Flowers come in an array of various colours, from pretty pinks to bright yellows and striking purples. But leaves are always green. Why? It's due to a pigment called chlorophyll. It's found in some plant cells that reflect green wavelengths of light. Its purpose is to absorb light energy to power the process of photosynthesis.
Definition of Chlorophyll
Let's start with the basics.
Chlorophyll is a pigment that absorbs and reflects specific wavelengths of light.
It is found inside the thylakoid membranes of chloroplasts. Chloroplasts are organelles (mini-organs) found in plant cells. They are the site of photosynthesis.
How Does Chlorophyll Make Leaves Green?
Although light from the sun appears yellow, it's actually white light. White light is a mixture of all the wavelengths of visible light. Different wavelengths correspond to different colours of light. For example, light with a wavelength of 600 nanometres is orange. Objects reflect or absorb light depending on their colour:
Black objects absorb all wavelengths
White objects reflect all wavelengths
Orange objects will only reflect the orange wavelengths of light
Chlorophyll does not absorb the green wavelengths of sunlight (between 495 and 570 nanometres). Instead, these wavelengths are reflected away from the pigments, so the cells appear green. However, chloroplasts aren't found in every plant cell. Only green parts of the plant (such as stems and leaves) contain chloroplasts within their cells.
Woody cells, roots and flowers do not contain chloroplasts or chlorophyll.
Chlorophyll isn't only found in terrestrial plants. Phytoplankton are microscopic algae that live in oceans and lakes. They photosynthesise, so they contain chloroplasts and thus chlorophyll. If there's a very high concentration of algae in a body of water, the water can appear green.
Eutrophication is the build-up of sediment and excess nutrients in bodies of water. Too many nutrients result in rapid algal growth. At first, the algae will photosynthesise and produce lots of oxygen. But before long, there will be overcrowding. Sunlight cannot penetrate the water so that no organisms can photosynthesise. Eventually, the oxygen gets used up, leaving behind a dead zone where few organisms can survive.
Pollution is a common cause of eutrophication. Dead zones are typically located near populated coastal areas, where excessive nutrients and pollution are washed into the ocean.
Figure 1 - Although they may look pretty, algal blooms have disastrous consequences for the ecosystem, and can even affect human health, unsplash.com
Chlorophyll Formula
There are two different types of chlorophyll. But for now, we'll focus on chlorophyll a. This is the dominant type of chlorophyll and an essential pigment found in terrestrial plants. It is necessary for photosynthesis to occur.
During photosynthesis, chlorophyll A will absorb solar energy and convert it into oxygen and a usable form of energy for the plant and for organisms that eat it. Its formula is imperative to making this process work, as it helps to transfer electrons during photosynthesis. The formula for chlorophyll A is:
C₅₅H₇₂O₅N₄Mg
This means it contains 55 carbon atoms, 72 hydrogen atoms, five oxygen atoms, four nitrogen atoms and just one magnesium atom.
Chlorophyll b is what is known as an accessory pigment. It is not necessary for photosynthesis to take place, as it does not convert light into energy. Instead, it helps broaden the range of light the plant can absorb.
Chlorophyll Structure
Just as the formula is vital for photosynthesis, how these atoms and molecules are organised is just as important! Chlorophyll molecules have a tadpole-shaped structure.
The 'head' is a hydrophilic (water-loving) ring. The hydrophilic rings are the site of light energy absorption. The centre of the head is home to a single magnesium atom, which helps to uniquely define the structure as a chlorophyll molecule.
The 'tail' is a long hydrophobic (water-repellent) carbon chain, which helps to anchor the molecule to other proteins found in the membrane of the chloroplasts.
The side chains make each type of chlorophyll molecule unique from one another. They are attached to the hydrophilic ring and help alter each chlorophyll molecule's absorption spectrum (see the section below).
Hydrophilic molecules have the ability to mix with or dissolve well in water
Hydrophobic molecules tend not to mix well with or repel water
Types of Chlorophyll
There are two types of chlorophyll: Chlorophyll a and Chlorophyll b. Both types have a very similar structure. In fact, their only difference is the group found on the third carbon of the hydrophobic chain. Despite their similarity in structure, Chlorophyll a and b have different properties and functions. These differences are summarised in the table below.
| Trait | Chlorophyll a | Chlorophyll b |
| How important is this type of chlorophyll for photosynthesis? | It is the primary pigment - photosynthesis cannot occur without Chlorophyll A. | It is an accessory pigment - it is not necessary for photosynthesis to take place. |
| What colours of light does this type of chlorophyll absorb? | It absorbs violet-blue and orange-red light. | It can only absorb blue light. |
| What colour is this type of chlorophyll? | It is bluish-green in colour. | It is olive green in colour. |
| What group is found at the third carbon? | A methyl group (CH3) is found at the third carbon. | An aldehyde group (CHO) is found at the third carbon. |
Chlorophyll Function
Plants don't eat other organisms for food. So, they have to make their own food using sunlight and chemicals - photosynthesis. The function of chlorophyll is sunlight absorption, which is essential for photosynthesis.
Photosynthesis
All reactions require energy. So, plants need a method of acquiring energy to power the process of photosynthesis. Energy from the sun is widespread and unlimited, so plants use their chlorophyll pigments to absorb light energy. Once absorbed, light energy is transferred into an energy storage molecule called ATP (adenosine triphosphate).
ATP is found in all living organisms. To learn more about ATP and how it is used during photosynthesis and respiration, check out our articles on them!
Plants use the energy stored in ATP to perform the reaction of photosynthesis.
Word equation:
carbon dioxide + water ⇾ glucose + oxygen
Chemical formula:
6CO2 + 6H2O ⇾ C6H12O6 + 6O2
- Carbon Dioxide: plants absorb carbon dioxide from the air using their stomata.
Stomata are specialised pores used for gas exchange. They are found on the underside of leaves.
- Water: plants absorb water from the soil using their roots.
- Glucose: glucose is a sugar molecule used for growth and repair.
- Oxygen: photosynthesis produces oxygen molecules as a by-product. Plants release oxygen into the atmosphere via their stomata.
A by-product is an unintended secondary product.
Briefly, photosynthesis is when plants release oxygen and take in carbon dioxide. This process presents two significant advantages for humans:
- The production of oxygen. Animals need oxygen to breathe, respire and live. Without photosynthesis, we wouldn't be able to survive.
- The removal of carbon dioxide from the atmosphere. This process reduces the effects of climate change.
Can Humans Use Chlorophyll?
Chlorophyll is a good source of vitamins (including Vitamins A, C and K), minerals, and antioxidants.
Antioxidants are molecules that neutralise free radicals in our bodies.
Free radicals are waste substances produced by cells. If left unchecked, they can harm other cells and affect our body's functions.
Because of chlorophyll's potential health benefits, some companies have begun incorporating it into their products. It's possible to buy chlorophyll water and supplements. However, the scientific evidence in its favour is limited.
Chlorophyll - Key takeaways
- Chlorophyll is a pigment that absorbs and reflects specific wavelengths of light. It is found in the membranes of chloroplasts, special organelles designed for photosynthesis. Chlorophyll is what gives plants their green hue.
- The formula for chlorophyll is C₅₅H₇₂O₅N₄Mg.
- Chlorophyll has a tadpole-like structure. The long carbon chain is hydrophobic. The hydrophilic ring is the site of light absorption.
- There are two types of chlorophyll: A and B. Chlorophyll A is the primary pigment necessary for photosynthesis. Chlorophyll A can absorb a greater range of wavelengths than Chlorophyll B.
- Chlorophyll absorbs light energy. Plants use this energy for photosynthesis.
1. Andrew Latham, How Do Plants Store Energy During Photosynthesis?, Sciencing, 2018
2. Anne Marie Helmenstine, The Visible Spectrum: Wavelengths and Colors, ThoughtCo, 2020
3. CGP, AQA Biology A-Level Revision Guide, 2015
4. Kim Rutledge, Dead Zone, National Geographic, 2022
5. Lorin Martin, What Are the Roles of Chlorophyll A & B?, Sciencing, 2019
6. National Geographic Society, Chlorophyll, 2022
7. Noma Nazish, Is Chlorophyll Water Worth The Hype? Here’s What The Experts Say, Forbes, 2019
8. Tibi Puiu, What makes things coloured – the physics behind it, ZME Science, 2019
9. The Woodland Trust, How trees fight climate change, 2022
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