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Questions and Answers
What is the primary function of fruit in plants?
Which type of fruit is formed from a single flower with multiple carpels?
What adaptation is typically associated with seed dispersal by wind?
What term describes an organism that contains a gene from another species?
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Which hormone is primarily responsible for promoting cell elongation?
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How does auxin typically influence plant growth?
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What physiological response occurs due to high levels of abscisic acid (ABA) in seeds?
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What is the 'triple response' in plants subjected to mechanical stress?
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What is the primary function of blue light photoreceptors in plants?
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Which of the following is NOT a way that plants can respond to water deficit?
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What role do phytochromes play in seed germination?
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Which phenomenon involves the response of plants to mechanical stress such as wind?
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What is the critical night length concept in relation to plants?
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What is the primary difference between macronutrients and micronutrients?
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Which of the following is NOT considered an essential macronutrient for plants?
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What are the three main elements commonly found in chemical fertilizers?
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In which part of the plant do mineral nutrients mainly need to be for effective root uptake?
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Which ion is crucial for establishing the membrane potential in plants?
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What is the primary driving force for water transport in plants according to the cohesion-tension hypothesis?
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What phenomenon occurs when the solute concentration increases in an area?
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What usually causes guard cells to open stomata in the morning?
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Which of the following are examples of a sugar sink in plants?
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What mechanism drives the transport of nutrients from the source phloem to the sink in plants?
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What distinguishes a gametophyte from a sporophyte in the plant life cycle?
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Which of the following describes the term heterospory?
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What is a major evolutionary advantage of having seeds?
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In seedless vascular plants, what distinguishes the gametophyte-sporophyte relationships compared to bryophytes?
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How many cotyledons does a typical eudicot seed possess?
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What develops inside the ovules during female gametophyte formation?
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Study Notes
Macronutrients and Micronutrients
- Macronutrients: Elements plants need in relatively large amounts.
- Micronutrients: Elements plants need in smaller amounts.
Essential Macronutrients
- Carbon (C)
- Hydrogen (H)
- Oxygen (O)
- Nitrogen (N)
- Phosphorus (P)
- Potassium (K)
- Calcium (Ca)
- Magnesium (Mg)
- Sulfur (S)
Chemical Fertilizer: The Big Three
- Nitrogen (N): Found as ammonium (NH₄⁺) or nitrate (NO₃⁻).
- Phosphorus (P): Found as phosphate (PO₄³⁻).
- Potassium (K): Found as potassium ions (K⁺).
Other Nutrient Sources
- Organic fertilizers: Compost, manure.
- Nitrogen-fixing bacteria: Convert atmospheric nitrogen into usable forms.
Acidic Soil Preference
- Most plants prefer slightly acidic soils (pH 6.0 to 6.5) for optimal mineral uptake.
- Acid conditions make nutrients more readily available.
Mineral Nutrient Availability
- Minerals need to be dissolved in the soil solution to be absorbed by roots.
Cation Exchange
- Negatively charged soil particles (clay and humus) bind to positively charged mineral ions (cations).
- This process makes minerals available in the soil solution, replenishing them as they are taken up by roots.
Long Distance Transport in Plants
- Xylem: Transports water and dissolved minerals from roots to leaves.
- Phloem: Transports sugars and other organic products throughout the plant.
Plant Parts and Resource Acquisition
- Roots: Absorb water and minerals from soil.
- Stems: Support the plant and transport water and nutrients.
- Leaves: Photosynthesis for food production; gas exchange.
Apoplast and Symplast
- Apoplast: The space outside the plasma membranes of plant cells, including cell walls and intercellular spaces.
- Symplast: The continuous network of cytoplasm connected by plasmodesmata, allowing the movement of substances through the cell interiors.
Routes of Transport
- Apoplastic Route: Substances move through cell walls and intercellular spaces, bypassing the plasma membrane.
- Symplastic Route: Substances move through plasmodesmata, traversing plasma membranes only once.
- Transmembrane Route: Substances move across multiple plasma membranes between adjacent cells.
Membranes Potential in Plants
- Potassium ions (K⁺) establish the membrane potential in plants.
- Hydrogen ions (H⁺): Drive co-transport with other substances, moving them against their concentration gradients.
Water Potential and Water Movement
- Water Potential (Ψ): The tendency of water to move from one area to another.
- Water moves from areas of higher water potential to areas of lower water potential.
- Solute Potential: The negative pressure or tension generated by the presence of solutes; drives water movement.
- Solutes within plant cells: Sugars, salts, and organic acids.
- Increasing solute concentration: Reduces the number of free water molecules.
Wilting
- When a plant loses more water than it absorbs, its turgor pressure decreases, causing the plant to wilt.
Bulk Flow
- Bulk Flow: The mass movement of a fluid driven by pressure gradients, facilitated by differences in water potential.
Endodermis Barrier
- The Casparian strip, a waterproof band within the endodermis, forces water and minerals to move through the plasma membranes of endodermal cells, preventing apoplastic movement, ensuring a controlled passage.
Cohesion-Tension Hypothesis
- Cohesion: Water molecules stick to each other due to hydrogen bonding.
- Tension: Transpiration pulls water up through the xylem.
Transpiration
- Transpiration: The process by which water vapor is lost from the leaves of plants through stomata.
Cohesion and Water Transport
- The cohesion of water molecules allows the continuous column of water to be pulled upward through the xylem, creating the transpiration stream.
Stomata Opening and Closing
- Opening: Increased guard cell turgor pressure caused by an influx of water, due to active potassium ion pump.
- Closing: Decreased guard cell turgor pressure caused by the loss of water.
Stomata Opening Cues
- Light: Blue light and red light activate stomata opening.
- Carbon Dioxide Concentration: Low CO₂ levels promote opening. -Internal Clock: Stomata open in the morning, even in the dark, controlled by circadian rhythms.
Sugar Source and Sink
- Source: Tissues where sugars are produced, e.g., leaves during photosynthesis.
- Sink: Tissues where sugars are used or stored, e.g., roots, flowers, and fruits.
Phloem Transport
- Pressure Flow Hypothesis: Sugars are loaded into the phloem at the source, creating a high turgor pressure. This drives the movement towards the sink, where sugars are unloaded, reducing pressure.
Charophyceae and Land Plant Similarities
- Cell Wall Structure: Cellulose synthesis in both.
- Chloroplast Structure: Similar chloroplast and chlorophyll.
- Peroxisome Enzymes: Similar enzymes for photorespiration.
- Flagellated Sperm: Both produce sperm with whiplike flagella.
Derived Key Traits of Land Plants
- Alternation of Generations: Life cycle with a multicellular haploid gametophyte and multicellular diploid sporophyte.
- Apical Meristem: Localized regions of active cell division at shoot (terminal bud) and root tips.
- Multicellular Dependent Embryo: Embryo develops within tissues of parent plant.
- Sporopollenin: A durable polymer that coats spores and pollen grains, protecting them from desiccation, decay and UV radiation.
- Cuticle: Waxy layer on the outer surface of the plant, preventing desiccation.
- Stomata: Pores on leaves that allow gas exchange.
Gametophyte and Sporophyte
- Gametophyte: The haploid, multicellular generation that produces gametes.
- Sporophyte: The diploid, multicellular generation that produces spores.
Bryophyte Phyla
- Liverworts (Hepatophyta)
- Hornworts (Anthocerophyta)
- Mosses (Bryophyta)
Gametophyte-Sporophyte Relationship: Bryophytes vs. Ferns
- Bryophytes: The dominant generation is the gametophyte; sporophyte is smaller and dependent on gametophyte.
- Ferns: The dominant generation is the sporophyte; the gametophyte is smaller and independent.
Seedless Vascular Plants
- Key Developmental Features: Vascular tissues (xylem and phloem), true roots, and leaves.
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Phyla:
- Lycophytes (Lycopodiophyta): Club mosses, spike mosses, and quillworts.
- Pterophytes (Pteridophyta): Ferns, horsetails, and whisk ferns.
Heterospory
- Heterospory: The production of two types of spores: megaspores (female) and microspores (male).
Ovule
- Ovule: A structure that contains a female gametophyte and is enclosed in the sporophyte tissue.
Advantages of Reduced Gametophytes
- Protection: Reduced gametophytes are protected from desiccation and environmental stresses within the sporophyte tissues.
- Efficient Reproduction: Reduced gametophytes allow for faster reproduction and more efficient fertilization.
Evolutionary Advantages of a Seed
- Dispersal: Seeds can be transported by wind, water, or animals, allowing long-distance dispersal.
- Dormancy: Seeds can remain dormant for long periods, ensuring survival in harsh conditions.
- Nutrition: Seeds contain a food supply for the developing embryo, enabling it to grow and establish itself independently.
Sporophyte Dominance in Higher Plants
- Lower Land Plants (Bryophytes): The sporophyte is dependent on the gametophyte for nutrition and survival.
- Higher Land Plants (Gymnosperms, Angiosperms): The sporophyte generation is dominant and independent, producing seeds that can disperse to new locations.
Female Reproductive Structure
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Pistil: The female reproductive structure consists of:
- Stigma: The sticky top part of the pistil that receives pollen.
- Style: The stalk that connects the stigma to the ovary.
- Ovary: The swollen base that houses the ovules.
Male Reproductive Structure
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Stamen: The male reproductive structure consists of:
- Anther: The pollen-producing sac.
- Filament: The stalk that supports the anther.
Inside the Ovary
- Ovules: Contain the megasporangium (structure that produces megaspores).
Female Gametophyte Formation
- Megaspores: Develop inside the ovule's megasporangium.
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Megaspore Development: One megaspore survives and undergoes mitosis to form a multicellular female gametophyte (embryo sac) with:
- Egg cell: The female gamete (1 cell).
- Two synergids: Assist in fertilization.
- Three antipodal cells: Function is unclear.
- Central cell: Contains two polar nuclei.
Synergid Cells
- Synergids: Assist in fertilization by guiding the pollen tube to the egg cell and releasing signaling molecules.
Pollen Sac
- Microsporangium: The pollen sac inside the anther of a stamen, where microspores are produced.
Mature Seed Contents
- Embryo: The young sporophyte.
- Endosperm: A food-storing tissue that nourishes the developing embryo.
- Seed Coat: Protective outer covering of the seed.
Endosperm Function
- Stored Food: Provides nutrients to the developing embryo.
Eudicot Seed Structure
- Eudicots: Have two cotyledons (seed leaves).
Germination Steps
- Imbibition: The absorption of water by the seed.
- Radicle: The embryonic root emerges first.
- Shoot: The embryonic shoot emerges second.
- Cotyledons: Seed leaves open and photosynthesize, providing nutrition to the seedling.
Fruit
- Fruit: A mature ovary, usually containing seeds, which develops after fertilization.
- Function: Protects and disperses the seeds.
Fruit Types
- Simple Fruit: Develops from a single ovary (e.g., apple, cherry, pea).
- Aggregate Fruit: Develops from multiple ovaries of a single flower (e.g., raspberry, strawberry).
- Multiple Fruit: Develops from multiple ovaries of a cluster of flowers (e.g., pineapple, fig).
- Accessory Fruit: Develops mainly from tissues other than the ovary, often with a small ovary encased within (e.g., apple, pear).
Fruit Dispersal Adaptation
- Water Dispersal: Fruits with buoyant or waterproof features (e.g., coconut).
- Wind Dispersal: Fruits with wings, hairs, or lightweight structures (e.g., dandelion).
- Sharp Spines: Attach to animal fur for dispersal.
- Edible Fruits: Animals eat the fruit and disperse the seeds in their droppings.
Plant Biotechnology
- Plant Breeding: Selectively crossing plants with desirable traits.
- Genetic Engineering: Directly altering a plant's DNA to introduce new traits.
- Transgenic: An organism whose DNA has been altered through genetic engineering.
Plant Biotechnology Advantages
- Improved Yield: Increased crop production (resistance to pests, diseases, and herbicides).
- Enhanced Nutrition: Increased nutrient content in food.
- Pest and Disease Resistance: Developing crops that are less susceptible to pests and diseases, reducing the need for pesticides and fungicides.
- Herbicide Resistance: Creating crops that can tolerate herbicides, allowing for more effective weed control.
- Increased Stress Tolerance: Developing crops that can withstand harsh environmental conditions, such as drought or salinity.
Plant Biotechnology Risks
- Unintended Consequences: Unforeseen effects on other organisms, ecosystems, and human health.
- Gene Flow: The transfer of genes from genetically modified crops to wild relatives, with potential impacts on biodiversity.
- Monoculture: Increased dependence on a few genetically similar crops, reducing genetic diversity and making crops more vulnerable to pests and diseases.
- Ethical Concerns: Concerns about the safety and long-term effects of genetically modified foods.
Hormones: Plant Growth Regulators
- Hormones: Chemical messengers produced within a plant that influence growth, development, and other responses.
- Multiple Effects: The specific response to a hormone can depend on the hormone concentration, the plant tissue, and the developmental stage.
Phototropism: Shoot and Root Responses
- Shoot: Bends towards light, due to auxin accumulation on the shaded side.
- Root: Bends away from light, due to auxin inhibiting root growth on the lighted side.
Plant Hormone Pioneers
- Charles Darwin and Francis Darwin: Demonstrated that the tip of a coleoptile (protective sheath of a grass seedling) is responsible for phototropic bending.
- Frits Went: Isolated auxin (chemical messenger) produced by the coleoptile tip.
- Kenneth Thimann: Identified the chemical structure of indoleacetic acid, a major form of auxin.
Auxin Effects
- Cell elongation: Promotes cell elongation in stems and roots.
- Apical Dominance: Inhibits the growth of lateral buds, promoting vertical growth.
- Fruit Development: Promotes fruit development and ripening.
Auxin Polar Transport
- Polar Transport: Auxin moves unidirectionally from the tip of the shoot to the base, not influenced by gravity.
- Mechanism: Auxin is actively transported across membranes by specialized proteins.
Auxin and Cell Growth
- Stimulated by Auxin: Cell wall loosening: Increased cell wall extensibility.
- Low Cell Wall pH: Activates expansins, enzymes that break down bonds in cell walls.
- Expansin Activity: Loosens cell walls, allowing for expansion.
- Increased Turgor Pressure: Water influxes into cells, increasing pressure against cell walls.
Auxin and Apical Dominance
- Reduced Auxin Flow: From the apical bud, weakens lateral buds and weakens their growth (apical dominance).
- Apical Bud Removal: Results in increased growth of lateral buds.
Cytokinin Functions
- Cell division: Promotes cell division in roots, shoots, and other tissues.
- Lateral Bud Development: Stimulates lateral bud development and shoot branching.
- Production: Primarily in roots.
Callus Differentiation Control
- High Cytokinin/Low Auxin Ratio: Promotes shoot formation.
- Low Cytokinin/High Auxin Ratio: Promotes root formation.
Apical Dominance Control
- Auxin: The apical bud produces auxin that inhibits lateral bud growth.
- Cytokinins: Produced by the roots, counteract auxin's effects, promoting lateral bud growth.
Gibberellin Effects
- Stem Elongation: Promotes stem elongation and leaf growth.
- Seed Germination: Triggers seed germination by stimulating the breakdown of food reserves.
- Fruit Development: Promotes fruit growth and development.
- Flowering: Can induce flowering in some plants.
Gibberellins and Seed Germination
- Environmental Cues: Water uptake and warmer temperatures initiate the release of gibberellins within the seed.
- Gibberellin Action: Promotes synthesis of enzymes (amylases) that break down starch into sugars, providing energy for growth.
Abscisic Acid Actions
- Dormancy: Promotes seed dormancy and bud dormancy.
- Stress Response: Helps in stress response, including drought tolerance and response to cold temperatures.
- Differentiation: Helps to control leaf formation and growth.
ABA in Seed Dormancy
- High ABA Levels: Maintains seed dormancy by preventing germination until conditions are favorable.
ABA and Wilt Response
- Wilt: ABA accumulates in leaves experiencing water stress, closing stomata, reducing transpiration.
Ethylene Effects
- Triple Response (Mechanical Stress): Inhibits stem elongation, promotes lateral swelling, and promotes curvature of the stem in response to mechanical stress (e.g., pushing a seedling through soil).
- Fruit Ripening: Promotes fruit ripening by stimulating the production of enzymes that soften fruits and break down cell walls.
- Senescence: Promotes senescence (aging) of leaves, flowers, and fruits.
- Leaf Abscission: Promotes leaf abscission (shedding) in autumn.
Ethylene in Fruit Ripening
- Storage in Paper Bag: Promotes ethylene production, triggering ripening.
Photomorphogenesis
- Photomorphogenesis: The development of a plant in response to light.
De-etiolation (Green up)
- Light Triggers: Seedlings grown in darkness exhibit etiolated growth (elongated stems, small leaves, and pale color).
- Light Triggers: De-etiolation, plant growth responds to light: stems thicken, leaves expand, and chlorophyll production increases.
Photoreceptors
- Photoreceptors: Light-sensitive pigment proteins that absorb specific wavelengths of light.
Blue Light Photoreceptors
- Blue Light: Controls phototropism, stomatal opening, and hypocotyl elongation.
Phytochromes and Seed Germination
- Red Light: Stimulates germination in many seeds.
- Far Red Light: Inhibits germination.
Phytochrome Toggle Switch
- Pr (inactive form): Absorbs red light (660 nm).
- Pfr (active form): Absorbs far red light (730 nm).
- Interconversion: Pr and Pfr are interconvertible.
- Red Light: Converts Pr to Pfr.
- Far Red Light: Converts Pfr to Pr.
Shade Avoidance
- Shade Trees: Allow more far-red light to pass through, increasing the Pr/Pfr ratio in plants underneath the shade tree.
- Shade Avoidance Response: Plants growing under shade trees grow taller, producing longer petioles and thinner leaves to reach more sunlight.
Circadian Rhythm
- Circadian Rhythm: Internally regulated, daily fluctuations in plant processes, such as opening and closing of stomata, leaf movement, and flower opening.
Photoperiodism
- Photoperiodism: The response of plants to the relative lengths of day and night.
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Controls:
- Flowering: Induction of flowering.
- Tuber Formation: Formation of underground storage organs.
- Leaf Abscission: Shedding of leaves.
- Dormancy: Entering a dormant state during winter.
- Environmental Stimulus: Day length (night length) is the primary environmental cue.
Photoperiodism: Plant Types
- Short-Day Plants: Flower when the night length is longer than a critical period (e.g., poinsettia).
- Long-Day Plants: Flower when the night length is shorter than a critical period (e.g., lettuce).
- Critical Night Length: The specific length of darkness that determines whether a plant will flower or not.
Gravitropism
- Gravitropism: The growth response of plants in response to gravity.
- Roots: Grow downwards (positive gravitropism).
- Shoots: Grow upwards (negative gravitropism).
Statioliths
- Statioliths: Dense starch-filled plastids found in root cap cells.
- Gravity Sensing: They settle to the bottom of cells, providing a signal for gravity sensing.
Thigmomorphogenesis
- Thigmomorphogenesis: The changes in plant growth and development in response to mechanical stimuli (touch or wind).
Thigmotropism
- Thigmotropism: Directional growth response to touch (e.g., vine tendrils wrapping around a support).
Plant Responses to Water Deficit
- Stomata Closure: Reduces transpiration.
- Leaf Rolling: Reduces surface area exposed to the sun, minimizing water loss.
- Root Growth: Increases root growth to access deeper sources of water.
- Hormonal Changes: Production of ABA stimulates stomatal closure and root growth.
Overwatering Damage
- Overwatering: Reduces oxygen availability in the soil, leading to root suffocation and death.
Salt Damage
- Osmotic Stress: High salt concentration draws water out of plant cells, leading to dehydration.
- Toxicity: High levels of salt ions can be toxic to plant cells.
Heat Stress
- Heat Harm: High temperatures damage proteins and enzymes, leading to cellular dysfunction.
- Heat Shock Proteins: Protect proteins from damage caused by high temperatures.
Cold Stress
- Cold Harm: Low temperatures damage membranes and can lead to freezing damage.
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Coping Mechanisms:
- Increased Levels of Unsaturated Fatty Acids: Increase membrane fluidity, improving resistance to cold.
- Production of Antifreezes: Protect cells from freezing.
Herbivore Defense Mechanisms
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Physical Defenses:
- Thorns: Sharp, pointed structures (e.g., rose).
- Spines: Modified leaves with sharp points (e.g., cactus).
- Trichomes: Hair-like structures that can irritate or deter herbivores.
- Tough Leaves: Difficult to chew or digest.
-
Chemical Defenses:
- Toxic Compounds: Poisons that deter herbivores.
- Digestive Inhibitors: Interfere with herbivores' digestive systems.
- Attractants: Attract predators or parasites of the herbivore.
- Alarm Signals: Chemicals released by damaged plants that warn other plants or attract natural enemies of the herbivore.
Plant Alarm Signals
- Lima Bean Plants: When attacked by spider mites, they release volatile compounds, signaling to neighbors to increase their defenses.
Effector-Triggered Immunity
- Effector-Triggered Immunity: A plant's immune response to specific molecules (effectors) produced by pathogens.
Hypersensitive Response
- Hypersensitive Response: A plant's localized defense response to pathogens, involving programmed cell death of infected cells.
- Mechanism: Confines the infection, preventing its spread.
Salicylic Acid for Defense
- Salicylic Acid: A plant hormone that activates systemic acquired resistance (SAR).
- SAR Activation: Induces a long-lasting, broad-spectrum resistance in the plant to future pathogens, making the plant primed for future infection.
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