Why is it important for signal transduction systems to trigger a rapid response within a cell and be able to be shut down quickly?
a. all signal transduction cascades are shut down immediately so as to prevent the cell from mounting a response.
b. cells are unable to respond quickly to hormones, and therefore signal transduction cascades always occur over a long time.
c. cells have to be able to function alone, because they are unable to work together in a coordinated fashion.
d. cells often have to respond very quickly to changes in their internal and external environments.

Mouse liver cells were homogenized and the homogenate subjected to equilibrium density-gradient centrifugation with sucrose gradients. fractions obtained from these gradients were assayed for marker molecules (i. e., molecules that are limited to specific organelles). the results of these assays are shown in the figure. the marker molecules have the following functions: cytochrome oxidase is an enzyme involved in the process by which atp is formed in the complete aerobic degradation of glucose or fatty acids; ribosomal rna forms part of the protein-synthesizing ribosomes; catalase catalyzes decomposition of hydrogen peroxide; acid phosphatase hydrolysis monophosphoric esters at acid ph; cytidylyltransferase is involved in phospholipid biosynthesis; and amino acid permease aids in transport of amino acids across membranes. a) name the marker molecule and give the number of the fraction that is most enriched for each of the following cell components: lysosomes; peroxisomes; mitochondria; plasma membrane; rough endoplasmic reticulum; smooth endoplasmic reticulum.