Menstruation does not occur if the
a. uterine lining thickens.
b. estrogen level falls.
c. progesterone level falls.
d. egg is fertilized.

Why do leaves change color in the fall? green pigments break down and no longer mask the color of chlorophyll. chlorophyll breaks down and no longer masks the colors of other pigments. red- and yellow-colored pigments grow and mask green-colored chlorophyll. green-colored chlorophyll breaks down and turns red and yellow.

Mrna decodes information from the original dna master plan to build proteins in the during the process of ribosomes.

What is it called when part of a cell membrane closes around a molecule to allow the molecule to enter the cell? a. passive transport b.diffusion c. endocytosis d. exocytosisc. endocytosis