Study Guide: The Molecules of Cells – Organic Compounds, Carbohydrates, Lipids, Proteins, and Nucleic Acids

Chapter 3: The Molecules of Cells

Introduction to Organic Compounds

Organic compounds are the foundation of all living organisms. They are primarily composed of carbon atoms bonded with hydrogen, oxygen, nitrogen, and other elements. Understanding their structure and function is essential for studying biology.

• Organic Compound: A molecule containing carbon atoms bonded to hydrogen and often other elements; typically found in living organisms.

• Example: Ethanol (C2H5OH) is an organic compound because it contains carbon and hydrogen atoms.

• Hydrocarbons: Molecules consisting only of carbon and hydrogen. Example: Methane (CH4).

• Structural Diversity: Carbon skeletons can vary in length, branching, presence of double bonds, and ring structures, leading to molecular diversity.

Properties of Carbon Skeletons

Carbon skeletons form the backbone of organic molecules and can differ in several ways:

• Length: Carbon chains can be short or long.

• Branching: Chains may be unbranched or branched.

• Double Bonds: Carbon atoms may be joined by single or double bonds.

• Rings: Some carbon skeletons form ring structures.

Example Table: Types of Carbon Skeletons

Type	Example	Description
Length	Ethane, Propane	Varies in number of carbons
Branching	Butane, Isobutane	Linear vs. branched chains
Double Bonds	1-Butene	Presence of C=C bonds
Rings	Cyclohexane, Benzene	Carbons arranged in a ring

Functional Groups and Isomers

Functional groups are specific groups of atoms within molecules that determine the chemical properties of those molecules. Isomers are compounds with the same molecular formula but different structures.

• Functional Groups: Hydroxyl (-OH), Carbonyl (C=O), Carboxyl (-COOH), Amino (-NH2), etc.

• Isomers: Molecules with the same formula but different arrangements of atoms.

Carbohydrates

Monosaccharides and Disaccharides

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically with the formula (CH2O)n. They serve as energy sources and structural materials.

• Monosaccharides: Simple sugars (e.g., glucose, fructose) that cannot be hydrolyzed into smaller carbohydrates.

• Disaccharides: Formed by joining two monosaccharides via dehydration synthesis (e.g., sucrose, lactose).

• Dehydration Synthesis: A reaction in which two molecules are joined by removing a molecule of water.

• Hydrolysis: A reaction in which a molecule is split into two by the addition of water.

Example Equation:

Formation of maltose from two glucose molecules:

$\text{C}_6\text{H}_{12}\text{O}_6 + \text{C}_6\text{H}_{12}\text{O}_6 \rightarrow \text{C}_{12}\text{H}_{22}\text{O}_{11} + \text{H}_2\text{O}$

Polysaccharides

Polysaccharides are large carbohydrates formed by linking many monosaccharides. They serve as energy storage or structural components.

• Starch: Storage form of glucose in plants.

• Glycogen: Storage form of glucose in animals.

• Cellulose: Structural component of plant cell walls.

Comparison Table: Monosaccharides and Polysaccharides

	Starch	Glucose	Cellulose	Glycogen
Monomer or Polymer	Polymer	Monomer	Polymer	Polymer
Function	Energy storage in plants	Immediate energy source	Structural support in plants	Energy storage in animals

Lipids

Characteristics and Types

Lipids are hydrophobic molecules, including fats, oils, phospholipids, and steroids. They are not polymers and are insoluble in water.

• Fats: Composed of glycerol and fatty acids; used for energy storage.

• Saturated Fatty Acids: No double bonds between carbon atoms; solid at room temperature.

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

• Steroids: Lipids with a carbon skeleton consisting of four fused rings (e.g., cholesterol).

Example: Olive oil contains unsaturated fats, while butter contains saturated fats.

Health Implications

• Trans Fats: Artificially hydrogenated fats associated with increased risk of heart disease.

• Omega-3 Fatty Acids: Essential fats beneficial for heart health.

Proteins

Structure and Function

Proteins are polymers made of amino acid monomers. Their structure determines their function, and even small changes can have significant effects.

• Amino Acids: Building blocks of proteins, each with a central carbon, amino group, carboxyl group, and side chain (R group).

• Polypeptides: Chains of amino acids linked by peptide bonds.

• Protein Shape: Determined by the sequence of amino acids; crucial for function.

• Example: Sickle cell disease is caused by a single amino acid change in hemoglobin, altering its shape and function.

Nucleic Acids

DNA and RNA

Nucleic acids store and transmit genetic information. DNA and RNA are polymers of nucleotides, each consisting of a sugar, phosphate group, and nitrogenous base.

• DNA (Deoxyribonucleic Acid): Double-stranded, stores genetic information.

• RNA (Ribonucleic Acid): Single-stranded, involved in protein synthesis.

• Gene: A segment of DNA that codes for a protein.

• Transcription: The process by which a gene's DNA sequence is copied to make an RNA molecule.

All nucleotides contain:

• A five-carbon sugar (deoxyribose in DNA, ribose in RNA)

• A phosphate group

• A nitrogenous base (adenine, thymine, cytosine, guanine in DNA; uracil replaces thymine in RNA)

Comparison Table: DNA vs. RNA

Feature	DNA	RNA
Strands	Double	Single
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Function	Genetic storage	Protein synthesis

Additional info: These notes are based on guided reading questions and diagrams from a General Biology textbook chapter on the molecules of cells, covering the structure and function of major biological macromolecules.