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The cell membrane review

Key terms

Cell membraneSpecialized structure that surrounds the cell and its internal environment; controls movement of substances into/out of cell
HydrophobicMolecule that repels water (“water-fearing”)
HydrophilicMolecule that is attracted to water (“water-loving”)
AmphipathicMolecule that contains both a hydrophobic and a hydrophilic end
PhospholipidAmphipathic lipid made of glycerol, two fatty acid tails, and a phosphate group
Phospholipid bilayerA biological membrane involving two layers of phospholipids with their tails pointing inward
Semipermeable membraneMembrane that allows certain substances to pass through

Structure and function of the cell membrane

The cell membrane is semipermeable (or selectively permeable). It is made of a phospholipid bilayer, along with other various lipids, proteins, and carbohydrates.
Image modified from OpenStax Biology.
Each phospholipid is amphipathic, with two hydrophobic tails and a hydrophilic head. The hydrophobic tails face inward towards one another, and the hydrophilic heads face outwards.
Image credit: OpenStax Biology.
The unique structure of the cell membrane allows small substances (like oxygen or carbon dioxide) to easily pass through.

Common mistakes and misconceptions

  • Hydrophobic tails face inward and hydrophilic heads face outward. If you get these two ends mixed up, think of the root word “phobia” which means “fear.” Hydrophobic tails fear the water, so they will always try to be as far as possible from the water solutions in and out of the cell.
  • The cell membrane contains a phospholipid bilayer, but the terms are not interchangeable. Part of the cell membrane is a phospholipid bilayer, made of two layers of phospholipid molecules. However, the cell membrane also contains other macromolecules like membrane proteins, and carbohydrates. Therefore, we can say that the cell membrane is made of a phospholipid bilayer, but it is not only made of it.

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