Final answer:
Neither mitochondria nor nucleus are receptor proteins; they are cellular organelles. 'Free-floating' receptor proteins more accurately describe internal receptors found in the cytoplasm, which are not anchored to the cell membrane.
Step-by-step explanation:
The question seems to be asking about which type of receptor proteins can be considered 'free-floating,' but the options given, mitochondria and nucleus, are not receptor proteins but cellular organelles.
Receptor proteins are associated with the cell membrane or are found within the cell cytoplasm.
However, within the context of receptor types and their localization, internal receptors, also known as intracellular or cytoplasmic receptors, are found in the cytoplasm and can be considered 'free-floating' as they are not anchored to the cell membrane.
These internal receptors bind to hydrophobic ligand molecules that can travel across the plasma membrane, in contrast to cell-surface receptors, which are bound to the cell membrane.
Cell-surface receptors are involved in signal transduction and do not 'float' within the cell as they are anchored to the cell membrane.
These include structural types such as free nerve endings in neurons for sensory reception and functional types like G-protein-coupled receptors.
Therefore, the concept of 'free-floating' receptors aligns more closely with internal receptors present in the cytoplasm rather than the mitochondria or nucleus.