Long ago, the ancestors of today’s plants discovered a special green molecule that helps them make food using sunlight. This molecule is called chlorophyll. We aren’t sure exactly how plants ended up with chlorophyll or why it’s green, especially since other colors might seem better at absorbing all kinds of light. But the fact that almost all plants use chlorophyll suggests there’s something special about it.
Plants need molecules like chlorophyll for two main reasons: to capture the sun’s energy and to transfer that energy to other parts of the leaf to make sugar. Interestingly, chlorophyll isn’t great at absorbing every type of light, but that’s not a big problem. This is because chlorophyll is spread out in layers throughout the leaf, not just on the surface. This setup allows the leaf to catch almost every bit of light that passes through.
Even though chlorophyll isn’t the best at absorbing green light, the layered structure of leaves helps them capture up to 80% of it. If a leaf absorbed all the green light right at the surface, it might take in too much energy too quickly, which could damage the leaf, similar to getting a sunburn.
By capturing energy throughout the leaf, plants don’t need to move it around. This is helpful because the real work begins when the plant uses sunlight to turn water and carbon dioxide into sugar. Chlorophyll is excellent at this job. While some bacteria can also make sugar, they aren’t as efficient as green plants.
The green sugar-making process is smaller and uses less energy to build. It also has a better system for using light energy, combining energy from two light particles to make more sugar. This makes the green method superior to other light-harvesting systems that have evolved over time.
We can see the success of this green method all around us. Green photosynthesis is responsible for more than 99.9% of the natural productivity on Earth. But was this outcome inevitable due to the physics and chemistry of “green,” or was it just a lucky coincidence? Could there be a planet somewhere with purple plants? We don’t know for sure, but here on Earth, chlorophyll’s benefits are clear, and that’s a great thing for our planet.
Conduct a simple experiment to extract chlorophyll from leaves. Use rubbing alcohol and a few leaves to see the green pigment in action. Observe the color and discuss why chlorophyll is essential for plants.
Use colored cellophane sheets to simulate different light conditions. Shine a flashlight through the sheets onto a leaf and observe how the leaf reacts. Discuss how chlorophyll absorbs different types of light and why green light is less absorbed.
Participate in a role-play activity where each student represents a part of the photosynthesis process. Act out how sunlight, water, and carbon dioxide are transformed into sugar and oxygen, emphasizing chlorophyll’s role.
Write a short story from the perspective of a leaf. Describe how it uses chlorophyll to capture sunlight and produce energy. Include details about the challenges and successes of being a green leaf.
Engage in a debate about whether green is the best color for photosynthesis. Consider the efficiency of chlorophyll and the possibility of other colors being more effective. Use scientific reasoning to support your arguments.
At some point in the distant past, the ancestors of today’s plants settled on a unique green molecule to help them make food using the sun’s energy. We don’t know exactly where this molecule came from, or why it was green—other colors might seem more efficient at absorbing all wavelengths of light. However, the fact that almost all plants on Earth still use chlorophyll must mean there’s more to it than meets the eye.
Plants need light-absorbing molecules like chlorophyll to do just two things: 1) capture the sun’s energy and 2) transfer that energy to other components of the leaf to fuel sugar production. Interestingly, having light-absorbing molecules that are less effective at absorbing certain wavelengths of light isn’t a significant issue. This is because chlorophyll molecules are not just on the surface of a leaf; they are spread in layers throughout. This arrangement allows the leaf ample opportunity to absorb every photon of light passing through.
For example, even though chlorophyll molecules are not very effective at absorbing green light, this layered structure enables a leaf to capture up to 80% of the green light that hits it. In fact, if a leaf were to absorb all that green light right at the surface, it could risk taking in too much energy too quickly, which could lead to damage similar to sunburn.
Another advantage of capturing energy in a distributed manner is that, once absorbed, the energy is already spread out—there’s no need to transport it throughout the leaf. This is beneficial because the real work is about to begin: using the sun’s energy to produce sugar from water and carbon dioxide. This is where chlorophyll truly excels. Other cellular systems, like those in certain bacteria, can also produce sugar, but not as efficiently as the green plants.
The green sugar-making machinery is both smaller (and thus less energy-intensive to build) and has a more effective system for utilizing light energy—it can combine the energy captured from two photons of light to fuel a larger-scale sugar synthesis. Thus, among the light-harvesting systems that evolved, the green method proved to be superior.
We can observe the results of this evolutionary competition every day around us—green photosynthesis accounts for more than 99.9% of the natural productivity on Earth. But did the physics and chemistry of “green” make this outcome inevitable, or was it simply a fortunate coincidence? Is there a planet somewhere in our galaxy dominated by purple plants? We don’t know, but what we do know is that here on Earth, chlorophyll’s advantages far outweigh its limitations, and for our planet, at least, that may be a very good thing.
Chlorophyll – A green pigment found in the chloroplasts of plants, algae, and some bacteria, essential for photosynthesis as it absorbs light energy. – Chlorophyll allows plants to capture light energy from the sun to produce food.
Plants – Living organisms that typically grow in soil and use sunlight to make their own food through the process of photosynthesis. – Plants are crucial for life on Earth as they produce oxygen and provide food for other organisms.
Energy – The ability to do work or cause change, which in biological systems is often derived from the sun and transformed through processes like photosynthesis. – During photosynthesis, plants convert light energy into chemical energy stored in sugars.
Light – A form of energy that is visible to the human eye and is necessary for photosynthesis in plants. – Light from the sun is absorbed by chlorophyll in plant leaves to start the process of photosynthesis.
Sugar – A type of carbohydrate that plants produce during photosynthesis, serving as a source of energy and building material for growth. – The sugar produced in photosynthesis is used by plants for energy and growth.
Photosynthesis – The process by which green plants and some other organisms use sunlight to synthesize foods with the help of chlorophyll, converting carbon dioxide and water into glucose and oxygen. – Photosynthesis is essential for life on Earth as it provides oxygen and organic compounds used by most living organisms.
Molecules – Groups of atoms bonded together, representing the smallest fundamental unit of a chemical compound that can take part in a chemical reaction. – Water molecules are split during photosynthesis to release oxygen and provide electrons for energy conversion.
Water – A transparent, tasteless, odorless, and nearly colorless chemical substance, essential for all known forms of life and a key reactant in photosynthesis. – Water is absorbed by plant roots and transported to leaves, where it is used in photosynthesis.
Carbon – A chemical element that is a fundamental component of all living organisms and is cycled through the environment in processes like photosynthesis and respiration. – Carbon dioxide from the air is used by plants during photosynthesis to produce glucose.
Green – The color associated with chlorophyll, which gives plants their characteristic color and plays a crucial role in absorbing light for photosynthesis. – The green color of leaves is due to the presence of chlorophyll, which is vital for capturing sunlight.
