subject guide notes

C3.1.17—Observations of tropic responses in seedlings

tropic movements are directional movements of plant in response to stimuli

• plants alter their growth so they can reach optimal conditions

tropic movements can be:

• positive (towards stimuli)

• negative (away from stimuli)

C3.1.18—Positive phototropism as a directional growth response to lateral light in plant shoots

phototropism is one type of tropic movements in plants

• is called positive phototropism, cuz stem is growing towards light

but roots exhibit negative phototropism cuz they grow away from the light

plants also exhibit geotropic movements

• roots typically display positive geotropism - grow downwards

• stems display negative geotropism - grow upwards

C3.1.19—Phytohormones as signalling chemicals controlling growth, development and response to stimuli in plants

phytohormones act as chemical messengers

• are plant hormones that regulate multiple physiological processes

there are 5 major types of phytohormones: auxins, cytokinins, gibberellins, abscisic acid & ethylene

C3.1.20—Auxin efflux carriers as an example of maintaining concentration gradients of phytohormones

auxin causes cell elongation

it is synthesized by the apical meristems (shoot tips) & are produced in coleoptile

tropic movements are caused by uneven distribution of auxin

when light is overhead, auxin produced at tip of shoot, diffuses evenly down the stem

• auxin is distributed evenly = cells grow at same rate = shoot grows vertically upwards

if photoreceptors in coleoptile detect light source from one direction, auxin molecules move towards shaded site of shoot

• increased concentration of auxin at shaded side = rapid cell elongation & growth on that side

• the uneven growth causes stem to bend towards light source

active directional cell-to-cell movement of auxin is a major transport of auxin & involves

• entry of auxin into cell passively or via membrane proteins called auxin influx carriers

• exit of auxin out of cell via membrane proteins called auxin efflux pumps

auxin efflux carriers create high concentration of auxin in intercellular space & low concentration in adjacent cell

• causes auxin to flow down its concentration gradient

C3.1.21—Promotion of cell growth by auxin

*can check kognity for this & slideshow

in order for plant cells to elongate, the cross-links between the cellulose molecules of cell wall hv to broken

C3.1.22—Interactions between auxin and cytokinin as a means of regulating root and shoot growth

combined actions of auxins & cytokinins regulate growth of roots & stems

meristems consist of undifferentiated cells

shoot meristems give rise to parts of plant that’re typically seen above ground

root meristems give rise to parts of plant below ground

auxins - synthesized by stem meristem while cytokinins synthesized by root meristems

• auxin then moves downwards towards root, resulting in geotropism

• cytokinin moves upwards towards the shoot

some studies show that high auxin to cytokinin ratio favours development of roots while a high cytokinin to auxin ration favours shoot & bud development

ratio also determines apical dominance

• as auxin transported down apical meristem, lateral bud formation is inhibited

• upward movement of cytokinins from root stimulates lateral bud formation

auxins lead to meristematic cell division while cytokinins lead to differentiation of cells

*check table for more comparisons

C3.1.23—Positive feedback in fruit ripening and ethylene production

ethylene (IUPAC name, ethene) is important in ripening fruit

• it causes all the changes the signal fruit is ripe

  • such as breaking down cell walls to soften fruit, causes green fruit to change color, breaks down starch into sugars

works on positive feedback mechanism

• its presence leads to synthesis of more ethylene

• as fruit ripens, larger amounts of ethylene are produced, speeding up the ripening process & ensuring ripening process is synchronised

production of ethylene depends on type of fruit