Certain mental problems are thought to be caused by not having enough neurotransmitters why would a lack of neurotransmitters cause problems

Lack of neurotransmitters may cause problems because neurotransmitters send signals from neuron to neuron. So neurons would not be able to receive sensory messages.

By water evaporation from the leaves is created a negative pressure at the top of the tree that promotes the ascension  of the water through channels that are called xylem. there are also two molecule forces that are involved: the intra-water molecules cohesion (water molecules stick to each other) and the adhesion when the water molecules stick to the channels they are running in. this is what enables water to go from the roots to the top of a tall tree.