Cellular respiration refers to the transfer of oxygen and carbon dioxide across the cell membrane. The cell membrane is the outer layer of all cells within the body; the barrier enclosing every cell from it’s surrounding tissue or fluid. While this transfer of gases is often referred to as an exchange, this is not strictly true.
Oxygen passes into a cell and carbon dioxide passes out based on their individual concentration levels, what is called the concentration gradient, rather than as an actual exchange. Both easily flow through the cell membrane, neither requiring any assistance from the multitude of transfer proteins situated in the cell membrane. Typically, oxygen levels within the cell are lower than those without, so oxygen diffuses into the cell. Carbon dioxide levels are normally higher within the cell, so they diffuse out.
This is because oxygen is required by the mitochondria within the cell to produce most of the adenine triphosphate (ATP) that acts as the energy supply for all cellular reactions requiring an energy input. This production process produces carbon dioxide as a byproduct. Therefore the energy requirements of every cell requires an inflow of oxygen and produces an outflow of carbon dioxide.
This is what is typically referred to as cellular respiration, because it emulates but is the reverse of the respiration that occurs between the alveolar (air sacks) and the capillaries (small blood vessels) of the lungs. The alveolar are the small sac-like structures at the outer surface of the lungs where the gases (air) breathed in, predominantly oxygen, diffuse into the neighboring small blood vessels (capillaries). Oxygen does this because the concentration within the air is higher than that in the blood arriving from the heart through the pulmonary circuit.
That same blood has a much higher concentration of carbon dioxide accumulated from its diffusion out of the cells than the concentration of carbon dioxide in air, so the carbon dioxide in the blood diffuses into the gas composition (air) in the lungs, to be expelled when we breathe out.
Diffusion of gases occurs due to Brownian motion. Basically, all gas molecules vibrate or move. The overall movement or change in concentration levels tends towards an evening of concentrations. While gas molecules move from areas of high concentration to low concentration and vice versa to the same degree, more molecules effectively move into the area of low concentration from high simply because there are more there. It is an averaging based on statistical norms.
While both oxygen and carbon dioxide can be carried dissolved within the plasma (liquid component) of blood, most is transported chemically bonded mostly to the hemoglobin central to every erythrocyte (red blood cell) or their cell membrane. Oxygen is primarily carried by the hemoglobin, while carbon dioxide generally forms more complex molecular compounds in the erthrocytes’ cell membrane.
Nevertheless, cellular respiration occurs only across the cell membrane, and purely based upon the concentrations of oxygen and carbon dioxide found on either side of that boundary. Oxygen almost always diffuses into each cell, while carbon dioxide diffuses out.