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LGS A-Level OCR Biology - Unit 5 - Plant and Animal Responses Part 1

Biology25 CardsCreated about 1 month ago

This deck covers key concepts from Unit 5 of the OCR Biology syllabus, focusing on plant and animal responses, including tropisms, plant hormones, and mechanisms of growth and adaptation.

Nastic responses

Plant responses in which the direction of the plant response is independent of the stimulus
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Key Terms

Term
Definition
Nastic responses
Plant responses in which the direction of the plant response is independent of the stimulus
Herbivory
Consumption of plants
Abiotic
Physical
Tropisms
Directional growth responses in plants
Phototropisms
Influenced by light e.g. plants grow towards light to photosynthesise due to auxin moving unilaterally
Geotropism
Influenced by gravity; Plants receive unilateral gravitational stimulus (downwards); Shoots are -vely geotropic and roots are +vely geotropic

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TermDefinition
Nastic responses
Plant responses in which the direction of the plant response is independent of the stimulus
Herbivory
Consumption of plants
Abiotic
Physical
Tropisms
Directional growth responses in plants
Phototropisms
Influenced by light e.g. plants grow towards light to photosynthesise due to auxin moving unilaterally
Geotropism
Influenced by gravity; Plants receive unilateral gravitational stimulus (downwards); Shoots are -vely geotropic and roots are +vely geotropic
Thigmotropism
Influenced by touch; Shoots of climbing plants e.g. ivy winding around other plants or solid structures for support
Chemotropism
Influenced by chemicals; Pollen tubes grow down the style of a plant towards the ovary where fertilisation takes place
Plant hormones
Cytokinins; Abscisic acid (ABA); Auxins; Giberellins; Ethene
Effects of cytokinins
Promote cell division; Delay leaf senescence - increases shelf life; Overcome apical dominance - lateral growth; Promote cell expansion
ABA
Inhibit seed germination and growth; Stimulate cold protective responses; Cause stomatal closure when the plant is stressed by low water availability
Effect of auxins
Promote cell elongation in roots - low conc; Promote shoot growth/ demote root growth - high conc; Inhibit growth of side shoots; Inhibit leaf abscission; Selective weedkiller; Promote cell division in cambium
Effect of gibberellins
Promote seed germination - break bud dormancy (works against ABA); Promote growth of stems - elongation of internodes; Develop seedless fruit and fruit setting; Acts synergistically w/ auxin
Effect of ethene
Promote fruit ripening - starch to sugar and breaks down chlorophyll and cell wall; Stimulates cells in abscission zone to expand and breaks cell wall causing leaf to fall off; Opp to auxin
How do plants avoid herbivores
Tannins - phenolic compounds; toxic to herbivores and microorganisms; Alkaloids - make plants taste bitter; Mimosa leaves fold up in response to touch - scares insects
Abscission in deciduous plants
Decreases production of auxin; More sensitive to ethene; Gene expression of enzymes in abscission zone; Cellulase breaks down cell walls in separation layer of abscission zone; Vascular bundles sealed off, fatty materials for neat, waterproof scar
Mechanism of seed germination
Seed absorbs H2O and activates embryo; Begins to produce gibberellins; Gene expression –> produces amylases and proteases to break down starch food stores; Glucose is used as a respiratory substrate and in protein synthesis
Mechanism of stomatal closure
Levels of soil water falls; Roots produce ABA; Transported and binds to guard cells; Increases pH, charged particles move out; Increases wp, water moves out; Loss of turgor closes stoma
Proof of gibberellins causing seed germination
Mutant varieties that lack gibberellin do not germinate but w/ external gibberellin they do; When gibberellin inhibitors are added to normal seeds they don't grow
Apical Dominance Effect
Auxin produced at the apex, inhibits growth of lateral buds
Experimental evidence for apical dominance
Removal of apical buds allows lateral bud to grow; Auxin/synthetic auxin placed on cut tip continues to inhibit the growth of side shoots; Plant 30 plants of same type, age, genotype and weight in same soil; Remove tip of 10 and apply auxin paste; Remove tip of another 10 and add paste w/out auxin; Leave last 10 as control; Sig. increase of no. of side shoots grown in first 10
Recent research on apical dominance
Auxin stimulates production of ABA (inhibits growth); When apex is removed as is the source of auxin, ABA levels decrease; Most cytokinins go to tip so when tip is removed cytokinins spread evenly around plant promoting growth
Where does growth occur in plants
Apical meristems; Lateral bud meristems
Mechanism of cell elongation by auxin
Tip produces auxins, diffuses down; Promotes active transport of H+ into cell walls; Lowers pH, optimum pH for expansins; Breaks H bonds within cellulose; Reduces rigidity and H2O enters
Confirming auxin as the hormone that causes growth
Impregnated agar blocks w/ diff conc. of auxin; Placed them on cut shoot tips; Same effects as in reg. shoots; Curvature is directly proportional to conc of auxin used