1. Carbohydrates: The Energy Providers

Carbohydrates are polyhydroxy aldehydes or ketones, or compounds that produce these upon hydrolysis. Their name, ‘carbo-hydrates,’ comes from their basic chemical structure, which can be represented as $(CH_2O)_n$.

Monosaccharides (Simple Sugars)

• Structure: Monosaccharides are the basic units of all carbohydrates. They are classified by the number of carbon atoms and by their functional group. Those with an aldehyde group are called aldoses (e.g., glucose, an aldohexose), and those with a ketone group are called ketoses (e.g., fructose, a ketohexose).
• Ring Structures: In solution, most monosaccharides exist in a cyclic form. Glucose forms a six-membered ring called a pyranose ring, while fructose typically forms a five-membered furanose ring.

Disaccharides

• Formation: A disaccharide is formed when two monosaccharides join through a glycosidic bond via a dehydration reaction.
• Key Examples:
  • Sucrose (table sugar) is a non-reducing sugar with an $\alpha$-1,2-glycosidic linkage.
  • Lactose (milk sugar) is formed by $\beta$-D-galactose and $\beta$-D-glucose with a $\beta$-1,4-glycosidic linkage.
  • Maltose (malt sugar) is formed by two $\alpha$-D-glucose units with an $\alpha$-1,4-glycosidic linkage.

Polysaccharides (Complex Carbohydrates)

• Starch: The energy storage polysaccharide in plants. It has two components: **amylose** (unbranched) and **amylopectin** (branched).
• Glycogen: The energy storage form in animals, stored mainly in the liver and muscles. It is structurally similar to amylopectin but more highly branched.
• Cellulose: A major structural component of plant cell walls, a linear polymer of $\beta$-D-glucose units, indigestible by humans.

2. Proteins: The Functional Workhorses

Proteins are large, complex molecules essential for virtually every process in a living organism. They are polymers of amino acids.

Amino Acids: The Monomers

• General Structure: An amino acid contains a central carbon atom bonded to an amino group ($−NH_2$), a carboxyl group ($−COOH$), a hydrogen atom, and a variable **R-group**.

• Peptide Bond: Amino acids link together through a covalent **peptide bond** formed by a dehydration reaction.

Levels of Protein Structure

• Primary Structure: The specific, linear sequence of amino acids.
• Secondary Structure: Localized folding into **$\alpha$-helix** and **$\beta$-pleated sheet**, stabilized by hydrogen bonds.
• Tertiary Structure: The overall three-dimensional shape of a single polypeptide chain, stabilized by interactions between R-groups (disulfide bridges, ionic bonds, etc.).
• Quaternary Structure: The arrangement of two or more polypeptide subunits. For example, **hemoglobin**.

3. Lipids: The Storage and Structural Components

Lipids are a diverse group of non-polar, hydrophobic molecules that are insoluble in water.

Fatty Acids

• Saturated: Contain only single bonds between carbon atoms. They are solid at room temperature.
• Unsaturated: Contain one or more double bonds, which cause a ‘kink’ in the chain. They are liquid at room temperature.

Phospholipids

• Structure: An **amphipathic** molecule with a hydrophilic (water-loving) head and two hydrophobic (water-fearing) tails.
• Function: The fundamental building blocks of **cell membranes**, where they form a **phospholipid bilayer**.