Homeostasis in plants

  • Created by: portia
  • Created on: 29-04-17 16:52
  • plants need to maintain a constant internal environment like animals e.g. mesophyll cells in leaves require a constant supply of carbon dioxide inorder to make best use of light energy for photosynthesis
  • stomata control the entry of carbon dioxide into the leaves
  • strictly speaking, stoma is the hole between the guard cells, but term is usually used to refer to the two guard cells and the hole between them
  • guard cells are highly specialised cells that respond to a wide range of environmental stimuli and thus control the internal atmosphere of the leaf

Stomata show daily rhythms of opening and closing, even when kept in constant light or constant dark.

  • Opening during the day maintains the inward diffusion of CO2 and the outward diffusion of oxygen.
  • It also allows the outward diffusion of water vapour in transpiration

The closure of stomata at night when photosynthesis cannot occur reduces rates of transpiration and conserves water.

Stomata respond to changes in environmental conditions.They open in response to:

  • increasing light intensity
  • low carbon dioxide concentrations in the air spaces within the leaf

When stomata are open, leaves gain CO2 for photosynthesis, but tend to lose much water in transpiration

Stomata close in response to:

  • darkness
  • high CO2 concentrations in the air spaces in the leaf
  • low humidity
  • high temperature
  • water stress, when the supply of water from the roots is limited and/or there are high rates of transpiration

The disadvantage of closing is that during daylight, the supply of CO2 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 closure of stomata

  • each stomatal pore is surrounded by two guard cells
  • guard cells 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
  • a decrease in water potential is needed before water can enter the cells by osmosis
  • this is brought about by the activities of the transporter proteins in their cell surface membranes
  • ATP-powered proton pumps in the membrane actively transport hydrogen ions H+, out of the guard cells
  • the decrease in H+ ion concentration inside the cells cause channel proteins in the cell surface membrane to open so that potassium ions, K+, move…


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