An absorption spectrum is a tool that measures how much light a substance absorbs across different wavelengths, providing a unique profile, much like a fingerprint, for different molecules. By analyzing this absorption pattern, scientists can identify substances and understand their properties. This method is particularly useful in biology for studying the pigments involved in photosynthesis.
Decoding an Absorption Spectrum Graph
An absorption spectrum graph visually represents how much light a substance absorbs at various wavelengths. The horizontal axis (x-axis) displays the spectrum of light, typically ranging from shorter wavelengths like violet and blue (around 400 nanometers) to longer wavelengths like red (around 700 nanometers). The vertical axis (y-axis) quantifies the amount of light absorbed.
The line plotted on the graph reveals which colors a substance preferentially absorbs. Where the line forms a peak, it indicates a high level of absorption at that specific wavelength. Conversely, where the line dips into a trough, it signifies that the substance absorbs very little light. Interpreting these patterns is how scientists understand the relationship between a substance and light.
Photosynthetic Pigments and Their Spectra
Plants and algae contain several types of pigments to capture light energy, each with a distinct absorption spectrum. The primary pigment is chlorophyll a, and its absorption pattern is often represented by a solid gray line. Chlorophyll a is central to photosynthesis because it is directly involved in converting light energy into chemical energy. Its absorption spectrum shows strong peaks in the blue-violet (around 430 nm) and red (around 662 nm) parts of the spectrum.
To maximize light capture, plants also use accessory pigments like chlorophyll b and carotenoids. Chlorophyll b assists by absorbing light in slightly different regions than chlorophyll a, with peaks around 453 nm and 642 nm. This allows the plant to harvest energy from a broader range of blue and red light.
Carotenoids are another group of accessory pigments that absorb light in the blue-violet and blue-green range (400-500 nm). By having a mixture of these pigments, photosynthetic organisms can absorb energy from a much wider portion of the visible light spectrum.
How Light Absorption Powers Photosynthesis
The energy captured by pigments from sunlight powers photosynthesis. When a pigment molecule absorbs a photon of light, its electrons are boosted to a higher, more energetic state. This captured energy is then funneled from one pigment molecule to another until it reaches a specialized area within the chloroplast called a reaction center. This initial stage of photosynthesis is known as the light-dependent reactions.
Inside the reaction center, light energy is used to initiate a series of chemical reactions. It powers the splitting of water molecules, which releases oxygen, and drives the formation of two energy-carrying molecules: ATP and NADPH. These molecules store the chemical energy converted from light, which is then used in the Calvin cycle to convert carbon dioxide into glucose, a sugar that serves as food for the plant.
The efficiency of photosynthesis at different wavelengths is shown on a graph known as an action spectrum. When an action spectrum is overlaid with an absorption spectrum, the patterns closely match. This confirms that the wavelengths of light most readily absorbed by the pigments are also the ones that drive the highest rates of photosynthesis.
Why Most Plants Appear Green
The color we perceive in an object is determined by the wavelengths of light it reflects. Plant leaves appear green because their pigments do not absorb all colors of light equally. As the absorption spectra for chlorophylls show, they are very effective at capturing blue and red light to power photosynthesis.
However, the graphs also reveal a significant trough in the green and yellow portions of the spectrum. This indicates that chlorophylls do not absorb these wavelengths efficiently. Instead, green light is largely reflected away from the leaf or transmitted through it.
It is this reflected green light that travels to our eyes, causing us to see the plant as green. The green appearance of the plant world is a visible signature of the wavelengths of light left behind after photosynthesis has taken its share.