Biological Molecules

Overview of Biological Molecules

Large biological molecules, also known as macromolecules, are essential for life and include carbohydrates, proteins, nucleic acids, and lipids. These molecules are responsible for a wide range of functions, including energy storage, structural support, catalysis, and information storage.

• Carbohydrates: Serve as fuel and building material.

• Proteins: Perform a vast array of functions, including catalysis, defense, and transport.

• Nucleic acids: Store and transmit genetic information.

• Lipids: Provide energy storage, insulation, and make up cell membranes.

Macromolecules: Polymers and Monomers

Structure of Polymers

Macromolecules are polymers, which are long molecules made up of repeating units called monomers. Carbohydrates, proteins, and nucleic acids are all polymers, while lipids are not true polymers.

• Polymer: A long molecule consisting of many similar or identical building blocks linked by covalent bonds.

• Monomer: The repeating unit that serves as the building block of a polymer.

The Synthesis and Breakdown of Polymers

Dehydration and Hydrolysis Reactions

Polymers are synthesized and broken down by specific chemical reactions:

• Dehydration reaction: Two monomers are covalently bonded together with the loss of a water molecule, forming a longer polymer.

• Hydrolysis: Polymers are disassembled into monomers by the addition of a water molecule, breaking the bond.

• Enzymes: Specialized macromolecules that speed up these chemical reactions.

Carbohydrates

Monosaccharides: Simple Sugars

Carbohydrates include sugars and polymers of sugars. The simplest carbohydrates are monosaccharides, which generally have molecular formulas that are multiples of CH2O. Glucose is the most common monosaccharide.

• Monosaccharides are classified by:

  • The location of the carbonyl group (aldose or ketose)

  • The number of carbons in the carbon skeleton (triose, pentose, hexose, etc.)

Ring Structures of Sugars

Although often drawn as linear skeletons, many sugars form rings in aqueous solutions. Monosaccharides serve as major fuel for cells and as raw material for building other molecules.

Disaccharides

A disaccharide is formed when a dehydration reaction joins two monosaccharides. The covalent bond formed is called a glycosidic linkage. Common disaccharides include maltose, lactose, and sucrose.

• Maltose: Found in germinating grains and processed foods.

• Lactose: Found in milk and dairy products.

• Sucrose: Found in sugar cane, sugar beets, fruits, and vegetables.

Polysaccharides

Polysaccharides are polymers of sugars and serve storage and structural roles. Their function is determined by their sugar monomers and the positions of their glycosidic linkages.

• Starch: Storage polysaccharide in plants, composed of glucose monomers. Stored as granules in chloroplasts and plastids.

• Amylose: The simplest form of starch (unbranched).

• Glycogen: Storage polysaccharide in animals, stored mainly in liver and muscle cells.

Structural Polysaccharides

Some polysaccharides serve structural roles:

• Cellulose: Major component of plant cell walls; polymer of glucose with β glycosidic linkages.

• Enzymes that digest starch (α linkages) cannot digest cellulose (β linkages).

• Cellulose in human food acts as insoluble fiber.

• Some microbes can digest cellulose, aiding herbivores in cellulose digestion.

Chitin is another structural polysaccharide, found in the exoskeleton of arthropods and in fungal cell walls.

Lipids

Overview of Lipids

Lipids are a diverse group of hydrophobic molecules that do not form true polymers. They consist mostly of hydrocarbon regions and include fats, phospholipids, and steroids.

Fats

Fats are constructed from glycerol and fatty acids. Glycerol is a three-carbon alcohol, and fatty acids are long hydrocarbon chains with a carboxyl group. Three fatty acids join to glycerol by ester linkages to form a triacylglycerol (triglyceride).

Fatty acids vary in length and in the number and location of double bonds:

• Saturated fatty acids: No double bonds; solid at room temperature; mostly animal fats.

• Unsaturated fatty acids: One or more double bonds; liquid at room temperature; mostly plant and fish fats.

Hydrogenation converts unsaturated fats to saturated fats and can create trans fats, which may be more harmful than saturated fats.

The major function of fats is energy storage. Adipose tissue stores fat, cushions organs, and insulates the body.

Phospholipids

Phospholipids consist of two fatty acids and a phosphate group attached to glycerol. The fatty acid tails are hydrophobic, while the phosphate group forms a hydrophilic head. Phospholipids self-assemble into bilayers, forming the basis of cell membranes.

Steroids

Steroids are lipids with a carbon skeleton consisting of four fused rings. Cholesterol is an important steroid in animal cell membranes and a precursor for other steroids. High cholesterol levels may contribute to cardiovascular disease.

Proteins

Functions of Proteins

Proteins are the most diverse macromolecules, accounting for more than 50% of the dry mass of most cells. They perform a wide range of functions, including catalysis (enzymes), defense, storage, transport, cellular communication, movement, and structural support.

Structure of Proteins

Proteins are polymers of amino acids, linked by peptide bonds to form polypeptides. Each protein has a unique sequence of amino acids, which determines its structure and function. A functional protein consists of one or more polypeptides folded into a specific three-dimensional shape.

• Amino acids: Organic molecules with amino and carboxyl groups, differing in their side chains (R groups).

• Polypeptide: A polymer of amino acids.

• Peptide bond: Covalent bond linking amino acids.

Protein Structure and Function

The sequence of amino acids determines a protein’s three-dimensional structure, which in turn determines its function. The function of a protein usually depends on its ability to recognize and bind to other molecules, often with high specificity due to complementary shapes.

Enzymes are proteins that act as catalysts, speeding up chemical reactions without being consumed. They can perform their functions repeatedly, making them essential for life processes.

Examples of protein functions:

• Enzymatic proteins: Digestive enzymes catalyze hydrolysis of bonds in food molecules.

• Defensive proteins: Antibodies inactivate and help destroy viruses and bacteria.

• Storage proteins: Casein in milk stores amino acids for baby mammals.

• Transport proteins: Hemoglobin transports oxygen in the blood.

Additional info: Protein structure is organized into four levels: primary (amino acid sequence), secondary (α-helices and β-sheets), tertiary (overall 3D shape), and quaternary (association of multiple polypeptides).