Cell surface receptors covering a cell membrane bind ligands in the extracellular matrix to allow communication between the outside and inside of the cell. Under normal physiological conditions,...

Cell surface receptors covering a cell membrane bind ligands in the extracellular matrix to allow communication between the outside and inside of the cell. Under normal physiological conditions, dynamic trafficking events, such as receptor synthesis, degradation, internalization, and recycling, occur concurrently with receptor/ligand binding on the cell surface, and can alter the number of receptors present and ligands remaining in the extracellular matrix. Once internalized from the cell membrane into the intracellular space, a receptor travels to an endosome, where receptor-ligand dissociation and sorting of the receptor occur. The endosome can recycle the receptor and send it back to the surface, or traffic the receptor to the lysosome for degradation. To increase the cell’s ability to respond to a particular ligand, the cell can also synthesize receptors to increase the number of cell surface receptors. Figure 3.16 shows a simplified model of receptor trafficking, where a single endosome directs all the receptors in the cell.

(a) Using Figure 3.16, draw a system boundary designed to count the receptors on the cell surface. Which trafficking processes would you need information about to find the rate of receptor internalization in this system?

(b) Draw a system boundary designed to count the receptors in the endosomal compartment. Which trafficking processes would you need information about to find the rate of receptor internalization in this system?

(c) Draw a system boundary to count the number of bound ligands.