Question

which of the following best describes the structure of the phospholipid bilayer in the cell membrane? * 1 point
two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails facing inward
a single layer of phospholipids arranged in a rigid structure
two layers of phospholipids with hydrophobic heads and hydrophilic tails
a thick layer of phospholipids that prevents all molecules from passing through

the term \semi - permeable\ means that the cell membrane: * 1 point
allows all substances to enter and exit freely
allows some substances to pass while blocking others
blocks all substances from entering the cell
only allows water molecules to pass

Explanation:

• For the first question: Phospholipids have hydrophilic (water - loving) heads and hydrophobic (water - fearing) tails. In a bilayer, the hydrophilic heads face outward (towards the aqueous environment inside and outside the cell), and the hydrophobic tails face inward (away from water). A single - layer is incorrect as it's a bilayer. Also, heads are hydrophilic and tails are hydrophobic, not the other way around. And the membrane is not a thick layer that blocks all molecules (it is semi - permeable).
• For the second question: The term semi - permeable means that the membrane has selective permeability. It does not allow all substances (so the first option is wrong), nor does it block all substances (third option is wrong). It is not only permeable to water (fourth option is wrong). It allows some substances (like small non - polar molecules, water in some cases) to pass while blocking others (like large polar molecules without transporters).

Answer:

• First question: Two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails facing inward
• Second question: Allows some substances to pass while blocking others