C3.1 INTEGRATION OF BODY SYSTEMS (PLANTS)

what is phototropism

• phototropism is the plants growth response to the light direction which helps the plant optimise their position for maximum light exposure

  • positive phototropism:

    • growth towards light (seen in shoots) like da leaves

  • negative phototropism:

    • growth away from light (seen in roots)

what are phytohormones

• phytohormones are plant hormones which are involved in chemical signalling in plants for growth, development, response and reproduction

• many phytohormones exist

  • focus on auxin, cytokin and ethylene

what is the role of auxin in positive phototropism

• positive phototropism is the growth of a plants part toward a light source (commonly observed in shoots and stems)

  • this behaviour ensures that the leaf receives maximum light for photosynthesis

• growth towards the light source is due to the unequal distribution of auxin

• auxin is produced in the shoot apex and is then transferred out of cells by auxin efflux carriers via active transport

• auxin enters cells through auxin influx carriers by facilitate diffusion which is a passive process

• in cases of when the light source is not directly above the plant but on the side, auxin efflux carriers can be positioned in the cell membrane on one side of the cell

  • this redistributes auxin away from the side with lateral light, creating a concentration gradient across the tissues

explain how auxin promotes cell elongation

• auxin causes cell elongation on the shaded side of the stem through the activation of hydrogen ion transport proteins or proton pumps

• the proton pumps secrete hydrogen ions into the apoplast (region of cell wall between cells), causing cellulose fibers to loosen as the pH decreases

  • this is due to the breakage of cross links between individual cellulose fibers aided by the activation of expansin proteins

  • this process allows for cell to fill with fluid and increase the turgor, expanding the cell causing a general growth and positive phototropism

briefly outline the role of cytokinin in phototropism

• influences root growth (idk how)

• promotes cell division

• stimulate lateral bud growth

• delay aging

• regulate nutrient allocation

  • idk if we have to know these but oh well

outline the differences between auxin and cytokinin

where hormone is synthesised:

• root tips produce cytokinin

• shoot tips produce auxin

movement in plant

• cytokinin is transported from the roots to the shoots

• auxin is transported from the shoots to the roots

ratio of phytohormone in plant

• high cytokinin : low auxin = increase root growth

• low cytokinin : high auxin = increase shoot growth

• equal levels of cytokinin and auxin = proportional growth in root and shoot

briefly outline the role of ethylene in fruit ripening

• ethylene / ethene is a gas phytohormone which causes fruit ripening through positive feedback

  • ethylene causes ripening which increases ethylene → positive feedback causes rapid ripening due to exponential increase in ethylene

• positive feedback as a result of ripening fruits cause synchronised ripening as the phoytohormone, ethylene is a gas which can affect nearby fruits

how does the ripening of the fruit caused by ethylene affect the taste, colour and texture of the fruit

• ethylene causes ripening of fruit which…

  • TASTE: increases the hydrolysis of starch into mono and disaccharides resulting in a sweeter taste

  • COLOUR: decreases chlorophyll activity which reveals other pigments, leading to a change in colour

  • TEXTURE: increases enzyme production to break down pectin, which is a substance holding the cell walls together → leads to a softer fruit