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A2 biology 5.4.1

OCR biology - plant responses

Why is it essential that plants can respond to external stimuli? To avoid abiotic stress e.g. phototropism in low light intensity, stomata close in drought. To avoid GRAZING/ predation. To ADAPT to changing conditions
Where does plant growth occur? Limited to meristem (TOTIPOTENT/ MITOSIS) - cell wall restricts division/elongation. Apical (roots/shoots, lengthening, mitosis/elongation), lateral bud (side shoots), lateral (edge of root/shoot, widening), intercalary (between nodes so leaves regrow)
Describe the production + action of plant growth SUBSTANCES Can be produced by most cells in a plant (i.e. not by specialised glands). Can act locally within the cell or can be trasnproted to target cell by simple diffusion, active transport, mass flow in xylem + phloem. Antagonistic + synergistic effects
What is a tropism + types? A directional growth response in which the direction of the response is determined by the direction of the external stimulus, positive = towards, negative = away. Photo (response to light), geo (gravity), thigmo (touch), chemo (chemicals
What is phototropism controlled by? Auxins eg. indole-3-acetic acid. Travel from apex to zone of elongation by diffusion/ MASS FLOW IN PHLOEM. Promotes cell elongation so shoots bend towards light. ALSO phototropin enzymes 1+2 which break down auxin
How does auxin stimulate cell elongation? Promotes active transport of H+ ions into cell wall by ATPase enzymes in plasma membrane, high [H+] low pH in cell wall, disrupts H bonding and makes optimum pH for expansin enzymes which break bonds, plasticity, cellulose fibres slide, cell expands
Why does a shoot bend if illuminated on one side? Auxin diffuses from apex to shaded side. Phototropin enzymes 1+2 break down auxin, activity is promoted by blue light. high [auxin] = faster rate of elongation on shaded side. shoot bends towards light for photosynthesis
What is geotropism? Response to gravity. Root grow towards pull of gravity - positive tropism - anchors plants in soil, maximises water absorption (photosynthesis, turgidity, cooling), mineral absorption. Shoots = negative tropism
What is thigmotropism + example? Response to touch - climbing plants wind around other structures/ plants for support, attach with hooks, pads, tendrils
What is chemotropism + example? REsponse to chemcials - pollen on stigma attracted to chemicals produced by ovary, pollen tube grows down style to fertilise ovule
Describe the process of leaf loss in deciduous plants Drop in [cytokinin]= senescence (aging, turning brown, dying), leaf no longer sink for phloem transport. Less auxin produced at apex =increased [ethene] +cells in abscission zone more sensitive to ethene. Increases cellulase production, digests cell walls
How do cytokinins prevent senescence? Make leaves a sink for phloem transport to ensure they receive a good supply of nutrients. Therefore, drop in cytokinin concentration causes senescence, which leads to abscission
What is apical dominance? Removal of shoot apex causes side branches to grow from lateral buds that were previously dormant
What is the function of apical dominance? Plants direct most of their sucrose to the shoot apex, plants grow tall and can compete for sunlight
What is the hypothesis for apical dominance + evidence to support it? Auxin inhibits lateral bud growth. Remove apex = lateral buds grow. Remove apex, apply auxin paste = don't grow. Thinmann/ Skoog: auxin inhibitor below apex = lateral buds grew. Plant upside down= lateral buds grow, auxin isn't transported against gravity
What evidence disproves this? Correlation doesn't always mean causation, Gocal: removal of apex of bean plant shoots increased auxin concentration in lateral buds
How do gibberellins control plant stem elongation + evidence? GA1 breaks down DELLA proteins that inhibit transcription factors, increase cell division/ elongation = stem growth. Tall pea plants have dominant Le allele, codes for enzyme that converts precursor -> GA1. Dwarf plants: homozygous recessive for le allele
What are commercial uses of auxin? Prevents PREMATURE ABSCISSION of leaves/fruit, promotes flowering, ends of cuttings =root growth, applied to unpollinated flowers= seedless fruit, herbicide (excessive SHOOT growth of weed, STEM unsupported, collapse)
What are commercial uses of gibberellins? Fruit production (delay senescence to increase shelf life, improve apple shape, elongate grape stalks), applied to germinating barley seeds (amylase, starch ->maltose, seeds are dried, malt ->beer), promotes seed production (selective breeding)...
Continued Sprayed on sugar cane to increase sugar yield - stimulates intermodal growth in intercalary meristem
WHat are commercial uses of cytokinins? Delay leaf senescence e.g. prevent lettuce leaves turning yellow. Stimulates growth of side shoots - increases yield, can be split into lots of small plants
What are commercial uses of ethene? PROMOTES ripening and fruit drop, PROMOTES female sex expression in cucumbers to reduce chance of self pollination which ruins taste, PROMOTES growth of side shoots - increases yield, can be split into lots of small plants
Created by: 11043



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