Here's why:
* Plasma membrane: This is the outer boundary of the cell, acting as a barrier between the cell's internal environment and the external environment. It is selectively permeable, meaning it allows some substances to pass through while blocking others.
* Oxygen: Oxygen is a small, nonpolar molecule, which allows it to easily diffuse across the lipid bilayer of the plasma membrane.
The journey of oxygen:
1. From the lungs: Oxygen enters the body through the lungs, where it diffuses into tiny blood vessels called capillaries.
2. Through the blood: Oxygen travels through the bloodstream, bound to hemoglobin in red blood cells.
3. To the tissues: Oxygen reaches the capillaries surrounding individual cells.
4. Across the plasma membrane: Oxygen diffuses from the capillaries, across the plasma membrane, and into the cell's cytoplasm.
5. Used by mitochondria: Inside the cell, oxygen is used by mitochondria during cellular respiration to produce energy (ATP).
