Plant Responses

• Explain why plants need to respond to their environment and list the different types of response they have with examples of each

  Need to respond to threats to survival to maximise their reproductive success...Physical/chemical protection against herbivoryMoving in response to touchProducing chemicals against pathogenic attackOpening and closing stomata as requiredGrowing appropriately in relation to abiotic factorsGerminating at the appropriate time or in the appropriate conditionsDropping leaves at the appropriate timePutting resources into growth of appropriate regionsFlowering at the appropriate timeRipening fruit appropriately Growing in the appropriate proportionsProduction of photosynthetic pigments appropriately
• Define the term "tropism"

  Directional growth response in which the direction of the response is determined by the direction of an external stimulus.
• Name, and give an example of, 4 different types of tropism.

  Phototropism, geotropism, chemotropism, thigmotropism
• Define the terms "plant hormone", and "target cells or target tissues"

  Chemical signals used to coordinate growth and responses.Those cells or tissues which have receptors specific for the plant hormone in question.
• Outline how plant hormones can be transported around plants and have their effect

  By diffusion or active transport across and between cells and by mass flow in xylem or phloem.Bind to receptors on target cells to trigger response.
• List 5 plant hormones and describe the effects of each of them

  Auxins - Control cell elongation, prevent leaf fall, maintain apical dominance, tropisms, fruit ripeningGibberellins - stem elongation, mobilisation of food stores in seed at germination, pollen tube growth in fertilisationCytokinins - promote cell divisionEthene - causes fruit ripenin, promotes ascissionABA (Abscisic acid) - Maintains dormancy of seeds and buds, stimulates cold protective responses, stimulates stomatal closing
• Draw a table to compare plant hormones and animal hormones.

  Plant: release from a variety of tissues, move by active transport, diffusion or mass flow in xylem/phloem, small molecules only, often act locally to site of production, varied effects particular in interaction with other hormones, decentralised regulation of production, few types of substanceAnimal: release from endocrine glands, move by mass flow in circulatory system, can be large proteins or small molecules, usually act at a distance from site of production, have specific effects, usually centralised control by CNS, many types of substance
• Define the term "meristem".


  Places in plants where growth can occur due to there being groups of immature, unspecialised cells that are still capable of dividing (stem cells)
• Name the 4 types of meristem in plants and describe where they are and their effect on the growth of the plant.

  Apical - tips of roots and shootsLateral bud - give rise to side shootsLateral - in cambium, responsible for roots and shoots getting widerIntercalary - between nodes, cause shoots to get longer
• Describe how auxins, such as IAA, lead to growth

  Auxin produced in apexDiffusion/active transport to zone of elongationAuxin binds to receptors on cell surface membraneTriggers second messenger cascadeActive transport of H+ by ATPase on plasma membrane into cell wallIncreased H+ conc. disrupts H-bonds and low pH optimum for expansins which break bonds within celluloseWalls less rigidExpand as cell takes in water
• Describe how a plant can grow towards light


  Auxin moves laterally across the tip of the shoot away from the light. This stimulates growth on the shady side, so the shoot grows asymmetrically towards the light.
• Describe the evidence showing how a plant grows towards light

  Shoot tip removed - no response - tip must either detect the stimulus or produce the messenger or bothLightproof cover placed over intact tip - no response - light stimulus must be detected by the tip.Mica on the illuminated side of the shoot allows the hormone to pass only down the shaded side where it increases growth and causes bending.Movement of chemical down shaded side is prevented by mica - no responseGelatin block inserted beneath tip- as gelatin allows chemicals to pass through it but not electrical messages, the bending which occurs must be due to a chemical from the tip.
• Design an experiment to investigate one factor affecting phototropism in seedlings/coleoptiles

  Germinate and grow seedlings in unilateral light with different colour filters to see which wavelengths of light trigger phototropic response.
• Design an experiment to investigate one factor affecting geotropism in shoots or roots

  Petri dishes stuck to wall and rotated at intervals as seeds germinate.
• Describe and explain the different geotropic and phototropic responses in shoots and roots.

  Roots - positively geotropic, (negatively phototropic)Shoots - positively phototropic, negatively geotropic
• List 4 physical defences plants have against herbivory

  Thorns, barbs, spikes, spiny leaves, fibrous and inedible tissue, hairy leaves, stings
• Describe 3 chemical defences plants have against herbivory

  Tannins - bitter taste deters herbivores, toxic to insectsAlkaloids - bitter tasting and toxicTerpenoids - toxins or repellents
• Define the terms "pheromone" and "volatile organic compounds (VOCs)"

  Airborne molecules which trigger a response in another organism of the same species (in case of plants can include different parts of same organism and other species)VOCs evaporate from plant producing them, travel through the air and are detected by receptors on different part of the plant/different plant
• Describe an example of plants releasing chemicals in response to herbivory.

  Releasing VOCs that attract a predator or parasitoid which then kills the herbivores.
• Describe how Mimosa pudica folds in response to touch

  When leaf is touched, electrochemical change causes active movement of potassium ions into cells on upper, flexor side of pulvinus while potassium ions are moved out of the lower, extensor side. Water follows the potassium ions by osmosis, so turgor in the top cells increases and in the lower cells decreases. As a result, the leaflet, or whole leaf, bends down.
• Define the terms "senescence", "leaf abscission" and "abscission zone".

  senescence - deterioration of with age - breaking down and reabsorbing useful moleculesleaf abscission - the fall of leavesabscission zone - region at the base of the leaf stalk made of two layers of cells sensitive to ethene.
• Describe the role of plant hormones in leaf loss in deciduous plants.

  Cytokinins inhibit senescence. Auxins inhibit abscission.Drop in cytokinins initiates senescence. Senescence causes reduction in auxin production. Reduction in auxin causes sensitivity of cells in abscission zone to ethene. Ethene initiates cellulase production which digests cell walls of cells in abscission zone.Abscission occurs.
• Draw a flow chart to show the process of germination in seeds including the role of plant hormones.

  Seed absorbs waterProduction of gibberellinsSwitch on genesProduction of enzymes e.g. amylase, proteaseBreak down of food storeATP productionGrow and break through seed coat
• Explain why a plant might close its stomata and describe this process including the role of plant hormones

  Closes stomata to reduce water lossABA binds to receptorSecond messenger systemFacilitated diffusion of Ca2+ in Facilitated diffusion of K+ and Cl- out down ECGIncreased water potential inside guard cellWater moves out by osmosisFlaccid Stomata close
• Define the term "apical dominance".

  The growth and dominance of the main shoot as a result of the suppression of lateral shoots by auxin.
• Describe the evidence supporting a link between auxin and apical dominance, and describe the evidence that suggests that this link is not directly causal.

  Decapitation - growth of lateral budsIAA put on cut end of shoot after decapitation - apical dominance maintainedAuxin transport inhibitor placed below apex - growth of lateral budsDecapitation and measure auxin levels - auxin levels in lateral buds increases
• Describe how gibberellins lead to growth.

  Causes cell elongation by loosening cell walls and cell division by stimulating production of a protein that controls the cell cycle.
• Describe the evidence supporting a link between gibberellins and stem elongation, and describe the evidence that showed that this link is directly causal.

  When applied to dwarf varieties they grow taller. Plants with higher conc. are taller.Shoot with blocked gibberellin earlier in synthesis grafted to plant that can't do last stage in synthesis. Shoot grows tall as can do last stage and produce gibberellins.
• Draw a table to compare the action of gibberellins and auxins in the growth of stems.

  Both increase stem length, increase cell length and are acidic.Auxins have no effect on cell division, make cell walls less rigid and stimulate apical meristemsGibberellins increase cell division, loosen cell walls and stimulate intercalary meristems.
• Describe the evidence supporting a link between gibberellins and seed germination.

  Mutant varieties unable to produce gibberellins do not germinate. If gibberellins then applied, germinate normally.Similarly when inhibitors added/removed.
• Describe how to investigate the effect of plant hormones on growth.

  Change concentrations - serial dilutionUse hydroponics - grow plants in water with added minerals etc. as soil hard to standardise.
• Describe the role of plant hormones in controlling the ripening of fruits.

  Peak of ethene production triggers a series of chemical reactions including greatly increased respiration rate. Ethene from ripe fruits will stimulate ripening of nearby fruit.
• Define the term "climacteric fruit".

  Fruits that continue to ripen after they have been harvested.
• Explain the benefits of using plant hormones to control ripening of fruit for sale in shops.

  Can be transported when hard - less easily damaged.Exposed to ethene in controlled conditions so all at same stage.Decreases waste.
• State the plant hormone in rooting powders and explain why rooting powders are useful.

  AuxinIncreases likelihood of successful cuttings, and large scale micropropagation.
• Describe an example of how a hormonal weed killer works and explain why it is useful in agriculture.

  Synthetic dicot auxins used. Absorbed by broad leaved weeds, increasing growth rate to unsustainable level so die. Does not affect narrow leaved (monocot) crops which are then free from competition.
• List 7 other commercial uses for named plant hormones.

  Auxins - production of seedless fruitEthene - promote fruit droppingCytokinikns - prevent ageing of ripened fruit, control tissue development in micropropagationGibberellins - delay ripening and ageing in fruit, improve the size and shape of fruit, speed up malting in beer brewing