Question

water (h₂o), carbon dioxide (co₂), and oxygen (o₂) are all quite small molecules, yet they move across cell membranes differently. both co₂ and o₂ passively diffuse across the cell membrane directly while h₂o moves across the cell membrane by facilitated diffusion. what can be concluded about these three molecules based on the way they react to the cell membrane? the water molecules are attracted to the lipid tails of the cell membrane, but the carbon dioxide and oxygen molecules are not. the water molecules are too small to pass between the lipid tails of the cell membrane, while the molecules of carbon dioxide and oxygen are not. the water molecules are repelled by the lipid tails of the cell membrane, but the carbon dioxide and oxygen molecules are not. the water molecules are too large to pass between the lipid tails of the cell membrane, while the molecules of carbon dioxide and oxygen are not.

Explanation:

To solve this, we analyze the properties of the molecules and the cell membrane. The cell membrane has a lipid bilayer with hydrophobic (water - repelling) tails. Water is a polar molecule, and the lipid tails repel polar molecules. $CO_2$ (non - polar) and $O_2$ (non - polar) can pass through the lipid tails directly via simple diffusion as they are non - polar and small enough. Water, being polar, is repelled by the lipid tails and thus moves via facilitated diffusion (through aquaporins), not directly through the lipid tails like $CO_2$ and $O_2$. Let's analyze each option:

• Option 1: Water molecules are not attracted to lipid tails (lipid tails are hydrophobic), so this is incorrect.
• Option 2: Water molecules are polar and the lipid tails are hydrophobic, so water can't pass between lipid tails. Also, $CO_2$ and $O_2$ are small and non - polar, so they can pass. This option misstates the reason for water's movement, so it's incorrect.
• Option 3: The lipid tails of the cell membrane are hydrophobic (water - repelling). Water, being polar, is repelled by these lipid tails, so it can't move through the lipid bilayer directly (hence uses facilitated diffusion). $CO_2$ and $O_2$ are non - polar and small, so they can pass through the lipid tails directly via simple diffusion. This matches the properties of the molecules and the cell membrane structure.
• Option 4: Water molecules are small (smaller than $CO_2$ in terms of molecular size in a sense relevant to membrane passage? No, the key is polarity, not size here. Also, the statement about water being too large is incorrect as water is a small molecule. So this option is incorrect.

Answer:

C. The water molecules are repelled by the lipid tails of the cell membrane, but the carbon dioxide and oxygen molecules are not.