Question

the properties of phospholipids determine their orientation within the lipid bilayer. the heads are polar and hydrophilic, so each head touches either the fluid in the outside environment or the cytoplasm inside the cell. the tails are nonpolar and hydrophobic, so they form the inner core of the membrane, away from fluid. image of phospholipid bilayer with extracellular environment (outside the cell), phospholipid with hydrophilic head and hydrophobic tails, intracellular environment (inside the cell) this arrangement prevents certain molecules from crossing the lipid bilayer on their own. the phospholipids are tightly packed, which means large molecules cannot fit between them. small nonpolar molecules can cross, but ions and most polar molecules cannot cross because they are repelled by the nonpolar fatty acid tails. some very small polar molecules, like water, are able to pass through, but they do so at a slow rate. which of the following molecules can pass through the lipid bilayer without assistance? select all that apply. sodium ion, a charged molecule; glucose, a large polar molecule; water, a very small polar molecule; oxygen, a small nonpolar molecule

Explanation:

1. For sodium ion (charged molecule): The text says ions are repelled by nonpolar tails and can't cross without assistance, so this is incorrect.
2. For glucose (large polar molecule): Large molecules can't fit between tightly packed phospholipids, so it can't cross without assistance, incorrect.
3. For water (very small polar molecule): The text states very small polar molecules like water can pass (slowly) without assistance, correct.
4. For oxygen (small nonpolar molecule): The text says small nonpolar molecules can cross without assistance, correct.

Answer:

water, a very small polar molecule; oxygen, a small nonpolar molecule