- The growing of plants toward light, called positive phototropism, is key to plant growth. In particular, green plants use light to make sugar and oxygen, so there is a distinct advantage to a biological capacity that allows a plant to turn into the strongest part of a light. This change is dependent on the direction and intensity of light. In cases of hormonal-driven phototropism, the plant will not "snap back" to its previous orientation in a subsequent period of darkness.
- The process begins with chemical photoreceptors in the plant called phototropins. Phototropins are biochemicals concentrated in various parts of the plant. Although there is some variation over the life of the plant, these phototropins are present as soon as a sprout comes out of the seed. When light shines on the phototropins, the UV light spectrum breaks certain chemical bonds that then literally unfold the phototropin protein. This begins a cascade that changes other proteins.
- Auxins are the next protein in the cascade and are the real actors in the changes that make the plant turn. Auxins are hormones that affect the organelles in the plant. They respond to the unfolding of the phototropins. They activate proton pumps that cause a chemical change at the cellular level that is the "how" of plant movement.
- Once the proton pumps begin to work, they remove positively charged particles from the immediate cellular environment. This reduces the pH on the dark side of the plant. This new, slightly acidic environment weakens an area of cell walls on the dark side of the plant. This weakening will make that part of the plant wall swell. This is the physical change that actually pushes the plant in the direction of the light
