Why is it not possible to completely convert thermal energy into mechanical energy

The second law of thermodynamics is a statement of the law of conservation of energy say that heat cannot be completely converted into mechanical energy  

Neurotransmitters are messengers responsible for communication between neurons. they are released from synaptic vesicles and are received by receptors.  the communication is referred to as neurotransmission. the following statement is true about neurotransmitters: a. they're stored in synaptic vesicles in the axon.