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Leaf Structure

  • the leaf structure is adapted for photosynthesis and gas exchange 
  • leaves have a thin, flat shape that provides a large surface area
  • the plants have lots of stomata mostly in the lower epidermis 
  • guard cells control the opening and closing of the stomata 
  • the upper epidermis protects against water loss by cuticle or by hairs on the surface
  • the lower epidermis has similar functions to the upper epidermis and contains the stomata 
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  • light - stomata open in light and close in the dark. when stomata are open water moves out of the leaf, which causes an increase in transpiration
  • temperature - increases the kinetic energy which increases the rate of evaporation of water and the rate of transpiration. 
  • humidity - the humidity of the air affects the water potential gradient, if there is a high humidity the gradient is reduced and the rate of transpiration is lower. 
  • air movement -  if there is any air movement the humid layer will be broken and the rate of transpiration will increase again. faster air movement increases the rate of transpiration. 
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  • during photosynthesis plant cells take in carbon dioxide and produce oxygen
  • when photosynthesis is taking place most of the carbon dioxide has to be taken from the air.
  • when photosynthesis is not taking place, oxygen diffuses into the leaf because it is being used by cells.
  • photosynthesis only occurs in the light.
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Cohesion Tension Theory

  • Cohesion - water molecules form hydrogen bonds between one another and stick together. The hydrogen is slightly positive and the oxygen slightly negative. 
  • The water forms a chain which can move in the xylem.
  • As water evaporates from the leaves, more water molecules are drawn up and move by cohesion.
  • The water is pulled up the xylem as a result of transpiration, this is called the transpiration pull.
  • This puts the xylem under pressure, giving the name cohesion tension theory
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  • Xerophytes are plants that do not have a large water supply so have developed to reduce water loss through transpiration. 
  • Thick cuticle - the waxy cuticle forms a waterproof barrier. The thicker the cuticle the less water can escape.
  • Rolling up leaves -  most leaves have their stomata on thier lower epidermis. They roll thier leaves to protect the lower epidermis and trap air within the rolled leaf. This pocket of air reduces the water potential gradient so there is a decrease in transpiration.
  • Hairy leaves - a thick layer of hairs, mainly on the lower epidermis, traps moist air near the surface. This reduces the water potential gradient and so transpiration.
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Standard Deviation

Standard deviation

  • calculate the mean value 
  • subtract the mean from all the measured values 
  • square all the numbers
  • add all the squared numbers together 
  • divide this by original number of measurements 
  • square root the answer 
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