Leaf Structure

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Leaf Structure Function
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Leaves

• Structure is optimized for
• Absorbing light
• Carrying out photosynthesis

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Leaves
Leaf Shapes Most of a leaf consists of a blade
attached to the stem by a petiole. The blade of a
simple leaf (left) can be different shapes. In a
compound leaf (right), the blade is divided into
many separate leaflets.
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Leaf Functions - Photosynthesis

• Mesophyll spongy palisade site of
  photosynthesis
• Stomata pore-like openings that allow CO2 and
  O2 to diffuse into and out of leaf
• Guard cells each stoma consists of two
  controls opening and closing of stomata as a
  response to water pressure

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Transpiration

• Water evaporates easily from leaf surfaces due to
  diffusion
• Transpiration is the loss of water through a
  plants leaves
• Water must be replaced by water drawn up from
  roots through the xylem vessels

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Gas Exchange

• Plants keep their stomata open just enough to
  allow photosynthesis to take place but not so
  much that they lose an excessive amount of water

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Gas Exchange

• When water pressure in guard cell is high, outer
  walls of cells forced into curved shape. This
  pulls inner walls away from one another, opening
  the stoma.
• Reverse happens when water pressure is low.
• Influenced by wind, temperature, time of day

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Guard Cells
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Stomates
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Guard Cells
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Rate of Photosynthesis
Amount of Water
Night Day Night
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Cellular Respiration
Rate of Photosynthesis
Night Day Night
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Water Transport in Plants

• Water is transported through plants by a
  combination of
• Capillary action
• Root pressure
• Transpiration

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Capillary Action

• Cohesion - water molecules are attracted to each
  other
• Adhesion attraction of unlike molecules
• Capillary action water molecules are attracted
  to walls of tube (xylem) and to each other.
  Thinner the tube the higher the water will rise.

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Capillary Action Capillary actionthe result of
water molecules ability to stick to one another
and to the walls of a tubecontributes to the
movement of water up the cells of xylem tissue.
As shown here, capillary action causes water to
move much higher in a narrow tube than in a wide
tube.
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Transpiration

• Loss of water from leaves pulls water up the
  plant transpirational pull.
• The hotter and drier the air, the windier the
  day, the more water is lost and the more water is
  pulled up from the roots
• If there is not enough water to replace water
  lost, wilting will occur

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Movement of Water Through a Plant.  Root
pressure, capillary action, and transpiration
contribute to the movement of water within a
plant. Transpiration is the movement of water
molecules out of leaves. The faster water
evaporates from a plant, shown in A, the stronger
the pull of water upward from the roots, shown in
B.