Final answer:
Oxygen moves from a red blood cell to a mitochondrion in a muscle cell through the process of diffusion, which is a passive transport mechanism that does not require energy.
Step-by-step explanation:
The movement of O2 from a red blood cell in a systemic blood capillary to a mitochondrion in a muscle cell can best be described as diffusion. This process is categorized under passive transport mechanisms, where oxygen molecules move from an area of higher concentration in red blood cells to an area of lower concentration within the mitochondria of muscle cells, driven by the inherent kinetic energy of molecules. This is essential for cellular respiration, where O2 is used in the mitochondria to produce ATP, the energy currency of the cell.
As cells use up oxygen, its concentration decreases inside the cell, leading to a concentration gradient. Oxygen then diffuses through interstitial fluid and the cell membrane directly into the cell's cytoplasm. This is a continuous process that does not require energy from the cell, thereby termed as a form of simple diffusion.