Phospholipids (SL IB Biology)

• Phospholipids form the basic structure of the cell membrane
  • Cell membranes are phospholipid bilayers
• Membranes are formed when a hydrophilic phosphate head bonding with two hydrophobic hydrocarbon (fatty acid) tails
• Phospholipids have a hydrophobic and a hydrophilic region
  • The phosphate head of a phospholipid is polar, so is hydrophilic and therefore soluble in water
  • The fatty acid tail of a phospholipid is nonpolar, so is hydrophobic and therefore insoluble in water
• Molecules with both polar/hydrophilic and non-polar/hydrophobic regions are said to be amphipathic

Phospholipid structure diagram

Phospholipids contain polar heads and non-polar tails, so are said to be amphipathic

• When placed in water, the hydrophilic phosphate heads of phospholipids orient themselves towards the water and the hydrophobic hydrocarbon tails orient themselves away from the water, causing them to form a phospholipid monolayer

Phospholipid monolayer diagram

Phospholipids can form monolayers on the surface of water

• When phospholipids are mixed with water, two-layered structures known as phospholipid bilayers can form; this is the basic structure of the cell membrane

Phospholipid bilayer diagram

A phospholipid bilayer is composed of two layers of phospholipids; their hydrophobic tails face inwards and hydrophilic heads face outwards

• The amphipathic nature of phospholipids means that the phospholipid bilayer acts as a barrier to most water-soluble substances
  • The non-polar fatty acid tails prevent polar molecules or ions from passing between them across the membrane
• This means that water-soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell and unwanted water-soluble molecules cannot get in

• Small, nonpolar molecules, such as O₂ and CO₂, are soluble in the lipid bilayer and can therefore easily cross cell membranes to be utilised by the cell
  • They do not need proteins for transport and can diffuse across quickly
• Other larger, non-polar molecules can also enter the cell across the lipid bilayer, e.g. steroid hormones
  • Steroid hormones contain cholesterol, a type of lipid
• The hydrocarbon region of cholesterol is non-polar, allowing it to cross lipid bilyars

Cholesterol structure diagram

Cholesterol has hydrophobic and hydrophilic regions

• Oestradiol and testosterone are two examples of steroid hormones formed from cholesterol
  • They are produced by gonadal tissues in the reproductive organs
• Due to their lipid structure they can cross the lipid bilayer and can readily travel into and out of cells and nuclei
  • Inside the nucleus these hormones alter and direct the process of transcription