Phospholipid Bilayer Study Guide


What is a lipid bilayer structure?

  • To expose the hydrophilic regions to water and hide the hydrophobic tail regions, the phospholipids arrange themselves in bilayers.

  • This arrangement is unpremeditated, as it is a natural process that does not require energy.

  • The lipid bilayer is a general element of all cell membranes, and it plays a pivotal role as its structural components furnish the obstacle that spots a cell’s limits.

  • The structure is termed a lipid bilayer because it comprises two layers of fat cells arranged in two sheets.

  • This structure shapes the layer that acts as a wall between the inside and outside the cell.

  • The average thickness of lipid bilayers is about five nanometers, and it surrounds all the cells providing the cell membrane structure.

Lipids and Phospholipids

  • Functioning as a barrier (being components of cell membranes) is the main aim of the lipid bilayer structure.
  • Fats like oil that are not soluble in water are known as lipids.
  • Every lipid molecule contains a hydrophilic region, a polar head region, and a hydrophobic or nonpolar region.
  • In cell membranes, phospholipid bilayers compose the permeability barriers within the aqueous compartments.
  • Also directly and indirectly, phospholipids control the functions of enzymes and proteins associated with them.

Cell Membrane and its role

  • The molecules in a cell can enter and leave, and not everything can get in and out of the cell.
  • The plasma membrane, also known as the cell membrane, forms a barrier between the cytoplasm present in the cell and the environment outside the cell.
  • The function of the cell membrane is to protect and support the cell and have control of things entering and leaving the cell.
  • This ability to permit only specific molecules to move in and out of the cell is termed selective permeability or semi-permeability.

What is a Phospholipid Bilayer?

  • Phospholipids mainly consist of fatty acids and alcohol, which are major constituents of a cell membrane (a phospholipid bilayer).
  • The water-resisting tails for each phospholipid molecule are towards the interior of the membrane, and the water accepting heads point outwards facing either the cytoplasm or the fluid surrounding the cell.

Phospholipid Bilayer

  • The cell membrane phospholipids permit entry to hydrophobic molecules as the middle part of the membrane is composed of similar hydrophobic tails.
  • Small lipids and steroids are readily permeable in the membrane as they are hydrophobic.
  • It is observed that hydrophilic molecules cannot pass through the cell membrane as the tail part of the phospholipids are hydrophobic in nature.
  • Molecules like glucose and ions of NA and k, which are hydrophilic, need the help of special proteins to cross the membrane.

Structure of a Phospholipid

Structure of a Phospholipid

  • The phospholipids consist of a polar head which is hydrophilic and is composed of glycerol and a phosphate molecule.
  • It also has two nonpolar tails that are hydrophobic and composed of fatty acids-hydrocarbon chains.
  • They are classed as amphipathic as they contain both hydrophilic, water-loving, and lipophilic, fat-loving regions.

The Functions of the Phospholipid Bilayer

The cell membranes have three basic functions,

  • To keep toxic substances of the cell out and restrict their entry within.
  • Having receptors and channels allows only specific molecules like ions, wastes, nutrients, and metabolic products. They mediate cellular and extracellular activities to move along with organelles and within the cell and outside the outside environment.
  • Separation of important but incompatible metabolic processes carried within the organelles


Biological membranes comprise a double layer of phospholipid molecules called a lipid bilayer and are selectively permeable in nature.


1. What holds the phospholipid together?

Weak hydrophobic interactions hold the bilayer together between the tails, and many substances cannot pass through hydrophilic/hydrophobic layers.

2. Is phospholipid bilayer the same cell membrane?

Cell membranes are composed of phospholipid bilayers, where the phospholipids in these membrane are arranged in two layers, called a phospholipid bilayer.

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