M3 S3: Transports in Plants

• The xylem vessels are made up of dead cells
• Phloem is the food-conducting tissue.
• Xylem is the water-conducting tissue.
• Multicellular Plants Need Transport Systems

  • Plants need substances like water, minerals and sugars to live
  • They also need to get rid of waste substances
  • Plants also need carbohydrates
• Plants are multicellular, so they have a small surface area: volume ratio
• Plants are relatively big with a relatively high metabolic rate
• Exchanging substances by direct diffusion from the outer surface to the cells would be too slow to meet their metabolic needs
• Plants need transport systems


  • To move substances to and from individual cells quickly
• Two Types of Tissue Involved in Transport in Plants

  • Xylem tissue
  • Phloem tissue
• Xylem tissue


  Transports water and mineral ions in solution, moving up the plant from the roots to the leaves
• Phloem tissue


  Mainly transports sugars (also in solution) both up and down the plant
• Xylem and phloem make up a plant's vascular system, found throughout the plant to transport materials to all parts
• Position of xylem and phloem in root, leaf and stem

  • In the root, the xylem is in the centre surrounded by phloem
  • In the stems, the xylem and phloem are near the outside
  • In a leaf, xylem and phloem make up a network of veins
• Dissecting plant stems


  1. Use a scalpel to cut a cross-section of the stem

  2. Use tweezers to place the cut sections in water

  3. Transfer each section to a dish containing a stain (e.g. toluidine blue O)

  4. Rinse the sections in water and mount each one onto a slide
• Xylem Vessels


  • They are very long tube-like structures formed from cells (vessel elements) joined end to end
  • There are no end walls, making an uninterrupted tube
  • The cells are dead, so they contain no cytoplasm
  • Their walls are thickened with a woody substance called lignin
  • The amount of lignin increases as the cell gets older
  • Water and ions move into and out of the vessels through small pits in the walls where there's no lignin
• Phloem Tissue


  • It's purely a transport tissue, not used for support
  • It contains phloem fibres, phloem parenchyma, sieve tube elements and companion cells
  • Sieve tube elements and companion cells are the most important cell types for transport
• Sieve tube elements


  • They are living cells that form the tube for transporting solutes through the plant
  • They are joined end to end to form sieve tubes
  • The 'sieve plates' are the end walls, which have lots of holes to allow solutes to pass through
  • They have no nucleus, a very thin layer of cytoplasm and few organelles
• Companion cells


  • They carry out the living functions for both themselves and their sieve tube elements
  • They provide the energy for the active transport of solutes
• Water enters a plant

  1. Water has to get from the soil, through the root and into the xylem to be transported around the plant

  2. Water enters through root hair cells and then passes through the root cortex, including the endodermis, to reach the xylem
• Osmosis


  Water is drawn into the roots via osmosis. This means it travels down a water potential gradient
• Water potential gradient

  Water always moves from areas of higher water potential to areas of lower water potential
• Soil around roots generally has a high water potential and leaves have a lower water potential because water constantly evaporates from them
• Water potential gradient


  This creates a water potential gradient that keeps water moving through the plant in the right direction, from roots to leaves
• Water moves through the root into the xylem

  1. Symplast pathway - goes through the living parts of cells, the cytoplasm

  2. Apoplast pathway - goes through the non-living parts of cells, the cell walls
• Symplast pathway


  • Water moves through the symplant pathway via osmosis
• Apoplast pathway


  • Water can simply diffuse through the cell walls, as well as pass through the spaces between them. The water can carry solutes and move from areas of high hydrostatic pressure to areas of low hydrostatic pressure
• The Casparian strip in the endodermis cells blocks the apoplast pathway, so the water has to take the symplast pathway
• Both pathways are used, but the main one is the apoplast pathway because it provides the least resistance
• Water transport up the xylem and out at the leaves


  1. Water leaves the xylem and moves into the cells mainly by the apoplast pathway

  2. Water evaporates from the cell walls into the spaces between cells in the leaf

  3. When the stomata open, the water diffuses out of the leaf down the water potential gradient into the surrounding air
• Transpiration


  The loss of water from a plant's surface
• Water moves up a plant against the force of gravity

  1. Cohesion and tension help water move up plants, from roots to leaves, against the force of gravity

  2. Water evaporates from the leaves at the top of the xylem (transpiration), creating a tension (suction), which pulls more water into the leaf

  3. Water molecules are cohesive (they stick together) so when some are pulled into the leaf others follow, moving the whole column of water in the xylem upwards

  4. Adhesion - water molecules are attracted to the walls of the xylem vessels, helping water to rise up through the xylem vessels
• Transpiration


  Evaporation of water from a plant's surface, especially the leaves
• Transpiration is a consequence of


  Gas exchange
• Gas exchange in plants

  1. Plant needs to open stomata to let in carbon dioxide for photosynthesis

  2. This also lets water out due to higher water concentration inside leaf than outside

  3. Transpiration is a side effect of the gas exchange needed for photosynthesis
• Four main factors affecting transpiration rate

  • Light
  • Temperature
  • Humidity
  • Wind
• Light


  The lighter it is, the faster the transpiration rate. Stomata open when it's light to allow CO2 in for photosynthesis
• Temperature


  The higher the temperature, the faster the transpiration rate. Warmer water molecules have more energy so they evaporate faster
• Humidity


  The lower the humidity, the faster the transpiration rate. Increased water potential gradient between leaf and air
• Wind


  The windier it is, the faster the transpiration rate. Increased air movement blows away water molecules from stomata
• Potometer


  Apparatus used to estimate transpiration rate by measuring water uptake by a plant