🔬General Biology I Unit 30 – Plant Form and Physiology

Plant Structure and Organization

• Plants exhibit a hierarchical organization consisting of cells, tissues, organs, and organ systems
  • Cells are the basic structural and functional units of plants
  • Tissues are groups of cells with similar structure and function (meristematic, dermal, ground, and vascular tissues)
  • Organs include roots, stems, and leaves, each with specific functions
  • Organ systems work together to maintain plant growth and development
• Plants have a modular body plan composed of repeating units called phytomers
  • Each phytomer consists of a node, internode, and one or more leaves
  • Phytomers are produced by apical meristems and contribute to plant growth
• Plant body is divided into two main systems: shoot system and root system
  • Shoot system includes stems, leaves, and reproductive structures (flowers and fruits)
  • Root system anchors the plant, absorbs water and nutrients, and stores food
• Plants exhibit indeterminate growth, meaning they can continue growing throughout their life cycle
  • Meristems are regions of actively dividing cells responsible for plant growth
    • Apical meristems located at the tips of shoots and roots contribute to primary growth (elongation)
    • Lateral meristems (cambium) located along the sides of stems and roots contribute to secondary growth (thickening)

Cell Types and Tissues

• Plant cells are eukaryotic and have several unique features compared to animal cells
  • Cell wall composed of cellulose provides structural support and protection
  • Large central vacuole stores water, nutrients, and waste products
  • Plastids, including chloroplasts, are organelles involved in photosynthesis and storage
• Meristematic tissues are composed of actively dividing cells and give rise to other plant tissues
  • Apical meristems found at the tips of shoots and roots
  • Lateral meristems (vascular cambium and cork cambium) found along the sides of stems and roots
• Dermal tissues cover and protect the plant body
  • Epidermis is the outermost layer of cells that provides protection and regulates gas exchange and water loss
    • Cuticle, a waxy layer on the surface of the epidermis, helps prevent water loss
  • Periderm (cork) replaces the epidermis in older stems and roots, providing additional protection
• Ground tissues fill the spaces between dermal and vascular tissues
  • Parenchyma cells are the most common ground tissue cells and perform various functions (photosynthesis, storage, and secretion)
  • Collenchyma cells provide structural support and have unevenly thickened cell walls
  • Sclerenchyma cells (fibers and sclereids) have thick, lignified cell walls and provide mechanical support
• Vascular tissues transport water, nutrients, and sugars throughout the plant body
  • Xylem tissue conducts water and dissolved minerals from roots to leaves
    • Tracheids and vessel elements are the conducting cells in xylem
  • Phloem tissue transports sugars and other organic compounds from leaves to other plant parts
    • Sieve elements and companion cells are the conducting cells in phloem

Roots, Stems, and Leaves

• Roots anchor the plant, absorb water and nutrients, and store food
  • Root system architecture varies among plant species (taproot vs. fibrous root systems)
  • Root hairs increase the surface area for water and nutrient absorption
  • Root cap protects the apical meristem and helps the root penetrate the soil
• Stems provide support, transport materials, and bear leaves and reproductive structures
  • Primary growth occurs at the apical meristem and results in stem elongation
  • Secondary growth occurs at the lateral meristems (cambium) and results in stem thickening
  • Nodes are points of leaf attachment and contain axillary buds that can give rise to branches
  • Internodes are the regions between nodes and vary in length depending on the plant species and environmental conditions
• Leaves are the main photosynthetic organs of plants and play a crucial role in gas exchange and transpiration
  • Leaf structure is adapted for efficient light capture and gas exchange
    • Epidermis with stomata regulates gas exchange and water loss
    • Mesophyll (palisade and spongy) contains chloroplasts for photosynthesis
    • Veins (xylem and phloem) transport water, nutrients, and sugars
  • Leaf arrangement (phyllotaxy) and shape vary among plant species and affect light interception and water loss
    • Alternate, opposite, and whorled leaf arrangements
    • Simple and compound leaf shapes

Transport Systems in Plants

• Xylem tissue transports water and dissolved minerals from roots to leaves
  • Water moves through xylem by transpirational pull and cohesion-tension mechanism
    • Transpirational pull is driven by the evaporation of water from leaves
    • Cohesion-tension mechanism relies on the cohesive properties of water molecules and the adhesion of water to xylem cell walls
  • Xylem cells (tracheids and vessel elements) are dead at maturity and have lignified cell walls for structural support
• Phloem tissue transports sugars and other organic compounds from leaves (sources) to other plant parts (sinks)
  • Sugars move through phloem by pressure flow mechanism
    • Osmotic pressure gradient is created by the loading of sugars into sieve elements at the source and unloading at the sink
  • Phloem cells (sieve elements and companion cells) are living but lack some organelles to facilitate transport
• Transpiration is the loss of water vapor from plant leaves through stomata
  • Stomata are pores in the leaf epidermis that regulate gas exchange and water loss
  • Guard cells surrounding the stomata change shape to open and close the pores in response to environmental factors (light, humidity, and CO2 concentration)
  • Transpiration helps drive the movement of water through the xylem and cools the plant through evaporative cooling
• Translocation is the long-distance transport of sugars and other organic compounds through the phloem
  • Sugars are loaded into sieve elements at the source (usually leaves) by active transport
  • Sugars are unloaded at the sink (growing regions, storage organs, or other plant parts) by diffusion or active transport
  • Translocation is bidirectional and can occur both upward and downward in the plant body

Photosynthesis and Energy Production

• Photosynthesis is the process by which plants convert light energy into chemical energy stored in sugars
  • Occurs in chloroplasts, which contain chlorophyll pigments that absorb light energy
  • Overall reaction: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2
• Light-dependent reactions (light reactions) occur in the thylakoid membranes of chloroplasts
  • Light energy is absorbed by chlorophyll and other pigments in photosystems (PSI and PSII)
  • Electron transport chain generates ATP and NADPH, which are used in the Calvin cycle
  • Oxygen is released as a byproduct of the light reactions (photolysis of water)
• Calvin cycle (light-independent reactions or dark reactions) occurs in the stroma of chloroplasts
  • CO2 is fixed by the enzyme RuBisCO to produce 3-phosphoglycerate (3-PGA)
  • ATP and NADPH from the light reactions are used to reduce 3-PGA to form sugars (glucose)
  • Sugars produced in the Calvin cycle are used for plant growth, development, and storage
• Factors affecting photosynthesis include light intensity, CO2 concentration, temperature, and water availability
  • Light intensity and CO2 concentration directly affect the rate of photosynthesis
  • Temperature affects enzyme activity, with optimal temperatures varying among plant species
  • Water availability affects stomatal opening and closure, which regulates CO2 uptake and transpiration
• C3, C4, and CAM photosynthesis are different pathways adapted to various environmental conditions
  • C3 photosynthesis is the most common pathway and occurs in temperate and tropical plants (rice, wheat, and soybeans)
  • C4 photosynthesis is an adaptation to hot, dry environments and involves a spatial separation of the light reactions and Calvin cycle (maize, sugarcane, and sorghum)
  • CAM (Crassulacean Acid Metabolism) photosynthesis is an adaptation to arid environments and involves a temporal separation of the light reactions and Calvin cycle (cacti and succulents)

Plant Growth and Development

• Plant growth is the irreversible increase in size and mass of a plant
  • Primary growth occurs at the apical meristems and results in the elongation of roots and shoots
  • Secondary growth occurs at the lateral meristems (cambium) and results in the thickening of stems and roots
• Plant development is the progression of a plant through its life cycle, from seed germination to senescence
  • Seed germination is the process by which a seed develops into a seedling
    • Requires water, oxygen, and suitable temperature
    • Hormones (gibberellins) play a crucial role in breaking seed dormancy and promoting germination
  • Vegetative growth is the period of active growth and development of leaves, stems, and roots
    • Influenced by environmental factors (light, temperature, and nutrients) and plant hormones (auxins, cytokinins, and gibberellins)
  • Reproductive growth involves the development of flowers, fruits, and seeds
    • Transition from vegetative to reproductive growth is triggered by environmental cues (photoperiod and temperature) and plant hormones (florigen)
    • Pollination and fertilization are essential for seed and fruit development
• Plant hormones (phytohormones) are chemical messengers that regulate plant growth and development
  • Auxins promote cell elongation, apical dominance, and root formation
  • Cytokinins promote cell division, delay senescence, and stimulate shoot formation
  • Gibberellins promote stem elongation, seed germination, and fruit development
  • Abscisic acid (ABA) regulates seed dormancy, stomatal closure, and stress responses
  • Ethylene is a gaseous hormone that promotes fruit ripening, leaf abscission, and senescence
• Senescence is the final stage of plant development, characterized by the degradation of tissues and organs
  • Nutrient remobilization occurs during senescence, with nutrients being transported from senescing tissues to developing organs (seeds and fruits)
  • Leaf abscission is the process by which leaves are shed from the plant, often in response to environmental cues (shorter days and cooler temperatures)

Plant Responses to Environment

• Plants are sessile organisms that must adapt to their environment to survive and reproduce
  • Respond to environmental stimuli through changes in growth, development, and physiology
  • Responses can be short-term (reversible) or long-term (irreversible) adaptations
• Phototropism is the growth response of plants to light
  • Shoots exhibit positive phototropism (grow towards light), while roots exhibit negative phototropism (grow away from light)
  • Mediated by the plant hormone auxin, which accumulates on the shaded side of the stem, promoting cell elongation
• Gravitropism (geotropism) is the growth response of plants to gravity
  • Roots exhibit positive gravitropism (grow downward), while shoots exhibit negative gravitropism (grow upward)
  • Mediated by the redistribution of auxin in response to gravity, with auxin accumulating on the lower side of the root or shoot
• Thigmotropism is the growth response of plants to touch or contact
  • Climbing plants (vines) exhibit positive thigmotropism, growing towards and wrapping around supports
  • Roots exhibit positive thigmotropism, growing around obstacles in the soil
• Photoperiodism is the response of plants to the relative lengths of day and night
  • Plants can be classified as long-day plants (flower when days are longer than a critical length), short-day plants (flower when days are shorter than a critical length), or day-neutral plants (flowering not affected by day length)
  • Phytochrome, a light-sensitive pigment, plays a crucial role in detecting day length and regulating flowering time
• Plant responses to abiotic stress include adaptations to drought, salinity, extreme temperatures, and nutrient deficiencies
  • Drought stress responses include stomatal closure, leaf rolling, and accumulation of osmolytes (sugars and proline)
  • Salt stress responses include ion exclusion, compartmentalization of ions in vacuoles, and synthesis of compatible solutes
  • Temperature stress responses include the production of heat shock proteins (HSPs) and cold-responsive proteins (CORs)
  • Nutrient deficiency responses include changes in root architecture, increased nutrient uptake efficiency, and remobilization of nutrients from senescing tissues

Practical Applications and Research

• Crop improvement through traditional breeding and genetic engineering
  • Selection of desirable traits (yield, disease resistance, and stress tolerance) in crop plants
  • Genetic modification of crops for enhanced nutrition (golden rice), herbicide resistance (Roundup Ready soybeans), and insect resistance (Bt cotton)
• Horticulture and ornamental plant production
  • Techniques for propagation, cultivation, and maintenance of ornamental plants
  • Development of new plant varieties with improved aesthetic qualities (flower color, fragrance, and plant architecture)
• Forestry and sustainable resource management
  • Silvicultural practices for the management of forests and wood production
  • Reforestation and afforestation efforts to restore degraded lands and mitigate climate change
• Bioremediation and phytoremediation
  • Use of plants to remove pollutants (heavy metals and organic compounds) from contaminated soils and water
  • Phytoremediation strategies include phytoextraction, phytodegradation, and phytostabilization
• Medicinal plants and natural product discovery
  • Identification and characterization of bioactive compounds from medicinal plants
  • Drug discovery and development based on plant-derived compounds (taxol from Pacific yew and artemisinin from sweet wormwood)
• Plant-microbe interactions and agricultural applications
  • Study of beneficial plant-microbe interactions (nitrogen-fixing rhizobia and mycorrhizal fungi)
  • Development of microbial inoculants and biofertilizers to improve plant growth and nutrient uptake
• Plant responses to climate change and environmental stress
  • Investigation of plant adaptations to changing environmental conditions (elevated CO2, temperature, and drought)
  • Development of stress-tolerant crop varieties through breeding and genetic engineering
• Space biology and plant growth in controlled environments
  • Study of plant growth and development in microgravity and controlled environmental systems
  • Development of life support systems for long-duration space missions (food production and air revitalization)