Lipids

Laura Armstrong

Teacher

Laura Armstrong

Recall Questions

This topic requires prior knowledge of biological molecules. You can test your knowledge on these below.

What is a reducing sugar?

A sugar that can donate electrons to another chemical. Examples include glucose, maltose, and lactose​.

Why do lipids form an emulsion in the emulsion test?

Lipids are insoluble in water but soluble in ethanol. When mixed with water, lipid droplets disperse, forming a white emulsion​.

What happens when proteins are present in the Biuret test?

Biuret reagent will change colour from light blue to purple​.

Topic Explainer Video

Check out this @LauraDoesBiology video that explains lipids or read the full notes below. Once you've gone through the whole note, try out the practice questions!

Introduction to Lipids

  • Lipids are non-polar, hydrophobic molecules made primarily of carbon, hydrogen, and oxygen.
  • They are insoluble in water but soluble in organic solvents such as alcohols​.

Structure of Triglycerides

  • Triglycerides are formed by the condensation of one glycerol molecule and three fatty acid molecules.
  • The bond formed between glycerol and each fatty acid is called an ester bond.

  • Reaction type: Condensation reaction (water is removed)​.

  • The fatty acids can be saturated: No C=C double bonds in the hydrocarbon chain; straight chains, solid at room temperature (e.g., animal fats).

  • The fatty acids can be unsaturated: One or more C=C double bonds in the hydrocarbon chain; bent chains, liquid at room temperature (e.g., plant oils).

Structure of Phospholipids

  • Phospholipids are formed by the condensation of one glycerol molecule and two fatty acid molecules.

  • The bond formed between glycerol and each fatty acid is called an ester bond.

  • Reaction type: Condensation reaction (water is removed)​.

  • Similar to triglycerides but one fatty acid is replaced with a phosphate group.

  • Structure:

    • Hydrophilic head (phosphate group and glycerol): Attracted to water.

  • Hydrophobic tails (fatty acids): Repel water.

  • Function: Forms the phospholipid bilayer in cell membranes, crucial for compartmentalisation in cells​.

Functions of Lipids in Living Organisms

Function

Description

Energy Storage

Lipids store twice as much energy per gram as carbohydrates, making them an efficient energy store​.

Insulation

Fat deposits under the skin provide thermal insulation in mammals (e.g., blubber in whales)​ which reduces heat loss.

Protection

Lipids cushion vital organs, protecting them from physical damage​.

Waterproofing

Waxes and oils prevent water loss in plants and animals (e.g., cuticle on leaves, sebum on skin)​.

Membrane Structure

Phospholipids form biological membranes, controlling substance transport into and out of cells.

Key Terms

  • Ester Bond: The bond formed between glycerol and fatty acids in a condensation reaction​.
  • Triglyceride: A lipid made of one glycerol and three fatty acids​.
  • Phospholipid: A lipid with glycerol, two fatty acids and a phosphate group, forming cell membranes​.
  • Hydrophobic: Repels water, like the fatty acid tails of lipids​.
  • Hydrophilic: Attracts water, like the phosphate head of phospholipids​.
No answer provided.

Exam Tips

Always link structure to function in lipid-related questions.

For phospholipids, describe how hydrophilic heads and hydrophobic tails lead to the formation of bilayers​.

No answer provided.

Describe the structure of a triglyceride and explain how its properties make it suitable for energy storage. (4 marks)

  • Triglycerides consist of one glycerol molecule and three fatty acids​.
  • Ester bonds form between glycerol and fatty acids via condensation reactions​.
  • High ratio of C-H bonds means triglycerides store more energy per gram than carbohydrates
  • Insoluble in water, so they do not affect the water potential in cells, making them efficient energy stores​.

Practice Question 1

Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!

Practice Question 2

If you want to try out another one, check this video out and see how you do!