Chlorophyll is a type of pigment that plays a crucial role in the process of photosynthesis, which converts light energy into chemical energy through the synthesis of organic compounds.
It is present in almost all photosynthetic organisms, such as green plants, algae, and cyanobacteria. Chlorophyll absorbs light energy and uses it to convert carbon dioxide into carbohydrates.
There are several types of chlorophyll, including chlorophylls a, b, c, d, e, and f. Chlorophyll a and b are the most common types found in higher plants and green algae, while chlorophylls c and d are often found in different types of algae. Chlorophyll e is a rare type found in certain golden algae, and bacterio-chlorophyll occurs in some bacteria.
In green plants, chlorophyll is located in thylakoids, which are membranous disk-like structures found in organelles called chloroplasts. The chlorophyll molecule is composed of a central magnesium atom surrounded by a porphyrin ring that contains nitrogen.
Attached to the ring is a phytol chain, which is a long carbon-hydrogen side chain. Minor modifications of certain side groups result in variations in chlorophyll structure.
Other pigments also play a role in determining a plant’s color. Anthocyanins, for example, are responsible for the red, purple, and blue colors of many fruits and flowers, while anthoxanthins contribute to the yellow and cream colors of some flowers and vegetables.
Carotenoids, such as beta-carotene, are responsible for the orange and yellow colors of carrots and pumpkins. The presence and relative amounts of these pigments, along with chlorophyll, determine a plant’s overall color.
Types of Chlorophyll
There are several types of chlorophyll, including chlorophylls a, b, c, d, e and f
Chlorophyll a: The Primary Pigment for Photosynthesis
Chlorophyll a, the preeminent chromophore utilized in photosynthesis, is ubiquitous among higher plants. It manifests a powerful absorption rate, enabling it to assimilate violet-blue and orange-red light while reflecting blue-green light. This mechanism expedites the efficient capture and conversion of light energy into chemical energy by plants.
Chlorophyll b: The Accessory Pigment
Chlorophyll b, a supplementary chromophore that complements the function of chlorophyll an in photosynthesis, is predominantly present in green algae and plants. This form of chlorophyll mostly absorbs orange-red light and reflects yellow-green light. The chlorin ring of chlorophyll b contains a CHO group, whereas the chlorin ring of chlorophyll-a encompasses a CH3 group.
Chlorophyll c: The Unusual Marine Pigment
Chlorophyll c is chiefly discerned in marine algae, such as brown algae, diatoms, and dinoflagellates. It is a distinctive chlorophyll pigment that has a porphyrin ring and is subcategorized into chlorophyll c1, c2, and c3. Each subtype differs in chemical composition and absorption rate.
Chlorophyll d: The Deep-Water Pigment
Chlorophyll d is exclusive to red algae and cyanobacteria that thrive in deep waters. These organisms use red light for photosynthesis, and chlorophyll d has adapted to effectively capture this type of light.
Chlorophyll e: The Rare Pigment
Chlorophyll e is an infrequent pigment that exists only in some golden algae, specifically the Xanthophytes (yellow-green algae).
Chlorophyll f: The Infrared Absorbing Pigment
Chlorophyll f, the most recently uncovered chlorophyll pigment, absorbs infrared light. Its light-absorbing ability extends beyond the visible range of light, but its precise function remains the subject of ongoing research.
Functions of Chlorophyll
Chlorophyll in the Biosynthesis of Sugars
Both types of chlorophyll are employed by plants to capture light energy. Chlorophyll is highly concentrated in the thylakoid membranes of chloroplasts, the organelles in which photosynthesis takes place.
These membranes consist of small sacs made of membranes that are stacked atop each other. Embedded in these membranes are various proteins that encircle chlorophyll and collaborate to channel the energy from light, via chlorophyll, into ATP bonds – the molecules that transport energy within cells.
ATP can then be utilized in the Calvin cycle, otherwise known as the dark cycle, to form sugars.
The chain of proteins that transmits energy from light and channels it toward the formation of sugars is identified as a photosystem. Photosynthesis, which incorporates both light and dark cycles, happens in plants, algae, and some bacteria.
These organisms take in carbon dioxide (CO2), water (H2O), and sunlight to produce glucose. They can employ this glucose in the process of cellular respiration to produce ATP or they can amalgamate the glucose into more intricate molecules for safekeeping.
Chlorophyll in the production of oxygen
As a result of photosynthesis, oxygen is generated as a by-product. Although plants utilize this oxygen in cellular respiration, they also discharge surplus oxygen into the atmosphere. This oxygen facilitates respiration for many non-plant organisms, thus sustaining life on our planet.
During the initial phase of the light-dependent reactions in photosynthesis, oxygen is produced. Plants fragment water molecules to generate electrons, hydrogen ions, and diatomic oxygen (O2).
The electrons are then used in the electron transport chain to drive ATP synthesis, and the oxygen is liberated into the air. Therefore, all the oxygen that we respire is manufactured through this process.
Benefits of Chlorophyll
The existence of life on Earth is solely dependent on the remarkable properties of chlorophyll. One of the key advantages of chlorophyll is the production of sugar through the process of ATP, driven by the very same chlorophyll.
Plants, as primary producers, form the foundation of the food chain, as all other organisms rely on the sugars they synthesize to sustain their livelihoods.
Although top predators may not consume plants directly, they depend on herbivores that solely consume plants to grow and form muscle by processing and utilizing plant nutrients. The accumulation of these nutrients in nature would not be possible without chlorophyll, which is a crucial component of the food chain.
Another critical benefit that all organisms derive from chlorophyll is oxygen. Although chlorophyll itself does not generate oxygen directly, it transfers electrons to molecules such as ATP and NADPH, which are capable of storing energy in their bonds.
This process necessitates the acquisition of electrons, which causes water molecules to be divided, generating oxygen in the process. This oxygen is subsequently released into the atmosphere.
The primary oxygen producers on our planet are plants, algae, and cyanobacteria. All other organisms, including most plants, rely on the availability of this essential oxygen to maintain their survival.
Senescence and the chlorophyll cycle
During plant senescence, chlorophyll degradation occurs through the action of enzymes like chlorophyllase. This enzyme breaks down the phytyl sidechain, which is the opposite of the reaction that produces chlorophylls from chlorophyllide a or b.
The conversion of chlorophyllide a to chlorophyllide b and the re-esterification of the latter to chlorophyll b allows for a continuous cycle between chlorophylls a and b. Additionally, chlorophyll b can be directly reduced to chlorophyll a through the reduction of 71-hydroxy chlorophyll a, which completes the cycle.
As the senescence progresses, chlorophyllides are transformed into nonfluorescent chlorophyll catabolites (NCCs), which are a group of tetrapyrroles that lack coloration.