AS-LEVEL AQA BIOLOGY NOTES
TOPIC 2: carbohydrates
Disaccharide = macromolecule consisting of 2 sugar molecules joined by a glycosidic bond
Polysaccharide = macromolecule consisting of more than 2 sugars - often a long chain polymer - joined together by glycosidic bonds
Two sugar molecules join together by a glycosidic bond (shown below):
Polysaccharide = macromolecule consisting of more than 2 sugars - often a long chain polymer - joined together by glycosidic bonds
Two sugar molecules join together by a glycosidic bond (shown below):
Glycosidic Bond
- Repeats between thousands of sugar monomers to form a polysaccharide
- A polysaccharide is not a sugar
- A polysaccharide is not a sugar
- If just two sugar monomers bond, they form a specific disaccharide
- Form by a condensation reaction
- Broken by hydrolysis
- Generally 1,4-glycosidic bonds
EXAM TIP You should be aware of the different combinations of sugar that form different disaccharides:
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There are two types of glucose molecule. They have the same chemical formula, but their structures are slightly different. The two versions are called alpha-glucose and beta-glucose, with reference to the first Carbon atom in the hexose ring. The difference is highlighted in the diagram, and involves simply the different direction of the H and OH groups attached to the Carbon atom.
Starch
- A mixture of amylose and amylopectin.
- Both amylose and amylopectin are long-chain a-Glucose polymers
- Both amylose and amylopectin are long-chain a-Glucose polymers
- The main form of carbohydrate used for energy storage in plants.
- Starch grains are stored in chloroplasts and are insoluble in water so they don’t affect water potential up and down the xylem - important for water and nutrient transport in the plant.
Amylose |
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Glycogen
- Multibranched a-Glucose polymer.
- Main form of carbohydrate used for energy storage in animals.
- Glycogen is stored in the muscles and in the liver.
- In times of high energy usage, these organs hydrolyse the glycogen stored and break it down into glucose molecules which can be used in respiration.
- Soluble in water.
- Compact shape makes it optimal for an energy storage molecule.
Cellulose
- Highly abundant in cell walls, making it most common molecule on the planet.
- Polymer of about 10,000 β-glucose molecules in a long unbranched chain called a microfibril.
- High stability due to structure - several polysaccharide chains running parallel to each other with cross links between them.
- The chain cross links are hydrogen bonds and contribute to the strength of the molecule.
- Stability renders plants strength and resistance to wind and rain.
Benedict’s test for reducing and non-reducing sugars
All monosaccharides are reducing sugars, while disaccharides can be either reducing or non-reducing.
To check whether a non-reducing sugar is present, we first need to break down the sugar into its reducing sub-parts. For instance, sucrose needs to be broken down into glucose and fructose before it will react with Benedict’s reagent. We can achieve this by:
Iodine test for starch
All monosaccharides are reducing sugars, while disaccharides can be either reducing or non-reducing.
- Benedict’s reagent is composed of copper-II sulphate in alkaline solution
- Add the reagent to the sample and heat in a water bath (80℃) for 3 minutes
- In the presence of a reducing sugar, Cu2+ ions are reduced to Cu+ and the sugar is oxidised
- Colour changes to red
- If no colour change present, this could indicate either:
- Absence of any sugar
- Presence of non-reducing sugar
- Absence of any sugar
To check whether a non-reducing sugar is present, we first need to break down the sugar into its reducing sub-parts. For instance, sucrose needs to be broken down into glucose and fructose before it will react with Benedict’s reagent. We can achieve this by:
- Heat solution with acid to hydrolyse any glycosidic bonds present
- Neutralise solution with sodium hydroxide
- Add benedict’s reagent and heat in water bath for 3 minutes
- If colour changes to orange/red then a non-reducing sugar was originally present
Iodine test for starch
- Use potassium iodide solution as source of iodine
- In the presence of starch, the solution will turn a dark blue-black colour
- This is due to iodine becoming trapped inside the coils of starch