Carbohydrates are the most abundant organic molecules in nature. They are primarily composed of Carbon (C), Hydrogen (H), and Oxygen (O), usually in the ratio of 1:2:1. In plants, they are produced through photosynthesis and serve as the primary source of energy and structural components.

Definition: Carbohydrates are chemically defined as polyhydroxy aldehydes or ketones, or substances that yield these compounds on hydrolysis.

Biological Significance

• Energy Source: Glucose is the primary "fuel" for cellular respiration.
• Storage: Plants store energy as Starch, while animals use Glycogen.
• Structure: Cellulose forms the structural framework of plant cell walls.
• Precursors: They serve as basic skeletons for the synthesis of fats and amino acids.

Monosaccharides are the simplest form of carbohydrates and cannot be hydrolyzed into smaller units. They are often called "simple sugars."

Structure and Properties

• General Formula: (CH2O)n, where n ranges from 3 to 7.
• Functional Groups: Either an aldehyde group (-CHO) called aldoses or a ketone group (>C=O) called ketoses.
• Optical Activity: Due to asymmetric carbon atoms, they exist in D and L isomeric forms.
• Cyclic Structure: In solution, sugars with 5 or more carbons usually form rings (Pyranose or Furanose).

Examples and Importance

Disaccharides consist of two monosaccharide units joined by a glycosidic bond formed via a dehydration reaction (loss of one water molecule).

Major Disaccharides

1. Sucrose (Table Sugar): Composed of Glucose + Fructose. It is the primary form in which sugar is transported in the phloem of plants.
2. Maltose (Malt Sugar): Composed of Glucose + Glucose. Produced during the breakdown of starch.
3. Lactose (Milk Sugar): Composed of Glucose + Galactose. Found primarily in animal milk.

Formula: C12H22O11 (Note: It is not simply C12H24O12 because one H2O is lost during bond formation).

Polysaccharides are long chains (polymers) of monosaccharides. They are classified based on the types of sugars they contain.

A. Homo-polysaccharides

Composed of only one type of monosaccharide unit.

• Starch: The main storage carbohydrate in plants. It consists of two components: Amylose (unbranched) and Amylopectin (branched).
• Cellulose: A linear polymer of beta-glucose. It provides high tensile strength to plant cell walls. Humans cannot digest it due to the lack of cellulase enzyme.
• Glycogen: "Animal starch" stored in liver and muscles. Highly branched structure for rapid mobilization.

B. Hetero-polysaccharides

Composed of two or more different types of monosaccharides.

• Hemicellulose: Found in plant cell walls along with cellulose.
• Pectin: Acts as "cement" in the middle lamella of plant cells.

Mucopolysaccharides

Also known as Glycosaminoglycans (GAGs). These are long unbranched polysaccharides containing repeating disaccharide units that include an amino sugar. Examples include Hyaluronic acid and Heparin.

Glycoproteins

These consist of oligosaccharide chains (glycans) covalently attached to amino acid side-chains in proteins. They play critical roles in cell-cell recognition and as membrane receptors.

6. Exam Focus: Tips & FAQs

Common Pitfalls

• Mistake: Writing the formula for starch as a simple multiple of glucose. Correction: Remember that water is removed for every bond formed.
• Mistake: Confusing Amylose with Amylopectin. Correction: Amylose is linear (alpha-1,4 bonds); Amylopectin is branched (alpha-1,4 and alpha-1,6 bonds).

Frequently Asked Questions

Q: Why is cellulose insoluble in water?
A: Due to its long linear chains and extensive hydrogen bonding between adjacent chains, which creates a very stable and hydrophobic structure.

Q: What is the significance of the "D" prefix in D-Glucose?
A: It refers to the orientation of the hydroxyl (-OH) group on the penultimate (second to last) carbon atom, indicating it is on the right side in a Fischer projection.