Molecules and Macromolecules: Structure and Function in Biology

Molecules in Biology

Why Study Molecules?

Understanding molecules is fundamental to biology because all living organisms are composed of chemical substances. The structure and function of biological molecules determine cellular processes, energy flow, and heredity.

• Molecules are the building blocks of cells and tissues.

• Biological functions depend on molecular interactions and properties.

• Studying molecules helps explain phenomena such as metabolism, genetics, and disease.

Water

Polar Covalent Bonds

Water molecules are held together by polar covalent bonds, which create partial charges on the atoms.

• Polar covalent bond: A bond where electrons are shared unequally, resulting in partial positive and negative charges.

• In water (H2O), oxygen is more electronegative than hydrogen, pulling electrons closer.

Hydrogen Bonds

Hydrogen bonds are weak attractions between the partial positive charge of hydrogen and the partial negative charge of oxygen in adjacent water molecules.

• Responsible for water's high cohesion, surface tension, and solvent properties.

• Essential for biological processes like DNA base pairing.

Macromolecules

Hydrocarbons and Functional Groups

Macromolecules are large molecules made from smaller units. Hydrocarbons are chains of carbon and hydrogen, and functional groups are specific clusters of atoms that confer unique properties.

• Hydrocarbon: Molecule consisting only of carbon and hydrogen.

• Functional group: Group of atoms that determine the chemical reactivity of a molecule (e.g., hydroxyl, carboxyl).

Definitions

• Macromolecule: Large molecule composed of repeating subunits.

• Monomer: Single unit that can join to form polymers.

• Dimer: Two monomers joined together.

• Polymer: Chain of monomers linked by covalent bonds.

Making and Breaking Polymers

• Dehydration synthesis: Formation of polymers by removing water.

• Hydrolysis: Breaking polymers into monomers by adding water.

• Enzymes catalyze both processes.

Carbohydrates

Monosaccharides

Monosaccharides are simple sugars, such as glucose, which serve as energy sources and building blocks.

• General formula: $\mathrm{C}_n\mathrm{H}_{2n}\mathrm{O}_n$

• Examples: glucose, fructose, galactose

Disaccharides

• Structure: Two monosaccharides joined by a glycosidic bond.

• Function: Transport and storage of energy (e.g., sucrose, lactose).

Oligosaccharides

Oligosaccharides are short chains of monosaccharides, often involved in cell recognition and signaling.

Polysaccharides

• Structure: Long chains of monosaccharides.

• Examples: starch, glycogen, cellulose, chitin

• Functions:

  • Starch: Energy storage in plants

  • Glycogen: Energy storage in animals

  • Cellulose: Structural component in plant cell walls

  • Chitin: Structural component in fungi and arthropods

Proteins

Amino Acids and R-Groups

Proteins are polymers of amino acids, each with a unique side chain (R-group) that determines its properties.

• Amino acid: Contains an amino group, carboxyl group, hydrogen, and R-group.

• R-groups can be polar, nonpolar, acidic, or basic.

Dipeptides and Polypeptides

• Dipeptide: Two amino acids joined by a peptide bond.

• Polypeptide: Long chain of amino acids.

Levels of Protein Structure

• Primary (1o) structure: Sequence of amino acids.

• Secondary (2o) structure: Local folding (alpha helix, beta sheet) stabilized by hydrogen bonds.

• Tertiary (3o) structure: Overall 3D shape due to interactions among R-groups.

• Quaternary (4o) structure: Association of multiple polypeptide chains.

Sickle Cell Anemia: Protein Folding Gone Wrong

• Mutation in hemoglobin gene causes abnormal folding.

• Leads to sickle-shaped red blood cells and health complications.

Enzymes

• How enzymes work: Enzymes are biological catalysts that speed up reactions by lowering activation energy.

• Physical conditions: Enzyme activity is affected by temperature and pH; extreme conditions can cause denaturation.

• Enzyme inhibitors: Molecules that reduce enzyme activity (competitive and noncompetitive inhibitors).

Lipids

What is a Lipid?

Lipids are hydrophobic molecules, including fats, oils, and steroids, that play roles in energy storage, membrane structure, and signaling.

Basic Functions of Lipids

• Energy storage

• Structural component of membranes

• Signaling molecules (e.g., hormones)

Classes of Lipids

Sterols

• Structure: Four fused carbon rings.

• Function: Component of cell membranes; precursor to steroid hormones.

• Lipoproteins: HDL (high-density) and LDL (low-density) transport cholesterol in blood.

Triglycerides (Triacylglycerols; Fats)

• Structure: Glycerol backbone + 3 fatty acids.

• Saturated vs. Unsaturated Fatty Acids:

  • Saturated: No double bonds; solid at room temperature.

  • Unsaturated: One or more double bonds; liquid at room temperature.

• Cis vs. Trans Fatty Acids: Cis have hydrogen atoms on the same side of the double bond; trans have them on opposite sides (trans fats are associated with health risks).

Phospholipids

• Structure: Glycerol + 2 fatty acids + phosphate-containing head group.

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

• Role in Membrane Structure: Form bilayers in water, creating cell membranes.

• Micelle: Spherical structure formed by phospholipids in water.

Relevant Vocabulary

• Covalent bond: Strong bond formed by sharing electrons.

• Polar covalent bond: Unequal sharing of electrons.

• Hydrogen bond: Weak attraction between polar molecules.

• Hydrocarbon: Molecule of carbon and hydrogen.

• Functional group: Reactive group of atoms.

• Polar: Uneven charge distribution.

• Nonpolar: Even charge distribution.

• Macromolecule: Large biological molecule.

• Monomer: Single unit of a polymer.

• Dimer: Two monomers joined.

• Polymer: Chain of monomers.

• Hydrolysis: Breaking polymers with water.

• Enzyme: Biological catalyst.

• Carbohydrate: Sugar molecule.

• Monosaccharide: Simple sugar.

• Disaccharide: Two sugars joined.

• Oligosaccharide: Short sugar chain.

• Polysaccharide: Long sugar chain.

• Cellulose: Plant structural polysaccharide.

• Glycogen: Animal storage polysaccharide.

• Starch: Plant storage polysaccharide.

• Chitin: Fungal/arthropod structural polysaccharide.

• Glucose: Common monosaccharide.

• Amino acid: Protein monomer.

• Protein: Polymer of amino acids.

• Polypeptide: Amino acid chain.

• Peptide bond: Link between amino acids.

• Hydrophobic: Repels water.

• Hydrophilic: Attracts water.

• Reactant: Starting material in a reaction.

• Product: Result of a reaction.

• Active site: Enzyme region where substrate binds.

• Denaturation: Loss of protein structure.

• Conformation: Protein shape.

• Lipid: Hydrophobic molecule.

• Triglyceride: Fat molecule.

• Phospholipid: Membrane lipid.

• Amphipathic: Both hydrophobic and hydrophilic.

• Micelle: Lipid sphere in water.

• Steroid: Lipid with four rings.

• Lipoprotein: Lipid-protein complex.

• Saturated (fat): No double bonds.

• Unsaturated (fat): Double bonds present.

Comparison Table: Types of Biological Macromolecules

Macromolecule	Monomer	Bond Type	Function	Example
Carbohydrate	Monosaccharide	Glycosidic bond	Energy storage, structure	Starch, cellulose
Protein	Amino acid	Peptide bond	Enzymes, structure, signaling	Hemoglobin, enzymes
Lipid	Fatty acid (not true polymer)	Ester bond	Energy storage, membranes	Triglyceride, phospholipid

Example: Enzyme Catalysis

• Enzyme binds substrate at the active site.

• Reaction occurs, converting substrate to product.

• Enzyme is unchanged and can catalyze more reactions.

• Equation for enzyme-catalyzed reaction: $\mathrm{E} + \mathrm{S} \rightarrow \mathrm{ES} \rightarrow \mathrm{E} + \mathrm{P}$

Additional info: Lipids are not true polymers because they are not formed from repeating monomer units in the same way as proteins and carbohydrates.