One reason that regulation of gene expression is important is that it saves energy and materials from being used when they are not needed. although most genes have more than one form of regulation acting upon them, at which point would regulation be most efficient in conserving energy and materials if the product of a gene is not needed?

Transcriptional Regulation  

Explanation:

Eukaryotic gene expression can be regulated during the time of transcription and RNA processing, which takes place in the nucleus of the cell and then during the time of protein translation in the cytoplasm.

Further regulation is being carried during the time of post transnational modification. The conservation of energy is very important because it saves the overall energy and space in the body.

Hence, the most important point for the regulation would be transcriptional regulation.

a- all cells come from pre-existing cells

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.