Back To CourseGED Science: Tutoring Solution
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When you hear of the word 'pigment' you probably think of colors like a red flower, green leaves, or a blue car. Pigments aren't just 'colors' though; they're unique chemical compounds that absorb certain wavelengths of light (hiding those colors from our eyes) and reflect other wavelengths of light (thus appearing that particular color to our eyes).
So, when you look at a red rose, you're actually seeing red wavelengths of light that the pigments in the petals are reflecting, while the pigments in the stem absorb red and reflect green wavelengths. This process of absorption and reflection isn't just important to understanding the way that we see color though - it's how autotrophs (organisms that produce their own food) convert 'light energy' to food.
If you have ever discussed photosynthesis (the process plants use to capture light energy and synthesize nutrients) then you've also talked about chlorophyll, which all autotrophs use to 'capture light'.
There are actually many forms of chlorophyll: chlorophyll-a, chlorophyll-b, chlorophyll-c1, chlorophyll-c2, chlorophyll-d, and divinyl chlorophyll-a, which all reflect light waves in the green spectrum. Due to minor differences in their particular molecular makeup, they each reflect different shades of green (yellow-green, lime green, forest green, blue-green, etc.). So, all of these various forms of chlorophyll are what give us all of the many amazing shades of green that you can see in nature.
Now, here's where it could get a little confusing, because not all chlorophylls are created equal. All of these various forms of chlorophyll, except chlorophyll-a, are considered accessory pigments because they, unlike chlorophyll-a, can't actually convert photons of light into energy; they 'assist' chlorophyll-a in the energy absorption process and then pass their absorbed energy on to chlorophyll-a for energy production. This makes chlorophyll-a 'alpha dog' in the process of photosynthesis because, without chlorophyll-a, plants couldn't actually access the light energy they absorb.
So, you may be wondering, if most trees have green leaves because of chlorophylls, why do they change color in the fall? Well, in the fall trees begin to prepare for the long, cold, and dark winter 'hibernating' months. They do this by breaking down their chlorophylls so they can conserve and store every molecule of their energy- it's going to be a long winter so they need all the energy they can. Now, as chlorophyll begins to break down, it starts revealing the yellows, fiery reds, and oranges of other accessory pigments in the leaves that were previously overshadowed by the high levels of chlorophylls.
Ok, so now that we understand what pigments are, how they differ from chlorophyll, and how plants use chlorophyll to access a range of wavelengths of light, we can discuss what role accessory pigments play in photosynthesis. Many plants, algae, and autotrophic bacteria have a secret weapon, called accessory pigments, that they use to increase their range of wavelength absorption and, in turn, their capacity for food production.
You're probably wondering how accessory pigments can do this since you already know that 'pigments', unlike chlorophyll, can't directly convert light into energy. This is true, but accessory pigments can pass their 'absorbed energy' on to chlorophyll for use in photosynthesis. Chlorophyll can and happily does accept the 'energy' absorbed by accessory pigments to increase its own rate of photosynthetic reactions. This means that, by using accessory pigments, plants can absorb wavelengths of light that chlorophyll alone doesn't give them access to.
There are many different types of non-chlorophyll accessory pigments, but some of the most common are carotenoids, phycocyanins, and phycoerythrins.
Carotenoids (caroten meaning 'carrot') are a group of some 600-700 different types of accessory pigments that reflect red, orange, and yellow wavelengths. These pigments are the source of much of the red, orange, and yellow coloration that you see in plant leaves and are also responsible for the red of tomatoes, the orange of carrots, and the yellow of corn.
Phycocyanins ('phyco' meaning seaweed and 'cyan' meaning dark blue) reflect blue ('cyan') wavelengths while absorbing longer yellow, orange, and red wavelengths. Cyanobacteria, a surface dwelling aquatic bacteria also known as blue-green bacteria, gets its name, as well as its blue appearance, from these accessory pigments. By using these phycocyanin pigments, along with chlorophyll, these bacteria dramatically increase their range of light wave absorption.
Some accessory pigments, such as phycoerythrins (again, 'phyco' meaning seaweed and 'erythrin' meaning red), which absorb blue, green, and some yellow wavelengths, allow organisms like marine red algae (also known as Rhodophyta or 'red algae') a greater range of habitation. Phycoerythrins enable red algae to live at greater depths of water because they exploit the fact that shorter wavelengths (such as blues, greens, and purples) are capable of penetrating water to greater depths than longer wavelengths.
Accessory pigments are chemical compounds that plants and photosynthesizing autotrophs use to increase their access to wavelengths of visible light that chlorophylls can't absorb.
These pigments get their name because they must be used as an 'accessory' to, rather than in place of, chlorophyll-a, as these pigments cannot directly pass their 'captured light energy' into photosynthesis. Pigments can, however, denote their captured energy to chlorophyll-a for increasing the plant's rate of photosynthesis.
While there are many accessory pigments that exist, the most common are carotenoids (responsible for the reds, oranges, and yellows in plants), phycocyanins (used by blue-green bacteria), and phycoerythrins (found in red algae). These pigments are vital as they enable autotrophs to absorb a greater range of light wavelengths and can also increase an organism's range of habitat. Aquatic autotrophs use these pigments to exploit absorption of shorter wavelengths, which penetrate water to greater depths than longer wavelengths.
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Back To CourseGED Science: Tutoring Solution
34 chapters | 724 lessons