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Accessory Pigments in Photosynthesis: Definition & Function

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  • 0:01 What Are Pigments?
  • 0:44 Photosynthesis & Chlorophyll
  • 2:40 Accessory Pigment's Role
  • 3:40 Types of Accessory Pigments
  • 4:55 Lesson Summary
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Lesson Transcript
Instructor: Sarah Phenix
In this lesson we will explore what a pigment is and the role of chlorophyll in photosynthesis. We will explore how accessory pigments increase a photosynthetic organism's ability to capture sunlight.

What Are Pigments?

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.

Photosynthesis and Chlorophyll

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.

Light Wave Absorption by Chlorophylls
Chlorophyll wavelength absorbance

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.

Accessory Pigment's Role

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.

Types of Accessory Pigments

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.

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