Photosynthetic Pigments

Photosynthetic Pigments

by Jasmine Chan 

Source: McManus, Alex. “HD Wallpaper of Plant.” The International Mentoring Network, 24 Mar. 2014, theimn.com/hd-wallpaper-of-plant/.

INTRODUCTION TO PHOTOSYNTHETIC PIGMENTS

Photosynthetic pigments are substances which capture light energy during light-depended reactions in photosynthesis. [1] There are a few types of photosynthetic pigments: chlorophyll and carotenoids. Different pigments absorb and reflect a different range of light. Photosynthetic pigments are found in the chloroplast of photosynthetic organisms. [2]

Fig.1 Example of the structure of a chloroplast.

Source: “Chloroplast.” DLPNG, dlpng.com/png/6626521.

CHLOROPHYLL

Fig.2 Chlorophyll under a microscope.

Source: Brookshire, Bethany. “Scientists Say: Chlorophyll.” Science News for Students, 3 Dec. 2019, www.sciencenewsforstudents.org/article/scientists-say-chlorophyll.

 

There are two main types of chlorophyll: chlorophyll-a and chlorophyll-b. All chlorophylls appear green to the eye and are soluble in lipids. They can be found in the thylakoid membranes of chloroplasts. The amount of chlorophyll found in a plant depends on the environment, soil conditions etc. [4] Chlorophyll contains a hydrophobic phytol chain which is embedded in the lipid membrane. The chain is linked to a tetrapyrrolic ring which is located out of the lipid membrane. The tetrapyrrolic ring is the structure of chlorophyll-a which absorbs energy from light. [2] This ring contains a prosthetic group - magnesium in the centre which allows the chlorophyll-a structure to accept or donate electrons readily. [3, 4] Chlorophyll is similar in structure to haemoglobin, a pigment found in red blood cells. [5]

Chlorophyll-a is the most abundant photosynthetic pigment in photosynthetic organisms such as plants. Chlorophyll-a absorbs blue (430nm) and red (662nm) light, and reflects the other wavelengths on the light spectrum. It reflects green light the most, causing photosynthetic organisms such as plants, to appear green to the human eye.

Chlorophyll-b is very similar to chlorophyll-a, structure-wise. It absorbs blue (453nm) and red (642) as well, and also reflects green light the most. Compared to chlorophyll-a, it is less abundant. 

Fig.3 Absorption spectrum of chlorophyll-a and chlorophyll-b.

Source: “Photosynthesis Pigments.” McDaniel College, www2.mcdaniel.edu/Biology/botf99/photo/p3igments.html. 

As chlorophyll-a and chlorophyll-b absorb slightly different wavelengths of light, it maximises the range of light a photosynthetic organism can get for ATP production which provides energy to the organism. [2]

There are rare types of chlorophyll knowns as chlorophyll-c, chlorophyll-d and chlorophyll-e. Chlorophyll-c and chlorophyll-d are most often found in non-green algae, and chlorophyll-e are found in golden algae. [5] 

Fig.4 Structures of chlorophyll-a and chlorophyll-b.

Source: Konwar, Mitali, and G. D. Baruah. A Possible Realization of Chlorophyll Laser. Centre for Laser & Optical Science, 2013.

CAROTENOIDS

Carotenoids are a class of accessory pigments since they are unable to transfer sunlight energy directly to the photosynthetic organism via the photosynthetic pathway, but they must pass on their absorbed energy to chlorophyll. [3] There are over 600 types of carotenoids. [6] They are hydrophobic and exist within the lipid membranes. Carotenoids can absorb light ranging from 460-550nm (yellow to red spectrum). [2] They are fat-soluble pigments which are often surrounded by chlorophyll, and or found in plastids of cells. They are soluble in fat due to their long carbon side chains, which allows carotenoids to partition into fatty regions of membranes, and makes the carotenoids more susceptible to oxidation. [4]

Fig.4 Absorption spectrum of chlorophyll-a, chlorophyll-b and carotenoids.

Source: “Photosynthesis: Light and Pigments.” Sciencemusicvideos, sciencemusicvideos.com/ap-biology/module-30-menu-photosynthesis/photosynthesis-3-light-and-pigments/.

They can also protect the plant from free radicals formed from ultraviolet radiation. [2] According to research, carotenoids also have anti-inflammatory benefits which can sometimes prevent cardiovascular diseases.

There are two types of carotenoids: carotenes and xanthophylls. Carotenes are hydrocarbons whereas xanthophyll contains carbon, hydrogen and oxygen. Xanthophylls are more yellow whereas carotenes are more orange, allowing them to absorb different wavelengths of light from yellow to red. [6] 

The most common carotenes are beta-carotene and alpha-carotenes. Beta-carotenes are the best carotenoids to convert into vitamin A after consumption of a photosynthetic organism. Vitamin A helps to maintain eye health and promotes a stronger immune system in humans. Beta-carotene can also protect against sunburn by increasing the protection level of the skin. Alpha-carotene is able to convert only half of the amount of vitamin A from beta-carotene in a given time. Alpha-carotene can help to reduce the risk of cancer and cardiovascular diseases. [6] 

Fig.5 Structures of alpha-carotene and beta-carotene.

Source: “Scientific Electronic Library Online.” SciELO, www.scielo.br/scielo.php?Script=sci_arttext.

Xanthophylls include lutein and zeaxanthin which are both heavily associated with the health of the eyes when digested from the photosynthetic organism. They are found in the macula of the eye. Zeaxanthin is found in the centre of the macula whereas lutein can be found nearer the outer areas of the macula. Lutein can protect against some types of cancer as well as enhance the immune system. Xanthophylls are able to work with carotenes increase antimutagenic and antitumor properties which reduce the risk of cancer. [7]

Fig.6 Structures of lutein and zeaxanthin.

Source: Arteni, Ana Andreea, and Mathias Fradot. Structure and Conformation of the Carotenoids in Human Retinal Macular Pigment. 2015.

BIBLIOGRAPHY

[1] “Photosynthetic Pigments.” Oxford Reference, www.oxfordreference.com/view/10.1093/oi/authority.20110803100324772. 

[2] “Photosynthesis Pigments.” McDaniel College, www2.mcdaniel.edu/Biology/botf99/photo/p3igments.html. 

[3] “Photosynthetic Pigments.” UC Museum of Paleontology, ucmp.berkeley.edu/glossary/gloss3/pigments.html. 

[4] Perera, Conrad O., and Anne D. Perera. “Technology of Processing of Horticultural Crops.” Handbook of Farm, Dairy and Food Machinery Engineering (Third Edition), Elsevier Inc., 2019, pp. 299–351. 

[5] “Chlorophyll.” Encyclopædia Britannica, www.britannica.com/science/chlorophyll. 

[6] Szalay, Jessie. “What Are Carotenoids?” LiveScience, Purch, 15 Oct. 2015, www.livescience.com/52487-carotenoids.html. 

[7] Tanumihardjo, S. A. “Carotenoids: Health Effects.” Encyclopedia of Human Nutrition (Third Edition), Elsevier Ltd., 2013, pp. 292–297.