The tiny structures inside of cells, such as Ribosomes or chloroplasts that have specialized roles are called organelles. These structures are found in the cytoplasm of eukaryotic cells and are enclosed with a membrane. These tiny structures are not present in prokaryotes like in bacteria.
Fats are used for energy after they are broken into fatty acids. Protein can also be used for energy, but the first job is to help with making hormones, muscle, and other proteins. Broken down into glucose, used to supply energy to cells. Extra is stored in the liver.
I'm not sure how exactly you wanted this question to be answered. You're either talking about the symmetry where animals would have two legs or two arms for example and thus producing a pair of each muscle on both sides of the body, or you're refering to the development of agonistic and antaonistic muscles where each of them served a different purpose; either extending or contracting.
During the process of cellular respiration, energy is converted and broken down from a monosaccharide of glucose to form energy containing molecules, that facilitate the production of ATP. The energy in the form of glucose is utilized within the cells of the organism, and the organelle responsible for this process, of manufacturing ATP, a usable energy molecule within the cell would be the mitochondria.