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Most unicellular green algae have light-sensitive transmembrane proteins called channelrhodopsins or ChRs that serve as sensory photoreceptors in the flagellar membrane.

The ChR is a cation channel with a photon-absorbing molecule or chromophore covalently bound within its transmembrane domain.

Upon absorption of light, the chromophore triggers a conformational change in the channel, allowing the cations to pass through.

This cation influx generates an electric current in the flagellum, causing it to propel the alga towards light.

Due to their light sensitivity, channelrhodopsins are often used as a research tool in animal models. 

For instance, a channelrhodopsin-expressing neuron can be triggered externally with just exposure to light and initiate transmission of a nerve impulse.

Using this technique, called optogenetics, scientists can map the activity within the neural circuit, even in the deeper areas of the brain without the use of any invasive equipment.