Oxygen crosses a plasma membrane
a. through facilitated diffusion.
b. by osmosis.
c. with the help of a nonspecific transporter.
d. through simple diffusion.
e. through porins.
The Correct Answer and Explanation is:
Correct Answer: d. through simple diffusion.
Explanation (300 words):
Oxygen (O₂) crosses a plasma membrane through simple diffusion, which is a passive transport process. Simple diffusion does not require energy (ATP) and occurs when molecules move from an area of high concentration to an area of low concentration directly through the lipid bilayer of the cell membrane.
The plasma membrane is made up of a phospholipid bilayer, with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward. This structure creates a selective barrier that allows certain small, nonpolar molecules to pass through easily, while blocking larger or polar substances.
Oxygen is a small, nonpolar gas molecule, which means it is lipid-soluble and can diffuse freely across the hydrophobic core of the membrane without the need for transport proteins or channels. This is in contrast to ions or polar molecules like glucose or water, which usually require protein channels or carriers.
Once oxygen diffuses into the cell, it is used by mitochondria during aerobic respiration to produce energy in the form of ATP. Because cells are constantly using oxygen, the concentration of oxygen inside the cell is typically lower than outside, maintaining a gradient that promotes continuous diffusion.
Let’s briefly clarify the incorrect choices:
- a. Facilitated diffusion: involves a carrier or channel protein; not needed for oxygen.
- b. Osmosis: specific to water movement, not oxygen.
- c. Nonspecific transporter: typically used for bulk movement of solutes, not applicable to oxygen.
- e. Porins: these are protein channels found mostly in the outer membranes of bacteria and mitochondria, not commonly used for oxygen in plasma membranes.
Thus, oxygen’s movement across the membrane is best described as simple diffusion, driven purely by its concentration gradient and its ability to dissolve in the lipid membrane.