A type of plant cell that acts like a hinge at joints to enable the movement of plant parts, such as the closing and opening of leaflets in response to light intensity (see nyctinasty
) or the rapid closure of a leaf in a carnivorous plant. Motor cells adjust their internal concentration of potassium ions (K+
) to alter their turgidity, and hence the cell shape. They can accumulate K+
via potassium channels in the plasma membrane, which promotes the osmotic uptake of water into the cell, making the cell swollen (turgid). Conversely, they can pump K+
out of the cell, which causes water to leave and the cell to shrink. The movements resulting from the changes in motor-cell turgor are relatively gradual, taking minutes or hours. However, in the case of carnivorous plants, such as Venus' flytrap, a very rapid leaf closure is required to trap insect prey. Here the motor cells along the midrib of the leaf become freely permeable to K+
, which surges out, causing water to follow and leading to near instantaneous collapse of the cells and swift closure of the leaf.