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Why do plants have green leaves and not red?

Why do plants have green leaves and not red?


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I know plants are green due to chlorophyll.

Surely it would be more beneficial for plants to be red than green as by being green they reflect green light and do not absorb it even though green light has more energy than red light.

Is there no alternative to chlorophyll? Or is it something else?


Surely it would be even more beneficial for plants to be black instead of red or green, from an energy absorption point of view. And Solar cells are indeed pretty dark.

But, as Rory indicated, higher energy photons will only produce heat. This is because the chemical reactions powered by photosynthesis require only a certain amount of energy, and any excessive amount delivered by higher-energy photons cannot be simply used for another reaction1 but will yield heat. I don't know how much trouble that actually causes, but there is another point:

As explained, what determines the efficiency of solar energy conversion is not the energy per photon, but the amount of photons available. So you should take a look at the sunlight spectrum:

The Irradiance is an energy density, however we are interested in photon density, so you have to divide this curve by the energy per photon, which means multiply it by λ/(hc) (that is higher wavelengths need more photons to achieve the same Irradiance). If you compare that curve integrated over the high energy photons (say, λ < 580 nm) to the integration over the the low energy ones, you'll notice that despite the atmospheric losses (the red curve is what is left of the sunlight at sea level) there are a lot more "red" photons than "green" ones, so making leaves red would waste a lot of potentially converted energy2.

Of course, this is still no explanation why leaves are not simply black - absorbing all light is surely even more effective, no? I don't know enough about organic chemistry, but my guess would be that there are no organic substances with such a broad absorption spectrum and adding another kind of pigment might not pay off.3

1) Theoretically that is possible, but it's a highly non-linear process and thus too unlikely to be of real use (in plant medium at least)
2) Since water absorbs red light stronger than green and blue light deep sea plants are indeed better off being red, as Marta Cz-C mentioned.
3 And other alternatives, like the semiconductors used in Solar cells, are rather unlikely to be encountered in plants…

Additional reading, proposed by Dave Jarvis:


I believe it is because of a trade off between absorbing a wide range of photons and not absorbing too much heat. Certainly this is a reason why leaves are not black - the enzymes in photosynthesis as it stands would be denatured by the excess heat that would be gained.

This may go some of the way towards explaining why green is reflected rather than red as you suggested - reflecting away a higher energy colour reduces the amount of thermal energy gained by the leaves.


There is quite a fun article here which discusses the colours of hypothetical plants on planets around other stars.

Stars are classified by their spectral type which is dictated by their surface temperatures. The Sun's is relatively hot, and it's spectral energy distribution peaks in the green region of the spectrum. However the majority of stars in the Galaxy are K and M type stars which emit mainly in the red and infrared.

This is relevant to this discussion since any photosynthesis on these worlds would have to adapt to these wavelengths of light in order to proceed. On planets around cool stars plant life (or its equivalent) might well be black!

OK, this is not entirely pie in the sky astrobiologist rubbish. It is actually quite relevant to the search for biosignatures and life on other planets. In order to model the reflectance spectrum of planets we observe (i.e. the light reflected from the primary star) we need to try and take into account any potential vegetation.

For example, if we take a reflectance spectrum of the Earth, we see a characteristic peak in the red "the red edge" which is due to surface plant life.

NASA also has a short page on this here.


There are two factors at play here. First is the balance between how much energy a plant can collect and how much it can use. It is not a problem of too much heat, but too many electrons. If it were a question of heat, a number of flowers selected for their black pigmentation would have their petals cooked off. ;)

If a plant does not have enough water, is too cold, is too hot, collects too much light, or has some other condition that prevents the electron transport chain from functioning properly, the electrons pile up in a process called photoinhibition.

These electrons are then transferred to molecules that they should not be transferred to, creating free radicals, wreaking havok within the plant's cells. Fortunately, plants produce other compounds that prevent some of the damage by absorbing and passing around the electrons like hot potatos. These antioxidants are also beneficial to us when we eat them.

This explains why plants collect the amount of light energy they do, but does not explain why they are green, and not grey or dark red. Surely there are other pigments that would be able to generate electrons for the electron transport chain.

The answer to that is the same as why ATP is used as the main energy transport molecule in organisms rather than GTP or something else.

Chlorophyll a and b were just the first things that came about that fulfilled the requirement. Certainly some other pigment could have collected the energy, but that region of parameter space never needed to be explored.


I know this question was asked and answered a number of years ago (with many great answers), but I couldn't help but notice that no one had approached this from an evolutionary perspective (like the answer to this question)…

Short Answer

Pigments appear as whatever color is not absorbed (i.e, they appear as whichever wavelength(s) of light they reflect).

Blue light was the most available wavelength of light for early plants growing underwater, which likely led to the initial development/evolution of chlorophyll-mediated photosytems still seen in modern plants. Blue light is the most available, most high-energy light that continues to reach plants, and therefore plants have no reason not to continue taking advantage of this abundant high energy light for photosynthesis.

Different pigments absorb different wavelengths of light, so plants would ideally incorporate pigments that can absorb the most available light. This is the case as both chlorophyll a and b absorb primarily blue light. Absorption of red light likely evolved once plants moved on land due to its increased abundance (as compared to under water) and its higher efficiency in photosynthesis.


Long Answer

Early Plants Develop Modern Photo-system

It turns out, just like the variability in transmittance of different wavelengths of light through the atmosphere, certain wavelengths of light are more capable of penetrating deeper depths of water. Blue light typically travels to deeper depths than all other visible wavelengths of light. Therefore, the earliest plants would have evolved to concentrate on absorbing this part of the EM spectrum.

However, you'll notice that green light penetrates relatively deeply as well. The current understanding is that the earliest photosynthetic organisms were aquatic archaea, and (based on modern examples of these ancient organisms) these archaea used bacteriorhopsin to absorb most of the green light.

Early plants grew below these purple bacteriorhopsin-producing bacteria and had to use whatever light they could get. As a result, the chlorophyll system developed in plants to use the light available to them. In other words, based on the deeper penetrative ability of blue/green light and the loss of the availability of green light to pelagic bacteria above, plants evolved a photosystem to absorb primarily in the blue spectrum because that was the light most available to them.

  • Different pigments absorb different wavelengths of light, so plants would ideally incorporate pigments that can absorb the most available light. This is the case as both chlorophyll a and b absorb primarily blue light.

  • Here's two example graphs (from here and here) showing the absorption spectrum of typical plant pigments:

So Why Are Plants Green?

As you can guess from the above paragraphs, since early under water plants received so little green light, they evolved with a chlorophyll-mediated photo-system that did not have the physical properties to absorb green light. As a result, plants reflect light at these wavelengths and appear green.

But Why Are Plants Not Red?…

Reason to ask this question:

This would seem to be equally plausible given the above information. Since red light penetrates water incredibly poorly and is largely unavailable at lower depths, it would seem that early plants would not develop a means for absorbing it and therefore would also reflect red light.

  • In fact, [relatively] closely related red algae did evolve a red-reflecting pigment. These algae evolved a photo-system that also includes the pigment phycoerythrin to help absorb available blue light. This pigment did not evolve to absorb the low levels of available red light, and so therefore this pigment reflects it and makes these organisms appear red.

    • Interestingly, according to here, cyanobacteria that also contain this pigment can readily change it's influence on the organism's observed color:

      The ratio of phycocyanin and phycoerythrin can be environmentally altered. Cyanobacteria which are raised in green light typically develop more phycoerythrin and become red. The same Cyanobacteria grown in red light become bluish-green. This reciprocal color change has been named 'chromatic adaptation'.

  • Further, (although it's still under debate) according to work by Moreira et al (2000) (and corroborated by numerous other researchers) plants and red algae likely have a shared photosynthetic phylogeny:

    three groups of organisms originated from the primary photosynthetic endosymbiosis between a cyanobacterium and a eukaryotic host: green plants (green algae + land plants), red algae and glaucophytes (for example, Cyanophora).

So what gives?

Answer:

The simple answer of why plants aren't red is because chlorophyll absorbs red light.

This leads us to ask: Did chlorophyll in plants always absorb red light (preventing plants from appearing red) or did this characteristic appear later?

  • If the former was true, then plants don't appear red simply because of the physical characteristics that the chlorophyll pigments evolved to have.

  • As far as I know, we don't have a clear answer to that question.

    • (others please comment if you know of any resources discussing this).
  • However, regardless of when red light absorption evolved, plants nevertheless evolved to absorb red light very efficiently.

    • A number of sources (e.g., Mae et al. 2000, Brins et al. 2000, and here) as well as numerous other answers to this question, suggest that the most efficient photosynthesis occurs under red light. In other words, red light results in the highest "photosynthetic efficiency."

      • This NIH page suggests the reason behind this:

      Chlorophyll a also absorbs light at discrete wavelengths shorter than 680 nm (see Figure 16-37b). Such absorption raises the molecule into one of several higher excited states, which decay within 10−12 seconds (1 picosecond, ps) to the first excited state P*, with loss of the extra energy as heat. Photochemical charge separation occurs only from the first excited state of the reaction-center chlorophyll a, P*. This means that the quantum yield - the amount of photosynthesis per absorbed photon - is the same for all wavelengths of visible light shorter than 680 nm.

Why Did Plants Remain Green?

So why have plants not evolved to use green light after moving/evolving on land? As discussed here, plants are terribly inefficient and can't use all of the light available to them. As a result, there is likely no competitive advantage to evolve a drastically different photosystem (i.e., involving green-absorbing pigments).

So earth's plants continue to absorb blue and red light and reflect the green. Because green light so abundantly reaches the Earth, green light remains the most strongly reflected pigment on plants, and plants continue to appear green.

  • (However, note that other organisms such as birds and insects likely see plants very differently because their eyes can distinguish colors differently and they see more of the strongly reflected UV light that ours cannot).

The biologist John Berman has offered the opinion that evolution is not an engineering process, and so it is often subject to various limitations that an engineer or other designer is not. Even if black leaves were better, evolution's limitations can prevent species from climbing to the absolute highest peak on the fitness landscape. Berman wrote that achieving pigments that work better than chlorophyll could be very difficult. In fact, all higher plants (embryophytes) are thought to have evolved from a common ancestor that is a sort of green alga - with the idea being that chlorophyll has evolved only once. (reference)

Plants and other photosynthetic organisms are largely filled with pigment-protein complexes that they produce to absorb sunlight. The part of the photosynthesis yield that they invest in this, therefore, has to be in proportion. The pigment in the lowest layer has to receive enough light to recoup its energy costs, which cannot happen if a black upper layer absorbs all the light. A black system can therefore only be optimal if it does not cost anything (reference).

Red and yellow light is longer wavelength, lower energy light, while the blue light is higher energy. It seems strange that plants would harvest the lower energy red light instead of the higher energy green light, unless you consider that, like all life, plants first evolved in the ocean. Seawater quickly absorbs the high-energy blue and green light, so that only the lower energy, longer wavelength red light can penetrate into the ocean. Since early plants and still most plant-life today, lived in the ocean, optimizing their pigments to absorb the reds and yellows that were present in ocean water was most effective. While the ability to capture the highest energy blue light was retained, the inability to harvest green light appears to be a consequence of the need to be able to absorb the lower energy of red light (reference).

Some more speculations on the subject: (reference)


There are several parts to my answer.

First, evolution has selected the current system(s) over countless generations through natural selection. Natural selection depends on differences (major or minor) in the efficiency of various solutions (fitness) in the light (ho ho!) of the current environment. Here's where the solar energy spectrum is important as well as local environmental variables such as light absorption by water etc. as pointed out by another responder. After all that, what you have is what you have and that turns out to be (in the case of typical green plants), chlorophylls A and B and the "light" and "dark" reactions.

Second, how does this lead to green plants that appear green? Absorption of light is something that occurs at the atomic and molecular level and usually involves the energy state of particular electrons. The electrons in certain molecules are capable of moving from one energy level to another without leaving the atom or molecule. When energy of a certain level strikes the molecule, that energy is absorbed and one or more electrons move to a higher energy level in the molecule (conservation of energy). Those electrons with higher energy usually return to the "ground state" by emitting or transferring that energy. One way the energy can be emitted is as light in a process called fluorescence. The second law of thermodynamics (which makes it impossible to have perpetual motion machines) leads to the emission of light of lower energy and longer wavelength. (n.b. wavelength (lambda) is inversely proportional to energy; long wavelength red light has less energy per photon than does short wavelength violet (ROYGBIV as seen in your ordinary rainbow)).

Anyway, chlorophylls A and B are complex organic molecules (C, H, O, N with a splash of Mg++) with a ring structure. You will find that a lot of organic molecules that absorb light (and fluoresce as well) have a ring structure in which electrons "resonate" by moving around the ring with ease. It is the resonance of the electrons that determine the absorption spectrum of a given molecule (among other things). Consult Wikipedia article on chlorophyll for the absorption spectrum of the two chlorophylls. You will note that they absorb best at short wavelengths (blue, indigo, violet) as well as at the long wavelengths (red, orange, yellow) but not in the green. Since they don't absorb the green wavelengths, this is what is left over and this is what your eye perceives as the color of the leaf.

Finally, what happens to the energy from the solar spectrum that has been temporarily absorbed by the electrons of chlorophyll? Since its not part of the original question, I'll keep this short (apologies to plant physiologists out there). In the "light dependent reaction", the energetic electrons get transferred through a number of intermediate molecules to eventually "split" water into Oxygen and Hydrogen and generate energy-rich molecules of ATP and NADPH. The ATP and NADPH then are used to power the "light independent reaction" which takes CO2 and combines it with other molecules to create glucose. Note that this is how you get glucose (at least eventually in some form, vegan or not) to eat and oxygen to breath.

Take a look at what happens when you artificially uncouple the chlorophylls from the transfer system that leads to glucose synthesis. http://en.wikipedia.org/wiki/Chlorophyll_fluorescence Notice the color of the fluorescence under UV light!

Alternatives? Look at photosynthetic bacteria.


Tobias Keinzler does a good job of explaining why black plants would not work, this is an explanation of why plants are green and not some other color.

Color of foliage is based on whatever the color is of bacteria (or archaea) that get incorporated to become chloroplasts. Or more specifically the color of their light absorbing pigments. there is a huge range in nature for color in photosynthetic organisms, plants are green becasue chlorophyll is green, it could have just as easily been red or purple. http://www.ucmp.berkeley.edu/glossary/gloss3/pigments.html

There is decent evidence that chloroplast ancestors absorb the margins of the visible spectrum becasue halobacterium absorb the major constituents, becasue the chlorophyll users did not compete with them directly instead absorbing the leftover light. It was only later when they got incorporated into larger cells that they came to dominate and eventually giving rise to plants. Plants are not green becasue green is better, plants are green becasue that is the first efficient photosynthetic pigment to evolve that did not compete with the dominate photosynthesizer.


Photosynthesis in Leaves That Aren’t Green

A: Photosynthesis (which literally means “light put together”) is that very elegant chemical process that jump-started life as we know it some 4 billion years ago. So to answer your question, we’ll need a short chemistry lesson. Basically six molecules of water (H2O) plus six molecules of carbon dioxide (CO2) in the presence of light energy produce one molecule of glucose sugar (C6H12O6) and emit six molecules of oxygen (O2) as a by-product. That sugar molecule drives the living world. Animals eat plants, then breathe in oxygen, which is used to metabolize the sugar, releasing the solar energy stored in glucose and giving off carbon dioxide as a by-product. That’s life, in a nutshell.

All photosynthesizing plants have a pigment molecule called chlorophyll. This molecule absorbs most of the energy from the violet-blue and reddish-orange part of the light spectrum. It does not absorb green, so that’s reflected back to our eyes and we see the leaf as green. There are also accessory pigments, called carotenoids, that capture energy not absorbed by chlorophyll. There are at least 600 known carotenoids, divided into yellow xanthophylls and red and orange carotenes. They absorb blue light and appear yellow, red, or orange to our eyes. Anthocyanin is another important pigment that’s not directly involved in photosynthesis, but it gives red stems, leaves, flowers, or even fruits their color.

Many plants are selected as ornamentals because of their red leaves— purple smoke bush and Japanese plums and some Japanese maples, to name just a few. Obviously they manage to survive quite well without green leaves. At low light levels, green leaves are most efficient at photosynthesis. On a sunny day, however, there is essentially no difference between red and green leaves’ ability to trap the sun’s energy. I have noticed the presence of red in the new leaves of many Bay Area plants as well as in numerous tropical species. The red anthocyanins apparently prevent damage to leaves from intense light energy by absorbing ultraviolet light. There is also evidence that unpalatable compounds are often produced along with anthocyanins, which may be the plant’s way of advertising its toxicity to potential herbivores. So red-leaved plants get a little protection from ultraviolet light and send a warning to leaf-eating pests, but they lose a bit of photosynthetic efficiency in dimmer light.

Botanists have been wondering about red versus green leaves for the past 200 years and there is still much research to be done in this arena. So you are in good company, Paul.


How Does a Plant With Red Leaves Support Itself Without Green Chlorophyll?

A. Some parasitic plants lack chlorophyll entirely and steal the products of photosynthesis from their green hosts, said Susan K. Pell, director of science at the Brooklyn Botanic Garden. Other plants, like a red-leafed tree, have plenty of chlorophyll, but the molecule is masked by another pigment.

Chlorophyll absorbs red and blue light, “reflecting, and thus appearing, green,” Dr. Pell said. Chlorophyll uses this electromagnetic energy, along with carbon dioxide and water, to make glucose and oxygen.

Most plants also have other pigments: carotenoids, which usually appear yellow to orange, and anthocyanins, which are red to purple. One pigment usually dominates. So a plant with red leaves probably has higher than usual amounts of anthocyanins, Dr. Pell said. But chlorophyll is still present and at work.

“We used to think that all fall foliage color change resulted from the revealing of already-present carotenoids and anthocyanins when chlorophyll was broken down in preparation for dormancy,” she said. We now know that leaves actually produce additional anthocyanins into old age, she said.

The evolutionary advantages are not fully understood, Dr. Pell said. One theory is that extra anthocyanins provide shade under which chloroplasts (structures within cells) can break down their chlorophyll, helping the plant reabsorb its building blocks, especially valuable nitrogen. Another theory is that anthocyanins, which are powerful antioxidants, protect the plants in preparation for winter.


Spring Green: Why Do New Leaves Have a Lighter Color?

(Inside Science) -- Spring has now officially arrived in the Northern Hemisphere. Already, many deciduous trees are shaking off their winter stupor and getting ready to unfurl delicate new leaflets.

"In the coming weeks, we're definitely going to start to have bright greens, all the way to a sage green," said Carrie Andresen, a park ranger at Catoctin Mountain Park in northern Maryland. The park's forests contain oak, maple, hickory, tulip poplar and other trees.

In general, the green of spring leaves is fresher and lighter than the deep verdant hues of summer's mature canopy.

The reasons, say scientists, have to do with the way foliage develops. Young leaflets' chloroplasts -- the part of the plant that contains the green pigment chlorophyll -- are still developing, so the leaves tend to be lighter. New leaves are also thinner, with fewer waxy or tough layers that can darken the green color.

When leaves start maturing they begin making additional pigments. Some of these molecules can give leaves the yellow and red colors you see in the fall.

Younger leaves generally have fewer accessory pigments, so the green of the chlorophyll that is present is not masked, said Gregory Moore, a plant scientist at the University of Melbourne in Australia. This is another reason why spring green can look brighter, he said.

However, some new leaves, like those of the red maple, are typically tinged red in the spring. This is because lots of sugar is pumped into the small, young leaves to fuel their growth, and the sugar is sometimes converted into the red pigment anthocyanin and stored in the leaf, giving it a reddish appearance, Moore explained. As the leaves mature, the extra anthocyanin is metabolized and the leaves turn green.

The reddish hue can have an extra benefit to the young plant: protection from sun damage, said Susan Ustin, an ecologist from the University of California, Davis.

Ustin has studied the way foliage appears to cameras mounted on airplanes, drones, or even satellites in space. In addition to visible light, these cameras can often "see" light in the infrared part of the spectrum, beyond what human eyes can detect. Plant leaves strongly reflect near-infrared light, so this extra information helps scientists estimate the type and density of vegetation in pictures sometimes taken from hundreds of miles away.

Ustin said a remote camera monitoring an agriculture field, where the plants are typically all one species, might be able to detect the subtle pigment changes that happen from spring to summer.

At an even larger scale, NASA satellites have captured the spring "green up" across giant swaths of Earth, showing, for example in these 2006 images of the Chesapeake Bay area (shown on the right), how an area with only a dusting of light green in April transforms into a lush landscape painted with deep green strokes in July.

The color change shown in the pictures is likely primarily due to the increase in the number and size of tree leaves as the spring progresses, as well as the development of crops in the agricultural fields between forest patches, said Jeff Masek, a scientist at NASA Goddard Space Flight Center in Greenbelt, Maryland who specializes in satellite imaging of vegetation.

Future satellites may carry cameras capable of capturing a more precise picture of leaf chemistry, Masek said. While a normal color camera like the one on your phone gathers only 3 channels of light (red, green and blue), so-called hyperspectral imaging cameras can capture hundreds of different channels across the electromagnetic spectrum. Hyperspectral cameras have been deployed on airplanes and sent to the International Space Station, and NASA is working on plans for a satellite with hyperspectral imaging capabilities that could launch in the next decade.

The cameras can gather a wealth of data about the health and diversity of plants, said Phil Townsend, a biologist at the University of Wisconsin-Madison who uses remote imaging to study the functioning of ecosystems. A satellite with hyperspectral imaging could measure the pigments and structure of plant leaves, monitor nitrogen compounds in plants, or detect the presence of molecules, such as compounds that some plants use to defend against insects, which are invisible to human eyes. All this information can help answer questions about plant biology on a large scale -- such as how healthy whole forests or fields are, or how well plants are exchanging nutrients, water, and gases like carbon dioxide and oxygen with the surrounding environment and the atmosphere.

Back at a more personal scale, visitors to Catoctin Mountain Park are getting up-close views of spring vegetation, Andresen said. "It's that sense of escape, like animals coming out of hibernation. We as humans are escaping the cabin fever, we're wanting to get out on the trails and really discover the changes."

She said that in addition to the delicate colors of emerging leaves, spring is marked by its diversity of hues, from the vibrant green of moss, to the red flowers on the redbud trees. The park offers visitors the chance to borrow special glasses designed to enhance the distinction between red and green colors for some people with colorblindness. "Spring and fall seem to be the most popular times for people to check them out because of the difference in bright colors during those times of year," she said.


Why are some plants not green?

Though plants are generally thought to be green, there are some that are not. If a plant appears another color, such as red, it is not necessarily because the plant does not contain chlorophyll. Other pigments may cover up the green pigment, making the plant appear a different color. In this case, the plant is still an autotroph (self-feeder), using photosynthesis to generate energy. However, the chlorophyll's hue is just being masked.

There are also plants that do not contain chlorophyll and therefore also do not appear green. These plants are called heterotrophs, meaning "other feeding." As their name suggests, they cannot make their own food and will either obtain nutrients from other plants or will feed on fungi. 

Examples of non green plants:


Why Are Plants Green? To Reduce the Noise in Photosynthesis.

Land plants are green because their photosynthetic pigments reflect green light, even though those wavelengths hold the most energy. Scientists finally understand why.

Olena Shmahalo/Quanta Magazine

Rodrigo Pérez Ortega

From large trees in the Amazon jungle to houseplants to seaweed in the ocean, green is the color that reigns over the plant kingdom. Why green, and not blue or magenta or gray? The simple answer is that although plants absorb almost all the photons in the red and blue regions of the light spectrum, they absorb only about 90% of the green photons. If they absorbed more, they would look black to our eyes. Plants are green because the small amount of light they reflect is that color.

But that seems unsatisfyingly wasteful because most of the energy that the sun radiates is in the green part of the spectrum. When pressed to explain further, biologists have sometimes suggested that the green light might be too powerful for plants to use without harm, but the reason why hasn’t been clear. Even after decades of molecular research on the light-harvesting machinery in plants, scientists could not establish a detailed rationale for plants’ color.

Recently, however, in the pages of Science, scientists finally provided a more complete answer. They built a model to explain why the photosynthetic machinery of plants wastes green light. What they did not expect was that their model would also explain the colors of other photosynthetic forms of life too. Their findings point to an evolutionary principle governing light-harvesting organisms that might apply throughout the universe. They also offer a lesson that — at least sometimes — evolution cares less about making biological systems efficient than about keeping them stable.

The mystery of the color of plants is one that Nathaniel Gabor, a physicist at the University of California, Riverside, stumbled into years ago while completing his doctorate. Extrapolating from his work on light absorption by carbon nanotubes, he started thinking of what the ideal solar collector would look like, one that absorbed the peak energy from the solar spectrum. “You should have this narrow device getting the most power to green light,” he said. “And then it immediately occurred to me that plants are doing the opposite: They’re spitting out green light.”

In 2016, Gabor and his colleagues modeled the best conditions for a photoelectric cell that regulates energy flow. But to learn why plants reflect green light, Gabor and a team that included Richard Cogdell, a botanist at the University of Glasgow, looked more closely at what happens during photosynthesis as a problem in network theory.

The first step of photosynthesis happens in a light-harvesting complex, a mesh of proteins in which pigments are embedded, forming an antenna. The pigments — chlorophylls, in green plants — absorb light and transfer the energy to a reaction center, where the production of chemical energy for the cell’s use is initiated. The efficiency of this quantum mechanical first stage of photosynthesis is nearly perfect — almost all the absorbed light is converted into electrons the system can use.

But this antenna complex inside cells is constantly moving. “It’s like Jell-O,” Gabor said. “Those movements affect how the energy flows through the pigments” and bring noise and inefficiency into the system. Quick fluctuations in the intensity of light falling on plants — from changes in the amount of shade, for example — also make the input noisy. For the cell, a steady input of electrical energy coupled to a steady output of chemical energy is best: Too few electrons reaching the reaction center can cause an energy failure, while “too much energy will cause free radicals and all sorts of overcharging effects” that damage tissues, Gabor said.

Gabor and his team developed a model for the light-harvesting systems of plants and applied it to the solar spectrum measured below a canopy of leaves. Their work made it clear why what works for nanotube solar cells doesn’t work for plants: It might be highly efficient to specialize in collecting just the peak energy in green light, but that would be detrimental for plants because, when the sunlight flickered, the noise from the input signal would fluctuate too wildly for the complex to regulate the energy flow.

Instead, for a safe, steady energy output, the pigments of the photosystem had to be very finely tuned in a certain way. The pigments needed to absorb light at similar wavelengths to reduce the internal noise. But they also needed to absorb light at different rates to buffer against the external noise caused by swings in light intensity. The best light for the pigments to absorb, then, was in the steepest parts of the intensity curve for the solar spectrum — the red and blue parts of the spectrum.

The model’s predictions matched the absorption peaks of chlorophyll a and b, which green plants use to harvest red and blue light. It appears that the photosynthesis machinery evolved not for maximum efficiency but rather for an optimally smooth and reliable output.

Cogdell wasn’t fully convinced at first that this approach would hold up for other photosynthetic organisms, such as the purple bacteria and green sulfur bacteria that live underwater and are named for the colors their pigments reflect. Applying the model to the sunlight available where those bacteria live, the researchers predicted what the optimal absorption peaks should be. Once again, their predictions matched the activity of the cells’ pigments.

“When I realized how fundamental this was, I found myself looking in the mirror and thinking: How could I be so dumb not to think about this before?” Cogdell said.

(There are plants that don’t appear green, like the copper beech, because they contain pigments like carotenoids. But those pigments are not photosynthetic: They typically protect the plants like sunscreen, buffering against slow changes in their light exposure.)

“It was extraordinarily impressive, I think, to explain a pattern in biology with an incredibly simple physical model,” said Christopher Duffy, a biophysicist at Queen Mary University of London, who wrote an accompanying commentary on the model for Science. “It was nice to see a theoretically led work that understands and promotes the idea that it is robustness of the system that seems to be the evolutionary driving force.”

Researchers hope the model can be used to aid in the design of better solar panels and other solar devices. Although the efficiency of photovoltaic technology has advanced considerably, “I would say it’s not a solved problem in terms of robustness and scalability, which is something that plants have solved,” said Gabriela Schlau-Cohen, a physical chemist at the Massachusetts Institute of Technology.

Gabor has also set his mind on someday applying the model to life beyond Earth. “If I had another planet and I knew what its star was like, could I guess what photosynthetic life might look like?” he asked. In the code of his model — which is publicly available — there is an option to do exactly that with any selected spectrum. For now, the exercise is purely hypothetical. “In the next 20 years, we probably will have enough data on an exoplanet to be able to [answer] that question,” Gabor said.


Why Do Different Plants Have Different Shades Of Green?

However, these aren&rsquot the only light-harvesting pigments present in leaves. To a much lesser degree, pigments that color orange, red, and yellow are also present in leaves, but they play a secondary role to chlorophyll. Carotenoids and xanthophyll are only two others of the numerous pigments within leaves that capture the energy of the sun. Carotenoids give vibrant orange and yellow hues. They are the pigments that give carrots their iconic orange. Xanthophylls are the reason sunflowers have such sunny yellow petals, as well as reds and oranges of various other fruits and vegetables.

The different concentrations of these pigment will dictate the color of green in the plants around us. The different types and amounts of pigment in different species of plant can reflect their evolutionary roots and reveal information about the plant&rsquos habitat, its nutritional status and needs, and its age.


Review Questions

Multiple Choice Question

1. Tick (✓) the appropriate answer:

(i) Identify the plant which has compound leaves:
(a) Banana
(b) Banyan
(c) Mango
(d) Rose

(ii) Which one of the following is not an insectivorous plant—
(a) Pitcher plant
(b) Venus flytrap
(c) Bladderwort
(d) Cactus

(iii) This leaf shows parallel venation:
(a) Banana
(b) Mango
(c) Banyan
(d) Guava

(iv) The point on the stem from where the leaf arises is:
(a) Petiole
(b) Lamina
(c) Node
(d) Trunk

(v) Which one of the following is essential for photosynthesis:
(a) Carbon dioxide
(b) Nitrogen
(c) Oxygen
(d) Soil

Question 2.

Name the following:Answer :

  1. The part of the plant which grows under the ground: root
  2. The part of the plant which grows above the soil: shoot

Question 3.


Differentiate between the following:

(i) Tap root and Fibrous root
Answer :
Tap root

  1. This root has one main primary root with many side secondary roots.
  2. It is found in dicot plants.
  3. e.g. mango, pea

Fibrous root

  1. These roots are clusters of same thickness and size, arising from the base of the stem.
  2. It is found in monocot plants,
  3. e.g. maize, wheat

(ii) Simple Leaf and compound leaf
Answer :
Simple Leaf

  1. The Lamina is uni divided and is a single piece.
  2. Example : mango,banana, banyan, etc.

Compound Leaf

  1. The leaf blade or lamina is divided into smalled units called leaflets.
  2. Example is rose.

(iii) Parallel venation and reticulate venation
Answer :
Parallel Venation

  1. In this type of venation,veins and veinlets are irregularly distributed in the lamina, forming a network.
  2. Examples are peepal, mango and guava leaves.

Reticulate Venation

  1. In this type of venation, veins are parallel to each other.
  2. Examples are banana, grass and wheat leaves.

Question 4.


What are the four functions of the roots ?
Answer :
The root serves the following functions :

  1. It fixes the plant in the soil.
  2. Absorbs water and minerals from the soil for the entire plant.
  3. It acts as a storage part for food materials for certain plants.
  4. It binds the soil together so that it does not get washed away during rain or blown over by the wind.

Question 5.


Mention the functions of the following :

(i) Spines
(ii) Tendril
(iii) Scale leaves
Answer :
(i) Spines—The leaves may be modified to form spines to reduce water loss by transpiration in desert plants.
(ii) Tendril — The stem may occur in the form of their thread – like leafless branch called tendril. It has the ten-dency to coil around any object and help the plant to climb it
(iii) Scale leaves — Scale leaves are present in some plants like onion and ginger. They are thin and dry or thick and fleshy and their function is to protect buds.

Question 6.


Define venation. What are the different types of ve-nation found in the leaves ?
Answer :
Venation: Arrangement of pattern of veins in a lanuina is called venation.
It is mainly of two types :

  1. Reticulate venation : Veins and veinlets are irregularly distributed in the lamina forming a network.
    Example: mango, guava.
  2. Parallel venation: Veins run parallel to each other
    Example: Banana, grass, wheat

Question 7.


Describe the modifications of leaf in any one insec-tivorous plant.
Answer :
Modification of leaves in Venus flytrap (an insectivorous plant)
The leaves of Venus flytrap have long pointed hair. It is divided into two parts having midrib in between like a hinge. When an insect visits the leaf, it closes its two parts and traps the insect. The insect is then digested by secreting digestive juices.

Question 8.


Write the two main functions of leaves.
Answer :
The two main functions of leaves are –

  1. Photosynthesis – Green leaves contain chlorophyll which, in presence of sunlight, manufacture food using carbon-dioxide and water.
  2. Transpiration – Surface of leaves have minute pores which help in loss of water by evaporation. It has cooling effect making roots absorb more water due to suction.

Question 9.

What is the modification seen in the Bryophyllum. Explain.
Answer :

  1. Bryophyllum is a plant whose leaves produce adventitious buds in their margin.
  2. The adventitious buds grow into new plants when they fall off from the parent plant.

Question 10.


Define:
(i) Photosynthesis
(ii) Tranpiration
Answer :
(i) Photosynthesis — The process by which plant leaf prepares or synthesises food from water and carbon dioxide in the presence of chlorophyll and sunlight is called photosynthesis.
(ii) Tranpiration — This is the process by which there is a loss of water in the form of vapour by evaporation from the surface of leaves. It has cooling effect, it causes suction force to make roots absorb more water with mineral ions.

Question 11.


Name the wide flat portion of the leaf
Answer :
The green, flat and broad part of the leaf is called ‘lamina’ or ‘leaf blade’.

Question 12.


What purpose is served by the spines horned on the leaves of cactus.
Answer :
Leaves are modified into spines to reduce water loss, like cactus. In prickly poppy, leaves bear spines on the margin.

Question 13.


Explain why leaf survival is so important to the plant?
Answer :
Because they perform two main function of photosynthesis and transpiration.

Question 14.

Give an example of the following and draw generalized diagrams for the same:
(i) Simple leaf and compound leaf.
(ii) Parallel venation and reticular venation.
Answer :
(i) Simple leaf and compound leaf.

  1. Simple leaf: In a simple leaf, the lamina is undivided and is a single piece, e.g., mango, banana, banyan, etc.
  2. Compound leaf: In a compound leaf, the leaf blade or lamina is divided into smaller units called leaflets e.g., rose.

(ii) Parallel venation and reticular venation.

  1. venation (Parallel ): In this type of venation, veins run and
    parallel to each other, e.g., banana, grass, maize and wheat leaves (monocot plants).
  2. Reticulate venation: In this type of venation, veins and veinlets are irregularly distributed in the lamina, forming a network, e.g. peepal, mango and guava leaves (dicot plants).

Question 15.

In list some of the advantages of transpiration to green plants.
Answer :
It helps to maintain the concentration of the sap inside the plant body:
The roots continue to absorb water from the soil. If excess water does not evaporate through transpiration, the sap will become dilute, preventing further absorption of water and minerals from the soil.

Cooling effect: In transpiration, water gets evaporated from the plant. The heat required for evaporation of water is obtained from the plant itself and thus, the plant cools itself when it is hot outside.

Question 16.

Why do some plants have to trap insects ?
Answer :
Insectivorous plants trap insect because they grow in a soil which is deficient in nitrogen and insects help in fulfilling the nitrogen requirement of plants.

Question 17.


Explain some of the modifications of leaves found in plants.
Answer :
Sometimes, the complete leaf or a part of the leaf is modified to perform a special function.
Some of these modifications include:

  1. Leaf tendril: In case of certain weak stemmed plants, leaves or leaflets are modified into wiry, coiled structures called tendrils. They are sensitive to touch. As they touch any object, they coil around it and support the plant to climb up. Eg., Sweet pea (upper leaflets are modified into tendrils).
  2. Spines: Leaves are modified into spines to reduce water loss, like cactus. In prickly poppy, leaves bear spines on the margin.
  3. Scale leaves: In some plants, like onion and ginger, thin and dry or thick and fleshy scale leaves are present.Their function is to protect buds.

Question 18.


What is a tendril ? Explain its use to the plant.

Answer :
A tendril is a specialized stem, leave or petrole with a thread like shop. They are sensitive to touch. As they touch any object, they coil around it and support the plant to climb up. Example : Sweet pea (upper leaflets are modified into tendrils).

Question 19.

Complete the cross word using the clues given below. Check your performance with the correct solutions given at the end of the chapter.


Nutrient Deficiencies

Too little nitrogen will cause a pepper plant's oldest leaves to turn yellow while the leaves on the rest of the plant may turn light green. Too little iron, manganese, molybdenum or zinc can also cause a lightening or yellowing of pepper leaves. Working a few inches of well-rotted compost, aged manure or another organic soil amendment into the site before planting, and side-dressing the plants with nitrogen several weeks after transplanting, will help to prevent nutrient deficiency problems. You may also need to use a supplemental micronutrient fertilizer. A soil or leaf test is the best way to determine with certainty that a nutrient deficiency is responsible for the pepper plant's light green color.


Meaning of Chlorophyll

Chlorophyll refers to a light-absorbing pigment molecule that reflects a green colour to the chloroplast containing tissues by absorbing light of longer wavelength (red) and light of shorter wavelength (blue) of the electromagnetic spectrum. Chlorophylls are significantly of two kinds, namely chlorophyll-a, and b. These two pigments differ by having different side-chain composition and the distinct absorption tendency.

  • Chlorophyll-a consists of a methyl group (CH3) in the side chain and tends to absorb more red light of the visible spectrum.
  • Chlorophyll-b consists of the aldehyde group (CHO) in the side chain and tends to absorb more violet-blue light of the visible spectrum.

History

Year of discoveryDiscovererDiscovery
1817Joseph Bienaime Caventou and Pierre Joseph PelletierIsolated and termed “Chlorophyll”
1864StokesThrough spectroscopy, demonstrated that chlorophyll is a mixture of two components (Chl-a and b)
1906-The presence of magnesium in chlorophyll has been detected
1906-1915Richard WillstatterIntroduced general structure of chlorophyll
1940Hans FischerIntroduced the structure of chlorophyll-a
1960Robert Burns WoodwardIntroduced synthesis of chlorophyll-a
1967Lan FlemingStudied the remaining stereochemical elucidation
1990Woodward and Co-authorsPublished an updated synthesis of chlorophyll
2010-Presence of chlorophyll-f has been detected in cyanobacteria

Why is Chlorophyll green?

Chlorophyll is a green pigment, which absorbs red and blue spectrum of the visible light and transmits green light. Due to the reflection of green light, all the chlorophyll-containing tissues or organelles appear green-coloured. Green colour of the leaves and stems is also due to this chlorophyll pigment.

Structure of Chlorophyll

A typical composition of chlorophyll comprises of a porphyrin head and a long phytol tail. Chlorophyll is a chelating ligand, which includes a central metal ion attached to the complex organic compound containing a mixture of carbon, nitrogen and hydrogen elements.

The structure of chlorophyll is characterized by:

  • The presence of magnesium (Mg 2+ ) as a central metal ion.
  • A varying side chain.
  • The presence of an extra fifth ring or isocyclic ring, fixed to the porphyrin head.


Porphyrin Head

It typically includes four pyrrole rings fixed to the coordinated central metal and called “Tetrapyrroles”. The first pyrrole ring is substituted with the side chain differing in both the chlorophyll pigments. Both the chlorophyll pigments, i.e. chl-a and b have a different side chain, CH3 and CHO respectively.

The porphyrin ring has a square planar arrangement, where the four nitrogen atoms join the four pyrrole rings to the central magnesium ion. Besides plants, the porphyrin ring also exists in haemoglobin and vitamin-B12 molecules that have a different central atom like iron and cobalt, respectively.

To the base of the porphyrin ring, an extra isocyclic ring is present. Porphyrin ring is a stable ring, around which an electron can migrate freely. It results in a high tendency of porphyrin ring to gain or lose electrons.

Phytol Tail

It associates with the porphyrin head via ester. It refers to the unsaturated hydrocarbon chain that contains 39 H-atoms and 20 C-atoms with two C-C double bonds.

A phytol chain is composed of four isoprene units with a chemical name (2-methyl-1, 3-butadiene). One isoprene unit has a molecular formula C5H8, as it consists of five carbon atoms and eight hydrogen atoms.

Types of Chlorophyll in Plants

Chlorophyll-a and Chlorophyll-b are the two pigments that are commonly present in the plants.

Chl-a serves as the primary light-absorbing pigment. Oppositely, chl-b works as an accessory pigment. Both the pigments absorb light of certain wavelength from the incoming white light emitted by the sun.

White light includes seven different colours like violet, indigo, blue, green, yellow, orange and red that we call “VIBGYOR”. Violet, blue, red and orange light are generally absorbed from the visible white light.

Chl-a shows great absorbencies towards the light of the red and orange spectrum, while chl-b shows great absorbencies towards the light of the violet and blue spectrum. Chlorophyll-a is a universal pigment present in all oxygenic photosynthetic organisms, while chlorophyll-b is ubiquitous in higher plants and some algae. In plant chlorophylls are embedded in the sac-like thylakoid membrane.

A thylakoid membrane involves many light-absorbing and accessory pigments that collectively form a Photosystem. An antenna or light-harvesting complex plus an active reaction centre consititute a photosystem. Chl-a is a primary pigment that absorbs the light energy (photons) from the sun (carries a bundle of photons) and passes it to the other pigment molecules till it outreaches a reaction centre.

In a photosystem, a reaction centre functions as an electron donor that transfers the photons to the electron acceptor molecule for the further cellular activities. Chlorophyll-b functions as an accessory pigment that expands the light-absorbing capacity of the light-absorbing particles.

Facts about Chlorophyll

There are some interesting facts about the chlorophyll that we must explore.

During plant senescence and fruit ripening

Plants degreen during the senescence stage and at the time of fruit ripening because during that period the chlorophyll pigments transform into colourless tetrapyrroles or NCC’s (Non-fluorescent chlorophyll catabolites).

Absorbing intensity

Chlorophyll has the highest absorbing capacity among the plant pigments, due to which it dominates or masks the leaf by its green colour. Once chlorophylls start to decompose, the colour of the leaves turn red, yellow, orange etc. For a plant to appear green, it must continuously replenish the chlorophyll.

Chemistry of chlorophyll

We can get a crystalline form of chlorophyll once the dried leaves are pulverized and treated with ethanol. We will get an amorphous form of chlorophyll when the dried leaves are pulverized and subjected to the treatment with reagents like ether or acetone. Chlorophyll is a mixture of two components, which includes a ratio of 3:1 of chl-a, and chl-b.

  • Solubility: Chlorophyll is a hydrophobic or fat-soluble organic compound that readily dissolves in lipids.
  • Acid treatment of chlorophyll causes a replacement of the magnesium with the two H-atoms and results in the formation of derivative “Phaeophytin” (olive-brown colour solid). Further hydrolysis of pheophytin causes splitting of phytol and results in the production of “Phaeophorbide”.
  • Base treatment of chlorophyll results in the formation of a series of phyllins and magnesium porphyrin compounds.
  • Denaturation: Prolonged cooking and steaming denature the conformation of chlorophyll.

Food sources of chlorophyll

Asparagus, bell peppers, broccoli, green cabbage, celery, kale, green olives, spinach, alfalfa etc.

Commercial use

Chlorophyll extracts of a plant are commercially used as additives in processed soaps, toothpaste, cosmetics, food products etc.

Medicinal uses

Chlorophyll has a wide range of medicinal uses. It serves as a natural body cleanser. Its regular uptake can reduce the faecal and urinary odour. Chlorophyll increases the bone, nail and teeth strength. It also provides immune support by increasing the RBCs count and reduces colon and liver cancer by interfering with the procarcinogens. Chlorophyll also detoxifies the blood by eliminating impurities from our body.