 Auxin diffuses, and is transported, downwards from the tip, with effects including suppressing growth of lateral buds.Auxins are active in light; when they are active they stimulate proton pumps in the cell wall which increases the acidity of the cell wall and activates expansin (an enzyme that breaks bonds in the cell wall structure) that weaken the cell wall and allow the cell to expand.Chloroplasts that have not been exposed to light are called etioplasts (see also plastids).