Life Study Guide

📖 Core Concepts Life’s defining traits – homeostasis, cellular organization, metabolism (anabolism > catabolism for growth), adaptation (evolution), response to stimuli, and reproduction. Thermodynamic view – living organisms are open systems that capture energy from environmental gradients to maintain order and copy themselves. Cell Theory – all living things are composed of cells; cells arise only from pre‑existing cells. DNA & the Central Dogma – DNA stores genetic information; information flows DNA → RNA → protein. Base pairing: A↔T, C↔G. Evolutionary mechanisms – natural selection, sexual selection, and genetic drift act on heritable variation, changing trait frequencies across generations. Classification hierarchy – Linnaean ranks (kingdom → species) and the modern three‑domain system (Bacteria, Archaea, Eukaryota) rooted in LUCA. Habitable Zone (HZ) – region around a main‑sequence star where liquid water could exist on a planetary surface; its size and stability depend on stellar mass and luminosity. --- 📌 Must Remember Homeostasis example: sweating cools the body. Metabolism: anabolism builds molecules; catabolism breaks them down. Cell differences: Prokaryotes lack a nucleus & membrane‑bound organelles; Eukaryotes have both. DNA composition: each nucleotide = base (A,T,C,G) + deoxyribose + phosphate. Three domains: Bacteria, Archaea, Eukaryota (LUCA at the root). Binary fission → prokaryotes; mitosis → diploid eukaryotes; meiosis → gametes. Habitable zone limits: inner edge (runaway greenhouse), outer edge (CO₂‑induced glaciation). Extremophiles thrive in > 100 °C, sub‑zero, high radiation, desiccation, etc. --- 🔄 Key Processes Metabolic balance (growth) Uptake of nutrients → anabolism (synthesis of macromolecules). Energy release from catabolism fuels biosynthesis. Net positive anabolic rate → increase in size/structure. Cell Division – Prokaryotes (Binary Fission) DNA replication. Attachment of replicated DNA to cell membrane. Cytokinesis splits the cell into two identical daughters. Cell Division – Eukaryotes (Mitosis) Prophase → chromosomes condense, spindle forms. Metaphase → chromosomes align at the equatorial plate. Anaphase → sister chromatids separate. Telophase → nuclear envelopes reform, followed by cytokinesis. Meiosis (Sexual Reproduction) One round of DNA replication → two rounds of division → four haploid gametes. Recombination (cross‑overs) increase genetic diversity. Evolutionary change (per generation) Mutation → new genetic variants. Genetic drift → random allele frequency changes (small populations). Natural/sexual selection → differential reproductive success → allele frequency shift. --- 🔍 Key Comparisons Prokaryote vs. Eukaryote Nucleus: absent vs. present. Organelles: none vs. mitochondria, chloroplasts, etc. → Cell size generally larger in eukaryotes. Virus vs. Living Cell Metabolism: none vs. self‑contained. Replication: hijacks host machinery vs. autonomous replication. Spontaneous Generation vs. Biogenesis (Pasteur) Life arising from non‑life vs. life only from pre‑existing life. Vitalism vs. Modern Chemistry “Vital force” required vs. organic compounds synthesized from inorganic precursors (e.g., Wöhler’s urea). Red‑dwarf HZ vs. Sun‑like HZ Smaller, closer, tidally locked, higher magnetic activity vs. wider, longer‑lasting, more stable. --- ⚠️ Common Misunderstandings Viruses are “alive.” They lack metabolism and cannot reproduce without a host → not cells. All organisms have nuclei. Only eukaryotes do; prokaryotes lack a true nucleus. Habitable zone guarantees life. It only indicates where liquid water could exist; other factors (atmosphere, chemistry) are required. Extremophiles are rare. They represent a large portion of microbial diversity and show life’s adaptability. DNA → protein is a one‑way street. Reverse transcription (RNA → DNA) exists in retroviruses, but the central dogma’s core flow remains DNA→RNA→protein for cellular organisms. --- 🧠 Mental Models / Intuition Life as a “gradient‑powered factory.” Think of a river (energy gradient) driving turbines (metabolic pathways) that keep the factory (cell) running and producing copies of itself. DNA as a library of blueprints. Each gene is a book; transcription copies the text (RNA), translation builds the product (protein). Evolution as a filter. Random variation creates many “drafts”; selection removes the unsuitable, leaving the best‑adapted designs. --- 🚩 Exceptions & Edge Cases Facultative sex: organisms switch between asexual and sexual reproduction depending on environmental stress. LUCA: not a single organism but the most recent common ancestor of all current life; its exact nature is inferred, not directly observed. Red‑dwarf planets: may be in HZ but face tidal locking and flare activity, challenging habitability. Synthetic biology organisms: engineered life forms may contain non‑canonical nucleotides or novel metabolic pathways not found in nature. --- 📍 When to Use Which Identify organism type: look for nucleus & organelles → eukaryote; lack thereof → prokaryote. Describe energy flow: use anabolism when discussing building macromolecules; catabolism for energy‑releasing breakdown. Choose classification level: for broad evolutionary relationships use domains; for species‑level identification use binomial nomenclature. Assess habitability: start with stellar mass → HZ size; then evaluate planetary atmosphere, magnetic field, and geological activity. --- 👀 Patterns to Recognize Range‑of‑tolerance curve: optimal zone (peak performance) flanked by stress zone → intolerance zone. Cellular organization pattern: membrane‑bound organelles → eukaryote; absence → prokaryote. Evolutionary pressure pattern: high environmental variability → increased sexual reproduction or recombination. Habitable‑zone scaling: \( \text{HZ radius} \propto \sqrt{L{\star}} \) (luminosity). --- 🗂️ Exam Traps “Viruses are cells.” Distractor: they contain genetic material but lack cellular structure and metabolism. Base‑pair mismatch: selecting A↔G or C↔T as correct pairings. Confusing binary fission with mitosis. Both divide cells, but only eukaryotes perform mitosis with spindle apparatus. Assuming all extremophiles are multicellular. Most are microorganisms; multicellularity is rare among extremophiles. Habitable zone vs. Galactic habitable zone. The former is stellar‑centric; the latter concerns metallicity, supernova rates, and orbital stability on a galactic scale. ---