Plasma Membrane Structure (College Board AP® Biology)

The Phospholipid Bilayer

Membranes

• Membranes form partially permeable barriers between the cell and its environment, between cytoplasm and organelles and also within organelles

• Substances can cross membranes by diffusion, facilitated diffusion, osmosis and active transport

• Membranes play a role in cell signalling by acting as an interface for communication between cells

Membranes of Eukaryotic Cells Diagram

Membranes formed from phospholipid bilayers help to compartmentalise different regions within the cell, as well as forming the cell surface membrane

Phospholipids

• Phospholipids form the basic structure of the membrane (the phospholipid bilayer)

• They are formed by a hydrophilic phosphate head bonding with 2 hydrophobic hydrocarbon (fatty acid) tails

• As phospholipids have a hydrophobic and hydrophilic part they are known as amphipathic

• The phosphate head of a phospholipid is polar (hydrophilic) and therefore soluble in water

• The fatty acid tail of a phospholipid is nonpolar (hydrophobic) and therefore insoluble in water

Phospholipid Structure Diagram

The generalized molecular structure of a phospholipid

• Due to their amphipathic properties, phospholipids display an emergent property when placed into water

• The hydrophilic phosphate heads orientate toward the water and the hydrophobic hydrocarbon tails orientate inward (away from the water)

  • They form a phospholipid monolayer

Phospholipid Monolayer Diagram

A phospholipid monolayer

• If phospholipids are mixed/shaken with water, they form spheres with the hydrophilic phosphate heads facing out toward the water and the hydrophobic fatty acid tails facing inward

  • This is called a micelle

Micelle Diagram

A micelle

• Alternatively, when there is a sufficient concentration of phospholipids present, then 2-layered structures may form

• These sheets are called phospholipid bilayers – this is the basic structure of the cell membrane

Formation of a Phospholipid Bilayer Diagram

A phospholipid bilayer is composed of 2 layers of phospholipids; their hydrophobic tails face inward and hydrophilic heads face outward

• The 2 layers of phospholipids are held together loosely by weak hydrophobic interactions between the hydrocarbon tails allowing some membrane fluidity

• The amphipathic properties result in the phospholipid bilayer acting as a barrier to most water soluble substances

  • The nonpolar fatty acid tails prevent polar molecules or ions from passing across the membrane

• This ensures that water soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell and that unwanted water soluble molecules cannot get in

The Fluid Mosaic Model

• The fluid mosaic model of membranes was first outlined in 1972 by Singer and Nicolson and it explains how biological molecules are arranged to form cell membranes

• The fluid mosaic model also helps to explain:

  • Passive and active movement between cells and their surroundings

  • Cell-to-cell interactions

  • Cell signalling

• The fluid mosaic model describes cell membranes as ‘fluid’ because:

  • The phospholipids and proteins can move around within their own layers

• The fluid mosaic model describes cell membranes as ‘mosaics’ because:

  • The scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above

• The fluid mosaic model of membranes includes four main components:

  • Phospholipids

  • Cholesterol

  • Glycoproteins and glycolipids

  • Integral and peripheral proteins

The fluid mosaic model diagram

The distribution of the proteins within the membrane gives a mosaic appearance and the structure of the proteins determines their position in the membrane