Botany Study Guide

📖 Core Concepts Botany – the scientific study of plants (land plants = embryophytes), covering anatomy, taxonomy, ecology, physiology, genetics, and biochemistry. Taxonomic hierarchy – Kingdom → Phylum/Division → Class → Order → Family → Genus → Species; binomial name = Genus species (e.g., Lilium columbianum). Photosynthesis – light energy captured by chlorophyll a/b creates ATP & NADPH, which power the Calvin cycle; Rubisco catalyses the first stable product (G3P). Plant hormones – auxins (cell elongation, tropisms), cytokinins (cell division), gibberellins (germination, stem elongation), abscisic acid (dormancy, stomatal closure), ethylene (ripening), jasmonates (wound response). Genetic inheritance – Mendelian laws (peas), self‑incompatibility, outcrossing vs. inbreeding, apomixis (asexual seed), polyploidy (auto‑ vs. allopolyploid). Ecological roles – oxygen production, carbon & water cycling, soil stabilization, primary food source, habitat formation (biomes). --- 📌 Must Remember Linnaean binomial system (1753) = genus + species. Primary photosynthetic pigments – chlorophyll a (blue‑violet) & chlorophyll b (orange/red). Key plant cell wall polymers – cellulose, pectin, xyloglucan; lignin strengthens secondary walls. Major hormone functions – IAA (auxin) → phototropism; cytokinin → shoot bud growth; GA → seed germination; ABA → drought response; ethylene → fruit ripening. DNA barcoding = rapid species ID using standardized gene regions. Agrobacterium tumefaciens Ti plasmid = natural vector for plant genetic engineering. Polyploidy outcomes – can create reproductive isolation & new species despite same geographic range. Mycorrhizal & rhizobial symbioses = nutrient exchange for plant‑derived carbon. --- 🔄 Key Processes Photosynthetic light reactions Light → chlorophyll excitation → electron transport → ATP + NADPH formation. Calvin cycle (dark reactions) CO₂ fixation by Rubisco → 3‑phosphoglycerate → reduction using ATP/NADPH → G3P (sugar precursor). Auxin‑mediated tropism Light → asymmetric auxin distribution → differential cell elongation → bending toward light. Seed germination (GA pathway) GA synthesis → degradation of DELLA repressors → activation of genes for enzyme production & growth. DNA barcoding workflow Extract DNA → amplify standard region (e.g., rbcL, matK) → sequence → compare to reference library. Agrobacterium transformation Infect plant cell → Ti plasmid T‑DNA integration → expression of transgene → regenerated transgenic plant. --- 🔍 Key Comparisons Autopolyploid vs. Allopolyploid Autopolyploid: chromosome doubling within one species → identical sets. Allopolyploid: hybridisation of two species → combined, divergent sets. C₃ vs. C₄ vs. CAM photosynthesis C₃: Calvin cycle only; prone to photorespiration. C₄: spatial separation of CO₂ fixation (mesophyll) and Calvin cycle (bundle‑sheath) → reduces photorespiration. CAM: temporal separation (night CO₂ uptake, day Calvin cycle) → water‑use efficient. Self‑incompatible vs. Self‑compatible plants Self‑incompatible: mechanisms prevent self‑fertilisation → promote outcrossing. Self‑compatible: can self‑fertilise → risk of inbreeding depression. --- ⚠️ Common Misunderstandings “All fungi are plants.” – Fungi belong to a separate kingdom; only embryophytes are true plants. “All algae are plants.” – Algae are a polyphyletic group; only the Charophyte lineage gave rise to land plants. “Auxin only promotes growth.” – Excess auxin can inhibit root elongation and cause epinasty; balance with other hormones is critical. “Polyploidy always increases vigor.” – While heterosis is common, polyploids can suffer reduced fertility if chromosome pairing is problematic. --- 🧠 Mental Models / Intuition “Plant as a factory” – Light energy (input) → ATP/NADPH (energy carriers) → sugars (product) → hormones (regulators) → growth & reproduction (output). “Genetic hierarchy” – DNA → epigenetic marks (methylation, histone mods) → transcription → protein → phenotype; think of epigenetics as “software settings” on the “hardware” DNA. “Ecological web” – Plants ⇄ soil microbes ⇄ animals ⇄ atmosphere; altering one node (e.g., mycorrhizae) ripples through the whole system. --- 🚩 Exceptions & Edge Cases Cryptogamia (group 24 in Linnaeus’ Systema Sexuale) includes non‑vascular plants and fungi—does not follow typical sexual organ counts. Apomixis bypasses meiosis & fertilisation, producing clonal seeds—rare but important for crop propagation. Totipotent parenchyma cells can regenerate whole plants, but highly lignified sclerenchyma or dead xylem cannot. C₄ photosynthesis evolved independently multiple times; not limited to grasses. --- 📍 When to Use Which Identify a plant species → start with morphology → confirm with DNA barcoding if morphology ambiguous. Study nutrient uptake → use root anatomy (hair density) for mineral absorption; use mycorrhizal assays for organic nutrient transfer. Improve crop yield → apply Mendelian breeding for single‑gene traits; use polyploid induction or transgenic approaches for complex traits (e.g., stress tolerance). Control weeds → select herbicides targeting auxin pathways (e.g., 2,4‑D) for broadleaf weeds; avoid in monocots where C₄ metabolism may confer resistance. --- 👀 Patterns to Recognize Hormone cross‑talk – auxin ↑ → cytokinin ↓ in roots (promotes root growth); GA ↑ → ABA ↓ during germination. Symbiotic nutrient exchange – mycorrhizae ⇄ phosphorus; rhizobia ⇄ nitrogen. Evolutionary trends – transition from water to land → development of cuticle, stomata, vascular tissue, seeds. Phylogenetic signals – DNA‑based trees often re‑arrange groups originally defined by morphology (e.g., APG reclassification of flowering families). --- 🗂️ Exam Traps “All plants have true roots.” – Bryophytes have rhizoids, not true roots. “All algae are photosynthetic.” – Some algae are heterotrophic or parasitic. “Auxin is the only hormone for phototropism.” – Cytokinins and gibberellins also modulate growth responses; ignoring them can lead to incomplete answers. “Polyploidy always leads to larger cells.” – While many polyploids exhibit gigantism, some show no size change; the effect is trait‑specific. “C₃ plants cannot survive in hot, dry climates.” – Many C₃ species employ CAM or other adaptations; answer choices that blanket‑state “C₃ = desert‑incompatible” are wrong. ---