Flavonoids as important molecules of plant interactions with the environment.Molecules, 19(10), 16240–16265.https://doi.org/10.3390/molecules191016240
New World Encyclopedia. (n. d.)Carotene.
Picklesimer, P. (2010)Pigments in Tomatoes Pack a Punch. Futurity.
Plants in Action. (n. d.)1.2.2 Chlorophyll absorption and photosynthetic action spectra.
Schulte, A. J.; Mail, M.; Hahn, L. A.; Barthlott, W. Beilstein. (2019). Ultraviolet patterns of flowers revealed in polymer replica – caused by surface architecture.Journal of Nanotechnology, 10, 459–466.DOI:10.3762/bjnano.10.45
Simms J. & Odle, T. (2018)Carotenoids. Gale Encyclopedia of Alternative Medicine.
WebExhibits.org. (n. d.)What pigments are in fruit and flowers?Causes of Color.
