General Biology: Core Concepts and Processes

Chemistry of Life

Essential Elements and Chemical Bonds

Living organisms are composed primarily of a few key elements and depend on chemical bonds for structure and function.

• Essential Elements: Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N) (abbreviated as CHON) make up the majority of living matter.

• Trace Elements: Required in small amounts (e.g., iodine for thyroid function, iron for hemoglobin).

• Atoms and Bonds: Atoms form ionic, covalent, or hydrogen bonds to achieve stable electron configurations.

• Water's Polarity: Water molecules are polar, enabling hydrogen bonding—critical for many biological processes.

• Acids and Bases: Influence the pH balance of biological systems.

Example: Table salt (NaCl) forms via ionic bonding between sodium and chloride ions.

Macromolecules and Cellular Structure

Major Biological Macromolecules

Cells are built from four classes of macromolecules, each with unique structures and functions.

• Carbohydrates: Sugars and polysaccharides; provide energy and structural support.

• Lipids: Fats, oils, and phospholipids; important for energy storage and membrane structure.

• Proteins: Polymers of amino acids; serve as enzymes, structural components, and signaling molecules.

• Nucleic Acids: DNA and RNA; store and transmit genetic information.

Protein Structure Levels

• Primary: Sequence of amino acids.

• Secondary: Local folding (α-helix, β-sheet) via hydrogen bonds.

• Tertiary: 3D shape due to side chain interactions.

• Quaternary: Association of multiple polypeptide chains.

DNA Base Pairing: Adenine (A) pairs with Thymine (T), Guanine (G) with Cytosine (C), held by hydrogen bonds.

Cells and Organelles

Cell Types and Structures

Cells are the basic units of life, classified as prokaryotic or eukaryotic based on internal organization.

• Prokaryotes: Lack a nucleus and membrane-bound organelles (e.g., bacteria).

• Eukaryotes: Have a nucleus and various organelles (e.g., plants, animals, fungi, protists).

Key Organelles and Functions

• Nucleus: Contains DNA; controls cell activities.

• Mitochondria: Site of ATP production via cellular respiration.

• Lysosomes: Contain digestive enzymes for breakdown of macromolecules.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Cell Membrane: Composed of phospholipids and proteins; selectively permeable.

Transport and Homeostasis

Movement Across Membranes

Cells regulate internal conditions by controlling the movement of substances across membranes.

• Diffusion: Passive movement of molecules from high to low concentration.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Active Transport: Movement against concentration gradient; requires ATP.

• Endocytosis: Uptake of large materials by engulfing them in vesicles.

• Exocytosis: Release of materials from the cell via vesicles.

Energy, Enzymes, and Metabolism

Cellular Energy and Catalysis

Cells use energy to drive metabolic reactions, often mediated by enzymes.

• ATP (Adenosine Triphosphate): Main energy currency; hydrolysis releases energy.

• Catabolic Reactions: Break down molecules, releasing energy.

• Anabolic Reactions: Build complex molecules, requiring energy input.

• Enzymes: Biological catalysts that lower activation energy and increase reaction rates.

• Optimal Conditions: Enzymes function best at specific pH and temperature.

Equation:

$\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P}_i + \text{energy}$

Cellular Respiration

Harvesting Energy from Glucose

Cellular respiration converts glucose into ATP through a series of metabolic pathways.

• Glycolysis: Occurs in cytoplasm; breaks glucose into pyruvate.

• Citric Acid Cycle (Krebs Cycle): Occurs in mitochondria; processes pyruvate, producing electron carriers.

• Oxidative Phosphorylation: Electron transport chain and chemiosmosis generate most ATP.

• Aerobic Respiration: Requires oxygen; yields more ATP than anaerobic processes.

Equation:

$\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{O}_2 \rightarrow 6\mathrm{CO}_2 + 6\mathrm{H}_2\mathrm{O} + \text{ATP}$

Photosynthesis

Converting Light Energy to Chemical Energy

Photosynthesis in plants and some protists transforms solar energy into chemical energy stored in glucose.

• Location: Occurs in chloroplasts.

• Light Reactions: Capture light energy to produce ATP and NADPH.

• Calvin Cycle: Uses ATP and NADPH to fix CO2 into sugars.

• Complementarity: Photosynthesis and respiration are interconnected energy processes.

Equation:

$6\mathrm{CO}_2 + 6\mathrm{H}_2\mathrm{O} + \text{light energy} \rightarrow \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{O}_2$

Cell Communication

Signaling and Cellular Responses

Cells communicate using chemical signals to coordinate activities and maintain homeostasis.

• Hormones: Chemical messengers; steroid hormones enter cells, peptide hormones bind to surface receptors.

• Signal Transduction: Process by which a signal is converted to a cellular response.

• Apoptosis: Programmed cell death; removes damaged or unnecessary cells.

Cell Cycle and Division

Growth and Reproduction of Cells

The cell cycle ensures accurate duplication and division of genetic material.

• Interphase: Includes G1 (growth), S (DNA synthesis), and G2 (preparation for division).

• Mitosis: Division of the nucleus into two identical daughter nuclei.

• Cytokinesis: Division of the cytoplasm.

• Cancer: Results from loss of cell cycle checkpoint control and uncontrolled cell division.

Meiosis and Heredity

Formation of Gametes and Genetic Variation

Meiosis reduces chromosome number and increases genetic diversity in sexually reproducing organisms.

• Meiosis: Two consecutive divisions (meiosis I and II) produce four haploid gametes.

• Genetic Variation: Created by crossing over and independent assortment.

• Separation: Homologous chromosomes separate in meiosis I; sister chromatids in meiosis II.

Mendelian Genetics

Inheritance Patterns and Genetic Mapping

Mendelian genetics explains how traits are inherited through dominant and recessive alleles.

• Genes and Alleles: Genes exist in different forms (alleles); dominant alleles mask recessive ones.

• Punnett Squares: Used to predict genetic outcomes of crosses.

• Linked Genes: Genes located close together on the same chromosome are inherited together.

• Recombination Frequency: Measures genetic map distance between genes.

DNA and Molecular Biology

Structure and Replication of Genetic Material

DNA encodes genetic information and is faithfully replicated during cell division.

• DNA Structure: Double helix of nucleotides (A, T, G, C).

• Replication: Semi-conservative process involving enzymes such as DNA polymerase, ligase, and primase.

• Key Experiments: Griffith's and Chargaff's experiments established the molecular basis of inheritance.

Big Picture: Energy and Life

Energy Flow in Biological Systems

Energy transformations connect all living organisms through the processes of photosynthesis and respiration.

• Photosynthesis: Stores energy in glucose molecules.

• Cellular Respiration: Releases energy from glucose for cellular work.

• Energy Flow: Producers (e.g., plants) capture energy; consumers (e.g., animals) utilize it.

Study Tips

• Use diagrams to visualize processes such as the ATP cycle, organelle structure, and cell division.

• Create flashcards for key terms and definitions.

• Practice connecting related processes (e.g., how photosynthesis supports respiration).