One of the jobs of lipids is to act as chemical messengers. This job is particularly important for the function of the A. heart.
B. brain.
C. lungs.
D. stomach.

B: Brain    

well thats the answer i have to write  over like 20 to get a good expiation but im not going to do that                                                            

biology is the study of life. it is the study of living organism.

before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.