nutrition and transport in plants

describe transpiration pull

water moves from xylem vessels into mesophyll cells by osmosis —> water continuously moves out of the mesophyll cells to form a thin film of moisture over their surfaces —> water evaporates from this thin film of moisture and water vapour moves into the intercellular air space —> water vapour diffuses through the stomata to the drier air outside the leaf, from a region of higher WV concentration to a region of lower WV. as water leaves the mesophyll cells, the water potential of the cell sap decreases. water molecules move from the cells deeper inside the leaf to the outer mesophyll cells by osmosis. these cells in turn remove water from the xylem, resulting in a suction force which pulls the whole column of water up the xylem vessel.

how does temperature of the air affect transpiration

rate of transpiration increases as temperature increases

increase temperature = increase rate of evaporation of water from thin film of mosiure surrounding mesophyll cells = increases WV concentration within the leaves, creating a steep WV concentration gradient between inside of the leaf and the environment = increases rate of diffusion of WV from intercellular air spaces through stomata

how does humidity affect rate of transpiration

rate of transpiration increases as humidity decreases

air inside the leaf is saturated with WV. when atmospheric humidity increases, the WV concentration gradient between the inside and outside of the leaf is less steep

WV diffuses out of leaf slower, leading to a lower rate of transpiration

how does wind speed affect transpiration

rate of transpiration increases was wind speed increases

wind blows away WV at the surface of the leaves outside stomata = decreases WV concentration outside leaf = WV concentration gradient between inside of leaf and environment is steeper = increased rate of diffusion of WV out of leaf

light intensity affect rate of transpiration

rate of transpiration increases as light intensity increases

light intensity affects guard cells and stomata

light intensity increase = guard cells become turgid and stomata are wider

more WV can diffuse out of the leave, so transpiration rate increases