Biological Macromolecules: Carbohydrates and Lipids

Biological Macromolecules

Overview of Macromolecules

Living things require four major classes of macromolecules, each with specific monomers (building blocks) and biological roles.

• Carbohydrates: Monomers are monosaccharides.

• Proteins: Monomers are amino acids.

• Nucleic Acids: Monomers are nucleotides.

• Lipids: Not true polymers; composed of different components depending on type.

Carbohydrates

Carbohydrates serve several essential functions in living organisms:

• Energy: Immediate and stored energy source.

• Structural Elements: Provide structural support in cell walls and exoskeletons.

Chemical Composition and Structure

• Composed mainly of carbon (C), hydrogen (H), and oxygen (O) in a 1:2:1 ratio.

• General formula: $C_nH_{2n}O_n$

• Monomers are called monosaccharides (simple sugars), typically containing 3–7 carbon atoms.

• Monosaccharides are carbon-based molecules with multiple hydroxyl (-OH) groups.

Classification by Carbon Number

• 2 carbons: Ethose (rare)

• 3 carbons: Triose

• 4 carbons: Tetrose

• 5 carbons: Pentose

• 6 carbons: Hexose

• 7 carbons: Heptose

Examples of simple sugars: glucose, fructose

Formation of Complex Carbohydrates

• Monosaccharides can be linked by glycosidic linkages (covalent bonds) via dehydration reactions (removal of water).

• Two monosaccharides: disaccharide

• Many monosaccharides: polysaccharide

Examples of Polysaccharides and Their Functions

Polysaccharide	Source	Function
Starch	Plants	Energy storage
Glycogen	Animals	Energy storage
Cellulose	Plant cell walls	Structural support
Chitin	Exoskeletons of arthropods, cell walls of fungi	Structural support

Digestibility of Polysaccharides

• Humans cannot digest cellulose due to the lack of enzymes to hydrolyze its beta linkages.

• Starch and glycogen can be digested for energy.

Recognition of Carbohydrates in Chemical Structures

• Look for a carbon backbone with multiple hydroxyl (-OH) groups and a 1:2:1 C:H:O ratio.

Lipids

Types of Lipids

• Fats (Triglycerides)

• Phospholipids

• Steroids

• Waxes

Chemical Structure and Elements

• Lipids are mainly composed of carbon, hydrogen, and oxygen (sometimes phosphorus in phospholipids).

• Fats are formed when a glycerol molecule (3-carbon alcohol with hydroxyl groups) bonds to three fatty acids via ester linkages.

Fatty Acids

• Saturated fatty acids: No double bonds between carbon atoms; found in animal fats (e.g., butter, lard); solid at room temperature.

• Unsaturated fatty acids: One or more double bonds; found in plant oils (e.g., olive oil); liquid at room temperature.

Triglycerides

• Composed of a glycerol backbone attached to three fatty acids.

• Function: Energy storage, insulation, and protection.

Bond Formation in Fats

• Dehydration reaction removes water to create ester bonds between glycerol and fatty acids.

Phospholipids

• Major component of cell membranes.

• Structure: Glycerol backbone, two fatty acid tails (hydrophobic), and a phosphate group (hydrophilic).

• Amphipathic: Both hydrophilic (phosphate head) and hydrophobic (fatty acid tails) regions.

Phospholipid Structure (Labeled)

• 1. Glycerol backbone

• 2. Fatty acid tails – hydrophobic

• 3. Phosphate group – hydrophilic

• 4. Additional molecule (e.g., choline) – hydrophilic

Biological Role of Phospholipids

• Form the phospholipid bilayer of cell membranes, creating a selective barrier between the cell interior and exterior.

Steroids

• Structure: Carbon skeleton with four fused rings.

• Examples: Cholesterol, steroid hormones.

• Function: Membrane structure (cholesterol), signaling molecules (hormones).

Summary Table: Lipid Types and Functions

Lipid Type	Main Components	Function
Fats (Triglycerides)	Glycerol + 3 fatty acids	Energy storage, insulation, protection
Phospholipids	Glycerol + 2 fatty acids + phosphate group	Cell membrane structure
Steroids	4 fused carbon rings	Membrane structure, hormones

Additional info: Lipids are hydrophobic due to their long hydrocarbon chains or rings, making them insoluble in water. The amphipathic nature of phospholipids is critical for membrane formation and function.