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Summary Homeostasis in plants
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Biology A level revision notes on homeostasis in plants
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Homeostasis in plantsStomata are the holes between the guard cells and control the entry of carbon
dioxide into leaves.
They show daily rhythms of opening and closing and even when they are kept in
constant light or dark the rhythms persist. Opening during the day maintains the
inward diffusion of carbon dioxide and the outward diffusion of oxygen and water
vapour (from transpiration). The closing of stomata at right reduces the rate of
transpiration and conserves water.
Stomata open Stomata close
Increasing light intensity Darkness
Blue light Low humidity
Low carbon dioxide concentrations in High carbon dioxide concentrations in
the air spaces within a leaf the air spaces within a leaf
High temperature
Water stress, when the supply of water
from the roots is limited or if there are
high rates of transpiration
The disadvantage of closing is that during daylight, the supply of carbon dioxide
decreases so the rate of photosynthesis decreases. The advantage is that water is
retained inside the leaf, which is important in times of water stress.
Opening and closing of stomata
Each stomatal pore is surrounded by two guard cells that open when they gain water
to become turgid and close when they lose water and become flaccid.
Guard cells gain and lose water by osmosis and a decrease in water potential is
needed before water can enter the cells.
ATP-powered protein pumps in the membrane actively transport hydrogen
ions out of the guard cells
The decrease in hydrogen ion concentration inside the cells causes channel
proteins in the cell surface membrane to open so that potassium ions move
into the cell. This happens because the removal of hydrogen ions leaves the
cell negatively charged compared with the outside, and as potassium ions are
positively charged, they are drawn down the electrochemical gradient
towards the negatively charged region
The potassium ions inside the guard cell lowers the solute potential and
therefore the water potential so water moves in by osmosis through the
aquaporins in the membrane
This increases the turgor pressure of the guard cells and the stomata open
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