37.2 Blue Light: the phototropic response

explain the phototrophic growth response in plants

coleoptile

a modified leaf that forms a sheath protecting the emerging shoots of young grasses

phototropism (“light-turn”)

directed movement in response to light

tropism

any directional movement that occurs in response to a directional stimulus

nastic movement

plant movements in which the direction of the response is not dependent on the direction of the stimulus

phototropins

photoreceptors that detect blue light and initiate phototropic responses

responses to a stimulated phototropin

chloroplast movement inside leaf cells to optimize light absorption

opening of stomata so CO2 can diffuse into cells as blue light triggers photosynthesis

auxin

a plant hormone that stimulates phototropism and cell elongation

acid growth hypothesis

the hypothesis that auxin triggers elongation of plant cells by increasing the activity of proton pumps, making the cell wall more acidic and leading to the expansion of the cell wall and an influx of water

two requirements for the cell to get larger

1. the cell wall has to loosen up to allow for an increase in cell size
2. water has to enter the cell and generate turgor pressure on the cell wall to trigger an increase in cell volume

expansins

proteins that are activated when the pH of the cell wall reaches 4.5 and is responsible for breaking the hydrogen bond that forms cellulose microfibrils and other polymers in the cell wall which loosens the structure

how does water enter the cell to increase turgor pressure to elongate

the proton pump pumps out protons, making the inside negatively charged, which favors the entry of potassium ions. The \[solutes\] increases inside and then water follows via osmosis

key point of elongation

pumping protons out brings water into the cell, which increases turgor pressure and pushes out the loosened cell wall.