in carnivorous plants, the leaf is not just used to photosynthesise, but also as a trap. changing the leaf shape to make it a better trap generally makes it less efficient at photosynthesis. for example, pitchers have to be held upright, so that only their opercula directly intercept light. the plant also has to expend extra energy on non-photosynthetic structures like glands, hairs, glue and digestive enzymes. to produce such structures, the plant requires atp and respires more of its biomass. hence, a carnivorous plant will have both decreased photosynthesis and increased respiration, making the potential for growth small and the cost of carnivory high.