OCR biology - plant responses
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| 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
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| 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)
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| 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
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| 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
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| 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
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| 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
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| 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
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| 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
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| What is thigmotropism + example? | Response to touch - climbing plants wind around other structures/ plants for support, attach with hooks, pads, tendrils
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| What is chemotropism + example? | REsponse to chemcials - pollen on stigma attracted to chemicals produced by ovary, pollen tube grows down style to fertilise ovule
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| 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
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| 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
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| What is apical dominance? | Removal of shoot apex causes side branches to grow from lateral buds that were previously dormant
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| 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
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| 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
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| What evidence disproves this? | Correlation doesn't always mean causation, Gocal: removal of apex of bean plant shoots increased auxin concentration in lateral buds
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| 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
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| 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)
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| 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)...
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| Continued | Sprayed on sugar cane to increase sugar yield - stimulates intermodal growth in intercalary meristem
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| 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
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| 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
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