Lesson 4: Plant systems Flashcards
What are tropic movements in plants?
Directional growth responses to external stimuli, involving differential growth — one side grows faster than the other.
What are auxins and their functions?
Control cell elongation, tropisms, and enhance apical dominance.
What do cytokinins do?
Promote cell division and meristem differentiation, delay ageing.
Role of gibberellins?
fruit/flower maturation and seed germination.
Function of abscisic acid?
Induces seed dormancy, prevents growth in wrong season.
Role of ethylene?
Stimulates fruit ripening.
What are the 5 plant hormones?
Auxins
Ethylene
Gibberellins
Abscisic acid
Cytokinins
How does auxin move between cells?
Enters cells via diffusion or influx carriers (passive)
Dissociates into IAA⁻, which can’t leave freely
Efflux carriers pump IAA⁻ out using ATP
Creates a high concentration gradient in the intercellular space.
Auxin diffuses into adjacent cell
How do auxins promote cell elongation?
Bind to receptors on the plasma membrane
Activate proton pumps → pump H⁺ into cell wall → lowers pH
Activates enzymes that loosen cell wall
K⁺ channels open → water enters by osmosis
Cell stretches → elongates
What is apical dominance?
Growth at the main shoot tip suppresses lateral buds.
How do auxins and cytokinins interact?
Auxins: made in stem meristems, move down
Cytokinins: made in root meristems, move up
Auxins inhibit lateral buds, cytokinins stimulate them
What determines root vs shoot formation?
High auxin:cytokinin → root formation
High cytokinin:auxin → shoot formation
How does ethylene affect fruit?
Activates ripening enzymes
Works via positive feedback: ethylene stimulates its own production
Unripe fruit = low ethylene → ripening triggers ethylene increase
What changes does ethylene cause in ripening fruit?
Sweetens taste (starch → sugars)
Color changes (chlorophyll breaks down, other pigments form)
