Plants contain a variety of pigments that are responsible for their characteristic green color and for absorbing light energy during photosynthesis. The main pigment found in plants is chlorophyll, which exists in several forms but chlorophyll a and chlorophyll b are the most abundant.
What is chlorophyll?
Chlorophyll is a green pigment found in the chloroplasts of plant cells and other photosynthetic organisms like algae. The chlorophyll molecule consists of a porphyrin ring coordinated to a magnesium ion surrounded by long hydrophobic hydrocarbon tail. It absorbs light most strongly in the blue and red regions of the visible light spectrum.
There are several different forms of chlorophyll that differ slightly in their chemical structure. The two most common forms are:
- Chlorophyll a – This is the most abundant form and has a blue-green color. Its absorption peaks are at 430 nm and 662 nm.
- Chlorophyll b – This is slightly yellowish-green. Its absorption peaks are at 453 nm and 642 nm.
Other less common forms include chlorophyll c, chlorophyll d, and chlorophyll f. All chlorophylls contain a network of alternating single and double bonds in the porphyrin ring which causes them to absorb light and appear green.
Function of chlorophyll
Chlorophyll’s main function is to absorb light energy during photosynthesis. The porphyrin head of the molecule absorbs photons and gets excited to a higher energy state when it is hit by light. This energy is then used in photosynthesis to drive the production of carbohydrates from carbon dioxide and water.
Specifically, chlorophyll is located in the thylakoid membranes inside chloroplasts. When a chlorophyll molecule absorbs light, it promotes one of its electrons to a higher energy level. This excited electron is shuttled through an electron transport chain, which pumps hydrogen ions into the thylakoid space. This creates a proton gradient that drives the production of the energy carrier molecule ATP and reducing power in the form of NADPH. ATP and NADPH are then used in the Calvin cycle to fix CO2 into sugar.
By absorbing light energy and channeling it into chemical energy, chlorophyll allows plants to utilize the energy from sunlight to power biochemical reactions. This process of photosynthesis provides energy for plants and also releases oxygen as a byproduct, which makes chlorophyll critically important for life on Earth.
Why do plants appear green?
Plants appear green because chlorophyll strongly absorbs red and blue light, but it reflects green light. When white light from the sun hits the leaves, the red and blue wavelengths are absorbed by chlorophyll, while the green wavelength is reflected back to our eyes. Since our eyes predominantly detect this reflected green light, it results in the green color we see.
Here is a more detailed explanation:
- The visible light spectrum that humans can see ranges from about 400-700 nm wavelength. Green light is around 500-570 nm.
- Chlorophyll a maximally absorbs blue light at 430 nm and red light at 662 nm.
- Chlorophyll b maximally absorbs blue light at 453 nm and red light at 642 nm.
- Therefore, chlorophyll absorbs very little light in the green range.
- When white light strikes a leaf, the blue and red wavelengths are absorbed, while the green is reflected back to the eye, making the leaf appear green.
In the fall, the green color fades as chlorophyll breaks down and other pigments like carotenoids become visible and create the yellow, orange, and red hues of autumn foliage.
Other plant pigments
In addition to chlorophyll, plants also contain various other pigment molecules that absorb light and aid photosynthesis. Some common ones include:
- Carotenoids – These are red, orange, and yellow pigments. The most common is beta-carotene. They help absorb blue-green and green light and protect chlorophyll from damage.
- Anthocyanins – Water-soluble purple and red pigments located in cell vacuoles. They absorb green light and protect leaves from UV damage.
- Phaeophytin – A gray-brown pigment created when chlorophyll loses its magnesium ion. It absorbs blue and red light.
- Xanthophylls – Yellow oxygenated carotenoids like lutein that absorb blue and green light.
While chlorophyll is the predominant pigment, plants evolved other accessory pigments like carotenoids and anthocyanins to broaden the spectrum of light they can absorb for photosynthesis. Each pigment absorbs different wavelengths of light.
Absorption spectra of plant pigments
The various pigments found in plants have different characteristic absorption spectra. Here is a comparison of the absorption of different plant pigments across wavelengths of visible light:
|Chlorophyll a||430 nm, 662 nm||Blue-green|
|Chlorophyll b||453 nm, 642 nm||Greenish-yellow|
|Beta-carotene||450 nm, 480 nm||Orange|
|Lutein||425 nm, 450 nm||Yellow|
|Anthocyanins||500-550 nm||Red, purple|
This range of absorption allows plants to harvest light across much of the visible spectrum for use in photosynthesis. The combined effect of all these pigments is why plant leaves look green.
Chlorophyll concentration in different plants
The concentration of chlorophyll varies widely among different plant species. Some factors that affect chlorophyll concentration include:
- Light exposure – Plants adapted to high-light conditions often have less chlorophyll, while shade-loving plants have more.
- Age of plant or leaf – Newly emerging leaves have lower chlorophyll until they mature.
- Nutrient availability – Especially nitrogen which is incorporated into chlorophyll.
- Water stress – Can reduce chlorophyll synthesis.
- Temperature – High temperatures may break down chlorophyll.
Here are some examples of chlorophyll concentrations measured in a few common plants:
|Plant||Chlorophyll concentration (mg/g leaf tissue)|
In general, chlorophyll content is much higher in leafy greens and herbs versus woody plants. Measuring chlorophyll content can provide useful information about the health and productivity of plants.
Biosynthesis of chlorophyll
Chlorophyll is synthesized from simple precursor molecules through a complex biosynthetic pathway. Here are the key steps:
- The precursor aminolevulinic acid (ALA) is made from glutamate.
- ALA condenses to form the pyrrole porphobilinogen (PBG).
- Four PBG molecules join to make tetrapyrrole ring uroporphyrinogen III.
- Additional modifications form protoporphyrin IX.
- Iron is inserted to make protoheme IX (heme).
- The heme is converted to chlorophyll by adding phytol chain and Mg2+ ion.
This multistep pathway involves chloroplast and mitochondrial enzymes as well as metabolic intermediates from other parts of the cell. Chlorophyll synthesis is tightly coordinated with synthesis of other photosynthetic components in the chloroplast.
Breakdown and loss of chlorophyll
Chlorophyll is constantly being broken down and resynthesized in plant leaves. It can degrade through various pathways:
- Chlorophyllase enzyme – Cleaves the phytol tail, creating chlorophyllide.
- Mg-dechelatase – Removes the Mg2+, forming pheophytin which appears olive-gray.
- Pheophorbide oxygenase – Opens the porphyrin ring in a multi-step pathway.
- Chlorophyll peroxidases – Oxidation by peroxides damages the chlorophyll molecule.
- Free radical reactions – Singlet oxygen and other reactive species degrade chlorophyll.
These breakdown products are either recycled, or accumulate as leaf litter. In autumn, chlorophyll is actively broken down allowing other leaf pigments like carotenoids to become visible, leading to the yellow and red fall colors.
Uses and applications of chlorophyll
Aside from its role in photosynthesis, chlorophyll has various applications due to its green color and chemical properties:
- Food coloring – Chlorophyll extracted from plants is used as a natural green dye in many foods like pasta, beverages, candy, etc.
- Antioxidant – Some research shows it may have antioxidant and anti-inflammatory activities when consumed.
- Odor remover – Chlorophyll can help bind and neutralize odors and is used in some topical ointments.
- Photodynamic therapy – Chlorophyll derivatives are studied for their ability to kill cancer cells when exposed to certain wavelengths of light.
Further research may uncover additional uses of chlorophyll in medicine, cosmetics, and other applications in the future.
In summary, the predominant pigment giving plants their characteristic green color is chlorophyll. It is essential for absorbing light energy during photosynthesis to power biochemical reactions. While chlorophyll is the most abundant plant pigment, other accessory pigments like carotenoids broaden the spectrum of light plants can utilize. Chlorophyll concentration varies widely between plant species and conditions. Elucidating the diverse functions and applications of chlorophyll and other plant pigments remains an active area of research.