Phospholipid — Science definition

A molecule with a phosphate 'head' and two fatty-acid 'tails' that is the main building block of cell membranes.

Where it is taught

In use

The membrane's building block is the phospholipid, a molecule with a split personality.

The Cell Membrane

Questions using phospholipid

Problem 1

In a phospholipid, the phosphate head is ___ and the fatty-acid tails are ___.
  1. hydrophilic; hydrophobic
  2. hydrophobic; hydrophilic
  3. both hydrophilic
  4. both hydrophobic
Answer and explanation

hydrophilic; hydrophobic

The phosphate head is water-loving (hydrophilic) and the fatty-acid tails are water-fearing (hydrophobic) — the split personality that lets a bilayer form.

Problem 2

Which molecule can pass most easily through the phospholipid bilayer WITHOUT a protein?
  1. A sodium ion (charged)
  2. Glucose (large, polar)
  3. A protein
  4. 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 3

In a cell membrane, the phospholipid tails face
  1. outward toward the watery surroundings
  2. inward, away from water, forming the membrane's interior
  3. toward the nucleus
  4. in random directions
Answer and explanation

inward, away from water, forming the membrane's interior

The nonpolar tails are excluded from water and cluster inward; the polar heads face the watery cytoplasm and exterior.

Problem 4

Which substance crosses a phospholipid bilayer most readily without help?
  1. A sodium ion
  2. A glucose molecule
  3. Oxygen
  4. 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.

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