In cell biology, oxygen molecules cross the plasma membrane of a cell by which basic transport process?

Introduction / Context:
Transport across cell membranes is a fundamental topic in biology. Different molecules use different mechanisms to move into and out of cells, depending on their size, polarity, and charge. Small nonpolar molecules such as oxygen and carbon dioxide are especially important for respiration and gas exchange. This question asks which basic transport process allows oxygen molecules to cross the plasma membrane.

Given Data / Assumptions:
The stem focuses on oxygen, a small nonpolar gas, and its movement across the plasma membrane. The options include osmosis, evaporation, facilitated diffusion, simple diffusion, and active transport. We assume the learner knows that osmosis refers specifically to water movement and that active transport requires energy in the form of ATP. The question is designed to test whether you can match the properties of oxygen with the correct transport mechanism.

Concept / Approach:
Oxygen is a small, nonpolar, and uncharged molecule. Such molecules can dissolve in the hydrophobic core of the phospholipid bilayer and move across it down their concentration gradient without assistance. This process is called simple diffusion. It does not require transport proteins or energy input. Facilitated diffusion is used by polar or charged molecules that need carriers or channels, and active transport moves substances against their gradient using energy. Therefore, the correct approach is to recall that oxygen crosses by simple diffusion through the lipid bilayer.

Step-by-Step Solution:
Step 1: Identify the nature of oxygen; it is a small, nonpolar gas molecule.Step 2: Recall that small nonpolar molecules can pass directly through the hydrophobic region of the cell membrane.Step 3: Remember that this direct movement down a concentration gradient is defined as simple diffusion.Step 4: Note that osmosis applies specifically to water, not to oxygen gas.Step 5: Choose the option that states simple diffusion directly through the lipid bilayer.

Verification / Alternative check:
You can verify by thinking about alveoli in the lungs, where oxygen diffuses from air into blood. This movement occurs down a concentration gradient without ATP use. The process is driven purely by differences in oxygen concentration between regions. If oxygen required active transport, cells would need to spend energy on every molecule of oxygen entering, which does not match known physiology. This real world example confirms that oxygen movement across membranes is by simple diffusion.

Why Other Options Are Wrong:
Osmosis, movement of water across a semipermeable membrane, is incorrect because osmosis refers only to water, not gases like oxygen. Evaporation, conversion of liquid water to vapour, is not a membrane transport process and does not describe how oxygen crosses membranes. Facilitated diffusion through specific membrane protein channels is used by polar molecules or ions that cannot cross the lipid bilayer easily; oxygen does not typically require such channels. Active transport using ATP driven membrane pumps moves substances against concentration gradients, which is unnecessary for oxygen moving down its gradient. Hence, these options do not describe oxygen transport correctly.

Common Pitfalls:
Students may confuse facilitated diffusion and simple diffusion because both are passive processes. The key difference is that facilitated diffusion uses proteins, while simple diffusion does not. Another pitfall is to assume that any important molecule must use active transport, but oxygen is an exception because its properties allow easy passage. To avoid errors, remember that small nonpolar gases use simple diffusion, water uses osmosis, and ions or polar molecules often rely on channels or carriers.

Final Answer:
Oxygen molecules cross the plasma membrane mainly by simple diffusion directly through the lipid bilayer.