Botany · Ch 13 — Photosynthesis
Absorption Spectrum and Action Spectrum
Absorption Spectrum and Action Spectrum
Two related but distinct graphs are used to characterise how a pigment or a photosynthesising leaf responds to light of different wavelengths. The absorption spectrum, covered in 13.6.1, plots how strongly a given pigment absorbs light at each wavelength - a purely physical/optical measurement of the pigment itself. The action spectrum, covered in 13.6.2, instead plots how effectively each wavelength actually drives the biological process of photosynthesis - a functional measurement of the whole photosynthetic system's output. Overlaying the two spectra, as Figure 13.8 does, is genuinely informative: the wavelengths chlorophyll a and b absorb most strongly (blue and red) are also the wavelengths that drive the highest rates of photosynthesis, which confirms that chlorophyll is indeed the primary light-capturing pigment - but the comparison also reveals subtle mismatches at the spe …
What this figure shows. A graph plotting photosynthetic rate / absorption rate (0-100, y-axis) against wavelength of light in nm (400-700, x-axis), with two overlaid curves: the action spectrum (rate of photosynthesis at each wavelength) and the absorption spectrum, each further broken into separate curves for chlorophyll a, chlorophyll b and carotene. Both curves show two peaks, one in the blue region and one in the red region, with a trough over the green-yellow region in between, illustrating that the wavelengths most strongly absorbed by chlorophyll a and b …