What are membrane bound receptors?

When hormones are bound to binding proteins, do they get rid of them when they reach their receptors?

• 2) can cortisol be either free in the plasma or bound to proteins? if yes, how can it be free if it is hydrophobic?

• Answer:

  both questions are the same : yes hormones (including cortison) bind to protein , but there is always some "free form" of the hormone .. it is not true that the hormone leave the protein at receptor site , no , the hormone that attach to protein will not be able to bind to any receptor , it is the free molecules that can bind to receptors .... why there are free molecules ? my idea is that because any reaction in the world happen in two directions, ( I think they are called reversible reactions ) ... but those reactions that look in one direction are merely having extremely predominant direction with weak opposite direction that is difficult to be detected by ordinary methods ... so imagine that there are two forces : one push the hormone to bind and the other push it to be free ... if the 1st was greater than the second it is said to be hydrophobic , and if the opposite it will be called hydrophilic ... just exactly similar to our skin color : you would be "white" when the melanin in your skin is low , but there is always some melanin , and when it is much you would be black similarly the "affinity" to bind to protein vs to be free in blood note : the free portion is the active form, and therefor to be more scientific we may ask to measure the free form , but it is so much sofisticated thing to do & therefor rarely done routinly ... we otherwise use the total concentration as a reflection of how much free form might be there (cause it is already proportional) so more total -> more free (& vice versa)