The Molecules of Cells: Structure and Function of Biological Macromolecules

The Molecules of Cells: The Basics

Introduction to Biological Molecules

All living organisms are composed of molecules that are essential for structure and function. The diversity and complexity of these molecules arise from the unique properties of carbon, which forms the backbone of most biological macromolecules.

• Organic molecules are primarily made of carbon atoms bonded to hydrogen, oxygen, nitrogen, and other elements.

• Hydrocarbons are organic molecules consisting only of carbon and hydrogen.

• Carbon's ability to form four covalent bonds allows for the construction of large, complex, and diverse molecules.

Chemical Structure of Large Biomolecules

Functional Groups

Functional groups are specific groups of atoms attached to carbon skeletons that participate in chemical reactions and determine the properties of organic molecules.

• Common functional groups include hydroxyl (-OH), carbonyl (C=O), carboxyl (-COOH), amino (-NH2), phosphate (-PO4), and methyl (-CH3).

• Most functional groups are polar and hydrophilic, increasing solubility in water.

• The arrangement and type of functional groups impact molecular shape and function.

The Four Classes of Biological Molecules

There are four major classes of biological macromolecules, each with distinct structures and functions:

• Carbohydrates

• Lipids

• Proteins

• Nucleic Acids

Most biological macromolecules are polymers, long chains of monomers linked by covalent bonds.

Polymer Formation and Breakdown

• Dehydration reactions (condensation reactions) join monomers by removing a water molecule, forming a new bond.

• Hydrolysis breaks polymers into monomers by adding a water molecule, breaking the bond between monomers.

Carbohydrates

Monosaccharides

Carbohydrates include sugars and their polymers. The simplest carbohydrates are monosaccharides, which serve as fuel and building blocks for other molecules.

• General formula: multiples of (CH2O), typically 3-7 carbon atoms.

• Examples: Glucose (C6H12O6), Fructose.

• Monosaccharides can form ring structures in aqueous solutions.

Disaccharides

• Formed by joining two monosaccharides via a dehydration reaction.

• Examples: Maltose (glucose + glucose), Sucrose (glucose + fructose), Lactose (glucose + galactose).

Polysaccharides

Polysaccharides are polymers of hundreds to thousands of monosaccharides joined by glycosidic linkages.

• Starch: Storage polysaccharide in plants, composed of glucose monomers; can be branched or unbranched.

• Glycogen: Storage polysaccharide in animals, highly branched and more compact than starch.

• Cellulose: Structural polysaccharide in plant cell walls; glucose monomers linked differently, forming straight, unbranched chains that hydrogen bond to form fibers.

Comparison of Starch, Glycogen, and Cellulose:

Polysaccharide	Function	Structure	Organism
Starch	Energy storage	Branched/unbranched glucose polymer	Plants
Glycogen	Energy storage	Highly branched glucose polymer	Animals
Cellulose	Structural support	Straight, unbranched glucose polymer	Plants

Lipids

Lipids are hydrophobic molecules that include fats, phospholipids, and steroids. They are not true polymers.

Fats (Triglycerides)

• Composed of glycerol and three fatty acids joined by ester linkages.

• Main function: energy storage.

• Saturated fatty acids: No double bonds, straight chains, solid at room temperature (e.g., butter).

• Unsaturated fatty acids: One or more double bonds, kinks in chains, liquid at room temperature (e.g., vegetable oil).

• Cis and trans fatty acids differ in the arrangement of hydrogen atoms around the double bond.

Phospholipids

• Similar to fats, but one fatty acid is replaced by a phosphate group.

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

• Form the basis of cellular membranes.

Steroids

• Lipids with a carbon skeleton of four fused rings.

• Cholesterol is a common steroid and precursor to other steroids, such as hormones and bile acids.

Proteins

Proteins are polymers of amino acids and are essential for structure and function in living organisms.

Amino Acids

• 20 different amino acids, each with a central carbon, amino group, carboxyl group, hydrogen atom, and variable R group.

• R group determines the properties (hydrophilic, hydrophobic, acidic, basic).

Polypeptides and Protein Structure

• Amino acids are linked by peptide bonds (formed by dehydration reactions).

• Proteins have four levels of structure:

  • Primary structure: Sequence of amino acids.

  • Secondary structure: Local folding into alpha helices and beta sheets, stabilized by hydrogen bonds.

  • Tertiary structure: 3D shape stabilized by interactions between R groups (hydrogen bonds, ionic bonds, disulfide bridges).

  • Quaternary structure: Association of multiple polypeptide subunits (not present in all proteins).

• Denaturation: Loss of protein's native structure due to changes in temperature, pH, or chemicals; results in loss of function.

Functions of Proteins

• Structural support (e.g., keratin in hair)

• Movement (e.g., actin, myosin in muscles)

• Storage (e.g., ovalbumin in eggs)

• Defense (e.g., antibodies)

• Transport (e.g., hemoglobin)

• Signaling (e.g., hormones, neurotransmitters)

• Catalysis (enzymes)

Nucleic Acids

Nucleic acids store and transmit genetic information. There are two types: DNA and RNA.

Nucleotides

• Monomers of nucleic acids, each consisting of a phosphate group, a pentose sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base.

• DNA bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C).

• RNA bases: Adenine (A), Uracil (U), Guanine (G), Cytosine (C).

Structure of DNA and RNA

• Nucleotides are joined by phosphodiester bonds, forming a sugar-phosphate backbone.

• DNA: Double-stranded helix, strands held together by hydrogen bonds (A with T, G with C).

• RNA: Usually single-stranded.

The Central Dogma of Molecular Biology

• Genetic information flows from DNA to RNA to protein.

• Transcription: DNA is transcribed into messenger RNA (mRNA).

• Translation: mRNA is translated by ribosomes to synthesize proteins.

Summary Table: Major Classes of Biological Molecules

Class	Monomer	Polymer	Main Functions
Carbohydrates	Monosaccharide	Polysaccharide	Energy storage, structure
Lipids	Fatty acids, glycerol	Not true polymers	Energy storage, membranes, signaling
Proteins	Amino acid	Polypeptide	Structure, catalysis, transport, signaling
Nucleic Acids	Nucleotide	Polynucleotide	Genetic information storage and transfer

Key Equations

• Dehydration reaction (general):

$\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Polymer} + \text{H}_2\text{O}$

• Hydrolysis (general):

$\text{Polymer} + \text{H}_2\text{O} \rightarrow \text{Monomer}_1 + \text{Monomer}_2$

Example: Complementary DNA Strand

• Given the DNA sequence: ATAGCT

• The complementary strand is: TATCGA

Additional info: This guide expands on the provided notes with definitions, examples, and tables for clarity and completeness, suitable for college-level General Biology students.