# The Glycosidic Bond ## Forming the Glycosidic Bond - Monosaccharides need to be linked together to make them more suitable for **transport and storage** - **Disaccharides** and **polysaccharides** form when two hydroxyl (-OH) groups on different saccharides interact

A **glycosidic bond** is the strong covalent bond (oxygen link) that holds two monosaccharides together

- Each glycosidic bond formation results in **one water molecule being removed** - Glycosidic bonds form through **condensation reactions** ### The condensation process - A hydroxyl (-OH) group from one monosaccharide bonds with the carboxyl group (-COOH) of another - An H from one saccharide combines with an OH from the other to make water ($\text{H}_2\text{O}$) - The remaining oxygen creates a bridge between the two sugar molecules **Example of maltose formation:** - Two α-glucose molecules approach each other - The -OH group from carbon 1 of the first glucose reacts with the -OH group from carbon 4 of the second glucose - This creates an α-1,4 glycosidic bond - One water molecule is released **Example of sucrose formation:** - α-glucose and β-fructose join together - The bond forms between different carbon positions due to the different sugar structures - Again, one water molecule is produced ### Key features - For **three fatty acids** joining to **one glycerol** to form a triglyceride → **three water molecules** released - The same principle applies to carbohydrates: more bonds = more water molecules released | **Molecule formed** | **Monomers joined** | **Water molecules released** | |---------------------|---------------------|------------------------------| | Disaccharide | 2 | 1 | | Polysaccharide (e.g. 100 monomers) | 100 | 99 | ## Breaking the Glycosidic Bond - The glycosidic bond **breaks** when water is **added** in a **hydrolysis reaction**

**Hydrolysis** means 'lyse' (to break) with 'hydro' (water)

- **Disaccharides** and **polysaccharides** break down through hydrolysis - Covalent bonds are broken as water is added ### The hydrolysis process - A water molecule is added to the glycosidic bond - The oxygen bridge between the two sugars breaks - The H from water attaches to one sugar - The OH from water attaches to the other sugar - This recreates the two original monosaccharides with their hydroxyl groups **Example with maltose:** - Maltose + water → glucose + glucose - The α-1,4 glycosidic bond breaks - Two separate glucose molecules are produced **Example with sucrose:** - Sucrose + water → glucose + fructose - The glycosidic bond breaks - One glucose and one fructose molecule result ### When hydrolysis occurs in organisms - **Digestion** of carbohydrates in the gut - Think of eating a piece of bread: the starch polymers must be broken down into glucose monomers for absorption - **Breaking down stored carbohydrates** for cellular respiration - When you exercise, glycogen in muscle and liver cells gets hydrolysed to release glucose for energy

Hydrolytic reactions are catalysed by **enzymes** different from those in condensation reactions

## Common Disaccharides - **Monosaccharides** join via condensation reactions to form **disaccharides** ### Three important disaccharides | **Disaccharide** | **Monosaccharides involved** | **Where found** | |------------------|------------------------------|-----------------| | **Maltose** | glucose + glucose | Formed during starch breakdown; present in germinating seeds like barley | | **Sucrose** | glucose + fructose | Main transport sugar in plants; table sugar extracted from sugar cane or sugar beet | | **Lactose** | glucose + galactose | Only found in milk; present in cow's milk, human breast milk | ### Maltose - Produced when starch is broken down by enzymes - Two α-glucose molecules joined by an α-1,4 glycosidic bond - Important in beer and whisky production where barley starch is converted to maltose ### Sucrose - The sugar transported through phloem vessels in plants - Composed of α-glucose and β-fructose - Forms when glucose produced in photosynthesis needs to be moved from leaves to other plant parts - Common in sweet fruits and vegetables like apples, carrots, and sweet corn ### Lactose - The sugar present in mammalian milk - Provides energy for young mammals - Some adult humans lack the enzyme lactase needed to digest it (lactose intolerance) ## Condensation vs Hydrolysis Summary ```text CONDENSATION: Monosaccharide + Monosaccharide → Disaccharide + Water (Building up - anabolic) HYDROLYSIS: Disaccharide + Water → Monosaccharide + Monosaccharide (Breaking down - catabolic) ```

For hydrolysis of non-reducing sugars, **acid must be added first** to break the glycosidic bond before testing with Benedict's reagent

### Energy considerations - **Condensation** requires energy input (endergonic) - Energy is needed to remove water and form the covalent bond - **Hydrolysis** releases some energy (exergonic) - Breaking bonds releases stored chemical energy - This is why carbohydrates are good energy stores ## Glycosidic Bond Types - Different types of glycosidic bonds form depending on: - Which carbon atoms are involved - Whether the monosaccharides are α or β forms - The orientation of the -OH groups ### Common glycosidic bond types | **Bond type** | **Found in** | **Structure** | |---------------|--------------|---------------| | α-1,4 | Starch (amylose), Glycogen | Between C1 of first glucose and C4 of second glucose | | α-1,6 | Starch (amylopectin), Glycogen | Creates branch points | | β-1,4 | Cellulose | Between C1 and C4 but with β-glucose (creates straight chains) | ### Why bond type matters - **α-1,4 bonds** allow chains to coil (helical structure) - Good for compact storage in starch and glycogen - **α-1,6 bonds** create branch points - More branch points = more sites for enzyme action = faster breakdown - **β-1,4 bonds** create straight chains - Perfect for structural strength in cellulose - The alternating orientation of β-glucose means adjacent chains can form many hydrogen bonds

Humans can digest α-1,4 and α-1,6 bonds but **not** β-1,4 bonds - this is why we cannot digest cellulose

## Practical: Identifying Glycosidic Bonds in Diagrams When looking at molecular diagrams: 1. **Find the oxygen bridge** connecting two ring structures 2. **Identify the carbon atoms** on either side of the oxygen 3. **Check the orientation** of groups above/below the plane of the rings 4. **Count which carbons** are involved (e.g., C1 to C4) The glycosidic bond itself is the **C-O-C** linkage between the two sugar rings. ### What to look for - Two hexagonal or pentagonal ring structures (the sugars) - An oxygen atom connecting them (the bridge) - No -OH groups at the connection points (they were removed as water) - The specific carbons involved determine the bond name