Biology Study Guide

📖 Core Concepts Biology – scientific study of life, encompassing all levels from molecules to ecosystems. Cell Theory – cells are the basic unit of life; all cells arise from pre‑existing cells. Macromolecules – polymers: carbohydrates (sugars), lipids (hydrophobic), proteins (amino‑acid chains), nucleic acids (nucleotides). Central Dogma – DNA → RNA → protein; reverse transcription (RNA → DNA) occurs in retroviruses. Metabolism – sum of all chemical reactions; catabolism breaks down molecules, anabolism builds them. Energy Flow – Sun → photosynthesis → chemical energy in carbohydrates → cellular respiration → ATP. Evolution – change in heritable traits of populations over generations; natural selection is the primary mechanism. Ecology – interactions among organisms and between organisms and abiotic environment; energy moves through trophic levels (≈10 % transfer efficiency). 📌 Must Remember Cell membrane – lipid bilayer with integral & peripheral proteins; semipermeable. Key organelles: nucleus (DNA), mitochondria (oxidative phosphorylation), chloroplasts (photosynthesis), ER/Golgi (protein/lipid processing). Glycolysis: net 2 ATP per glucose; occurs in cytoplasm. Krebs cycle: produces NADH, FADH₂, and 1 ATP per acetyl‑CoA. Oxidative phosphorylation: electron transport chain creates proton gradient; ATP synthase makes 34 ATP per glucose. Photosynthesis: light reactions → NADPH + ATP; Calvin cycle fixes CO₂ into sugars. Mendelian inheritance – segregation (one allele/gamete) & independent assortment (unless linked). DNA replication – semiconservative; each daughter helix contains one parental strand. Gene regulation – positive (activators) vs negative (repressors); operons (prokaryotes) & epigenetics (eukaryotes). Speciation – reproductive isolation (pre‑zygotic & post‑zygotic) leads to lineage splitting. Trophic efficiency – 10 % of energy passes to next level; 90 % lost as heat/waste. 🔄 Key Processes Cellular Respiration Glycolysis (cytoplasm): Glucose → 2 pyruvate + 2 ATP + 2 NADH. Pyruvate → Acetyl‑CoA (mitochondrial matrix): produces NADH + CO₂. Krebs Cycle (matrix): each acetyl‑CoA → 3 NADH + FADH₂ + 1 ATP + 2 CO₂. Oxidative Phosphorylation (inner membrane): electrons travel through ETC, pumping protons → ATP synthase makes 34 ATP. Photosynthesis Light absorption (thylakoid chlorophyll) → water splitting → O₂ + electrons. Electron transport (PSII → PSI) → NADPH formation + proton gradient. Photophosphorylation: ATP synthase uses gradient → ATP. Calvin Cycle (stroma): CO₂ + ATP + NADPH → G3P → glucose. DNA Replication (Semiconservative) Origin → helicase unwinds → primase adds RNA primer → DNA polymerase synthesizes new strands (5'→3') → ligase joins Okazaki fragments → proofreading corrects errors. Gene Expression (Prokaryotic Operon) Repressor bound → transcription OFF. Inducer binds repressor → repressor releases → RNA polymerase initiates → gene products made. Cell Cycle (Mitosis) Prophase → Metaphase → Anaphase → Telophase followed by cytokinesis → two identical daughter cells. 🔍 Key Comparisons Prokaryote vs Eukaryote Nucleus: absent vs present. DNA: circular nucleoid vs linear chromosomes. Organelles: none vs membrane‑bound (mitochondria, ER, etc.). Catabolism vs Anabolism Energy flow: release vs consume. Typical pathways: glycolysis vs fatty‑acid synthesis. Lactic vs Alcoholic Fermentation End product: lactate (animals) vs ethanol + CO₂ (yeast). NAD⁺ regeneration: both, but different enzymes. Autocrine vs Paracrine vs Endocrine signaling Target distance: same cell vs nearby cells vs distant cells via bloodstream. ⚠️ Common Misunderstandings “Water is polar, so it dissolves everything.” – Only other polar or ionic substances dissolve well; non‑polar molecules (e.g., oils) are insoluble. “All proteins are enzymes.” – Proteins have structural, signaling, transport, and regulatory roles besides catalysis. “DNA → RNA → protein is a one‑way street.” – Reverse transcription and RNA editing show bidirectional flow. “All mutations are harmful.” – Mutations can be neutral or beneficial; some confer adaptive advantage. “All cells in a multicellular organism have the same DNA expression.” – Gene regulation creates diverse phenotypes without DNA sequence changes. 🧠 Mental Models / Intuition “Cell as a factory” – Membrane = security gate; nucleus = control room; mitochondria = power plant; ribosomes = assembly line. “Energy ladder” – Sunlight → photons → electron flow (photosynthesis) → chemical bonds (carbohydrates) → ATP (cellular respiration) → work. “Inheritance as a deck of cards” – Each parent shuffles their alleles; gametes draw one card per gene; offspring receive a random mix. “Ecological pyramid” – Base (primary producers) supports decreasing energy/biomass up the levels; think of a leaky bucket losing 90 % each step. 🚩 Exceptions & Edge Cases Linkage – Genes on same chromosome may not assort independently; recombination frequency < 50 % indicates linkage. Alternative splicing – One gene can produce multiple protein isoforms, expanding functional diversity. Anaerobic respiration – Some microbes use electron acceptors other than O₂ (e.g., nitrate) → not covered in outline but a known exception to classic aerobic pathway. Endosymbiotic gene transfer – Mitochondrial and chloroplast genomes have transferred many genes to the nucleus. 📍 When to Use Which Identify energy source → Use photosynthesis if organism is autotrophic; use cellular respiration for heterotrophs. Determine inheritance pattern → Apply Mendelian rules for single‑gene traits; consider linkage or polygenic models if ratios deviate. Predict metabolic fate of glucose → In presence of O₂ → aerobic respiration; absence of O₂ → fermentation (lactic or alcoholic). Choose regulatory model → Prokaryotes: operon logic; Eukaryotes: transcription factors + epigenetic marks. 👀 Patterns to Recognize “Redox ladder” – Electrons flow from high‑energy donors (e.g., NADH) to low‑energy acceptors (O₂) in respiration; similar pattern in photosynthetic electron transport (water → NADP⁺). “Coupled reactions” – ATP hydrolysis drives unfavorable steps; ATP synthesis is coupled to proton gradient dissipation. “Dominant vs recessive” – Dominant phenotype appears in heterozygotes; recessive only when both alleles are recessive. “Trophic level loss” – Expect 10 % energy transfer; large biomass at producer level, much less at apex predators. 🗂️ Exam Traps Confusing “oxidative phosphorylation” with “photosynthetic phosphorylation.” – Only mitochondria use oxidative phosphorylation; chloroplasts use photophosphorylation. Misreading “linkage” as “independent assortment.” – Linked genes do NOT segregate 1:1; recombination frequency < 50 % indicates linkage. Assuming all “enzymes” are proteins. – Ribozymes (RNA enzymes) exist; not highlighted in outline but a possible distractor. Mixing up “autocrine” and “paracrine” signals. – Autocrine acts on the same cell; paracrine acts locally on neighbors. Over‑generalizing “all mutations are detrimental.” – Many are neutral; some confer advantages (e.g., antibiotic resistance). --- Use this guide to skim core ideas, recall high‑yield facts, and spot classic exam pitfalls. Good luck!