AP BIOLOGY

Detailed explanation-1: -A signal molecule binds to surface-membrane receptors if it is large, hydrophilic, or cannot readily cross the plasma membrane. A signal molecule binds to an intracellular receptor if it is small, hydrophobic or can readily cross the plasma membrane.

Detailed explanation-2: -The specific receptors a cell has determines which signals it can detect and thus respond to. After the signal has been perceived by the receptors, transduction occurs. Transduction is when the external signal is detected by the cell and converted to an internal change.

Detailed explanation-3: -Cells have proteins called receptors that bind to signaling molecules and initiate a physiological response. Different receptors are specific for different molecules.

Detailed explanation-4: -The neurotransmitters are hydrophilic molecules that bind to cell surface receptors. Because the neurotransmitters are hydrophilic molecules, they are unable to cross the plasma membrane of their target cells.