BY2 - Transport in Plants

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  • Transport in Plants
    • Transpiration
      • DEFINITION: The evaporation of eater from the inside of the leaves and into the atmosphere.
      • Evaporation of water molecules creates the transpiration stream - water is lost from the leaves which causes more water molecules to be 'pulled' up the xylem from the roots.
        • All plants have to balance water loss and water uptake.
          • If a plant loses more water that it absorbs then it will wilt.
            • If a plant loses an excess of water it cannot retain its turgor and cells become flaccid, causing the plant tot die.
          • As long as there is a concentration gradient between the inside of the leaf and the atmosphere water vapour will diffuse out.
      • Factors Affecting the Rate of Transpiration
        • The rate at which water is lost is called the transpiration rate.
          • It is dependent upon environmental factors.
            • Any factor that causes an increase in the concentration gradient between the inside of the leaf and the atmosphere will increase the rate of transpiration.
        • Higher Temperatures
          • Increases the rate of transpiration.
            • Water molecules have increased kinetic energy so more water molecules evaporate from the air space, through the stomata and into the surrounding atmosphere.
        • Humidity (Percentage of water vapour in the air)
          • High Humidity
            • Large percentages of water vapour in the air.
            • Causes the rate of transpiration to decrease.
          • Low Humidity
            • Little to no water vapour in the air.
            • Causes the rate of transpiration to increase due to a large concentration gradient between the inside and outside of the leaves.
        • Air Movement
          • Increased air movement increases the rate of transpiration.
            • Water vapour is carried away from the diffusion shell and this increases the diffusion concentration gradient.
        • Light Intensity
          • Increasing light intensity causes the rate of transpiration to increase.
            • This is because it results in stomatal opening.
      • Potometer
        • Measures the loss of water vapour (or the evaporation of water) from the leaves.
        • Set Up
          • 1. Shoot cut is placed under the water to prevent air entering.
            • Avoid wetting the leaves because the rate of transpiration will be reduced due to the concentration gradient changing.
            • 2. Place shoot into the bung and seal with Vaseline to prevent air entering.
              • 3. Allow time for the apparatus to settle and adapt to the new conditions.
                • 4. Ensure the bubble is set at an appropriate position on the scale.
                  • 5. Record the movements of the bubble per unit of time (minute/hour/day etc.) and this will give and approximation of the transpiration rate.
                    • When explaining the method, remember to state how you would alter the environmental conditions.
                      • E.G. Number of Fans
                      • E.G. Number of Lamps
                      • E.G. Temperature (number of heaters).
        • Measures the distance travelled by the bubble over a set period of time.
        • Control Variable = Removing shoot leaves.
    • Adaptation of Different Plants to Differing Water Availability
      • Plants can be classified depending on water availability.
        • Xerophytes
          • Plants that live in conditions where water is scarce.
            • Low water availability.
            • E.G. Marram Grass (grow in sand dunes).
          • Curled Leaf
            • Stomata are inside the leaf.
            • This decreases surface area so less water is lost.
          • Thick Waxy Cuticle
            • Reduces water loss.
          • Sunken Stomata
            • Found deep inside the leaf.
            • This reduces the concentration gradient between the leaf and the atmosphere (a less significant difference).
          • Hairs
            • Traps water vapour.
            • Reduces concentration gradient between the inside and outside of the leaf.
          • Roots
            • Really large roots to absorb water.
        • Hydrophytes
          • Plants that live in water.
            • They grow submerged or partially submerged in water.
            • E.G. Water Lily
          • Large Air Spaces
            • Provides buoyancy.
          • Stomata
            • On the upper surface of the leaf to allow gas exchange to take place.
          • Thin Cuticle
            • Little water vapour is lost.
          • Little Support Tissue
            • Not needed as the plant is supported by the water.
        • Mesophytes
          • Plants that live where there is an adequate supply of water.
    • Translocation
      • Takes place in the phloem.
        • Phloem Structure
          • Consists of several types of cells.
            • Main ones are Sieve Tube Cells and Companion Cells.
              • Sieve Tube cells are stacked on top of one another.
                • End walls of these cells contain pores known as sieve plates.
                • These cells do not contain a nucleus.
                  • Organelles have disintegrated.
              • Each Sieve Tube cell is linked to one or more companion cells by the plasmodesmata.
              • Companion cells contain dense cytoplasm, nucleus and mitochondria.
      • DEFINITION: It is the transport of soluble organic material (e.g. sucrose or amino acids).
      • The SOURCE is the site of photosynthesis and the synthesis or organic materials.
        • The SOURCE will always be the leaf.
        • Sucrose / Amino Acids will be transported away from the source and they will be transported to all other parts of the plant where they will be used for storage and growth.
          • These areas are known as SINKS.
      • Theories of Translocation
        • Main theory is the Mass Flow theory.
          • This suggests that there is a passive flow of sugars from the phloem to the leaf, where it is of high concentrations to the growing regions.
            • This is because the source (leaf) has a higher water potential / higher hydrostatic pressure than in the roots.
              • Consequently the sucrose moves on mass from source to sink.
          • Arguments Against Mass Flow Theory
            • It doesn't explain the existence of sieve plates.
            • Rate of transport in the phloem is 10,000 times faster than by diffusion.
            • Sucrose and amino acids flow at different rates and in different directions - this is not explained by mass flow theory.
            • Companion cells contain mitochondria and produce energy - the role of these cells is not explained in the theory.
        • Recent Theories
          • An active process is involved.
          • Cytoplasmic streaming is responsible for the bi-directional flow.
          • Solutes travel along different routes.
            • E.G. Protein filaments.
      • Experimental evidence to prove that organic substances are transported in the phloem:
        • 1. Ringing Experiment
          • Bark was removed and the contents of the phloem were examined.
            • This proved the phloem transports sucrose.
        • 2. Aphid Experiment
          • Aphid attaches to phloem - body is cut away leaving the mouthpart and phloem sap is analysed.
            • Proved the sap is sucrose / amino acids.
              • Also proved that the sucrose / amino acids are transported at a rapid rate - too rapid to be explained by diffusion.
        • 3. Radioisotopes - C14
          • C14 is supplied to the leaf. C14 is a fixed part of glucose which is converted into sucrose in the leaves and is translocated from the leaves to the sink (e.g. roots).
            • This proved that sucrose is transported bi-directionally to the sinks.

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