What is a leaf?
Leaves are the photosynthetic organ of a tree or plant. They are responsible for converting sunlight into energy that the tree can use to grow and survive. Leaves are typically flat and thin and are attached to the branches of a tree by a stem called a petiole
Leaves work by using a complex process called photosynthesis to convert sunlight, carbon dioxide and water into glucose and oxygen.
Photosynthesis
Photosynthesis occurs in specialized structures within the leaf called chloroplasts, which contain the pigment chlorophyll. Chlorophyll is responsible for capturing energy from the sun and this energy is then used as fuel for the chemical reactions involved in photosynthesis.
Photosynthesis can be split into two stages : light-dependent reactions and light-independent reactions.
In light-dependent reactions, light energy is absorbed by the chlorophyll to generate high energy molecules (Such as ATP and NADPH) that the tree uses as fuel.
This fuel is then used in the light-independent reaction to convert carbon dioxide into glucose, releasing oxygen as a byproduct.
Why Green?
The pigment chlorophyll is green, and since chloroplasts are filled with it, most leaves appear green. Chlorophyll molecules are extremely effective at absorbing light in the blue and red parts of the visible spectrum, but they reflect green light, which is why they appear green to the human eye.
The green color is an evolutionary adaptation that allows plants and other photosynthetic organisms like bacteria to efficiently absorb the energy they need to produce food, while reflecting the excess light that could potentially damage their delicate photosynthetic machinery.
If chloroplasts were a different color, then they would absorb different wavelengths of light and would not be able to carry out photosynthesis in the same way. But the color we perceive does not cause the absorption, rather, because it absorbs certain colors and reflects others, we see the colors it reflects, causing us to perceive leaves as green. The green color is not the critical factor, but the specific wavelengths of light that are absorbed is important for photosynthesis to occur. Chloroplasts can perform photosynthesis if they are colored differently, but then they would need the correct pigments to absorb the appropriate wavelengths of visible light.
But the Sun looks red, why does it absorb blue light?
The Sun appears reddish due to the scattering of light in the Earth’s atmosphere, which causes longer wavelength light to be more visible, however, the Sun emits light across the entire visible spectrum, including blue light.
Blue light may seem like it’s just more light, but that is not the case. When a chlorophyll molecule absorbs a photon of blue light, it becomes excited and energized. This energy leads to the production of ATP and is transferred to the photosynthetic reaction centers, where it is used to power the conversion of carbon dioxide and water into glucose and oxygen.
Blue light is important as it provides the energy that is needed to power the process that produces food for the tree and oxygen for us.
If the Sun emits light across the spectrum, why aren’t leaves black? Then all light would be absorbed!
Whatever light is not absorbed by any object, we perceive as color. Black is the absence of any reflection of light, so a black leaf would absorb all visible light it receives and reflect none.
The Sun does emit light across the entire visible spectrum, including green light, so it makes logical sense that a black leaf would absorb the most light and produce the most glucose.
However, while it seems like leaves would be more efficient if they were black and could absorb all wavelengths of light, this isn’t necessarily the case. Absorbing too much light can actually be harmful to the photosynthetic machinery within chloroplasts, and may damage or reduce the efficiency of photosynthesis.
Additionally, leaves need to strike a balance between photosynthetic efficiency and heat buildup. Like machines, this natural process generates heat, and if it gets too hot, the plant would need to lose energy to dissipate the heat. This makes the system inefficient, so plants have evolved to absorb certain wavelengths of light to optimize efficiency and not raw production.
So while it might seem counterintuitive, the green color of leaves is actually an important adaptation that allows them to balance their need for light absorption with their need to avoid excess heat buildup. By reflecting green light and absorbing primarily blue and red light, leaves are able to efficiently capture the light energy they need for photosynthesis, while also avoiding the negative consequences of excess light absorption.
