Answer:
During exercise your cells need energy to function. in order to do so they harness it through ATP. In order to get ATP the cells convert it to ADP usually through aerobic respiration. Aerobic respiration requires oxygen to function and the cells will use up all their oxygen stores for it. Cells will get more oxygen when the exercise has been completed through the pulmonary system delivering oxygenated blood to the cells and taking away the used CO2 rich blood.
Plasma Membrane is the answer
Our genes can be affected by the environment and change the way our traits are displayed, called the phenotype. However, changes in phenotype have an underlying genotypic source. Epigenetics or epigenetic changes are changes in the way traits are expressed as an effect of the environment (i.e. food we ate, chemicals in the body, environmental stresses), without changes in the DNA. Simply put, in epigenetics, some parts of the DNA are turned on or off in response to environmental conditions.