Pigments in plants capture the energy of light for the synthesis of carbohydrates. Pigments such as chlorophyll enable plants to harness the energy of the sun.

What is special about the light that activates pigments?

Light is energy that travels in waves, like other forms of electromagnetic energy such as x-rays, radio waves, and microwaves. All of these forms of energy collectively comprise the electromagnetic spectrum.

Every wave has a wavelength, the distance from one crest to the next crest. All electromagnetic radiation travels at the same speed, the speed of light, so longer wavelengths carry less energy than shorter wavelengths. In the visible spectrum, for example, red light, with a wavelength of about 700 nanometers, carries less energy than blue light, which has a wavelength of about 380 nanometers.

Pigments absorb and reflect specific wavelengths of light. When a pigment absorbs light, it captures the energy from that light. When it reflects light, it sends that light to an adjacent plant tissue or outside the plant altogether.

The set of wavelengths that can be absorbed by a pigment, and that activate it to absorb energy, are known as its absorption spectrum. In plants, there are three pigments most responsible for the absorption of sunlight, chlorophyll a, chlorophyll b, and beta-carotene.

  • Chlorophyll a absorbs mostly blue light with a wavelength of about 430 nanometers, and red light with a wavelength of about 662 nanometers. It looks green because it reflects green light.
  • Chlorophyll b also absorbs blue and red light, but responds to different wavelengths with peak absorption. It is most strongly activated by blue light with a wavelength of about 392 nanometers and red light with a wavelength of about 692 nanometers. It also reflects green light, so it looks green.
  • Beta-carotene absorbs blue-green light with wavelengths between 450 and 475 nanometers. It reflects orange light, so it looks orange to us.

All plants capable of photosynthesis, as well as algae and photosynthetic bacteria, contain chlorophyll a. Algae don’t contain chlorophyll b, but photosynthetic plants do. Plants that contain beta-carotene can use it in photosynthesis, and to make other plant pigments such as lycopene, lutein, and zeaxanthin.

Activation of plant pigments occurs within a range of frequencies that humans associate with single colors. But the range of frequencies is very small. Only a photon with the right amount of energy can “bump” an electron in a plant pigment into a higher-energy orbit that is farther from the nucleus of the atom it orbits. 

Atoms with these “excited” electrons transfer this energy to nearby molecules which enter a cycle that fixes carbon from the carbon dioxide in the atmosphere into carbohydrates the plant can use to make new tissues or to release needed energy, with the release of oxygen.

Modern Biology Provides Hands-On Learning Experiences for Your Students to Explore Photosynthesis

Modern Biology makes the concepts of photosynthesis come alive for your students. Our experiment IND-29: Electrophoretic and Chromatographic Analysis of Photosynthetic Pigments from Blue-Green Algae gives your students opportunities to explore the photosynthetic pigments in easily maintained, inexpensive Cyanobacteria.

Cyanobacteria, also known as blue-green algae, are among the oldest photosynthetic organisms on Earth. There are two classes of photosynthetic pigments in Cyanobacteria. One of them is phycocyanin. This pigment is blue. Cyanobacteria also contain both kinds of chlorophyll and several carotenoid pigments.

Your students first isolate the phycocyanin pigment with electrophoresis. They then dissolve the other pigments with alcohol and separate them with paper chromatography.

This experiment integrates critical laboratory skills with visually striking results that your students will find memorable.

Every experiment from Modern Biology is designed to help your students develop a scientific mindset. And every experiment from Modern Biology has been teacher-tested with safety, economy, efficiency, and ease of supervision in mind.

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