Living Solar Panels

Cacti photosynthesize differently than other plants. Most plants absorb CO2 during the day and immediatly use it in photosynthesis to make sugars to grow. When a plant absorbs CO2, they lose water. Because cacti grow in hot, dry places, they evolved to hold on to as much water as possible by only absorbing CO2 at night when temperatures are lower and the rate of water loss is low. 

Plants need sunlight to photosynthesize, so the cactus must hold onto the CO2 throughout the night. In order to maximize the amount of CO2 they can uptake, they store it as a compound called malic acid. The acid is then broken down during the day so the CO2 can be used in photosynthesis. This process is called Crassulacean Acid Metabolism.

Most plants are somewhat transparent, meaning sunlight passes through from the top to underside of the leaf. Cacti, however, are opaque, meaning light that hits one side of the stem does not pass through to the other side. This means that each side of the stem photosynthesizes independently of one another. 

The stem side facing the sun is photosynthesizing at a higher rate than the shaded side, meaning the sun-facing side is breaking down the malic acid faster than the shaded side. This means the sun-facing side is becoming less acidic and the pH is rising while the shaded side is maintaining a more acidic pH for longer. This creates a pH gradient, also known as an electrochemical gradient. We can measure this as a voltage and harvest this as electricity.