The role of photosynthetic pigments
Absorption, reflection and transmission Although some of the light striking a leaf is reflected and some is transmitted, most of it is absorbed. Of this absorbed light, however, only a small part is used for photosynthesis. The rest is converted to heat and lost (e.g. by radiation)
Absorption and action spectra. The degree of absorption of light of different wavelengths can be measured using a spectrometer. Each pigment’s absorption spectrum can then be accurately plotted:
An action spectrum charts the effectiveness of different wavelengths of light at bringing about the process of photosynthesis:
Selective absorption Chlorophyll a and b are structurally complex molecules containing magnesium and absorb light mainly from the red and blue wavelengths of the spectrum. Accessory pigments (e.g. xanthophyll and carotene) absorb light from other parts of the spectrum and pass the energy on to chlorophyll.
Since each pigment absorbs different wavelengths of light, the total quantity of light absorbed by the plant, and therefore available for photosynthesis, is greater than if only one pigment was present in plants.
Fate of light striking a leaf • Copy figure 5.1 from page 27 of Higher Biology by Torrance • (Back of jotter) Answer question 2 on page 31 – hint: this relates to absorption and action spectra that we were working on last time
Calculating Rf value Substance /pigment
Distance travelled
Solvent
100mm
Orange
96mm
Dark green
58mm
Blue-green
48mm
Greyyellow
44mm