Phospholipid bilayer — Science definition
The double layer of phospholipids — heads facing the water outside and inside, tails tucked together in the middle — that forms the membrane.
Where it is taught
- The Cell Membrane — Cells, Membranes & Exchange, Life Science: Biology, Grade 10
In use
Because a cell is watery on the inside and bathed in fluid on the outside, the molecules line up in two rows — a phospholipid bilayer.
The Cell Membrane
Questions using phospholipid bilayer
Problem 1
Which molecule can pass most easily through the phospholipid bilayer WITHOUT a protein?
- A sodium ion (charged)
- Glucose (large, polar)
- A protein
- Oxygen (small, nonpolar)
Answer and explanation
Oxygen (small, nonpolar)
Small nonpolar molecules like oxygen dissolve straight through the hydrophobic core; charged or large molecules need transport proteins.
Problem 2
Which substance crosses a phospholipid bilayer most readily without help?
- A sodium ion
- A glucose molecule
- Oxygen
- A protein
Answer and explanation
Oxygen
Small nonpolar molecules such as oxygen and carbon dioxide dissolve in the nonpolar interior and cross freely. Ions and large polar molecules require transport proteins.