3.3 Plant Transport

• Smaller organisms have a:

  • large surface area : volume ratio
  • short diffusion distant
  • lower levels of activity therefore smaller metabolic demand
• Larger organisms have:

  • larger diffusion and transport distances
  • smaller surface area : volume ratio
  • increasing levels of metabolic activity
  • therefore they require a specialised mass transport system
• Adaptations:

  • branching body shape
  • flat and thin leaves
  • root hairs in the roots
  • specialised mass transport system which enables efficient transport of nutrients and waste
• Mass transport systems help to:

  • bring substances quickly from one exchange site to another
  • maintains the diffusion gradients at exchange sites + between cells and their fluid surroundings
  • also ensures effective cell activity by keeping the immediate fluid environment within a suitable metabolic range
• Function of the xylem is to carry dissolved minerals + water up the plant, provides structural support and food storage
• Xylem tissue is found:

  vascular bundle found in the centre of the roots and xylem is found in the centre core

  in the stems the vascular bundle is located around the outside and xylem tissue is found on the inside

  in the leaves the vascular bundle is spread from the centres and the xylem tissue is found on the upper side closest to the upper epidermis
• Function of the phloem tissue is to transport organic compounds like sucrose from the source (leaf) to the sink (roots)
• Cells of phloem tissues:

  • sieve tube elements
  • companion cells
  • parenchyma cells for storage and strengthening fibres
• Cells of the xylem tissue:

  • Tracheid - long, narrow tapered cells with pits
  • vessel elements - large with thickened cell walls + no end plates
  • xylem parenchyma cells
  • sclerenchyma cells
• Structure + Function of xylem vessel elements

  • lignifies cell walls - adds strength to withstand the hydrostatic pressure so the vessels do not collapse
  • no end plates - allows the mass flow of water + dissolved solutes as cohesive and adhesive forces are no impeded
  • no protoplasm - doesn't impede the mass flow of water + dissolved solutes
  • pits in wall - lateral movement of water which allows continual flow in case of air bubbles
  • small diameter of vessels - helps prevent the water column from breaking + assists with capillary action
• Structure + Function of sieve tube elements

  • line up end to end to form a continuous tube
  • sieve plates with sieve pores - allows for the continuous movement of organic compounds
  • cellulose cell wall - strengthens the wall to withstand the hydrostatic pressures that move the assimilates
  • no nucleus, vacuole or ribosomes - maximises the space for the translocation for the assimilates
  • thin cytoplasm - reduces friction to facilitate the movement of the assimilates
• Structure + Function of companion cells

  • each sieve tube element is accompanied with a companion cells which controls the metabolism
  • they play a part in loading + unloading of sugars into phloem
  • do have organelles - provide metabolic support to sieve tubes + help with the loading and unloading
  • transport proteins - moves assimilates in and out of the sieve tube elements
  • large no of mitochondria - provide ATP for the active transport of assimilates in and out of companion cells
  • plasmodesmata - links the sieve tube elements together
• Advantages of transpiration:

  • means of cooling via evaporation (thermoregulation)
  • uptake of mineral ions
  • provides turgor pressure of the cells which provides support to leaves and stems
• Transpiration stream refers to the movement of water from the roots to the leaves. It is the gradient in the water potential which is the driving force permitting the movement of water from the soil (high water potential) to the surrounding atmosphere (low water potential)
• Factors affecting rate of transpiration

  • Wind
  • Temperature
  • Light intensity
  • Humidity
• Effect of wind on transpiration rate

  1. Lower concentration of water molecules in air outside of leaves

  2. Less wind means water molecules can accumulate near the leaf surface

  3. Increases humidity in that area

  4. Reduces concentration gradient

  5. Decreases rate of transpiration
• Effect of temperature on transpiration rate

  1. Increase in temperature

  2. Increase in kinetic energy

  3. Increases rate of transpiration as water molecules move out of the leaf at a faster rate
• Effect of light intensity on transpiration rate

  1. Low light levels mean stomata are closed

  2. No transpiration

  3. Increase in light opens up stomata

  4. Rate of transpiration increases
• Effect of humidity on transpiration rate

  1. High humidity means increase in concentration of water outside

  2. Reduces concentration gradient between inside the leaf and the air outside

  3. Decreases rate of transpiration
• Apoplastic pathway 


  majority of water travels through this pathway which is where the water travels through series of spaces running through the cellulose cell walls and intercellular spaces vis diffusion. when the water reaches the endodermis it hits the Casparian strip which contains suberin which is impermeable so it forms a barrier. this forces water to take the symplastic pathway
• Symplastic Pathway 


  less water travels through this pathway. this is where water travels through the cytoplasm + plasmodesmata of cells via osmosis
• Cohesion-tension Theory 


  this is the main factor responsible for the movement of water up the xylem from the roots to the leaves. transpiration pull puts the xylem under tension (negative pressure within the xylem) and because of the cohesive nature of water there is a continuous stream of water being pulled across the mesophyll cells and up the xylem
• Movement of water through leaves

  1. water moves up the xylem vessels to replace the water lost from the leaf

  2. water leaves xylem vessel through non-lignified areas called pits either through the symplastic or apoplastic pathway

  3. water moves through the mesophyll cell wall (apoplastic) or out of the mesophyll cytoplasm (symplastic) into the cell wall

  4. water evaporates from a mesophyll cell wall into the air spaces creating a transpiration pull

  5. water vapour diffuses from air spaces through a stoma by a process called transpiration which lowers the water potential
• Translocation

  • transport of assimilates from source to sink
  • source of assimilates could be the leaves and stem, storage organs or food stores
  • the sink could be meristems, roots and any part of storage in the plant
  • loading and unloading of the sucrose from the source to the phloem and phloem to the sink is an active process and requires ATP
• Carbohydrates are transported in plants in form of sucrose as it is a disaccharide and so contains more energy. this allows for efficient energy transfer + increases energy storage. it is also less reactive than glucose as its a non-reducing sugar and therefore no intermediate reactions occur as it is being transported
• Active loading of assimilates

  1. Hydrogen Ions (protons) are actively pumped using ATP out of the companion cells into the surrounding area, increasing the proton concentration outside the cell

  2. This creates a proton concentration gradient, so the protons will want to mov back into the companion cells down the concentration gradient via the co-transporters

  3. Co-transporters only work when protons and sucrose are transported together. This increases the sucrose concentration in the companion cells. Co-transporters use facilitated diffusion

  4. The sucrose the diffuses through the plasmodesmata from the companion cells into the sieve tube down the concentration gradient

  5. Diffusing sucrose molecules into the sieve tube lowers the water potential in the sieve tube elements

  6. This causes water molecules from the xylem to move in via osmosis

  7. This generates turgor pressure, which causes the mass flow of assimilates along the phloem to areas of lower pressure (sinks)
• Unloading of assimilates

  1. sucrose is unloaded at any point where it is needed and so there will be a diffusion gradient
  2. sucrose will diffuse from the sieve tube into the companion cells then into other cells by diffusion
  3. sucrose is constantly being used so concentration gradient is always maintained
  4. the loss of sucrose from the sieve tubes raises the water potential so water moves out, into the xylem via the pits and the companion cells via osmosis
  5. this reduces the turgor pressure at the sink