transport in plants

• Structure of xylem vessels

  • Long uninterrupted tubes of dead cells with no end walls

  → reduces resistance to flow of water

  • Vessel walls → contain lignin for support and waterproofing

  → spiral lignin thickening pattern allows flexibility

  → have non-lignified pits to allow movement of water and ions from xylem vessels into other cells

  → impermeable except through the pits
• Location of transport tissues

  • Xylem vessels → provide mechanical support to the plant

  → transport water and mineral ions e.g. nitrates

  • Phloem vessels → transport solutes e.g. sucrose

  • In stems and roots there is a layer of cells between the xylem and phloem called the cambium. This is meristem tissue (a source of undifferentiated stem cells for growth and repair of tissues).
• Water transport up the xylem vessels

  1. Water evaporates from leaves through stomata (transpiration)

  2. Water potential of the mesophyll cells lowers so water is drawn out the xylem by osmosis

  3. This creates upward tension on the column of water in the xylem vessels

  4. Water molecules are cohesive due to hydrogen bonding, so are attracted to each other

  5. Water molecules are adhesive to the xylem walls (due to hydrogen bonding) so water can move up by capillary action

  6. The whole chain of water moves up the xylem vessel

  7. More water enters through the root cortex by osmosis
• Cohesion-tension theory of water transport

  Explanation for how water is transported up the xylem vessels
• Water into the xylem

  • Root hair cells have a large surface area to increase the rate of water and mineral ion absorption

  • Water is absorbed into the root hair cell by osmosis down a water potential gradient
• Symplast pathway

  Water moves by osmosis through the cytoplasm and plasmodesmata (gaps in the cell walls which connect the cytoplasm)

  → does not cross cell-surface membranes

  • Water must cross one cell-surface membrane in the root hair cell, so this pathway has more resistance than the apoplast pathway
• Apoplast pathway

  • Water (carrying solutes) diffuses through the absorbent cell walls so does not cross a cell-surface membrane

  • Water moves by mass flow down a hydrostatic pressure gradient

  • Most water takes this route due to less resistance
• When water reaches the impermeable Casparian strip in the endodermis, it must cross a cell-surface membrane and enter the symplast pathway to reach the xylem vessels. This means that the phospholipid bilayer is selective about what can pass through. Charged particles and ions will not reach the xylem unless via a channel protein.
• Water out of the xylem

  • Water enters the leaf mesophyll cells from the xylem by osmosis

  • Travels via the apoplast pathway (through cell walls) through the leaf

  • Water evaporates from the cell walls into the air spaces

  • Water vapour diffuses out of the leaf through the stomata
• To dissect a plant stem, cut a thin transverse (or longitudinal) cross-section.

  Toluidine blue O stain will stain the lignin in the xylem vessel walls a blue-green colour to observe on a microscope slide.
• Transpiration

  • Evaporation of water from a plant, mainly the leaves

  • Cell walls are moist → water evaporates from mesophyll cell walls

  • Stomata open to let in CO2 → water diffuses out down water potential gradient between air spaces and atmosphere

  • Transpiration is a consequence of gas exchange
• Rate of transpiration

  • Lighter = faster rate → stomata open to allow CO2 in for photosynthesis so water vapour can leave

  • Warmer = faster rate → water evaporates more quickly because the molecules have more energy, so the water potential gradient of water between the inside and outside of the leaf is greater

  • Less humid = faster rate → water potential gradient is steeper if there is less water vapour in the surrounding air

  • Windy weather = faster rate → water potential gradient is maintained because water molecules are blown away from the stomata
• Potometer


  Used to estimate rate of transpiration
• Potometer assumes that rate of water uptake is the same as the transpiration rate
• Setting up a potometer


  1. Cut stem underwater

  2. Set up apparatus underwater

  3. Need a continuous column of water in the xylem vessels

  4. Seal everything tightly

  5. Dry the leaves after removing from the water
• Using a potometer

  1. Keep end of tube submerged in water so no additional air bubbles enter

  2. Use the tap to reset the air bubble so it does not reach the stem
• Measuring with a potometer

  • Measure the distance the air bubble travels per unit of time
  • Calculate the volume of water uptake by multiplying the distance by the cross sectional area of the capillary tube
• Estimating surface area of a leaf

  • Might want to measure rate of transpiration per cm2 of leaf area

  • Press the leaf flat onto graph paper with 1cm3 squares and draw around the leaf

  • Count the number of squares that were covered to find the surface area

  → have a consistent method to account for partially covered squares e.g. only count squares that were over half covered

  • Double the area if the surface area of both sides of the leaf is required
• Xerophytes


  Plants that live in dry, windy, warm conditions and need to be adapted to conserve water
• Xerophytic adaptations

  • Reduce the water potential gradient between the inside and outside of the leaf
  • Thick waxy cuticle increases diffusion distance and reduces transpiration
  • Can have spines instead of leaves which reduces surface area : volume ratio e.g. cacti
  • Hairs around stomata to trap water vapour
  • Stomata sunken into pits to trap water vapour
  • Curled leaves to trap water vapour
  • Fewer stomata
  • Ability to store carbon dioxide so the stomata only need to open at night
• Specific examples of xerophytes

  • Marram grass grows on sand dunes and has curled leaves
  • Cacti grow in deserts and have spines
• Hydrophytic adaptations

  • Hydrophytes live in water → need to be adapted to survive in low oxygen conditions

  • Stomata on the upper leaf surface of floating leaves to maximise gas exchange

  • Airs sacs allow leaves to float so they can absorb light and exchange gases

  • Wide flat leaves to increase light absorption for photosynthesis

  • Less structural support in the stems because they are supported by water

  • Flexibility allows movement with currents

  • Specific example → water lilies
• Structure of phloem

  • Sieve tubes → made of living cells called sieve tube elements which have no nucleus and few organelles

  → end walls called sieve plates allow solutes (also called assimilates) to pass through

  • Companion cells → next to sieve tube elements

  → carry out living functions for themselves and neighbouring sieve tube element

  → lots of mitochondria
• Translocation

  • Movement of solutes from source to sink through the phloem

  → requires energy from ATP (active process)

  → source = somewhere the solute is made e.g. sucrose is made at the leaves

  → sink = somewhere the solute is used up or changed e.g. sucrose is converted to starch by enzymes for storage in tubers, meristem in roots and shoots needs sucrose for growth

  → some areas can be both a source and a sink e.g. roots can store carbohydrates and release them when needed depending on the time of year
• Active loading at the source

  • Companion cell uses ATP to actively transport H+ ions from companion cell into surrounding tissue

  • H+ ions diffuse back into companion cell down a concentration gradient and bring sucrose by co-transport against the sucrose concentration gradient (using a co-transporter protein)

  • Same process can happen to get sucrose into sieve tubes from companion cells, but that can also happen by passive diffusion through plasmodesmata
• Mass flow hypothesis

  • At source → sucrose actively transported into sieve tube by companion cells

  → water potential of sieve tube is lowered so water is drawn in by osmosis from the xylem

  → hydrostatic pressure is increased

  • At sink → sucrose diffuses from phloem into surrounding tissue to be converted to glucose or starch

  → water potential of sieve tube is raised so water is forced out by osmosis into the xylem

  → hydrostatic pressure is decreased

  • Sucrose moves down hydrostatic pressure gradient from source to sink → this is mass flow
• Sucrose is more suitable than glucose as a transport molecule because it is relatively metabolically inactive so will not be used in respiration during transport.