Introduction: Cells & Life

Defining Life and Biological Systems

Understanding what constitutes life and how biological systems differ from non-living systems is foundational in biology. This includes distinguishing between organic and inorganic molecules and recognizing the unique properties of living organisms.

• Vitalism: The outdated idea that living organisms possess a 'vital force' not found in non-living matter. Modern biology rejects this concept.

• Organic Molecules: Compounds primarily made of carbon atoms in living organisms, often containing hydrogen, oxygen, nitrogen, and other elements.

• Inorganic Molecules: Molecules not based on carbon-hydrogen frameworks, such as water, salts, and minerals.

Structure & Function of Molecules #1: Water & Carbohydrates

Properties of Water and Carbohydrates

Water and carbohydrates are essential molecules in biological systems, each with unique structures and functions that support life.

• Polar Covalent Bond: A type of chemical bond where electrons are shared unequally, resulting in partial charges (e.g., in water molecules).

• Hydrogen Bond: A weak bond between a hydrogen atom and an electronegative atom (like oxygen), crucial for water's properties.

• Cohesion & Adhesion: Cohesion is the attraction between water molecules; adhesion is the attraction between water and other substances.

• Hydrophilic vs. Hydrophobic: Hydrophilic substances interact well with water; hydrophobic substances do not.

• Carbohydrates: Organic molecules composed of carbon, hydrogen, and oxygen, serving as energy sources and structural components.

Example: Glucose (C6H12O6) is a monosaccharide carbohydrate used for energy in cells.

Structure & Function of Molecules #2: Carbohydrates & Lipids

Types and Roles of Carbohydrates and Lipids

Carbohydrates and lipids are two major classes of biomolecules with diverse structures and functions.

• Carbohydrates: Include monosaccharides (glucose, fructose), disaccharides (sucrose), and polysaccharides (starch, cellulose, glycogen).

• Lipids: Hydrophobic molecules such as fats, oils, phospholipids, and steroids.

• Bilayer: Structure formed by phospholipids in cell membranes, with hydrophilic heads and hydrophobic tails.

• Triglyceride: A lipid formed from glycerol and three fatty acids, used for energy storage.

Example: Cellulose provides structural support in plant cell walls, while glycogen stores energy in animal cells.

Structure & Function of Molecules #3: Lipids & Proteins

Proteins and Lipids in Biological Systems

Proteins and lipids are fundamental to cell structure and function, participating in membrane formation, signaling, and catalysis.

• Amino Acid: Building block of proteins, containing an amino group, carboxyl group, and side chain (R group).

• Polypeptide: A chain of amino acids linked by peptide bonds.

• Phospholipid: A lipid with a phosphate group, forming the main component of cell membranes.

• Cholesterol: A steroid important for membrane fluidity and as a precursor for hormones.

• Protein Structure: Includes primary (sequence), secondary (alpha-helix, beta-sheet), tertiary (3D folding), and quaternary (multiple polypeptides) levels.

Example: Hemoglobin is a protein with quaternary structure that transports oxygen in blood.

Proteins & DNA

Structure and Function of Proteins and Nucleic Acids

Proteins perform a wide range of functions, while DNA stores genetic information. Their structures are closely related to their functions.

• Enzyme: A protein that catalyzes biochemical reactions.

• Denaturation: Loss of protein structure and function due to environmental changes.

• Nucleotide: Building block of DNA and RNA, consisting of a sugar, phosphate, and nitrogenous base.

• DNA vs. RNA: DNA contains deoxyribose and is double-stranded; RNA contains ribose and is usually single-stranded.

Example: DNA polymerase is an enzyme that synthesizes new DNA strands during replication.

DNA, Organelles, and Endomembranes

Cellular Structures and Genetic Material

Cells contain various organelles, each with specialized functions. DNA is housed in the nucleus (eukaryotes) or nucleoid (prokaryotes).

• Organelle: Specialized structure within a cell (e.g., nucleus, mitochondria, chloroplast).

• Endomembrane System: Includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, and vesicles.

• Prokaryote vs. Eukaryote: Prokaryotes lack a nucleus and membrane-bound organelles; eukaryotes have both.

• Ribosome: Site of protein synthesis.

Example: The mitochondrion is the site of cellular respiration and ATP production.

Organelles and Endomembranes 2

Membrane Transport and Cell Structure

Cells regulate the movement of substances across membranes using various mechanisms, maintaining homeostasis and communication.

• Carrier, Channel, Pump: Proteins that facilitate or actively transport molecules across membranes.

• Diffusion & Osmosis: Passive movement of molecules; osmosis refers specifically to water.

• Facilitated Diffusion: Passive transport via membrane proteins.

• Active Transport: Movement against a concentration gradient, requiring energy (ATP).

• Endocytosis & Exocytosis: Processes for bulk transport into and out of cells.

• Isotonic, Hypertonic, Hypotonic: Terms describing relative solute concentrations and their effects on cells.

Example: Sodium-potassium pumps maintain ion gradients in animal cells.

Enzymes and Metabolism

Catalysis and Regulation of Metabolic Pathways

Enzymes accelerate chemical reactions, and metabolism encompasses all chemical processes in cells. Regulation ensures efficiency and responsiveness.

• Catabolism vs. Anabolism: Catabolism breaks down molecules for energy; anabolism builds complex molecules from simpler ones.

• Gibbs Free Energy (): Determines whether a reaction is spontaneous () or requires energy input ().

• Allosteric Regulation: Enzyme activity is modulated by molecules binding at sites other than the active site.

Example: Phosphofructokinase is allosterically regulated in glycolysis.

Glycolysis and Respiration

Energy Production in Cells

Cells extract energy from glucose through glycolysis, the Krebs cycle, and oxidative phosphorylation, primarily in mitochondria.

• Glycolysis: The breakdown of glucose to pyruvate, producing ATP and NADH.

• Krebs Cycle (Citric Acid Cycle): Completes the oxidation of glucose derivatives, generating NADH, FADH2, and ATP.

• Electron Transport Chain: Series of protein complexes that transfer electrons, creating a proton gradient for ATP synthesis.

• Substrate-level Phosphorylation: Direct formation of ATP in glycolysis and the Krebs cycle.

• Oxidative Phosphorylation: ATP production using energy from the electron transport chain.

Example: Each glucose molecule yields up to 38 ATP molecules through cellular respiration.

Electron Transport and Fermentation

ATP Synthesis and Anaerobic Pathways

Cells can generate ATP in the absence of oxygen through fermentation, while aerobic respiration uses the electron transport chain and chemiosmosis.

• Anaerobic vs. Aerobic: Anaerobic processes do not require oxygen; aerobic processes do.

• ATP Synthase: Enzyme that synthesizes ATP using the proton gradient across the mitochondrial membrane.

• Fermentation: Process that regenerates NAD+ in the absence of oxygen, producing lactate or ethanol.

• Chemiosmosis: Movement of protons across a membrane to drive ATP synthesis.

Example: Muscle cells perform lactic acid fermentation during intense exercise.

Key Comparison Table: Prokaryotic vs. Eukaryotic Cells