# Lipids & Ester Bonds ## Lipids - Lipids are macromolecules containing carbon, hydrogen and oxygen atoms - Unlike carbohydrates, lipids contain a **lower proportion of oxygen** - Lipids are **non-polar** and **hydrophobic** (insoluble in water) - There are **two groups of lipid** to know: - **Triglycerides** (the main component of fats and oils) - **Phospholipids** - Lipids play important roles in: - Energy yield - Energy storage - Insulation - Hormonal communication ## Triglycerides - Triglycerides are non-polar, hydrophobic molecules - The **monomers** that make up triglycerides are **glycerol** and **fatty acids** ### Glycerol - Glycerol is an **alcohol** (an organic molecule containing a hydroxyl group bonded to a carbon atom) ### Fatty Acids - Fatty acids contain a **methyl group** at one end of a hydrocarbon chain (known as the **R group**) - Chains of hydrogens bonded to carbon atoms - Typically 4 to 24 carbons long - At the other end is a **carboxyl group** - The shorthand chemical formula for a fatty acid is **RCOOH** - Fatty acids can vary in two ways: - **Length** of the hydrocarbon chain (R group) - The fatty acid chain may be **saturated** or **unsaturated** ### Saturated vs Unsaturated Fatty Acids

Saturated fatty acids have no double bonds between carbon atoms in the hydrocarbon chain

- **Saturated fatty acids** are mainly found in animal fat - All carbon-carbon bonds are single bonds - The chain is "saturated" with hydrogen atoms

Unsaturated fatty acids have one or more double bonds between carbon atoms in the hydrocarbon chain

- **Unsaturated fatty acids** are mainly found in vegetable oils - Exceptions include coconut and palm oil - Can be **mono-unsaturated** (one double bond) or **poly-unsaturated** (multiple double bonds) - **Cis-fatty acids**: - H atoms are on the **same side** of the double bond - Can be metabolised by enzymes - **Trans-fatty acids**: - H atoms are on **opposite sides** of the double bond - Cannot form enzyme-substrate complexes - Not metabolised - 'Trans-fat' is linked with coronary heart disease | **Feature** | **Saturated Fatty Acid** | **Unsaturated Fatty Acid** | |-------------|-------------------------|---------------------------| | **Double bonds** | None (all C-C bonds are single) | One or more C=C double bonds | | **Shape** | Straight chains | Kinked/bent chains due to double bonds | | **Main source** | Animal fats | Plant oils (mainly) | | **State at room temperature** | Usually solid | Usually liquid | ## Phospholipids - Phospholipids are a type of lipid formed from glycerol and fatty acids - Unlike triglycerides, there are only **two fatty acids** bonded to a glycerol molecule - One fatty acid has been replaced by a **phosphate ion** ($\text{PO}_4^{3-}$) ### Structure of Phospholipids - The **phosphate** group is **polar** → soluble in water (**hydrophilic**) - The **fatty acid 'tails'** are **non-polar** → insoluble in water (**hydrophobic**)

Phospholipids are amphipathic (they have both hydrophobic and hydrophilic parts)

### Behaviour in Water - As a result of having hydrophobic and hydrophilic parts, phospholipid molecules form **monolayers or bilayers** in water - The hydrophilic phosphate heads face the water - The hydrophobic fatty acid tails face away from water (towards each other) | **Structure** | **Arrangement** | |---------------|----------------| | **Monolayer** | Forms at water-air interfaces (e.g. surface tension) | | **Bilayer** | Forms in aqueous environments (e.g. cell membranes) | ## The Ester Bond ### Forming Triglycerides - Triglycerides contain **three ester bonds** between the fatty acids and the glycerol molecule

An ester bond forms when a hydroxyl (-OH) group from glycerol bonds with the carboxyl (-COOH) group of a fatty acid

### Condensation Reaction - An H from glycerol combines with an OH from the fatty acid to make **water** - The formation of an ester bond is a **condensation reaction** - For **each ester bond** formed, **one water molecule** is released - Three fatty acids join to one glycerol molecule to form a triglyceride - Therefore, for one triglyceride to form, **three water molecules** are released **General equation:** $$\text{Glycerol} + 3 \times \text{Fatty Acid} \xrightarrow{\text{condensation}} \text{Triglyceride} + 3\text{H}_2\text{O}$$ ### Key Points - The carbon atom from the carboxyl group (with the double-bonded oxygen) bonds to the oxygen atom from the hydroxyl group - This creates the ester linkage: **C-O-C** (with a C=O attached) ### Breaking Down Triglycerides - The ester bonds can break during **breakdown** of the triglyceride molecule - e.g. during digestion

This process is a hydrolysis reaction – three water molecules are added to reverse the condensation

**Hydrolysis equation:** $$\text{Triglyceride} + 3\text{H}_2\text{O} \xrightarrow{\text{hydrolysis}} \text{Glycerol} + 3 \times \text{Fatty Acid}$$ ## Summary Table: Triglycerides vs Phospholipids | **Property** | **Triglyceride** | **Phospholipid** | |--------------|------------------|------------------| | **Number of fatty acids** | 3 | 2 | | **Phosphate group present** | No | Yes | | **Polarity** | Non-polar (entirely hydrophobic) | Amphipathic (hydrophilic head, hydrophobic tails) | | **Water molecules released during formation** | 3 | 3 | | **Main function** | Energy storage | Cell membrane structure | | **Solubility in water** | Insoluble | Partially soluble (forms bilayers) | ## Formation of Phospholipids - Phospholipids also form through **condensation reactions** involving ester bonds - Two fatty acids bond to glycerol via ester bonds - The phosphate group bonds to the third position on glycerol - **Three water molecules** are released in total during formation

Although lipids contain glycerol and fatty acids, they are NOT polymers because the fatty acids do not join to each other

## Key Distinctions ### Why Lipids Are Not Polymers - In **polymers**, monomers join together in chains (e.g. amino acids → polypeptides) - In **triglycerides**, each fatty acid joins to glycerol independently - The fatty acids do **not** form bonds with each other - Separate lipid molecules (e.g. triglycerides) are not held together by covalent bonds - Therefore, **lipids cannot be classed as polymers** ## Worked Example: Ester Bond Formation Consider the formation of a triglyceride from glycerol and three fatty acids: 1. **Glycerol** has three hydroxyl (-OH) groups 2. Each **fatty acid** has one carboxyl (-COOH) group 3. During condensation: - The -OH from glycerol reacts with -COOH from fatty acid - Forms C-O-C ester linkage - Releases H₂O **Result:** Three ester bonds form, releasing three water molecules ## Practical Applications ### Testing for Lipids - The **emulsion test** can detect lipids in a sample - Method involves: - Adding ethanol to dissolve lipids - Adding water - Lipids form a milky emulsion if present ### Importance in Biology - **Energy storage**: Triglycerides store more energy per gram than carbohydrates - **Membrane structure**: Phospholipids form the bilayer of all cell membranes - **Insulation**: Lipids provide thermal and electrical insulation - **Protection**: Adipose tissue cushions organs - **Signaling**: Some lipids act as hormones or hormone precursors