Final answer:
The composition of the substance with 2/3 water and 1/3 protein with the highest protein content in the body is muscle tissue, consisting mainly of contractile proteins that enable movement.
Step-by-step explanation:
The composition of the substance in the body with 2/3 water and 1/3 protein that has the highest protein content is likely referencing muscle tissue. Muscles are composed of fiber-like proteins that are insoluble in water, serving structural and motor functions in the body.
These proteins, including myosins, are capable of contraction and extension and are predominantly protein once water is removed. In the context of plasma proteins, albumin is the most abundant and contributes to various functions like transporting substances and maintaining osmotic pressure. However, muscle tissue surpasses the protein composition in albumin or other plasma proteins.