Oxygen enters a cell via:
Active transport
Diffusion
Filtration
Osmosis

Oxygen and carbon dioxide move across cell membranes via simple diffusion, a process that requires no energy input and is driven by differences in concentration on either side of the cell membrane.

Explanation:

Simple diffusion is responsible for the spread of an air freshener through a room, or of a few drops of dye to eventually turn a glass of water a uniform color. Molecules naturally disperse from areas of higher concentration to lower concentration.

As oxygen-rich (and carbon dioxide-poor) blood travels by a cell the oxygen diffuses through the cell membrane to the area of lower concentration inside the cell. It can do this easily because the oxygen molecule (O2) is very small and has no charge or polarity. The oxygen is used up rapidly by mitochondria. This rapid consumption causes oxygen to constantly move into the cell from the blood.

The mitochondria creates carbon dioxide (CO2) as a waste product of cellular respiration (the process that makes energy for your body). Because the CO2 is of a higher concentration in the cell than in the blood passing by, this gas continually diffuses out of the cell. It too is small and uncharged so it can pass through cell membranes easily.

These movements require no energy (in the form of ATP) on behalf of the cell.

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explanation: