Photosynthesis and Photophosphorylation

Overview of Photosynthesis

Photosynthesis is a fundamental biochemical process in which light energy is converted into chemical energy by autotrophic organisms, such as plants and some bacteria. This process is essential for the production of organic molecules and oxygen, supporting life on Earth.

• Heterotrophs produce CO2 and H2O from carbohydrate metabolism, and consume O2 and H2O.

• Autotrophs produce O2 and carbohydrates, and heterotrophs consume carbohydrates and O2.

• Examples: Plants (autotrophs), animals (heterotrophs).

Photosynthetic Pigments

Photosynthetic organisms contain specialized pigments that absorb light energy for use in photosynthesis.

• Chlorophyll a: Contains a porphyrin ring with Mg2+ at its center; absorbs light maximally at 680 nm.

• Accessory pigments: Include chlorophyll b and carotenoids, which broaden the absorption spectrum and protect the organism from photo-damage.

• Carotenoids: Consist of compounds like β-carotene and xanthophylls (e.g., lutein), which absorb light in regions where chlorophylls are less efficient.

Key Pigment Structures

• Chlorophyll a: Main pigment in oxygenic photosynthesis.

• β-Carotene: An orange accessory pigment.

• Lutein: A xanthophyll (oxygenated carotenoid) accessory pigment.

Absorption and Action Spectra

The absorption spectrum shows the wavelengths of light absorbed by pigments, while the action spectrum demonstrates the relative effectiveness of different wavelengths in driving photosynthesis.

• Absorption spectrum: Indicates which wavelengths of light are absorbed by each pigment.

• Action spectrum: Plots the rate of photosynthetic activity as a function of wavelength.

Graphical Representation

• The absorption spectra of chlorophyll a, chlorophyll b, and carotenoids show distinct peaks, indicating their optimal light absorption regions.

• The action spectrum closely matches the combined absorption spectra of all pigments, highlighting the wavelengths most effective for photosynthesis.

Key Equations

• General equation for oxygenic photosynthesis:

Summary Table: Pigments and Their Properties

Additional info: Accessory pigments help maximize the range of light energy that can be used for photosynthesis and protect the photosynthetic apparatus from damage by excess light.