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Step 1

The Electromagnetic Spectrum
electromagnetic spectrum

Visible light is a small portion of the electromagnetic spectrum. Light can be reflected, absorbed, or transmitted by objects. When light wavelengths between 380-750 nanometers are reflected off surfaces, we see a range of colors from violet to red.

Certain plant pigments absorb wavelengths of light and convert light energy to chemical energy during photosynthesis.

Step 2.1

Light and Chloroplasts
diagram of light absorbtion of chloroplasts

Chloroplasts contain several pigments that absorb light of different wavelengths. Chlorophylls absorb mostly blue-violet and red light, and reflect green light. That’s why leaves look green to our eyes. Other pigments, such as carotenoids, absorb and reflect different wavelengths that might damage chlorophyll.

Step 2.2

Excited Electrons
diagram of chlorophyll when light hits it

When light strikes chlorophyll, electrons from chlorophyll are boosted to higher energy levels. As they fall back down to their ground state, they may emit a glow (fluorescence) and lose some energy as heat.

Step 3

Photosystem Overview

Chlorophyll molecules are bound to proteins and arranged in clusters called photosystems in the thylakoid membrane. When light energy excites the electrons of chlorophyll, the electrons are passed to an electron acceptor instead of falling back to their ground state.

Step 4

Photosystems I and II
Photosystems I and II

Photosystem II contains chlorophyll P680, which absorbs wavelength 680. The excited electrons (gold arrows) travel through an electron transport chain made up of protein carriers (purple). Electrons from the splitting of water replenish the electrons lost from Photosystem II. Electrons from Photosystem II replenish the electrons lost from Photosystem I.

Photosystem I contains chlorophyll P700, which absorbs wavelength 700. The excited electrons are captured by NADP+, reducing it to NADPH.

Step 5

diagram of chemiosmosis

Chemiosmosis drives the formation of ATP in both mitochondria (cellular respiration) and chloroplasts (photosynthesis). When electrons travel down the electron transport chain, the energy is used to create a hydrogen ion concentration gradient across the thylakoid membrane.

The concentration gradient forces hydrogen ions to pass through ATP synthase, which catalyzes the phosphorylation of ADP. This process is called photophosphorylation in plants. The final step of the process is the reduction of NADP+ to NADPH.