Light Energy and Pigments*#
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The sun emits an enormous amount of electromagnetic radiation (solar energy) that spans a broad swath of the electromagnetic spectrum, the range of all possible radiation frequencies. When solar radiation reaches Earth, a fraction of this energy interacts with and may be transferred to the matter on the planet. This energy transfer results in a wide variety of different phenomena from influencing weather patterns to driving a myriad of biological processes. In Bis2a we are largely concerned with the latter and we discuss some very basic concepts related light and its interaction with biology below.
First, however we need to refresh a couple of key properties of light.
- Light in a vacuum travels at the constant speed of 299,792,458 m/s. We often abbreviate the speed of light with the variable "c".
- Light has properties of waves. A specific "color" of light has a characteristic wavelength.
4. Finally, each frequency (or wavelength) of light is associated with a specific energy. We'll call energy "E". The relationship between frequency and energy is:
\[E = h \,f\]
where \(h\) is a constant called the Planck constant (~6.626x10-34 Joule•second when frequency is expressed in cycles per second). Given the relationship between frequency and and wavelength you can also write E = h*c/Ⲗ. Therefore, the larger the frequency (or shorter the wavelength) the more energy is associated with a specific "color". Wave 2 in the figure above has greater energy than wave 1.
The Light we See
The visible light seen by humans as white light is composed of a rainbow of colors, each with a characteristic wavelength. Certain objects, such as a prism or a drop of water, disperse white light to reveal the colors to the human eye. In the visible spectrum, violet and blue light have shorter (higher energy) wavelengths while the orange and red light have longer (lower energy) wavelengths.
Figure 4: The colors of visible light do not carry the same amount of energy. Violet has the shortest wavelength and therefore carries the most energy, whereas red has the longest wavelength and carries the least amount of energy. (credit: modification of work by NASA)
Absorption by Pigments
The interaction between light and biological systems occurs by several different mechanisms, some of which you may learn about in upper-division courses in cellular physiology or biophysical chemistry. In Bis2a we are mostly concerned about the interaction of light and biological pigments. These interactions can initiate a variety of light dependent biological process that can be grossly grouped into two functional categories: cellular signaling and energy harvesting. Signaling interactions are largely responsible for perceiving changes in the environment (in this case changes in light). An example of a signaling interaction might be the interaction between light and the pigments expressed in an eye. By contrast, light/pigment interactions that are involved in energy harvesting are used for - not surprisingly - capturing the energy in the light and transferring it to the cell to fuel biological processes. Photosynthesis, which we will learn more about soon, is one example of an energy harvesting interaction.
At the center of the biological interactions with light are groups of molecules we call organic pigments. Whether in the human retina, chloroplast thylakoid, or microbial membrane, organic pigments often have specific ranges of energy or wavelengths that they can absorb. The sensitivity of these molecules for different wavelengths of light is due to their unique chemical makeups and structures. A range of the electromagnetic spectrum is given a couple of special names because of the sensitivity of some key biological pigments: The retinal pigment in our eyes, when coupled with an opsin sensor protein, “sees” (absorbs) light predominantly between the wavelengths between of 700 nm and 400 nm. Because this range defines the physical limits of the electromagnetic spectrum that we can actually see with our eyes, we refer to this wavelength range as the "visible range". For similar reasons - plants pigment molecules tend to absorb wavelengths of light mostly between 700 nm and 400 nm - plant physiologists refer to this range of wavelengths as "photosynthetically active radiation".
Two Key Types of Pigments We Discuss in Bis2a
Chlorophylls (including bacteriochlorophylls) are part of a large family of pigment molecules. There are five major chlorophyll pigments named: a, b, c, d, and f. Chlorophyll a is related to a class of more ancient molecules found in bacteria called bacteriochlorophylls. Chlorophylls are structurally characterized by ring-like porphyrin group that coordinates a metal ion. This ring structure is chemically related to the structure of heme compounds that also coordinate a metal and are involved in oxygen binding and/or transport in many organisms. Different chlorophylls are distinguished from one another by different "decorations"/chemical groups on the porphyrin ring.
Figure 5: The structure of heme and chlorophyll a molecules. The common porphyrin ring is colored in red. Attribution: Marc T. Facciotti (original work)
Carotenoids are the red/orange/yellow pigments found in nature. They are 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) — which are used as biological "advertisements" to attract seed dispersers (animals or insects that may carry seeds elsewhere). In photosynthesis, carotenoids function as photosynthetic pigments. In addition, when a leaf is exposed to full sun, that surface is required to process an enormous amount of energy; if that energy is not handled properly, it can do significant damage. Therefore, many carotenoids help absorb excess energy in light and safely dissipate that energy as heat.
Flavonoids ENTER DESCRIPTION HERE
Each type of pigment can be identified by the specific pattern of wavelengths it absorbs from visible light. This characteristic is known as the pigment's absorption spectrum. The graph in the figure below shows the absorption spectra for chlorophyll a, chlorophyll b, and a type of carotenoid pigment called β-carotene (which absorbs blue and green light). Notice how each pigment has a distinct set of peaks and troughs, revealing a highly specific pattern of absorption. These differences in absorbance are due to differences in chemical structure (some are highlighted in the figure). Chlorophyll a absorbs wavelengths from either end of the visible spectrum (blue and red), but not green. Because green is reflected or transmitted, chlorophyll appears green. Carotenoids absorb in the short-wavelength blue region, and reflect the longer yellow, red, and orange wavelengths.
Figure 6: (a) Chlorophyll a, (b) chlorophyll b, and (c) β-carotene are hydrophobic organic pigments found in the thylakoid membrane. Chlorophyll a and b, which are identical except for the part indicated in the red box, are responsible for the green color of leaves. Note how the small amount of difference in chemical composition between different chlorophylls leads to different absorption spectra. β-carotene is responsible for the orange color in carrots. Each pigment has (d) a unique absorbance spectrum.
Importance of having multiple different pigments
Not all photosynthetic organisms have full access to sunlight. Some organisms grow underwater where light intensity and available wavelengths decrease and change, respectively, with depth. Other organisms grow in competition for light. Plants on the rainforest floor for instance must be able to absorb any bit of light that comes through, because the taller trees absorb most of the sunlight and scatter the remaining solar radiation. To account for these variable light conditions, many photosynthetic organisms have a mixture of pigments whose expression can be tuned to improve the organism's ability to absorb energy from a wider range of wavelengths than would be possible with one pigment alone.