mass transport in plants

• Adaptation of the xylem

  NO CYTOPLASM = reduces resistance to flow of water to ensure a continuous column of water

  
  

  VESSELS ARE STACKED ON TOP OF EACH OTHER = ensures continuous column of water

  
  

  LIGNIN IN CELL WALLS = walls are waterproof/and allows them to withstand tension

  
  

  XYLEM IS DEAD = no water potential created by a living cell

  
  

  NARROW TUBES = increases surface area for adhesion

  
  

  XYLEM HAVE PORES/PITS = enable the sideways flow of water

  
  

  NO END WALLS = reduces resistance to flow of water so continuous column of water
• Explain how xylem tissue is adapted to its function (4 marks)


  - long cells with no end walls so allows water to move in a continuous column

  - no cytoplasm so allows easier water flow

  - thick wall containing lignin so provides support

  - pits in walls so allows lateral movement
• Cohesion-tension theory


  - water evaporates from the leaf through the open stomata (transpiration)

  - this leads to a lower water potential in the mesophyll cells than in the xylem creating a water potential gradient

  - water moves from xylem into mesophyll cells by osmosis which creates tension + pulls water up in the xylem

  - water molecules cohere together by hydrogen bonds to form a continuous column of water

  - adhesion of water molecules to walls of xylem. this causes the xylem to be moved inwards
• Transpiration - light intensity

  - Increased rate of transpiration

  -> more stomata are open for gas exchange for photosynthesis meaning more water evaporates
• Transpiration - humidity

  - Decreased rate of transpiration

  -> air is more saturated so there is a reduced water potential gradient between mesophyll cells and air so rate of diffusion of water vapour out of the leaf is reduced
• Transpiration - Temperature

  - Increased rate of transpiration

  -> water particles have increased kinetic energy meaning that there is an increased rate of evaporation
• Transpiration - Air movement

  - Increased rate of transpiration

  -> air movement moves the water vapour away from the stomatal pores, increasing the water potential gradient for increased evaporation
• Measuring the rate of transpiration with a potometer


  - a leafy shoot is cut underwater

  - potometer is filled completely with water, making sure there are no air bubbles

  - using a rubber tube, the leafy shoot is fitted to the potometer under water

  - potometer is removed from under the water + all joints are sealed with waterproof jelly

  - an air bubble is introduced into the capillary tube

  - distance moved by the air bubble in given time is measured. the rate of water uptake + therefore the rate of transpiration can be calculated

  - to repeat experiment, air bubble must be be pushed back to the start of the scale by opening the tap to allow water from the reservoir to enter the capillary tube

  - experiment can be repeated to compare the rates of water uptake under different conditions
• How to make the potometer experiment reliable:

  - seal joints/ensure airtight

  - cut shoot underwater

  - dry off leaves

  - ensure no air bubbles are present

  - shut tap

  - note where bubble is at start position
• A potometer actually measures water uptake, not water loss by transpiration. Why?


  - not all water lost via transpiration

  - water is used for support/turgidity

  - water used in photosynthesis

  - water produced in respiration
• Using a potometer, a student wanted to calculate the rate of water uptake by the shoot in cm^3 per minute. What measurements did she need to make?


  - distance travelled by bubble (cm^3)

  - time taken for distance travelled (minutes)

  - volume (cm^3)

  - diameter of capillary tube
• What is the function of the phloem?


  transports organic (contains carbon) substances (mainly sucrose) in plants
• Phloem consists of two types of living cells:


  - sieve tube elements have perforated end walls (sieve plates) to allow continuous flow of organic substances = they have no nucleus, very little cytoplasm + few organelles

  
  

  - companion cells (one for each sieve tube element) carry out the living functions for the sieve tube elements eg. provide ATP for active transport of solutes
• Explain how phloem tissue is adapted for its function (3)


  - sieve tubes have no nucleus, little cytoplasm and few organelles so it allows unobstructed flow of solutes

  
  

  - end walls of sieve tubes perforated with holes so it allows continuous flow of substances through sieve tube

  
  

  - companion cells contain many organelles eg. mitochondria so it carries out the functions for the sieve tubes eg. mitochondria make ATP for active transport
• Translocation


  - movement of organic substances (eg. sucrose, amino acids) in the plant from source to sink

  
  

  - sources produce carbohydrates like sucrose during photosynthesis eg. leaves

  
  

  - sinks are areas which need energy such as growing regions (roots) or storage organs (potatoes)

  
  

  - translocation is an energy requiring process = it can move in both directions up + down the phloem
• Mass flow hypothesis


  - at source, sucrose is actively transported by companion cells into the sieve tube elements in the phloem

  - this lowers water potential inside the phloem, causing water to move from the xylem to phloem by osmosis

  - this creates a higher hydrostatic pressure at the source compared to the sink which causes mass flow towards the sink

  - at sink, sucrose is used in respiration or stored as starch, creating a concentration gradient between the sink cells and phloem, so sucrose is removed from phloem into sink cells (facilitated diffusion/active transport)

  - removal of sucrose, increases water potential in phloem so water moves back into xylem via osmosis, creating a lower hydrostatic pressure at sink, forming a pressure gradient
• Suggest what would happen to the rate of translocation if the plant was treated with excessive heat


  rate of translocation would decrease as heat denatures the carrier proteins so no active transport so there is no movement of sucrose/translocation in phloem
• Describe the transport of carbohydrate in plants

  - sucrose is actively transported by companion cells into the sieve tube elements of the phloem

  - which lowers water potential inside phloem causing water to move from xylem by osmosis

  - this creates a higher hydrostatic pressure at the source

  - causing mass flow towards the sink where sucrose is used in respiration
• The highest rate of translocation is midday. Explain your answer (5)


  - light intensity + temperature is at its highest, so there will be the highest rate of photosynthesis

  - more sucrose is produced at source/ leaves

  - meaning more sucrose is actively transported into phloem by companion cells, so more water enters phloem by osmosis due to lowered water potential

  - this creates high hydrostatic pressure at source compared to sink, increasing pressure gradient down phloem

  - more sucrose moves by mass flow down phloem