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.

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.

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.

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

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).

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.

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).

^{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}