Step 1: Understanding the Concept:
Carbohydrates are the most abundant organic molecules in nature and serve as primary energy sources and structural components in plants.
They are classified based on the degree of polymerization (the number of sugar units):
- Monosaccharides: Single sugar unit (e.g., Glucose).
- Disaccharides: Two sugar units (e.g., Sucrose).
- Oligosaccharides: 3 to 10 sugar units.
- Polysaccharides: Long chains of hundreds or thousands of sugar units.
Step 2: Detailed Explanation:
Starch is the most important storage carbohydrate in plants, concentrated in seeds (grains), tubers (potatoes), and roots (cassava).
Biochemically, starch is not a single molecule but a mixture of two polymers of \(\alpha\)-D-glucose:
1. Amylose: A linear, unbranched chain of glucose units linked by \(\alpha\)-1,4-glycosidic bonds. It typically makes up \(20-30%\) of starch.
2. Amylopectin: A highly branched polymer where glucose units are linked by \(\alpha\)-1,4 bonds in the chains and \(\alpha\)-1,6 bonds at the branch points. It makes up \(70-80%\) of starch.
Because starch consists of massive chains of glucose monomers, it is the quintessential example of a Polysaccharide.
In the human diet and animal nutrition, starch is broken down by amylase enzymes into glucose to provide energy. In plants, it is stored in specialized organelles called amyloplasts.
Step 3: Final Answer:
As starch is a high-molecular-weight polymer consisting of thousands of glucose units, it is classified as a polysaccharide.
Therefore, the correct option is (D).