2.6 Photosynthesis, Light and Pigments

Photosynthesis transfers light energy into chemical energy in carbon compounds. Plants, algae and some prokaryotes use carbon dioxide and water as inputs, produce organic molecules and release oxygen. The familiar glucose equation is an overall summary rather than one direct reaction. Oxygen released in photosynthesis originates from water, while carbon dioxide supplies the carbon incorporated into carbohydrate.

Visible light is a range of electromagnetic wavelengths. A pigment appears the color it reflects or transmits; absorbed wavelengths are the ones available to excite electrons. Chlorophylls absorb strongly in blue-violet and red regions but weakly in green, so leaves commonly appear green. Accessory pigments broaden the wavelengths that can contribute to photosynthesis and can transfer excitation energy toward reaction centers.

An absorption spectrum shows how strongly a pigment or pigment mixture absorbs each wavelength. An action spectrum shows the biological response—such as oxygen production or carbon dioxide uptake—at each wavelength. Their shapes should correspond because absorbed light drives photosynthesis, but they are not identical: an action spectrum integrates all pigments, energy transfer, tissue optics and downstream limitations.

Engelmann placed a filamentous alga with aerobic bacteria under light separated into wavelengths. Bacteria clustered where the alga released most oxygen, especially in blue and red regions. The bacteria served as biological oxygen indicators. The design linked wavelength to photosynthetic oxygen production, but interpretation depends on controls for light intensity and on the assumption that bacterial distribution reflects oxygen rather than another attraction.

Paper or thin-layer chromatography separates pigments because they differ in solubility in the mobile solvent and attraction to the stationary phase. A pigment that is more soluble in the solvent and less strongly attracted to the stationary material travels farther. Separation is not reliably explained by molecular mass alone. The origin must remain above the solvent surface so the sample does not simply dissolve into the reservoir.

An $R_f$ value helps identify a pigment only when compared under the same solvent, stationary phase and conditions. Pigment color and multiple standards add evidence. If the solvent front travels 80 mm and a pigment center travels 52 mm, its $R_f$ is 0.65. Measuring from the bottom of the plate rather than the origin, or measuring the edge of a diffuse band inconsistently, introduces systematic error.