Notes for B1.1.8 Hydrophobic properties of lipids - IB | RevisionDojo
B1.1.8 Hydrophobic properties of lipids Notes
What Makes Lipids Hydrophobic?
Dominance of Non-Polar Bonds
Lipids predominantly contain long hydrocarbon chains or rings, consisting of carbon (C) and hydrogen (H) atoms.
These non-polar covalent bonds lack partial charges, meaning they cannot form hydrogen bonds with polarwater molecules.
Exclusion from Water Network
Water molecules form a cohesive hydrogen-bonded network.
Lipids are pushed out of this network, causing them to cluster and separate in aqueous environments (e.g., oil droplets in water).
Analogy
Think of water molecules as a group of friends holding hands at a party.
Lipids, being strangers who don’t know the handshake, are left to cluster together on the sidelines.
Classes of Lipids and Their Hydrophobicity
Fats and Oils (Triglycerides)
Composed of three fatty acid chains bonded to glycerol.
Fats are typically solid at room temperature (more saturated chains).
Oils are liquid at room temperature (richer in unsaturated chains).
Both readily form droplets in water due to hydrophobic tails.
Waxes
Longer hydrocarbon chains than fats, highly water-repellent.
Found as protective coatings (e.g., plant cuticles, bird feathers).
Steroids
Have four fused rings of hydrocarbons (e.g., cholesterol, testosterone).
Mostly non-polar, though small polar functional groups (like –OH) can slightly reduce hydrophobicity.
Note
Cholesterol, for example, has a small hydrophilic hydroxyl group, but its large hydrophobic structure dominates, making it insoluble in water.
Example
Think of butter (a fat) and olive oil (an oil). Their inability to mix with water is a direct result of their hydrophobic triglyceride structure.
Biological Significance of Hydrophobic Lipids
Energy Storage
Lipids’ non-polar nature lets them form compact, water-excluding droplets in adipose tissue.
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Questions
Recap questions
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Question 1
Recap question
Triglycerides store more energy per gram than carbohydrates primarily because:
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Note
Lipids are a diverse group of biological molecules that share a common characteristic: they are hydrophobic, meaning they do not mix well with water. This property is crucial for many of their biological functions.
The term hydrophobic comes from Greek words meaning "water-fearing."
Unlike many other biological molecules, lipids lack polar groups that can interact with water.
This makes them insoluble in water but soluble in non-polar solvents like oils.
AnalogyThink of lipids like oil in a salad dressing - they always separate from the water-based vinegar because they can't mix together.
DefinitionHydrophobic refers to substances that repel or do not mix with water.
