Describe the difference in crude protein and digestible protein. Why is there a difference?

What is the difference between regular protein and muscle building protein?

Protein is protein - as long as you eat (or supplement with) the most common ones (meat, fish, poultry, or, in the case of supplements - whey is probably the most versatile) - you are going to support muscle-building, provided that you actually exercise to trigger muscle synthesis (obviously, if all you do is eat, you are unlikely to have any meaningful gains, other than fat).

Protein is broken down to amino acids by your digestive system, before these amino acids are used to assemble various proteins used in different tissues in your body, including muscle tissue. Some amino acids may also be synthesized inside your body - they are called non-essential. Some cannot and have to be consumed with food - those are called essential amino acids. If the protein you cosume does not have essential amino acids (there are 9 of them), your body cannot properly synthesize some of the tissues.

Your muscles actually contain several types of protein, but at least three of the essential amino acids (leucine, isoleucine and valine) are known to play a big role in muscle synthesis. So, technically speaking, any protein containing these essential amino acids is “muscle building”, but it doesn’t mean that other types of protein have no impact - again, there are many types of protein in a muscle tissue, including collagen, elastin, actin, myosin, etc.

From that perspective, unless you are limiting yourself to only non-essential or poorly absorbed amino acids (like when consuming plant proteins, for instance) - most protein you eat would play a role in building muscle.

Explanation:

b) the heart is a pump made of muscle and that is difficult to grow in the lab.