8. Transport In Plants

• The Xylem & Phloem

  • Plants contain two types of transport vessel:
  • Xylem vessels – transport water and minerals (pronounced: zi-lem) from the roots to the stem and leaves
  • Phloem vessels – transport food materials (mainly sucrose and amino acids) made by the plant from photosynthesising leaves to non-photosynthesising regions in the roots and stem (pronounced: flow-em)
  • These vessels are arranged throughout the root, stem and leaves in groups called vascular bundles

  
• Adaptations of Xylem Vessels: Extended

  • Function: transport tissue for water and dissolved mineral ions
  • Adaptations:
  • Cells joined end to end with no cross walls to form a long continuous tube
  • Cells are essentially dead, without cell contents, to allow free passage of water
  • Outer walls are thickened with a substance called lignin, strengthening the tubes, which helps support the plant

  
• Root Hair Cells

  • Root hairs are single-celled extensions of epidermis cells in the root
  • They grow between soil particles and absorb water and minerals from the soil
  • Water enters the root hair cells by osmosis
  • This happens because soil water has a higher water potential than the cytoplasm of the root hair cell
  • The root hair increases the surface area of the cells significantly
  • This large surface area is important as it increases the rate of the absorption of water by osmosis and mineral ions by active transport
• Structure of the root

  
• Pathway Taken by Water

  1. Osmosis causes water to pass into the root hair cells

  2. Water passes through the root cortex

  3. Water enters the xylem vessels

  4. Water is carried up to the leaves

  5. Water enters the mesophyll cells
• Pathway of water

  • root hair cell
  • root cortex cells
  • xylem
  • leaf mesophyll cells
• Investigating Water Movement in Plants

  1. Place a plant (like celery) into a beaker of water that has had a stain added to it (food colouring)

  2. After a few hours, observe the leaves of the celery turning the same colour as the dyed water

  3. Cut a cross-section of the celery, observe certain areas of the stalk being stained the colour of the water
• Investigating water movement in plants

  • Proves that water is being taken up by the celery
  • Shows that the water is being carried in specific vessels through the stem - the xylem vessels
• Pathway of water into and across a root

  
• Investigating water movement in plants using a stain

  
• Transpiration

  The loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by diffusion of water vapour through the stomata
• Water movement in plants

  1. Water travels up xylem from the roots into the leaves of the plant

  2. To replace the water that has been lost due to transpiration
• Xylem

  • Substance called lignin is deposited in the cell walls which causes the xylem cells to die
  • These cells then become hollow and join end-to-end to form a continuous tube for water and mineral ions to travel through from the roots
  • Lignin strengthens the plant to help it withstand the pressure of the water movement
  • Movement in xylem only takes place in one direction - from roots to leaves (unlike phloem where movement takes place in different directions)
• Functions of transpiration in plants

  • Transporting mineral ions
  • Providing water to keep cells turgid in order to support the structure of the plant
  • Providing water to leaf cells for photosynthesis
  • Keeping the leaves cool
• The conversion of water (liquid) into water vapour (gas) as it leaves the cells and enters the airspace requires heat energy. The using up of heat to convert water into water vapour helps to cool the plant down.
• Water uptake, transport and transpiration

  
• Transpiration in plants

  
• Transpiration rate


  The rate at which water evaporates from the leaves of a plant
• Investigating the effect of temperature and wind speed on transpiration rate

  1. Cut a shoot underwater to prevent air entering the xylem and place in tube

  2. Set up the apparatus as shown in the diagram and make sure it is airtight, using vaseline to seal any gaps

  3. Dry the leaves of the shoot (wet leaves will affect the results)

  4. Remove the capillary tube from the beaker of water to allow a single air bubble to form and place the tube back into the water

  5. Set up the environmental factor you are investigating

  6. Allow the plant to adapt to the new environment for 5 minutes

  7. Record the starting location of the air bubble

  8. Leave for a set period of time

  9. Record the end location of air bubble

  10. Change the wind speed or temperature (only one - whichever factor is being investigated)

  11. Reset the bubble by opening the tap below the reservoir

  12. Repeat the experiment
• Relationship between transpiration rate and environmental factors

  The further the bubble travels in the same time period, the faster transpiration is occurring and vice versa
• Environmental factors investigated


  • Temperature: Temperature of room (cold room and warm room)
  • Wind speed: Use an electric fan to mimic different wind speeds
• As temperature increases

  The rate of transpiration also increases
• As wind speed increases


  The rate of transpiration also increases
• An experimental setup for testing the effect of light intensity on transpiration rates. The apparatus can be modified to test the effects of temperature and wind speed

  
• Water Vapour Loss: Extended

  • Evaporation takes place from the surfaces of spongy mesophyll cells
  • The many interconnecting air spaces between these cells and the stomata create a large surface area
  • This means evaporation can happen rapidly when stomata are open
• Transpiration Stream

  1. Water molecules are attracted to each other by cohesion - creating a continuous column of water up the plant

  2. Water moves through the xylem vessels in a continuous transpiration stream from roots to leaves via the stem

  3. Transpiration produces a tension or 'pull' on the water in the xylem vessels by the leaves

  4. As water molecules are held together by cohesive forces (each individual molecule 'pulls' on the one below it), so water is pulled up through the plant

  5. If the rate of transpiration from the leaves increases, water molecules are pulled up the xylem vessels quicker
• Explaining the Effects of Temperature, Wind Speed & Humidity: Extended

  • Wind speed, humidity and temperature all have an effect on the rate at which transpiration occurs
  • The table below explains how these factors affect the rate of transpiration when they are all high; the opposite effect would be observed if they were low
  • A potometer can be used to investigate the effect of environmental factors on the rate of transpiration

  
• Wilting: Extended

  • If more water evaporates from the leaves of a plant than is available in the soil to move into the root by osmosis, then wilting will occur
  • This is when all the cells of the plant are not full of water, so the strength of the cell walls cannot support the plant and it starts to collapse
• A wilted plant cannot support itself and starts to collapse

  
• Translocation


  The transport of sucrose and amino acids in the phloem, from regions of production to regions of storage or use
• Soluble products of photosynthesis


  • Sugars (mainly sucrose)
  • Amino acids
• Phloem tubes


  • Made of living cells (as opposed to xylem vessels which are made of dead cells)
  • Cells are joined end to end and contain holes in the end cell walls (called sieve plates) which allow easy flow of substances from one cell to the next
• Translocation

  1. Transport in the phloem goes in many different directions depending on the stage of development of the plant or the time of year

  2. Dissolved food is always transported from the source (where it's made) to sink (where it's stored or used)
• During winter, when many plants have no leaves

  The phloem tubes may transport dissolved sucrose and amino acids from the storage organs to other parts of the plant so that respiration can continue
• During a growth period (eg during the spring)


  The storage organs (eg roots) would be the source and the many growing areas of the plant would be the sinks
• After the plant has grown (usually during the summer)

  The leaves are photosynthesizing and producing large quantities of sugars; so they become the source and the roots become the sinks – storing sucrose as starch until it is needed again
• Translocation through the phloem

  
• Comparison between Xylem and Phloem Tissue Table