Plant transport

• The need for transport systems in multicellular plants

  • large plants have a smaller SA:V
  • short diffusion distance
  • diffusion rate not fast enough to transport sucrose around plant
  • systems allow movement of nutrients to reach all tissues allowing a high metabolic rate
• Adaptations of xylem vessels (1)

  • made from dead cells aligned in a continuous, column
  • tubes are narrow so water column does not break easily and capillary action is effective
  • bordered pits in lignified walls enable water to move laterally between vessels
• Adaptations of xylem vessels (2)

  • lignin deposited in spiral patterns: allows xylem to stretch as plant grows/ stem to bend
  • lignin prevents vessels from collapsing when water is in short supply
  • no organelles so hollow
• Adaptations of phloem vessels

  > sieve tube elements - carry sap up and down and is separated by porous sieve plates

  • contain no nucleus and little cytoplasm leaving space for mass flow
  • thin walls - short diffusion distance of substances

  > companion cells - between sieve tubes

  • contain nucleus, cytoplasm and many mitochondria for ATP production for AT (sucrose loading)
• Transverse section - across

  Longitudinal section - straight lengths
• dicotyledonous plants - flowering plants (2 seed leaves)
• Xylem vessel only carries substance up
• Vascular bundles contain collenchyma for strength and support in the plant.
• Distribution of vascular bundles in the stem

  found near outer edge

  • provides strength and flexibility to withstand bending forces
  • cambium (between X-P) - layer of meristems which divide to produce new xylem and phloem

  A) vascular bundle

  B) xylem

  C) phloem

  
• Distribution of vascular bundles in the roots

  found at centre

  • X shape provides strength to withstand pulling forces
  • around bundle is the endodermis
  • inside endodermis is a layer of meristem cells

  A) phloem

  B) xylem

  
• Distribution of vascular bundles in the leaf

  A) xylem

  B) phloem

  
• Plasmodesmata - gaps in plant cell walls containing cytoplasm that connects two cells
• 3 types of pathways taken by water -

  Apoplast, symplast, vacuolar
• Apoplast pathway:

  • water passes through spaces in cell walls & between cells
  • moves water & dissolved minerals ions by mass flow
• Symplast pathway:

  • water passes through plasma membrane and enters cell cytoplasm
  • it passes through plasmodesmata to another cell
• Vacuolar pathway

  • water is not confined to cytoplasm of cells and is able to pass through vacuole
• Less negative means the water potential is higher

  More negative means the water potential is lower
• In osmosis, water moves down the concentration gradient
• Where is the main site of photosynthesis in plants - palisade mesophyll
• adhesion - attraction between water molecules and walls of the xylem vessel
• cohesion - attraction of between water molecules caused by hydrogen bonds
• Transpiration stream - movement of water from roots, up the xylem and out of the leaf.
• Water uptake in the roots

  1> mineral ions actively absorbed from soil via root hair cells

  2> decreases WP inside cell, water moves down conc. gradient by osmosis

  3> water moves across root cortex down conc. by osmosis to endodermis of vascular bundle

  4> water enters symplast pathway as blocked by casparian strip in the endodermis

  5> ions moved to medulla via AT, decreasing WP causing water to follow by osmosis

  . pressure increases forcing water into xylem pushing water up
• Role of the endodermis

  • ensures water and nitrates pass into cell cytoplasm through plasma membrane
  • membrane contains transporter proteins which actively pump ions from cytoplasm of cortex into medulla and xylem
  • causing water to follow by osmosis
  • water cannot pass back into cortex as apoplast pathway is blocked by the Casparian strip
• Water is moved up the xylem by capillary action
• Mechanism of capillary action in plants

  • water molecules are attracted to each other via cohesion
  • holds them together in a column
  • as water evaporates due to transpiration, chain is pulled up due to low hydrostatic pressure creating tension on xylem walls
  • as vessels are narrow, water molecules are attracted to sides of walls via adhesion, enabling water to move up by mass flow
• Evidence to support the cohesion-tension theory in plants

  > when plant stem is cut air is sucked into xylem

  • creates tension in vessels
  • air prevents cohesion between water molecules
  • so water movement stops

  > tree trunk diameter decreases when transpiration is at its maximum

  transpiration pull generates a negative tension in xylem
• How stomata control rate of transpiration

  • when open, water vapour can diffuse out of the leaf
  • K+ can enter guard cell lowering WP
  • water moves into cells by osmosis
  • cells become turgid
  • enables diffusion of water vapour out of leaf
• How are terrestrial plants adapted to reduce water loss?

  • waxy cuticle layer on upper epidermis on leaves
  • stomata found on under surface of leaves - reduces evaporation from direct heat from sun
  • stomata closed at night - no light for photosynthesis
  • deciduous plants lose leaves when temperatures are too low for photosynthesis
• Examples of xerophytes

  marram grass and cacti
• Xerophytes - adapted to dry conditions

  they have a low WP inside leaf cells by maintaining a high salt conc.
• How does Marram grass reduce water loss

  • leaves rolled longitudinally with stomata inside - traps moist air
  • thick waxy cuticle on upper epidermis - reduces evaporation
  • long roots - extend deep into sand, increasing SA for water absorption
  • spongy mesophyll is dense with few air spaces - decreases SA for evaporation of water
• How does cacti reduce water loss?

  • store water in ribbed stems - become swollen and expand when water is available
  • leaves are spines - reduces SA and water loss by transpiration
  • stem is green for photosynthesis
  • widespread roots - absorbs any rain that falls
• Hydrophytes - plants adapted to living in water
• Adaptations of water lilies:

  • large spaces in leaf- keeps leaves afloat so they are in air and can absorb sunlight
  • stomata on upper epidermis -exposed to air to allow gaseous exchange
  • leaf stem has many large air spaces- buoyancy and allows o2 to diffuse quickly to roots for aerobic respiration
  • have hydathodes at tips of leaves - release water droplets which evaporate from leaf surface
• What happens if a water cannot leave a plant due to high humidity?

  the transpiration stream stops and plant cannot transport mineral ions up to the leaves needed for growth and photosynthesis
• Transpiration - the loss of water vapour from the stomata and is a consequence of gaseous exchange.
• Why is transpiration important?

  • replaces water lost via water vapour at leaves
  • maintains cell turgidity
  • transports useful mineral ions up plant
  • supplies water to keep plant cool on a hot day
• • Water evaporates from the cell walls of the spongy mesophyll
  • water leaves via diffusion out of the leaf through open stomata
  • requires a higher water vapour potential inside the leaf than outside
• Environmental factors that influence transpiration

  1. Light intensity
  2. Water availability
  3. Wind
  4. Relative humidity
  5. Temperature