Lipids
Laura Armstrong
Teacher
Contents
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.
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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.
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The bond formed between glycerol and each fatty acid is called an ester bond.
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Reaction type: Condensation reaction (water is removed).
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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).
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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
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Phospholipids are formed by the condensation of one glycerol molecule and two fatty acid molecules.
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The bond formed between glycerol and each fatty acid is called an ester bond.
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Reaction type: Condensation reaction (water is removed).
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Similar to triglycerides but one fatty acid is replaced with a phosphate group.
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Structure:
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Hydrophilic head (phosphate group and glycerol): Attracted to water.
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Hydrophobic tails (fatty acids): Repel water.
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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. |
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Protection |
Lipids cushion vital organs, protecting them from physical damage. |
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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!