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Course: Biology library > Unit 13
Lesson 2: The light-dependent reactionsLight and photosynthetic pigments
Properties of light. How chlorophylls and other pigments absorb light.
Introduction
If you've ever stayed out too long in the sun and gotten a sunburn, you're probably well aware of the sun's immense energy. Unfortunately, the human body can't make much use of solar energy, aside from producing a little Vitamin D (a vitamin synthesized in the skin in the presence of sunlight).
Plants, on the other hand, are experts at capturing light energy and using it to make sugars through a process called photosynthesis. This process begins with the absorption of light by specialized organic molecules, called pigments, that are found in the chloroplasts of plant cells. Here, we’ll consider light as a form of energy, and we'll also see how pigments – such as the chlorophylls that make plants green – absorb that energy.
What is light energy?
Light is a form of electromagnetic radiation, a type of energy that travels in waves. Other kinds of electromagnetic radiation that we encounter in our daily lives include radio waves, microwaves, and X-rays. Together, all the types of electromagnetic radiation make up the electromagnetic spectrum.
Every electromagnetic wave has a particular wavelength, or distance from one crest to the next, and different types of radiation have different characteristic ranges of wavelengths (as shown in the diagram below). Types of radiation with long wavelengths, such as radio waves, carry less energy than types of radiation with short wavelengths, such as X-rays.
The visible spectrum is the only part of the electromagnetic spectrum that can be seen by the human eye. It includes electromagnetic radiation whose wavelength is between about 400 nm and 700 nm. Visible light from the sun appears white, but it’s actually made up of multiple wavelengths (colors) of light. You can see these different colors when white light passes through a prism: because the different wavelengths of light are bent at different angles as they pass through the prism, they spread out and form what we see as a rainbow. Red light has the longest wavelength and the least energy, while violet light has the shortest wavelength and the most energy.
Although light and other forms of electromagnetic radiation act as waves under many conditions, they can behave as particles under others. Each particle of electromagnetic radiation, called a photon, has certain amount of energy. Types of radiation with short wavelengths have high-energy photons, whereas types of radiation with long wavelengths have low-energy photons.
Pigments absorb light used in photosynthesis
In photosynthesis, the sun’s energy is converted to chemical energy by photosynthetic organisms. However, the various wavelengths in sunlight are not all used equally in photosynthesis. Instead, photosynthetic organisms contain light-absorbing molecules called pigments that absorb only specific wavelengths of visible light, while reflecting others.
The set of wavelengths absorbed by a pigment is its absorption spectrum. In the diagram below, you can see the absorption spectra of three key pigments in photosynthesis: chlorophyll a, chlorophyll b, and β-carotene. The set of wavelengths that a pigment doesn't absorb are reflected, and the reflected light is what we see as color. For instance, plants appear green to us because they contain many chlorophyll a and b molecules, which reflect green light.
Most photosynthetic organisms have a variety of different pigments, so they can absorb energy from a wide range of wavelengths. Here, we'll look at two groups of pigments that are important in plants: chlorophylls and carotenoids.
Chlorophylls
There are five main types of chlorophylls: chlorophylls a, b, c and d, plus a related molecule found in prokaryotes called bacteriochlorophyll.
In plants, chlorophyll a and chlorophyll b are the main photosynthetic pigments. Chlorophyll molecules absorb blue and red wavelengths, as shown by the peaks in the absorption spectra above.
Structurally, chlorophyll molecules include a hydrophobic ("water-fearing") tail that inserts into the thylakoid membrane and a porphyrin ring head (a circular group of atoms surrounding a magnesium ion) that absorbs lightstart superscript, 1, end superscript.
Although both chlorophyll a and chlorophyll b absorb light, chlorophyll a plays a unique and crucial role in converting light energy to chemical energy (as you can explore in the light-dependent reactions article). All photosynthetic plants, algae, and cyanobacteria contain chlorophyll a, whereas only plants and green algae contain chlorophyll b, along with a few types of cyanobacteriastart superscript, 2, comma, 3, end superscript.
Because of the central role of chlorophyll a in photosynthesis, all pigments used in addition to chlorophyll a are known as accessory pigments—including other chlorophylls, as well as other classes of pigments like the carotenoids. The use of accessory pigments allows a broader range of wavelengths to be absorbed, and thus, more energy to be captured from sunlight.
Carotenoids
Carotenoids are another key group of pigments that absorb violet and blue-green light (see spectrum graph above). The brightly colored carotenoids found in fruit—such as the red of tomato (lycopene), the yellow of corn seeds (zeaxanthin), or the orange of an orange peel (β-carotene)—are often used as advertisements to attract animals, which can help disperse the plant's seeds.
In photosynthesis, carotenoids help capture light, but they also have an important role in getting rid of excess light energy. When a leaf is exposed to full sun, it receives a huge amount of energy; if that energy is not handled properly, it can damage the photosynthetic machinery. Carotenoids in chloroplasts help absorb the excess energy and dissipate it as heat.
What does it mean for a pigment to absorb light?
When a pigment absorbs a photon of light, it becomes excited, meaning that it has extra energy and is no longer in its normal, or ground, state. At a subatomic level, excitation is when an electron is bumped into a higher-energy orbital that lies further from the nucleus.
Only a photon with just the right amount of energy to bump an electron between orbitals can excite a pigment. In fact, this is why different pigments absorb different wavelengths of light: the "energy gaps" between the orbitals are different in each pigment, meaning that photons of different wavelengths are needed in each case to provide an energy boost that matches the gapstart superscript, 4, end superscript.
An excited pigment is unstable, and it has various "options" available for becoming more stable. For instance, it may transfer either its extra energy or its excited electron to a neighboring molecule. We'll see how both of these processes work in the next section: the light-dependent reactions.
Want to join the conversation?
Is the lumen really space or does it just absorb all other wavelengths and appear empty? How would you tell?(14 votes)
The lumen is very much so a space. Our Intestines have a lumen. By definition, the lumen is simply an internal body cavity encapsulated or enclosed by something. In this case, the thylakoid membrane (the 3rd membrane of chloroplasts) encloses the lumen. The lumen would contain all of the reactants and intermediates of the light-dependent reactions. As the video displayed, water molecules are broken and the H+ protons are pumped into the lumen. So overall, the lumen is a cavity that contains the organic molecules (H+) that have diffused or transported across the thylakoid membrane. Hope this helps!(38 votes)
- Why are leaves green even though other pigments are present?(7 votes)
Because chlorophyll is most abundant pigment so it masks other pigments.(11 votes)
- Plants contain chlorophyll a which absorbs green lights but in the light dependent stage,
light of wavelengths 680nm and 700nm which are red lights are absorbed why is that so please.(5 votes)
Chlorophyll looks green not because it absorbs green light, but actually because it doesn't absorb it. The green is left unabsorbed, so it can reach your eyes.(8 votes)
why are wavelengths and pigments important for photosynthesis?(4 votes)- They're responsible for what light is absorbed and what light is reflected.(8 votes)
For clarification, do all plants have both chlorophyll a and b (I presume in the leaves?), or do all plants have chlorophyll a and only some have chlorophyll b? Thanks!(6 votes)
No that'she not right. In algae and cyanobacteria there is chlorophyll a and chlorophyll b occurs in green algae and plants.
Source: www.ucmp.berkeley.edu/glossary/gloss3/pigments.html(2 votes)
- Can we conduct photosynthesis under monochromatic light rays such as green?(5 votes)
- Interesting question! Whether the plant would be able to live or not depends both upon the plant itself and the wavelength of the light. Different types of pigments absorb different wavelengths of light, and some plants have more of one type than others. If a plant has more carotene, for example, it would better absorb orange light. No pigment really absorbs green light best, which is why its reflected and most plants are green or greenish. Scientists can create "action spectrums" that show what wavelengths of light result in the most oxygen produced (to measure the amount of photosynthesis).
It turns out that green light is actually very useful for plants, and although it is the most reflected light it does serve a purpose, with the plant still managing to use most of the green light thrown at it. Green light, for some reason, penetrates better into lower-lying leaves and allows them to photosynthesize better.
Under a monochromatic light source, the plants obviously wouldn't do as well. Red light is the most important, as chlorophyll a, the most common type, absorbs light best in the red area of the spectrum. The blues are second, and green comes in last. It's hard to tell for sure if the plant will photosynthesize enough to thrive under green light, but it definitely will at least a little.(4 votes)
How do carotenoids dissipate the excess energy as heat?(5 votes)
The simple answer is that due to their structure carotenoids are able to convert chemical potential energy into vibrational energy.
This seems like a reasonably informative page:
https://biology.appstate.edu/fall-colors/hidden-colors-leaves-what-are-functions-those-yellow-and-orange-pigments-we-see-fall
• the portions most relevant to your question are about half-way down starting with "Carotenoids are More than Just Accessory Pigments"(3 votes)
Just asking a question here, when an acid interacts with chlorophyll, which part of the chlorophyll gets stripped off? Thanks!(5 votes)- The acid removes the magnesium ion replacing it with two hydrogen atoms giving an olive-brown solid, pheophytin-a.(2 votes)
- what is the role of pigments during photosynthesis?(2 votes)
- I suggest you reread this article, since that is a big part of what it discusses.
e.g. the section titled "Pigments absorb light used in photosynthesis".(4 votes)
- At which wavelength is chlorophyll a most effective? Is it when the line on the graph peaks? Or when it is at its lowest point?(3 votes)
- Chlorophyll a absorbs the maximum wavelength at the peaks indicated by its graph, i.e,at the blue and red regions(450-470nm and 660nm). So at the blue and red wavelenghts chlorophyll a is most efficient.(2 votes)
