Transport in plants

• Xylem vessels


  Transport water and minerals 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
• Vascular bundles


  • Groups of xylem and phloem vessels arranged throughout the root, stem and leaves
• Plants contain two types of transport vessel: xylem and phloem
• Xylem and phloem vessels are arranged in vascular bundles throughout the root, stem and leaves
• Xylem is always on the inside and phloem is always on the outside in a vascular bundle
• Xylem vessels


  • 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 lignin to strengthen the tubes and support the plant
• Root hair cells


  Single-celled extensions of epidermis cells in the root that grow between soil particles and absorb water and minerals
• Water enters the root hair cells by osmosis because soil water has a higher water potential than the cytoplasm of the root hair cell
• Root hair cells


  • Increase the surface area of the root significantly, which increases the rate of absorption of water by osmosis and mineral ions by active transport
• Pathway Taken by Water

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

  2. Through the root cortex

  3. Into the xylem vessels
• Once the water gets into the xylem, it is carried up to the leaves where it enters mesophyll cells
• Pathway of water into and across a root

  1. Root hair cell

  2. Root cortex cells

  3. Xylem

  4. Leaf mesophyll cells
• Investigating Water Movement in Plants

  1. Place plant (like celery) into beaker of water with stain

  2. After a few hours, leaves turn same colour as dyed water

  3. Cut cross-section of celery, only certain areas of stalk are stained, showing water is carried in xylem vessels
• Xylem


  • Substance called lignin is deposited in the cell walls which causes the xylem cells to die
  • Cells 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
• Transpiration


  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
• Investigating the Effect of Temperature & Wind Speed on Transpiration Rate

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

  2. Dry the leaves of the shoot

  3. 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

  4. Set up the environmental factor you are investigating

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

  6. Record the starting location of the air bubble

  7. Leave for a set period of time

  8. Record the end location of air bubble

  9. Change the wind speed or temperature

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

  11. Repeat the experiment
• Transpiration rate


  The further the bubble travels in the same time period, the faster transpiration is occurring and vice versa
• Transpiration has several functions in plants: transporting mineral ions, providing water to keep cells turgid, providing water to leaf cells for photosynthesis, keeping the leaves cool
• Experiment to investigate the effect of light intensity on transpiration rates

  1. Record distance of air bubble at start

  2. Leave for set period of time

  3. Record end location of air bubble + calculate distance travelled

  4. Reset air bubble using tap of reservoir if necessary

  5. Repeat experiment after changing factor being investigated (e.g. light intensity)
• Experimental setup


  • Can be modified to test the effects of temperature and wind speed
• As temperature increases


  The rate of transpiration also increases
• As wind speed increases


  The rate of transpiration also increases
• When designing an investigation to ensure a fair test, all factors must be kept the same other than the one being investigated
• Evaporation


  Takes place from the surfaces of spongy mesophyll cells
• 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, 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
• Adhesion occurs between the cellulose and water in the xylem vessels
• Cohesion between the water molecules helps create a tension enabling water to be pulled upwards (cohesion-tension theory)
• Factors affecting transpiration rate

  • Wind speed
  • Humidity
  • Temperature
• Wind speed


  More good airflow removes water vapour from the air surrounding the leaf, increasing water loss
• Humidity


  Less transpiration when air is saturated with water vapour as the concentration gradient is weaker
• Temperature


  More transpiration at higher temperatures as water molecules evaporate from mesophyll and diffuse away faster
• A potometer can be used to investigate the effect of environmental factors on the rate of transpiration
• Wilting


  Occurs when more water evaporates from the leaves than is available in the soil to move into the root by osmosis, so the cells are not full of water and the plant collapses
• Translocation


  The transport of sucrose and amino acids in the phloem, from regions of production to regions of storage or use
• Substances transported in the phloem

  • Sucrose
  • Amino acids
• Direction of phloem transport

  1. Varies depending on stage of plant development and time of year

  2. Dissolved food is always transported from the source (where it's made) to sink (where it's stored or used)
• Examples of phloem transport direction

  • During winter, from storage organs to other parts for respiration
  • During growth period, from storage organs to growing areas
  • During summer, from photosynthesizing leaves to roots for storage
• Phloem tubes are made of living cells joined end to end with holes in the end cell walls (sieve plates) to allow easy flow