Carbohydrates

Definition: Monosaccharides are the simplest form of carbohydrates. They act as the monomers (building blocks) for larger carbohydrate molecules.

• Examples: Glucose, Galactose, and Fructose are common monosaccharides.

  • Glucose is especially important in biology, acting as a primary energy source for cells. It has two isomers:

    1. α-glucose (alpha-glucose)

    2. β-glucose (beta-glucose)

The small structural difference between α-glucose and β-glucose has major implications for the type of polysaccharide that is formed.

Assessed diagram:

The structure of α-glucose and β-glucose.

Disaccharides

When two monosaccharides join together in a condensation reaction, a glycosidic bond is formed, and a water molecule is released.

• Maltose = Glucose + Glucose

• Sucrose = Glucose + Fructose

• Lactose = Glucose + Galactose

These are all examples of disaccharides, with each pair of monosaccharides joined by a glycosidic bond.

The formation of a glycosidic bond by condensation reaction. In this example two α-glucose molecules react to form maltose.

Polysaccharides

Polysaccharides are formed by the condensation of multiple glucose units. The arrangement of glucose molecules and the type of glucose isomer (α or β) determine the structure and function of the polysaccharide.

1. Glycogen (in animals)

   • Formed by the condensation of α-glucose.

   • Highly branched structure allows for rapid release of glucose—essential for meeting high metabolic demands in animals.

2. Starch (in plants)

   • Also formed by α-glucose.

   • Comprised of two main components:

     • Amylose: Mostly unbranched, coiled structure (helical), good for compact energy storage.

     • Amylopectin: Branched, allowing for quicker release of glucose when needed.

3. Cellulose (in plant cell walls)

   • Formed by the condensation of β-glucose.

   • Straight, unbranched chains that run parallel, linked by hydrogen bonds, forming strong microfibrils. This makes cellulose rigid and ideal for providing structural support in plant cell walls.

Structure-Function Relationship

• Glycogen and Starch are both used primarily for energy storage (glycogen in animals, starch in plants). Their branching patterns help release energy quickly when needed.

• Cellulose is:

• Strong and rigid: Many hydrogen bonds form between many cellulose strands to form microfibrils. Collectively this confers great strength and rigidity, thus preventing plant cells from bursting when water enters by osmosis. 

• Fully permeable: Large gaps remain between different microfibrils which ensures substances are able to move close to the cell membrane where selective absorption can take place.

The basic structure of amylose, amylopectin and glycogen. Each is made of repeating α-glucose molecules.

 The structure of a cellulose microfibril.

Biochemical tests

Benedict’s Test for Reducing and Non-Reducing Sugars

1. Reducing Sugars (e.g., Glucose, Maltose)

   • Method:

     • Add Benedict’s solution (blue) to the sample.

     • Heat in a water bath.

   • Result:

     • If a reducing sugar is present, the solution changes color from blue to green, yellow, orange, or brick-red, depending on concentration.

2. Non-Reducing Sugars (e.g., Sucrose)

   • Method:

     • First perform a negative reducing sugar test (solution remains blue).

     • Boil a fresh sample with dilute acid (e.g., HCl) to hydrolyse the glycosidic bond.

     • Neutralise the mixture with an alkali (e.g., NaOH).

     • Now add Benedict’s solution and heat again.

   • Result:

     • If a non-reducing sugar was originally present (e.g., sucrose), the solution will now turn brick-red, indicating the presence of monosaccharides released by hydrolysis.

3. Iodine / Potassium Iodide Test for Starch

• Method: Add a few drops of iodine in potassium iodide solution to the sample.

• Result: If starch is present, the sample changes to a blue-black color.

Definitions