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Biology AS AQA

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  1. 1-1-biological-molecules-carbohydrates
    11 主题
  2. 1-2-biological-molecules-lipids
    3 主题
  3. 1-3-biological-molecules-proteins
    5 主题
  4. 1-4-proteins-enzymes
    12 主题
  5. 1-5-nucleic-acids-structure-and-dna-replication
    8 主题
  6. 1-6-atp-water-and-inorganic-ions
    4 主题
  7. 2-1-cell-structure
    7 主题
  8. 2-2-the-microscope-in-cell-studies
    4 主题
  9. 2-3-cell-division-in-eukaryotic-and-prokaryotic-cells
    8 主题
  10. 2-4-cell-membranes-and-transport
    9 主题
  11. 2-5-cell-recognition-and-the-immune-system
    7 主题
  12. 2-6-vaccines-disease-and-monoclonal-antibodies
    6 主题
  13. 3-1-adaptations-for-gas-exchange
    6 主题
  14. 3-2-human-gas-exchange
    14 主题
  15. 3-3-digestion-and-absorption
    5 主题
  16. 3-4-mass-transport-in-animals
    6 主题
  17. 3-5-the-circulatory-system-in-animals
    4 主题
  18. 3-6-mass-transport-in-plants
    6 主题
  19. 4-1-dna-genes-and-chromosomes
    10 主题
  20. 4-2-dna-and-protein-synthesis
    3 主题
  21. 4-3-genetic-diversity-mutations-and-meiosis
    7 主题
  22. 4-4-genetic-diversity-and-adaptation
    6 主题
  23. 4-5-species-and-taxonomy
    4 主题
  24. 4-6-biodiversity
    9 主题
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Exam code:7401

Phospholipids: structure & function

  • Phospholipids are a type of lipid molecule that are a major component of cell membranes. They are amphipathic, meaning they contain both:

    • a hydrophilic head

    • hydrophobic tails

Structure

  • A typical phospholipid consists of:

    • one glycerol molecule

    • two fatty acid chains

      • These are non-polar and hydrophobic

    • one phosphate group

      • This is polar and hydrophilic

      • Sometimes attached to an additional polar group (e.g. choline)

  • The fatty acids are bonded to glycerol by ester bonds, while the phosphate group is attached to glycerol via a phosphoester bond

Diagram of a phospholipid showing hydrophilic phosphate head, glycerol, hydrophobic fatty acid tails, and ester bonds. Labels highlight polar and non-polar regions.
The structure of a phospholipid

Monolayers and bilayers

  • As a result of having hydrophobic and hydrophilic parts, phospholipid molecules form monolayers or bilayers in water

Diagram of a phospholipid monolayer and bilayer; shows hydrophilic heads, hydrophobic tails, extracellular and intracellular regions, and hydrophobic core.
In the presence of water, due to the hydrophobic and hydrophilic parts, phospholipids will form monolayers or bilayers.

Function

  • Hydrophobic fatty acid tails create a hydrophobic core when a phospholipid bilayer forms

    • This acts as a barrier to water-soluble molecules

  • The hydrophilic phosphate heads form H-bonds with water, allowing the cell membrane to be used to compartmentalise

    • This enables the cells to organise specific roles into organelles, helping with efficiency

  • The composition of phospholipids contributes to the fluidity of the cell membrane

    • If there are mainly saturated fatty acid tails, then the membrane will be less fluid

    • If there are mainly unsaturated fatty acid tails, then the membrane will be more fluid

  • Phospholipids control membrane protein orientation

    • Weak hydrophobic interactions between the phospholipids and membrane proteins hold the proteins within the membrane but still allow movement within the layer

Examiner Tips and Tricks

Make sure you also learn which parts of phospholipids are hydrophobic and which parts are hydrophilic, as this is what gives these molecules their useful properties in cells.

You also need to be able to explain the different properties of phospholipids.

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