Intro to Botany Unit 6 Review: Plant Biochemistry & Metabolism

Key Concepts and Definitions

• Photosynthesis converts light energy into chemical energy (glucose) using carbon dioxide and water
• Cellular respiration breaks down glucose to release energy (ATP) for plant growth and development
• Metabolism encompasses all chemical reactions involved in maintaining plant life
  • Includes anabolism (building complex molecules) and catabolism (breaking down molecules)
• Enzymes act as biological catalysts accelerating chemical reactions without being consumed
• Plant hormones (auxins, gibberellins, cytokinins) regulate growth, development, and responses to stimuli
• Biochemical pathways involve series of enzymatic reactions transforming substrates into products
• Environmental factors (light, temperature, water, nutrients) influence plant biochemistry and physiology

Photosynthesis Basics

• Occurs in chloroplasts containing chlorophyll pigments that absorb light energy
• Light-dependent reactions capture light energy to produce ATP and NADPH
  • Photosystems I and II work in tandem to transfer electrons and generate ATP
• Calvin cycle (light-independent reactions) uses ATP and NADPH to fix carbon dioxide into glucose
• C3, C4, and CAM photosynthesis pathways adapted to different environmental conditions
  • C4 plants (corn, sugarcane) have higher photosynthetic efficiency in hot, dry climates
• Photorespiration competes with photosynthesis, reducing efficiency by consuming oxygen and releasing carbon dioxide
• Photosynthetic pigments (chlorophylls, carotenoids) absorb different wavelengths of light
• Factors affecting photosynthesis include light intensity, carbon dioxide concentration, temperature, and water availability

Cellular Respiration in Plants

• Occurs in mitochondria to break down glucose and release energy (ATP)
• Glycolysis breaks down glucose into pyruvate in the cytoplasm
• Citric acid cycle (Krebs cycle) oxidizes acetyl-CoA to produce NADH and FADH2
  • Occurs in the mitochondrial matrix
• Electron transport chain transfers electrons from NADH and FADH2 to oxygen, generating ATP through chemiosmosis
• Fermentation (anaerobic respiration) occurs in the absence of oxygen, producing ethanol or lactic acid
• Respiration rate influenced by temperature, oxygen availability, and substrate concentration
• ATP produced during respiration powers various cellular processes (growth, development, transport)

Plant Metabolism Pathways

• Primary metabolism involves essential processes for plant survival and growth
  • Includes photosynthesis, respiration, amino acid synthesis, and lipid metabolism
• Secondary metabolism produces compounds not directly involved in growth and development
  • Includes phenolics, terpenes, and alkaloids that serve defensive or attractive roles
• Glycolysis breaks down glucose into pyruvate, generating ATP and NADH
• Pentose phosphate pathway produces NADPH and ribose-5-phosphate for biosynthetic processes
• Fatty acid synthesis builds long-chain hydrocarbons for energy storage and membrane components
• Amino acid metabolism synthesizes and degrades amino acids for protein synthesis and nitrogen metabolism
• Nitrogen assimilation incorporates inorganic nitrogen (nitrate, ammonium) into organic compounds
• Sulfur metabolism incorporates sulfur into amino acids (cysteine, methionine) and other compounds

Enzymes and Biochemical Reactions

• Enzymes lower activation energy of reactions, increasing reaction rates
• Active site of enzyme binds to substrate, forming enzyme-substrate complex
• Cofactors (coenzymes, metal ions) assist enzymes in catalyzing reactions
  • Examples include NAD+, FAD, and iron-sulfur clusters
• Enzyme activity regulated by pH, temperature, substrate concentration, and inhibitors
• Michaelis-Menten kinetics describes relationship between substrate concentration and reaction rate
  • $V = \frac{V_{max}[S]}{K_m + [S]}$
• Allosteric regulation involves binding of effectors at sites other than active site, modulating enzyme activity
• Feedback inhibition occurs when end product of pathway inhibits enzyme catalyzing earlier step

Plant Hormones and Signaling

• Auxins (indole-3-acetic acid) stimulate cell elongation, apical dominance, and root formation
• Gibberellins promote stem elongation, seed germination, and fruit development
• Cytokinins regulate cell division, delay senescence, and promote shoot formation
• Abscisic acid (ABA) mediates stomatal closure, seed dormancy, and stress responses
• Ethylene is a gaseous hormone that promotes fruit ripening and leaf abscission
• Brassinosteroids regulate cell expansion, vascular differentiation, and stress tolerance
• Hormone crosstalk involves interactions between different hormones to coordinate plant responses
• Signal transduction pathways convert hormone signals into cellular responses
  • Includes receptor binding, second messengers (calcium, cAMP), and gene expression changes

Environmental Factors Affecting Plant Biochemistry

• Light influences photosynthesis, photomorphogenesis, and circadian rhythms
  • Phytochromes and cryptochromes are photoreceptors that detect light signals
• Temperature affects enzyme activity, membrane fluidity, and metabolic rates
  • Plants adapt to temperature stress through production of heat shock proteins and compatible solutes
• Water availability impacts turgor pressure, stomatal conductance, and nutrient uptake
  • Osmotic adjustment helps maintain cell turgor under drought stress
• Nutrient deficiencies alter plant metabolism and growth
  • Example: nitrogen deficiency reduces chlorophyll content and photosynthetic capacity
• Air pollution (ozone, sulfur dioxide) can damage plant tissues and impair biochemical processes
• Biotic stresses (pathogens, herbivores) trigger defense responses mediated by hormones (jasmonic acid, salicylic acid)

Practical Applications and Research

• Genetic engineering modifies plant metabolism to improve crop yield, nutritional content, and stress tolerance
  • Golden Rice engineered to produce beta-carotene (provitamin A) to combat vitamin A deficiency
• Metabolomics studies plant metabolites using analytical techniques (mass spectrometry, NMR)
  • Helps identify biomarkers for stress responses and discover new natural products
• Biofuels produced from plant biomass (cellulose, starch) offer renewable energy sources
• Phytoremediation uses plants to remove pollutants from soil and water
  • Hyperaccumulators (Thlaspi caerulescens) can accumulate high levels of heavy metals
• Plant-based pharmaceuticals produce therapeutic compounds in plant systems
  • Example: artemisinin (antimalarial drug) produced in engineered tobacco plants
• Sustainable agriculture practices optimize plant biochemistry for improved yield and environmental health
  • Includes precision fertilization, integrated pest management, and cover cropping