Carbohydrates and Lipids: Monosaccharides to Polysaccharides

Posted on Mar 25, 2025 in Biology

Carbohydrates and Lipids

Monosaccharide monomers are linked together by condensation reactions to form disaccharides and polysaccharide polymers.

Monosaccharides	Glucose	Energy molecule used in aerobic respiration
	Galactose	Nutritive sweetener in foods
	Fructose	Fruit sugar
Disaccharides	Maltose	Malt sugar found in barley, consists of 2 glucose molecules
	Lactose	Sugar found in milk
	Sucrose	Transport sugar found in plants because of its solubility
Polysaccharides	Starch	Storage carbohydrate in plants
	Glycogen	Storage carbohydrate in animals
	Cellulose	Main component in plant cell walls

When two monomers combine together, they form a dimer. When many monomers combine together, they form a polymer.
Condensation Reactions: The building of large macromolecules (polymers) by the removal of water molecules when monomers combine. Each time two monomers combine, one water molecule is removed.

For example: Glucose is a monosaccharide that is used to build up large storage molecules (polysaccharides) in plants and animals. In plants, many glucose molecules combine through condensation reactions to form the polysaccharide starch. In animals, glucose molecules are combined to form the polysaccharide glycogen through condensation reactions.

When a plant or an animal needs to use energy stored in polysaccharide molecules, the opposite reaction to condensation takes place. This breakdown of larger polysaccharides into smaller monosaccharides through the addition of water is called hydrolysis.
Starch and glycogen are broken down by the addition of water into glucose molecules (the energy molecule used in aerobic respiration).
In lipids, the polymer is called a triglyceride.
Hydrolysis of a triglyceride uses water to break apart the lipid into glycerol C₃H₅(OH)₃ and 3 fatty acids.

Structure and function of cellulose and starch in plants and glycogen in humans.

Polysaccharides are long chains of monosaccharides held together with glycosidic linkages made by condensation reactions.
Starch, cellulose, and glycogen are all polysaccharides that are made from long chains of glucose; however, they differ in their structure and the type of glucose, which leads to different functions.

Starch – a long chain of α (alpha) glucose molecules used as a glucose storage by plants.
Starch consists of two types of molecules: amylose, which is linear, and amylopectin, which is branched.
Since the bonds in starch are α-glucose, the –OH groups from the glucose molecules are always pointed down, causing starch to have a curved appearance. This makes starch a good molecule for storing glucose in plants.

Even though glucose is hydrophilic, starch is too large to be soluble in water at room temperature.
Cellulose are unbranched straight chains of β (beta) glucose molecules, held together with glycosidic bonds.
Since the –OH groups point out in opposite directions and every other β glucose is flipped 180 degrees, cellulose forms a nice straight chain.
These straight chains also allow cellulose to form bundles linked by H-bonds.
This is essential for cellulose’s function, which is to provide strength for cell walls in plant cells (high tensile strength).
Notice the up and down alternating glycosidic bonds between the glucose molecules.

by glycosidic bonds

It is highly branched, making the molecule more compact and a perfect molecule for energy storage.
It is stored in the liver and some muscles of humans.