🌿 Photosynthesis

What Is Photosynthesis?

Photosynthesis is the process by which photoautotrophs convert light energy into chemical energy stored in carbohydrates.

General equation:

> 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂

Key ideas:

• Occurs mainly in chloroplasts
• Uses light energy, water, and CO₂
• Produces glucose and oxygen

It underpins almost all food chains and drives global oxygen production. 🌎

Chloroplast Structure

Chloroplasts are double‑membrane organelles.

Internal components:

• Thylakoids: flattened sacs; stacked into grana
• Thylakoid lumen: internal space
• Stroma: fluid around grana, contains enzymes, DNA, ribosomes

Functional separation:

• Light reactions: thylakoid membranes
• Calvin cycle: stroma

This compartmentalization enables proton gradients and precise enzyme control.

Types of Photosynthetic Pigments

Main pigments:

• Chlorophyll a: primary; reaction center
• Chlorophyll b: accessory; broadens absorption
• Carotenoids (carotenes, xanthophylls): accessory; photoprotection

They form antenna complexes that capture photons and transfer excitation energy to reaction centers, where photochemistry occurs.

Absorption Spectra vs Action Spectra

• Absorption spectrum: wavelengths a pigment absorbs
• Action spectrum: wavelengths that effectively drive photosynthesis

For chloroplasts, both peak in blue (~430–470 nm) and red (~640–680 nm), with a trough in green (reflected, so leaves appear green).

The similarity between spectra supports pigments’ role in energy capture.