The Molecules of Cells

Introduction

This section explores two major classes of biological molecules: lipids and nucleic acids. These molecules play essential roles in cell structure, energy storage, information transfer, and regulation of biological processes.

Lipids

Overview of Lipids

Lipids are a diverse group of hydrophobic molecules that do not dissolve in water. Unlike carbohydrates, proteins, and nucleic acids, lipids are not polymers and are generally smaller in size.

• Composition: Made primarily of carbon (C), hydrogen (H), and oxygen (O), with less oxygen than carbohydrates; some lipids also contain phosphorus (P).

• Types of Biologically Important Lipids: Triglycerides, phospholipids, steroids, eicosanoids, and waxes.

Triglycerides

Triglycerides (fats and oils) are the main form of stored energy in animals.

• Structure: Composed of three fatty acids covalently bonded to a glycerol molecule.

• Formation: Created by dehydration synthesis; broken down by hydrolysis.

• Functions: Long-term energy storage, cushioning of organs, and insulation.

• Energy Content: Fats store more energy per gram than carbohydrates due to their reduced (more hydrogen-rich) structure.

Fatty Acids: Saturated vs. Unsaturated

Fatty acids vary in chain length and the presence of double bonds.

• Saturated Fatty Acids: No double bonds between carbon atoms; saturated with hydrogen. Straight chains, solid at room temperature (e.g., butter).

• Unsaturated Fatty Acids: One or more double bonds cause bends in the chain, making them liquid at room temperature (e.g., oils).

Health Implications: Diets high in saturated fats are linked to atherosclerosis and heart disease. Unsaturated fats are generally considered healthier.

Trans Fats

• Definition: Unsaturated fats that have been chemically hydrogenated to behave like saturated fats (solid at room temperature).

• Labeling: Often listed as "partially hydrogenated oils" on food labels.

• Health Risks: Associated with increased risk of cardiovascular disease.

Phospholipids

Phospholipids are major components of cell membranes.

• Structure: Two fatty acids and a phosphate group attached to glycerol.

• Amphipathic Nature: Have a hydrophilic (polar) phosphate "head" and hydrophobic (nonpolar) fatty acid "tails".

• Function: Form bilayers in aqueous environments, creating the fundamental structure of cell membranes.

Steroids

Steroids are lipids with a characteristic four-ring hydrocarbon structure.

• Cholesterol: The parent steroid; component of cell membranes and precursor for steroid hormones.

• Steroid Hormones: Include hormones that regulate salt balance, reproduction, and stress responses.

• Transport: Cholesterol is packaged in lipoproteins for transport in blood.

Anabolic Steroids

• Definition: Synthetic molecules similar to testosterone, mimicking its muscle-building effects.

• Consequences: Use is associated with negative behavioral and physiological effects.

• Examples of Effects: Behavioral: aggression; Physiological: liver damage, hormonal imbalances.

Nucleic Acids

Overview of Nucleic Acids

Nucleic acids are polymers of nucleotides, which store and transmit genetic information.

• Components of a Nucleotide: Phosphate group, five-carbon sugar (ribose or deoxyribose), and a nitrogenous base.

• Types of Nucleic Acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

Structure of Nucleotides

• DNA: Contains deoxyribose sugar; bases are adenine (A), guanine (G), cytosine (C), and thymine (T).

• RNA: Contains ribose sugar; bases are adenine (A), guanine (G), cytosine (C), and uracil (U).

Polymerization of Nucleic Acids

• Nucleotides are joined by dehydration synthesis, forming a sugar-phosphate backbone.

• In DNA, two strands form a double helix held together by hydrogen bonds between complementary bases (A pairs with T, C pairs with G).

• RNA is usually single-stranded.

Functions of Nucleic Acids

• DNA: Long-term storage of genetic information; encodes instructions for protein synthesis.

• RNA: Transfers genetic information from DNA and is involved in protein assembly (translation).

Summary Table: Comparison of DNA and RNA

Central Dogma of Molecular Biology

• Gene Expression: DNA is transcribed into RNA, which is then translated into protein.

• Process:

  1. Transcription: DNA → RNA

  2. Translation: RNA → Protein

Key Equations and Concepts

• Dehydration Synthesis (Condensation Reaction): Formation of a covalent bond with the removal of a water molecule.

• Hydrolysis: Breaking of a covalent bond with the addition of a water molecule.

Additional info:

• Some content, such as the detailed structure of nucleotides and the full list of behavioral/physiological effects of anabolic steroids, was inferred or expanded for completeness.

• References to figures and videos were omitted, but key concepts were included in the text.