Macromolecules: Structure and Function in Cells

Chapter 3.3: Cells Make Large Molecules from a Limited Set of Small Molecules

Main Classes of Macromolecules

Cells construct large, complex molecules (macromolecules) from a small set of simple molecules. These macromolecules are essential for life and are classified into four main groups:

• Carbohydrates: macromolecules made of sugar monomers

• Proteins: macromolecules made of amino acid monomers

• Lipids: not true polymers, but large molecules with hydrophobic properties

• Nucleic acids: macromolecules made of nucleotide monomers

Formation and Structure of Macromolecules

• Macromolecule: a giant molecule formed by the joining of smaller molecules, usually by a dehydration reaction.

• Polymer: a large molecule consisting of many identical or similar monomers linked together by covalent bonds.

• Monomer: the subunit that serves as a building block of a polymer.

• Dehydration reaction: a chemical reaction in which two molecules become covalently bonded to each other with the removal of a water molecule.

• Hydrolysis: a chemical reaction that breaks bonds between two molecules by the addition of water; an essential part of digestion.

• Enzyme: a macromolecule, usually a protein, that serves as a biological catalyst, changing the rate of a chemical reaction without being consumed by the reaction.

Diversity of polymers arises from:

• Different arrangements or variations in the sequence of monomers

• Variety and uniqueness of each organism

• Monomers are essentially universal

Chapter 3.4: Monosaccharides are the Simplest Carbohydrates

Carbohydrates

Carbohydrates are one of the main classes of biological macromolecules, consisting of:

• Monosaccharides: single-monomer sugars (e.g., glucose)

• Disaccharides: two-monomer sugars

• Polysaccharides: polymers of monosaccharides

Monosaccharide: a simple sugar with a molecular formula that is generally some multiple of CH2O.

• Glucose: a six-carbon monosaccharide that serves as a building block for many polysaccharides and whose oxidation in cellular respiration is a major source of ATP for cells ($C_6H_{12}O_6$).

Chapter 3.7: Polysaccharides are Long Chains of Sugar Units

Polysaccharides

Polysaccharides are carbohydrate polymers of many monosaccharides (sugars) linked by dehydration reactions. They serve as storage molecules or structural compounds.

• Starch: storage polysaccharide in plants; polymer of glucose; stored in roots and tubers; may be unbranched or branched (e.g., potatoes and grains).

• Glycogen: extensively branched glucose storage polysaccharide found in liver and muscle cells in animals; the animal equivalent of starch.

• Cellulose: structural polysaccharide of plant cell walls; composed of glucose monomers; molecules are linked by hydrogen bonds into cable-like fibrils; indigestible by most animals.

Other polysaccharides:

• Chitin: structural polysaccharide found in many fungal cell walls and in the exoskeletons of arthropods.

All carbohydrates are hydrophilic due to the many hydroxyl groups attached to their sugar monomers.

Chapter 3.8: Fats are Lipids that are Mostly Energy-Storage Molecules

Lipids

Lipids are organic compounds consisting mainly of carbon and hydrogen atoms linked by nonpolar covalent bonds, making them mostly hydrophobic. Lipids include fats, phospholipids, and steroids, and are insoluble in water.

• Hydrophobic: "water-fearing"; substances that do not mix with water.

• Unsaturated fatty acid: a fatty acid that has one or more double bonds between carbons in the hydrocarbon tail and thus lacks the maximum number of hydrogen atoms; liquid at room temperature.

Chapter 3.12: Proteins Have a Wide Range of Functions and Structures

Proteins

Proteins are functional biological molecules consisting of one or more polypeptides folded into a specific three-dimensional structure. They are structurally and functionally the most elaborate and varied of all life's molecules.

• Enzymes: speed and regulate virtually all chemical reactions in cells.

• Transport proteins: move sugar molecules and other nutrients into cells.

• Defensive proteins: antibodies of the immune system.

• Signal proteins: hormones and other chemical messengers that help coordinate the body's activities.

• Receptor proteins: receive and transmit signals into cells.

• Contractile proteins: supply amino acids to developing embryos.

• Structural proteins: tendons and ligaments.

• Storage proteins: store amino acids.

"Globular" and "fibrous" refer to a protein's general shape; each protein has a much more specific shape.

The dependence of protein function on a protein's shape becomes clear when a protein is altered.

• Denaturation: a process in which a protein unravels, losing its specific structure and hence function; can be caused by changes in pH, salt concentration, or by high temperature. Also refers to the separation of the two strands of the DNA double helix, caused by similar factors.

• If a protein doesn't fold properly, an accumulation of misfolded proteins occurs, which results in diseases such as Alzheimer's and Parkinson's.

• Prions: infectious misfolded proteins that are associated with serious degenerative brain diseases.

Chapter 3.13: Proteins are Made from Amino Acids Linked by Peptide Bonds

Amino Acids and Peptide Bonds

Amino acids are organic molecules containing a carboxyl group and an amino group; they serve as the monomer of proteins.

• Cells join amino acids together in a dehydration reaction that links the carboxyl group of one amino acid to the amino group of the next amino acid; a water molecule is removed, resulting in a peptide bond (the covalent bond between two amino acid units in a polypeptide).

• Polypeptide: a polymer (chain) of amino acids linked by peptide bonds, formed by a dehydration reaction.

Summary Table: Major Classes of Biological Macromolecules

Class	Monomer	Polymer	Function	Example
Carbohydrates	Monosaccharide	Polysaccharide	Energy storage, structure	Starch, cellulose, glycogen
Proteins	Amino acid	Polypeptide	Catalysis, structure, transport, defense	Enzymes, antibodies
Lipids	Glycerol, fatty acids	Not true polymers	Energy storage, membranes, signaling	Fats, phospholipids, steroids
Nucleic acids	Nucleotide	Polynucleotide	Information storage, transmission	DNA, RNA

Additional info: The above notes expand on the original content by providing definitions, examples, and a summary table for clarity and completeness. All key terms and processes are explained in the context of General Biology.