What is meant by half unit membrane, double unit membrane and single unit membrane?

hydrogenosomes are new organelles bound by double unit membrane they are bound by a half bilayer and occur in abundance in the alleurone grains which of the following has a single unit membrane they produce hydrogen extensively spherosomes were observed they are bound by a half bilayer and occur in abundance in the aleurone grains the peroxisomal membrane is permeable to pyridine nucleotides the permeability to amino acides and uric acid