13.1: Introduction

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• Brad Basehore, Michelle A. Bucks, & Christine M. Mummert
• Harrisburg Area Community College

Introduction

Have you ever wondered why leaves change colors in the fall but remain green during other seasons of the year? Leaves contain multiple types of pigments that are involved in many different functions in the plant, including photosynthesis, protection from UV radiation, and even attracting pollinators. Pigment molecules are stored inside of plastids, a class of cellular organelles that includes chloroplasts, the organelles responsible for photosynthesis. Found inside chloroplasts, chlorophyll is the most common pigment in a leaf. It comes in two varieties (chlorophyll a and chlorophyll b). During the spring and summer months, when day length is longest and the sunlight is most direct, plants produce a large amount of chlorophyll, which is used to capture sunlight energy that drives photosynthesis.

All of the wavelengths of energy emitted by the sun are collectively called the electromagnetic spectrum. Waves at the low end of the spectrum (those with the longest wavelengths), like radio waves and microwaves, emit less energy, while waves at the high end (those with the shortest wavelengths), like x-rays and gamma rays, emit higher energy. The very small section of the electromagnetic spectrum that humans can see is called the visible light spectrum. Plants can use these same wavelengths of light, from red at the low end to violet at the high end of the visible light spectrum, to power photosynthesis (Fig. 13.1.1).

Figure \(\PageIndex{1}\): Photosynthesis Credit: [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)

Photosynthesis is the process by which plants use energy from the sun and water from the soil to convert atmospheric carbon dioxide into sugar (glucose) that can be used for energy production (Fig. 13.1.2).

Figure \(\PageIndex{2}\): Chemical Equation for Photosynthesis.

Different pigments absorb and reflect different wavelengths of light and therefore appear different colors to us. During the spring and summer months, when plants are most productive, they produce a huge amount of chlorophyll molecules. Chlorophyll a and chlorophyll b absorb mostly at the red-to-orange and blue-to-violet ends of the visible light spectrum; they reflect the rest of the wavelengths. The reflected wavelengths, mostly in the yellow-to-green range, are the wavelengths that are detected by the human eye. This is why plants appear green to us (Fig. 13.1.3).

Figure \(\PageIndex{3}\): Absorption spectrum of chlorophyll a and chlorophyll b Credit: "File:Chlorophyll Absorption Spectrum.svg" by Serge Helfrich is licensed under CC BY-SA 4.0

Another class of pigments found in plants is the carotenoids. Carotenoids include carotenes, which appear yellow-orange, and xanthophylls, which appear mostly yellow. These pigments are found in the plant all year round but are largely masked by the abundant chlorophyll molecules that are present during the spring and summer. Carotenoids play a minor role in photosynthesis, but they play a larger role in protecting the plant tissues from damage caused by UV radiation from the sun.

As summer gives way to fall, day length gets shorter, temperatures decrease, and water may become less available. Plants stop producing new chlorophyll molecules, and the remaining chlorophyll molecules are broken down and reabsorbed to be used again in the following spring. Carotenoids break down more slowly, which is why we can see their yellow and orange colors showing through as chlorophyll is lost from the leaves.