Chlorophyll Function, Use & Examples - Video & Lesson Transcript

Chlorophyll

Chlorophyll

Paul May

School of Chemistry, University of Bristol

VRML, Jmol, and Chime versions

Chlorophyll is the molecule that absorbs sunlight and uses its energy to synthesise carbohydrates from CO 2 and water. This process is known as photosynthesis and is the basis for sustaining the life processes of all plants. Since animals and humans obtain their food supply by eating plants, photosynthesis can be said to be the source of our life also.

Chlorophyll is the green coloration in leaves.

Photosynthesis

In 1780, the famous English chemist Joseph Priestley (right) found that plants could "restore air which has been injured by the burning of candles." He used a mint plant, and placed it into an upturned glass jar in a vessel of water for several days. He then found that "the air would neither extinguish a candle, nor was it all inconvenient to a mouse which I put into it". In other words, he discovered that plants produce oxygen.

A few years later, in 1794, the French chemist Antoine Lavoisier (left), discovered the concept of oxidation, but soon after was executed during the French Revolution for being a Monarchist sympathiser. The judge who pronounced sentence said "The Republic has no need for scientists".

So it fell to a Dutchman, Jan Ingenhousz (left), who was court physician to the Austrian empress, to make the next major contribution to the mechanism of photosynthesis. He had heard of Priestley's experiments, and a few years later spent a summer near London doing over 500 experiments, in which he discovered that light plays a major role in photosynthesis.

"I observed that plants not only have the faculty to correct bad air in six to ten days, by growing in it...but that they perform this important office in a complete manner in a few hours; that this wonderful operation is by no means owing to the vegetation of the plant, but to the influence of light of the sun upon the plant".

Very soon after, more pieces of the puzzle were found by two chemists working in Geneva. Jean Senebier, a swiss pastor, found that "fixed air" (CO 2 ) was taken up during photosynthesis, and Theodore de Saussure discovered that the other reactant necessary was water. The final contribution to the story came from a German surgeon, Julius Robert Mayer (right), who recognised that plants convert solar energy into chemical energy. He said:

"Nature has put itself the problem of how to catch in flight light streaming to the Earth and to store the most elusive of all powers in rigid form. The plants take in one form of power, light; and produce another power, chemical difference."

The actual chemical equation which takes place is the reaction between carbon dioxide and water, powered by sunlight, to produce glucose and a waste product, oxygen. The glucose sugar is either directly used as an energy source by the plant for metabolism or growth, or is polymerised to form starch, so it can be stored until needed. The waste oxygen is excreted into the atmosphere, where it is made use of by plants and animals for respiration.

Chlorophyll as a Photoreceptor

Chlorophyll is the molecule that traps this 'most elusive of all powers' - and is called a photoreceptor. It is found in the chloroplasts of green plants, and is what makes green plants, green. The basic structure of a chlorophyll molecule is a porphyrin ring, co-ordinated to a central atom. This is very similar in structure to the heme group found in hemoglobin , except that in heme the central atom is iron, whereas in chlorophyll it is magnesium.

Click for 3D structure file

There are actually 2 main types of chlorophyll, named a and b. They differ only slightly, in the composition of a sidechain (in a it is -CH 3 , in b it is CHO). Both of these two chlorophylls are very effective photoreceptors because they contain a network of alternating single and double bonds, and the orbitals can delocalise stabilising the structure. Such delocalised polyenes have very strong absorption bands in the visible regions of the spectrum, allowing the plant to absorb the energy from sunlight.

The different sidegroups in the 2 chlorophylls 'tune' the absorption spectrum to slightly different wavelengths, so that light that is not significantly absorbed by chlorophyll a, at, say, 460nm, will instead be captured by chlorophyll b, which absorbs strongly at that wavelength. Thus these two kinds of chlorophyll complement each other in absorbing sunlight. Plants can obtain all their energy requirements from the blue and red parts of the spectrum, however, there is still a large spectral region, between 500-600nm, where very little light is absorbed. This light is in the green region of the spectrum, and since it is reflected, this is the reason plants appear green. Chlorophyll absorbs so strongly that it can mask other less intense colours. Some of these more delicate colours (from molecules such as carotene and quercetin) are revealed when the chlorophyll molecule decays in the Autumn, and the woodlands turn red, orange, and golden brown. Chlorophyll can also be damaged when vegetation is cooked, since the central Mg atom is replaced by hydrogen ions. This affects the energy levels within the molecule, causing its absorbance spectrum to alter. Thus cooked leaves change colour - often becoming a paler, insipid yellowy green.

As the chlorophyll in leaves decays in the autumn, the green colour fades and is replaced by the oranges and reds of carotenoids.

Chlorophyll in Plants

The chlorophyll molecule is the active part that absorbs the sunlight, but just as with hemoglobin, in order to do its job (synthesising carbohydrates) it needs to be attached to the backbone of a very complicated protein. This protein may look haphazard in design, but it has exactly the correct structure to orient the chlorophyll molecules in the optimal position to enable them to react with nearby CO 2 and H 2 O molecules in a very efficient manner. Several chlorophyll molecules are lurking inside this bacterial photoreceptor protein (right).

References:

Chlorophyll Definition and Role in Photosynthesis

Chlorophyll is the name given to a group of green pigment molecules found in plants, algae, and cyanobacteria. The two most common types of chlorophyll are chlorophyll a, which is a blue-black ester with the chemical formula C 55 H 72 MgN 4 O 5 , and chlorophyll b, which is a dark green ester with the formula C 55 H 70 MgN 4 O 6 . Other forms of chlorophyll include chlorophyll c1, c2, d, and f. The forms of chlorophyll have different side chains and chemical bonds, but all are characterized by a chlorin pigment ring containing a magnesium ion at its center.

Key Takeaways: Chlorophyll Chlorophyll is a green pigment molecule that collects solar energy for photosynthesis. It's actually a family of related molecules, not just one.

Chlorophyll is found in plants, algae, cyanobacteria, protists, and a few animals.

Although chlorophyll is the most common photosynthetic pigment, there are several others, including the anthocyanins.

The word "chlorophyll" comes from the Greek words chloros, which means "green", and phyllon, which means "leaf". Joseph Bienaimé Caventou and Pierre Joseph Pelletier first isolated and named the molecule in 1817.

Chlorophyll is an essential pigment molecule for photosynthesis, the chemical process plants use to absorb and use energy from light. It's also used as a food coloring (E140) and as a deodorizing agent. As a food coloring, chlorophyll is used to add a green color to pasta, the spirit absinthe, and other foods and beverages. As a waxy organic compound, chlorophyll is not soluble in water. It is mixed with a small amount of oil when it's used in food.

Also Known As: The alternate spelling for chlorophyll is chlorophyl.

Role of Chlorophyll in Photosynthesis

The overall balanced equation for photosynthesis is:

6 CO 2 + 6 H 2 O → C 6 H 12 O 6 + 6 O 2

where carbon dioxide and water react to produce glucose and oxygen. However, the overall reaction doesn't indicate the complexity of the chemical reactions or the molecules that are involved.

Plants and other photosynthetic organisms use chlorophyll to absorb light (usually solar energy) and convert it into chemical energy. Chlorophyll strongly absorbs blue light and also some red light. It poorly absorbs green (reflects it), which is why chlorophyll-rich leaves and algae appear green.

In plants, chlorophyll surrounds photosystems in the thylakoid membrane of organelles called chloroplasts, which are concentrated in the leaves of plants. Chlorophyll absorbs light and uses resonance energy transfer to energize reaction centers in photosystem I and photosystem II. This happens when energy from a photon (light) removes an electron from chlorophyll in reaction center P680 of photosystem II. The high energy electron enters an electron transport chain. P700 of photosystem I works with photosystem II, although the source of electrons in this chlorophyll molecule can vary.

Electrons that enter the electron transport chain are used to pump hydrogen ions (H+) across the thylakoid membrane of the chloroplast. The chemiosmotic potential is used to produce the energy molecule ATP and to reduce NADP+ to NADPH. NADPH, in turn, is used to reduce carbon dioxide (CO 2 ) into sugars, such as glucose.

Other Pigments and Photosynthesis

Chlorophyll is the most widely recognized molecule used to collect light for photosynthesis, but it's not the only pigment that serves this function. Chlorophyll belongs to a larger class of molecules called anthocyanins. Some anthocyanins function in conjunction with chlorophyll, while others absorb light independently or at a different point of an organism's life cycle. These molecules may protect plants by changing their coloring to make them less attractive as food and less visible to pests. Other anthocyanins absorb light in the green portion of the spectrum, extending the range of light a plant can use.

Chlorophyll Biosynthesis

Plants make chlorophyll from the molecules glycine and succinyl-CoA. There is an intermediate molecule called protochlorophyllide, which is converted into chlorophyll. In angiosperms, this chemical reaction is light-dependent. These plants are pale if they are grown in darkness because they can't complete the reaction to produce chlorophyll. Algae and non-vascular plants don't require light to synthesize chlorophyll.

Protochlorophyllide forms toxic free radicals in plants, so chlorophyll biosynthesis is tightly regulated. If iron, magnesium, or iron are deficient, plants may be unable to synthesize enough chlorophyll, appearing pale or chlorotic. Chlorosis may also be caused by improper pH (acidity or alkalinity) or pathogens or insect attack.

Chlorophyll Function, Use & Examples - Video & Lesson Transcript

Additional Information

What is Chlorophyll?

Chlorophyll is the pigment that gives plants their green color. This pigment is necessary in order for plants to make their own food during the process of photosynthesis.

Chlorophyll is referred to as a photoreceptor. Photoreceptors are specially designed proteins that receive and respond to light. Since chlorophyll is a photoreceptor, it is able to detect light. When light from the sun hits these photoreceptors, the photoreceptors are able to absorb energy from sun and carry out photosynthesis.

History

During the 18th century, scientists discovered new information about plants and photosynthesis.

1774- Joseph Priestley placed a sprig of mint plant and burning candle into a closed container. The candle continued to burn until all oxygen was consumed. After several days, Priestly noticed he was able to relight the candle. Priestley concluded the plant produced the substance (oxygen) needed for the candle to burn.

1779- Jan Ingenhousz submerged plants in a container of water. After exposure to light, small bubbles appeared on the underside of the leaves. After light was removed, bubbles stopped appearing. Ingenhousz noted bubbles only seemed to appear on green parts of the plant. Ingenhousz concluded that the green part of the plant was responsible for the production of a gas.

By the end of the 18th century, scientists knew the individual roles water, air, and sunlight played in the survival of plants but did not know how they worked together.

1817- Joseph Bienaimé Caventou and Pierre-Joseph Pelletier isolated and named the molecule necessary for photosynthesis- chlorophyll. The word chlorophyll comes from the Greek khloros (green) and phullon (leaf).

(green) and (leaf). 1883- Julius von Sachs determined that chlorophyll was located in specialized plant structures called chloroplasts. He was also given credit for proving that chlorophyll was necessary for photosynthesis.

1915- Richard Willstatter received a Nobel Prize in Chemistry for figuring out the structure of the chlorophyll molecule.

Chlorophyll Location and Structure

Chloroplasts are specialized structures found in all photosynthetic plants including algae and cyanobacteria. Here is a magnified view of a single strand of chloroplasts.

Chloroplasts in a Plant Cell

Inside a chloroplast are pancake-like structures called thylakoids. It is in the membrane of these thylakoids where chlorophyll is found.

Thylakoid Sacs

Molecular Structure of Chlorophyll

Chlorophyll has the chemical formula C55 H72 MgN4 O5 and is referred to as a chelating agent. Chelating agents are compounds that react with metal ions to form a substance that is water-soluble (able to dissolve in water).

A molecule of chlorophyll contains a central magnesium (Mg) ion bonded to a larger organic molecule referred to as a chlorin ring. Attached to the ring is a long carbon-hydrogen phytol chain.

Chlorophyll

Chlorin rings are similar to a porphyrin ring except they have been partially hydrogenated, meaning they contain more hydrogen. Notice the difference in ring structure between the chlorin ring above and the porphyrin ring here.

Porphyrin Ring

It may be surprising to find an important molecule in our blood called hemoglobin has a molecular structure similar to chlorophyll. Hemoglobin is a protein found in red blood cells and is responsible for transporting oxygen from the lungs to the rest of the body. There are a few main differences between hemoglobin and chlorophyll molecules. In hemoglobin, the central metal atom is iron instead of magnesium. The phytol chain is also different.

Hemoglobin

Types of Chlorophyll

There are two main types of chlorophylls found in photosynthetic organisms:

Type of Chlorophyll Location Chlorophyll a Almost all photosynthetic organisms (plants, algae, cyanobacteria, and aquatic species) Chlorophyll b Higher plants and green algae

Source: Fruit and Vegetable Phytochemicals: Chemistry and Human Health

Role of Chlorophyll in Plants

Plants are referred to as producers because they make their own food during photosynthesis. Since plants do not obtain energy from other organisms they often form the foundation of many food chains. The energy stored in plant tissue is transferred to animals that eat them.

Have you ever stood in the hot sun with a black shirt on or walked on something that was dark in color during the heat of the day? If so, you have probably experienced how various colors absorb energy from the sun differently. Natural pigments absorb and reflect various wavelengths of light.

Chlorophyll a absorbs light in the blue-violet region and reflects green light.

Chlorophyll b absorbs red light and reflects green light.

Notice that both chlorophyll a and chlorophyll b reflect green light.

Chlorophyl Reflects Green Wavelengths of Light

Photosynthesis

Since light absorption is needed for photosynthesis to occur, it makes sense we would find the light-absorbing pigment, chlorophyll, inside chloroplasts (the specialized cells in plants responsible for photosynthesis).

Notice the different elements represented in the chemical reaction for photosynthesis:

Photosynthesis Chemical Reaction

During photosynthesis, carbon dioxide (CO2), water (H2 O), and sunlight must be available. The two byproducts of photosynthesis are oxygen (O2), which is released into the environment, and glucose (a carbohydrate that serves as food for the plant).

Chlorophyll Function

This is an image of a thylakoid. The image has been zoomed in so you are able to see the thylakoid membrane where the light-dependent reactions of photosynthesis take place. Since light is captured here, it should come as no surprise that this is where we find chlorophyll.

The image above shows two photosystems (PSI and PSII). Photosystems are protein complexes in plants involved in photosynthesis. Chlorophyll can be found in and around these photosystems. Notice PSI comes second in the process of photosynthesis. This is because PSI was discovered and named before PSII. The reaction center of PSI is referred to as P700. This reaction center is affected by light with wavelengths greater than 680 nanometers. PSII is referred to as P680. The pigments in this photosystem are affected by light with wavelengths less than 680 nanometers

Steps of the Light-Dependent Reactions in Photosynthesis

A photon is a discrete unidirectional packet of light that is absorbed by chlorophyll.

During the light-dependent reactions: