Chloroplasts are the organelles in plant and algae cells that conduct photosynthesis. They absorb sunlight and use it to synthesize foods from CO2 and water. Chloroplasts have a green color because they contain the pigments chlorophyll a and chlorophyll b. These chlorophyll pigments are essential for photosynthesis as they absorb light energy. The chlorophylls have a specific molecular structure that makes them appear green. Understanding why chloroplasts are green provides insight into how photosynthesis works at the molecular level.
What are chloroplasts?
Chloroplasts are plastids, which are organelles found in plants and algae. They are considered the “kitchens” of plant and algae cells where photosynthesis takes place. Chloroplasts have two membranes – an outer membrane and an inner membrane. Inside the inner membrane is a fluid called the stroma, which contains enzymes, DNA, ribosomes, and other components involved in photosynthesis. Also inside the inner membrane are stacked disc-shaped sacs called thylakoids. The thylakoid sacs contain chlorophyll pigments and other photosynthetic components. During photosynthesis, light energy is absorbed by the pigments in the thylakoid membranes. Chloroplasts transform this light energy into chemical energy that the plant or algae can use. Overall, chloroplasts provide plants and algae with their green color and allow them to synthesize food.
Why are chloroplasts green?
Chloroplasts have a green color because they contain pigments called chlorophylls. There are two main types of chlorophyll – chlorophyll a and chlorophyll b. These chlorophylls are located in the thylakoid membranes within chloroplasts. They have a specific molecular structure that makes them maximally absorb wavelengths of violet-blue and orange-red light. However, because green wavelengths are the least absorbed by chlorophylls, they are reflected back. This gives chloroplasts and leaves their characteristic green color.
Specifically, chlorophyll molecules contain a network of alternating single and double bonds between carbon and nitrogen atoms. This network is referred to as a conjugated system. The conjugated system allows excitation of electrons to a higher energy state when chlorophyll absorbs light. The absorbed energy is then used in photosynthesis. The specific conjugated system and molecular structure of chlorophylls a and b result in maximal absorption of violet-blue and orange-red wavelengths. Green wavelengths are the least absorbed and are reflected, giving rise to the green color.
Types of chlorophyll
There are several major types of chlorophyll pigments:
- Chlorophyll a – This is the most abundant chlorophyll pigment. It is found in all photosynthesizing organisms such as plants, algae, and cyanobacteria. Chlorophyll a strongly absorbs violet-blue and orange-red wavelengths but reflects green wavelengths.
- Chlorophyll b – This type is found in green algae and higher plants. It absorbs blue and orange-red light and reflects green light similarly to chlorophyll a.
- Chlorophyll c – There are several forms of this chlorophyll found in different algae groups. Chlorophyll c absorbs blue and red-orange light but reflects green light.
- Chlorophyll d – This pigment is found in certain red algae species. It absorbs blue-green and orange-red light while reflecting yellow-green light, giving some red algae their reddish color.
The different chlorophyll pigments work together to maximize light absorption for photosynthesis. Chlorophyll a is the most critical, while other types extend the range of light that can be utilized. However, all chlorophylls reflect and transmit green wavelengths, giving chloroplasts and leaves their green color.
Absorption spectrum of chlorophyll
The green color of chloroplasts is directly related to the specific wavelengths of light absorbed by chlorophyll pigments. The absorption spectrum of chlorophyll shows peaks in the violet-blue and orange-red regions of the visible light spectrum.
As seen in the table, chlorophylls maximally absorb light in the violet to blue and orange to red wavelength ranges. Green wavelengths are absorbed much more weakly. This causes green light to be reflected by chloroplasts rather than absorbed, giving rise to their green color.
Why green wavelengths are reflected
The molecular structure of chlorophyll causes it to absorb light most strongly at the blue and red ends of the visible spectrum. Light from the middle green wavelengths is not absorbed efficiently. There are a few reasons why green light is reflected rather than absorbed:
- The specific alternating single and double bonded conjugated system in chlorophyll does not readily accept the energy from green wavelengths.
- Excitation to higher energy states requires a precise amount of energy, which is not provided by the green photons.
- Green light lacks the energy to excite electrons across chlorophyll’s molecular orbitals.
- The carotenoid and xanthophyll pigments in chloroplasts also reflect green light.
In essence, the green photons are the wrong energy for the molecular electronic transition. The conjugated system and molecular orbitals of chlorophyll are “tuned” to the blue and red wavelengths instead.
From an evolutionary perspective, chlorophyll absorbing maximal blue and red light, while transmitting green, may have provided advantages for early plants:
- Blue and red light penetration to lower plant layers – By not absorbing green wavelengths, chlorophyll allows deeper penetration of light required for photosynthesis in lower leaves.
- Reflected green light provides contrast with purple-red background – The reflected green color provided better visual contrast for attracting insect pollinators and reproduction.
- Green light is most abundant in plant habitats – Absorbing maximal green light would not have left enough of the other wavelengths.
Therefore, some researchers hypothesize that the evolution of green light-reflecting chlorophyll helped ancient plants flourish by enabling better light capture, visual contrast, and interaction with pollinators.
Other photosynthetic pigments
In addition to chlorophyll, chloroplasts contain other associated pigments that aid in photosynthesis by broadening the spectrum of light absorption. Key accessory pigments include:
- Carotenoids – These orange and yellow pigments absorb in the 400-500 nm blue region. Carotenoids transfer the absorbed energy to chlorophyll.
- Xanthophylls – These yellow pigments absorb light in the 400-500 nm blue region similarly to carotenoids. They protect against photodamage.
- Phycobilins – These red and blue pigments are found in cyanobacteria and red algae. They extend absorption to 550-660 nm to capture more light.
The variety of photosynthetic pigments allows chloroplasts to absorb light energy across much of the visible spectrum. However, all the pigments transmit and reflect some green light, resulting in the green chloroplast color.
Structural color contribution
In addition to pigment color, the physical structure of chloroplasts also contributes to their green appearance. Thylakoids are arranged in stacks called grana within chloroplasts. These grana form layered structures that can reflect and scatter light. Studies show that chloroplast structural color can interact with pigment color to fine-tune the final observed green color. This effect may help chloroplasts reflect more green light than expected from pigments alone.
Consequences of changing chloroplast color
Because the green color of chloroplasts is so interconnected with chlorophyll pigments, it is difficult to engineer chloroplasts that absorb maximal green light. Some scientists have mutated chloroplasts to reduce chlorophyll production, resulting in yellow or orange color. However, this also decreases photosynthetic efficiency. Researchers have also investigated fluorescent proteins to make chloroplasts appear red when exposed to blue light. But again, photosynthesis is disrupted without the normal pigments and structures. Overall, changing the chloroplast color has proven challenging without impairing the chloroplast’s ability to conduct photosynthesis. Evolution has optimized chloroplasts for both optimal light absorption by chlorophyll pigments and reflecting green light.
Chloroplasts have a green color because they contain chlorophyll pigments that optimally absorb violet, blue, and red light while reflecting green light. The specific molecular structure and conjugated system of chlorophylls is tuned to the blue and red wavelengths for maximal photosynthetic light harvesting. Green light lacks the proper energy to excite electrons in chlorophyll and is mostly reflected. Accessory pigments and structural grana also contribute to reflecting green light. The green color likely provided advantages for early plant evolution. Overall, the green chloroplasts illustrate an elegant optimization by evolution of light absorption for photosynthesis along with reflecting light that enhances adaptability and survival. Understanding the chloroplast’s green color provides insight into the intricate design and function of the molecular machines that support virtually all life on Earth.