GCE AQA BIOL - Lipids

3.1.3 Substances are exchanged between organisms and their environment by passive or active transport across exchange surfaces. The structure of plasma membranes enables control of the passage of substances across exchange surfaces.
Plasma membranes

• Glycerol and fatty acids combine by condensation to produce triglycerides.
• The R-group of a fatty acid may be saturated or unsaturated. In phospholipids, one of the fatty acids of a triglyceride is substituted by a phosphate group.
• The emulsion test for lipids.

Lipids are another group of macromolecules - along with carbohydrates and proteins - that form an important part of our diet 🍲 and that you will need to know for GCE AQA Biology.

• They contain carbon, hydrogen and oxygen 
• There are two classes you should know: 
  • Triglycerides (fats/oils)
  • Phospholipids 
• Lipids serve a variety of important functions 
  • Waterproofing 🌊
  • Shock absorber 💥
  • Insulator 🌡️
  • Store of energy 🔋
  • Source of energy ⚡

Triglycerides 

• Triglycerides are (as their name suggests) made of three fatty acids plus one glycerol 
• The reaction between these components is called condensation because it produces water as a side product 
  • The water comes from a hydrogen of the glycerol combined with the hydroxyl group of the fatty acid 
  • The condensation reaction producing 1 triglyceride produces 3 water molecules as well 

• There are different types of fatty acids, which then cause the resultant triglycerides to be different: 
  • Saturated = no carbon-carbon double bond 
  • Mono-unsaturated = 1 carbon-carbon double bond 
  • Polyunsaturated = >1 carbon-carbon double bond 

Phospholipids

• Made of 1 glycerol, 2 fatty acids, and a phosphate molecule
• They have a unique property of being amphipathic
  • The phosphate head is polar and thus hydrophilic
  • The fatty acid tails are non-polar and thus hydrophobic
  • This helps them serve their purpose in forming plasma membranes
    • The phosphate heads orient themselves to face water (outwards)
    • The fatty acid tails face each other, away from the water
    • This forms the phospholipid bilayer that constitutes the plasma membrane

Test for lipids
For GCE AQA Biology, you should know the test for lipids -- the emulsion test.
Procedure

1. Add 5 cm³ of ethanol to 2 cm³ of the sample in a test tube
2. Shake the test tube
3. Add 5 cm³ of water and shake.

Positive (lipids present): Cloudy-white colour - lipids were emulsified
Negative (no lipids): Solution remains clear

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Written by Justine | Biology Specialist @ Tuttee

References
Toole, G., & Toole, S. (2015). Aqa biology A level. Oxford: Oxford University Press.
Images from

1. https://alevelbiology.co.uk/notes/lipids-structure-functions/
2. https://www.pinterest.ca/pin/401313016769558496/ 
3. https://www.pinterest.com/pin/422564377536350033/
4. https://keystagewiki.com/index.php/Lipid