Final answer:
When all available receptors have ligand bound to them, they are described as being saturated, indicating that no additional ligands can bind at that moment. This is related to a cellular response where receptors, being proteins, have specific sites that are fully occupied by ligands.
Step-by-step explanation:
When all available receptors have ligand bound to them, we say the receptors are saturated. This saturation implies that the receptors' binding sites are fully occupied, and no additional ligands can bind until some of the ligands dissociate. Receptors, which are typically proteins on or within a cell, have specific binding sites for ligands. Ligands can be various molecules such as hormones, neurotransmitters, or drugs that facilitate a biological response upon binding with the receptor. In the context of internal receptors, these ligands are typically hydrophobic and can traverse the plasma membrane to directly influence gene expression.
In contrast, cell-surface receptors are involved in pathways where the ligand binding to the extracellular domain leads to a conformational change that activates the receptor's intracellular domain. These receptors, such as tyrosine kinase receptors and G-protein-linked receptors, play pivotal roles in the signal transduction pathways, where the binding of ligands to receptors initiates a sequence of events leading to cellular responses.