The cell membrane allows some small molecules like oxygen to cross. Why is important for oxygen to be able to cross the cell membrane?
Oxygen is required for the process of cellular respiration. It is this process through which cells convert food to energy. Since cells do not generate oxygen themselves, they need oxygen from elsewhere to enter into the cells, which can only be done by passing through the semi-permeable cell membrane.
During the process of cellular respiration, the food is oxidized and converted to simpler molecules, along with release of adenosine triphosphate or ATP molecules. ATP molecules are also called energy molecules because they can be converted to ADP and AMP molecules; loss of each phosphate bond in this process releases energy. It is this energy that is used by cells to performance various functions, including growth, reproduction, sustenance, maintenance, etc. If the cellular respiration takes place in absence of oxygen, the process produces much less energy. In simplest terms, aerobic respiration is as much as 15 times more energy producing than anaerobic respiration. Hence cells prefer oxygen, which needs to cross the cell membrane and enter the cell to participate in this process of food breakdown and energy generation.
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