(A) The tip of the outer segment of a rod cell in the retina regularly sheds and needs to be replaced. Anabolic activity (green) in the same cell builds the macromolecules needed to replace the lost material. Two glycolytic enzymes, called PKM2 and LDHA (light blue circles), are thought to drive the anabolic metabolism of rod cells. (B) Reducing the expression of either of the genes for these enzymes results in rod cells with shorter outer segments, most likely because there is not enough anabolic activity to counteract the shedding. (C) Keeping mice in constant darkness suppresses the shedding and means that rod cells that lack LDHA or PKM2 still have outer segments of a normal length.