Transport Process in Plants

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Chapter 31

• Transport Process in Plants

2
The Movement of Water and Minerals

• Transpiration
• More than 90 percent of the water entering a
  plants roots is released into the air as water
  vapor.
• This loss of water vapor from the plant body is
  known as transpiration.

3
The Cohesion-Tension Theory

• Transpiration of a water molecule results in a
  negative pressure in the leaf cells, inducing the
  entrance from the vascular tissue of another
  water molecule, which, because of the cohesive
  property of water, pulls with it a chain of water
  molecules extending up from the cells of the root
  tip.
• This process is powered by the energy of the sun,
  not from the plant.

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Factors Influencing Transpiration

• Temperature- the rate of evaporation doubles for
  every 10C increase in temperature.
• Humidity
• The regulatory effect exercised by the opening
  and closing of the stomata.

5
The Mechanism of Stomatal Movements

• The osmotic movement of water is involved in the
  opening and closing of the stomata.
• Each stoma has two surrounding guard cells.
• When the guard cells are turgid, they blow out,
  opening the stoma. When they lose water, they
  relax and the stoma closes.

6
The Uptake of Minerals

• Plant cells require a number of different
  chemical elements, which are found in the earth
  in the form of minerals.
• A mineral is a naturally occurring inorganic
  substance, usually solid, with a definite
  chemical composition.
• Mineral ions are taken up in the water solution
  by the roots and travel through the xylem in the
  transpiration stream .
• Minerals ions are brought into plant cells by
  active transport.

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Mineral Requirement of Plants

• Plants require minerals for a number of
  functions. (the most critical is the regulation
  of water balance)
• Specific elements may be an essential component
  of a critical biological molecule which will not
  function properly in its absence
• Non Specific elements are those that have
  interchangeable roles.
• Some minerals, phosphorus and calcium, are
  required constituents of cell membranes and
  membrane permeability.

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• Other minerals are indispensable components of a
  variety of enzyme systems that catalyze chemical
  reactions in the cell

9
The Movement of Sugars Translocation

• The process by which the product of
  photosynthesis are transported to other tissues
  is known as translocation.

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Evidence for the Phloem

• 300 years ago botanists recognized that water is
  transported in the tracheids and vessels of the
  xylem.
• There was debates about the role of the phloem in
  the movement of sugar.

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The Pressure Flow Hypothesis

• The movement of sugars and other organic solutes
  in translocation follows what is known as a
  source-to-sink pattern.
• The most widely accepted explanation for this is
  the pressure-flow hypothesis.
• The pressure-flow hypothesis states that the
  solutes move in solutions that move because of
  differences in water potential caused by
  concentration gradients of sugar.

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Model of the Pressure-Flow Hypothesis
13
Factors Influencing Plant Nutrition

• Soil Composition
• Soil is composed of rock fragments associated
  with organic material , both living and in
  various stages of decomposition.
• The mineral content of soil depends in part on
  the parent rock from which the soil was formed.
• Mineral contents is more dependant on biological
  factor
• The size of the soil particles are another
  factor. The larger the particles, the faster
  water and minerals drain through the soil.

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• Under natural conditions, the soil and the
  availability of nutrients and water to plant
  roots, are constantly changing.
• This is because plants secrete hydrogen ions,
  which help to degrade rock surfaces and release
  positively charged ions form those surfaces.
• As they decay, plant parts constantly add to the
  humus, changing the content of the soil, its
  texture, and its capacity to hold minerals and
  water.
• In return, plants are dependant upon the mineral
  content of the soil and its holding capacity. As
  theses improve, plants increase in both number
  and size and sometimes change in kind. This
  produces changes in the soil.

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The Role of Symbiosis

• Mycorrhizae- the association between fungi and
  roots. The fungi extract nutrients form the soil
  and make them available to plants.
• Plans have the greatest requirement for nitrogen
  but because they cannot use gaseous nitrogen,
  plants are dependant upon nitrogen containing
  ions, ammonium and nitrate ,from the soil.
• The nitrogen-containing compounds are returned to
  the soil by the death of the plant and are
  reprocessed by soil organisms, taken up by plant
  roots in the form of nitrate dissolved in water
  and reconverted to organic compounds.

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• The lost nitrogen is returned to the soil by
  nitrogen fixation, the process by which
  atmospheric nitrogen is incorporated into organic
  nitrogen-containing compounds.
• The symbiotic bacteria are by far the most
  important in terms of total amounts of nitrogen
  fixed.
• The most common of the nitrogen fixing symbiotic
  bacteria is Rhizobium.

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Recombinant DNA and Nitrogen Fixing

• Genetic mapping of a free-living nitrogen-fixing
  bacterium has shown that the 17 genes known to be
  involved in nitrogen fixation are clustered on
  one portion of the chromosome.
• Experiments have tried to show that nitrogen
  fixing genes can be transferred to the cells of
  plants such as corn, which could then, in effect,
  make their own nitrogen-containing-compounds.
• Such experiments have not worked out yet but show
  great potential in the near future.