The Building Blocks of Carbohydrates
Starch is a classic example of a polysaccharide, a complex carbohydrate made up of many smaller, repeating units called monosaccharides, or simple sugars. The single monomer that constructs starch is glucose. Glucose is a hexose, meaning it contains six carbon atoms, and serves as the primary energy source for most living organisms. In plants, excess glucose from photosynthesis is converted into starch for long-term storage.
The process of joining these individual glucose monomers is called polymerization, which links them via glycosidic bonds. When we consume starchy foods, our digestive system reverses this process through hydrolysis, breaking these glycosidic bonds with the help of enzymes to release the individual glucose molecules for our body to use as fuel.
The Two Components of Starch
All starch is not created equal; it is actually a mixture of two different glucose polymers: amylose and amylopectin. The ratio and structure of these two components significantly influence the properties of the starch, such as its solubility and how it is digested.
Amylose: The Linear Chain
Amylose is a linear, unbranched chain of D-glucose units linked by α-1,4-glycosidic bonds. Due to these specific linkages, the chain naturally coils into a helical structure, much like a spiral staircase. This coiled structure is more compact and makes amylose less accessible to digestive enzymes compared to amylopectin, often classifying it as a resistant starch. Foods with higher amylose content, such as basmati rice, tend to be less sticky when cooked.
Amylopectin: The Branched Structure
Amylopectin, on the other hand, is a highly branched molecule. While its main chains are also connected by α-1,4-glycosidic bonds, its branching points are formed by α-1,6-glycosidic bonds. These branches occur approximately every 24 to 30 glucose units, giving it a tree-like shape. The branched structure provides a high surface area for digestive enzymes to act upon, making amylopectin more rapidly digestible than amylose. This is why starchy foods with more amylopectin, like glutinous rice, become sticky and gelatinous when cooked.
The Digestion of Starch
Digestion of starch begins in the mouth, where salivary amylase starts breaking down the glucose chains into smaller pieces. The process continues in the small intestine with pancreatic amylase and other brush border enzymes like maltase, which ultimately break the starch down into its final glucose units, ready for absorption into the bloodstream.
Starch vs. Cellulose: A Structural Tale
It is fascinating to note that cellulose, the primary component of plant cell walls, is also a polysaccharide made from glucose monomers. However, a key structural difference dictates its function and our ability to digest it. While starch consists of α-glucose, cellulose is made of β-glucose, which results in β-1,4-glycosidic bonds. Humans lack the necessary enzymes to break these bonds, which is why cellulose is indigestible fiber in our diet, unlike the easily metabolized starch. This subtle difference in bonding orientation fundamentally changes the molecule's properties and biological role.
| Feature | Amylose | Amylopectin |
|---|---|---|
| Structure | Linear, unbranched chain | Highly branched chain |
| Key Linkages | Primarily α-1,4-glycosidic bonds | α-1,4-glycosidic bonds and α-1,6-glycosidic bonds at branch points |
| Digestibility | Slower to digest; some can be resistant | Rapidly digested due to more enzyme access points |
| Solubility | Less soluble in water | Soluble in water, especially hot water |
| Cooking Properties | Contributes to a fluffier, separated texture in foods | Contributes to a gelatinous and sticky texture |
Conclusion
In summary, the smaller units called in starch are glucose monomers, which link together to form the larger polysaccharide molecule. Starch is not a single entity but a blend of two distinct glucose polymers: the linear amylose and the branched amylopectin. Their structural differences, created by specific glycosidic bonds, determine how they are digested and their physical properties in food. This understanding of starch's composition is key to appreciating its role as a vital energy source in both plants and animals.
For more information on the structural chemistry of carbohydrates, you can visit the Biology LibreTexts page on Carbohydrates.
Key takeaways on what are smaller units called in starch:
- Monosaccharide Monomer: The fundamental building block of starch is a simple sugar called glucose.
- Two Polymer Types: Starch is comprised of two distinct glucose polymers: amylose and amylopectin.
- Amylose Structure: Amylose is a linear chain of glucose linked by α-1,4 bonds, which coils into a helical shape.
- Amylopectin Structure: Amylopectin features a highly branched structure due to both α-1,4 and α-1,6 glycosidic bonds.
- Digestibility Difference: Amylopectin is digested faster due to its many branches, while amylose can be a resistant starch.
- Cellulose vs. Starch: Though both are glucose polymers, the differing α- and β-glycosidic bonds make starch digestible by humans, while cellulose is not.
