Mass transport in plants

• How does water enter a plant cell?

  It moves down a water potential gradient across the partially permeable membrane

  By osmosis
• What is an advantage of transpiration?


  The constant stream of water also transports ions around the plant

  The evaporation has a cooling effect, stopping the enzymes being denatured if the plant overheats
• Why is there cohesion between water molecules?

  They are polar molecules (diploe - delta +ve on hydrogen and delta -ve on oxygen)

  Hydrogen bonding
• How are organic substances moved through a plant?

  By translocation

  In the phloem
• Describe the phloem


  Elongated cells joined end to end

  Holes in end walls - sieve plates

  In bundles in the steam

  Very few organelles/cytoplasm
• Why is there a higher rate of respiration when there is a high concentration of sucrose at the source?

  Sucrose --> glucose

  Glucose can be respired
• Why do phloem cells need companion cells?

  They have lost many of their organelles e.g nucleus when they are specialised

  It helps them repair and maintain themselves

  Produces ATP for active transport of H+
• What is a sink?

  A part of the plant that uses organic molecules (sucrose) e.g. respiring tissue
• What is a source?

  A part of the plant that produces organic molecules (Sucrose) e.g. leaf
• Complete the sentence

  Translocation moves organic molecules from to


  From sources to sinks
• What does ' there is positive pressure in the phloem' mean?

  The organic molecules flow from a high pressure to a low pressure
• Describe mass flow in plants

  1. Sucrose actively transported into phloem at source, by companion cells

  2. Lowers water potential of the phloem

  3. Water moves from xylem into phloem by osmosis

  4. Volume and pressure increase

  5. Sucrose unloaded from phloem at sink where cells are respiring/ it is stored

  6. Lowers water potential of sink cells

  7. Water moves out of phloem by osmosis
• Describe how sucrose enters the phloem

  1. Sucrose moves by facilitated diffusion with H+ into the companion cell from the source cell

  2. H+ actively transported out

  3. Sucrose moves into phloem sieve element by facilitated diffusion
• How do experiments with aphids support the mass flow hypothesis?

  Aphids allowed to feed on plants (phloem)

  Mouthparts removed from aphids

  Sap flows OUT

  Supports idea of positive pressure
• What are the 3 pieces of evidence which do not support the mass flow hypothesis?

  1. Sieve plates don't make sense (reduce rate of translocation)

  2. Not always to sink with lowest water potential

  3. Organic molecules move at different speeds
• How do metabolic inhibitors support the mass flow hypothesis?

  Inhibit respiration

  Translocation does not occur

  Shows that it required ATP and active transport
• How do ringing experiments support the mass flow hypothesis?

  Ring of bark removed from tree (removes phloem NOT xylem)

  Sucrose cannot be transported so collects

  Lowers water potential

  Water moves out of the xylem and bulges

  Shows phloem important in transport of organic molecules
• What are the 4 pieces of evidence which support the mass flow hypothesis?

  1. Ringing experiments

  2. Aphids - mouthparts removed

  3. Radioactive tracers

  4. Metabolic inhibitors
• How do we know there is a concentration gradient of sucrose from source to sink?

  The sap from the source has a higher concentration thank from the sink
• How do we know photosynthesis is needed for translocation?


  A plant virus will travel up the plant in the light and not the dark
• What is the xylem?

  A hollow tube of dead cells
• What is the function of the xylem?

  Transport water and mineral ions in solution
• Where does the xylem transport water and mineral ions?

  Through the stem and leaves of plants
• On a graph of rate of water movement against time, why would the curve for upper branches of a tree differ from the trunk?

  Rate of movement increases in the upper branches

  Then increases in the trunk

  Water moving in branches creates tension/pull
• Why does light intensity affect the rate of transpiration?

  Higher light intensity

  Increases rate of photosynthesis

  Stomata open to allow more CO2 to diffuse into leaf

  More water evaporates
• Why does increasing temperature affect the rate of transpiration?

  Molecules have more kinetic energy

  Faster rate of diffusion/evaporation of water
• Why is the rate of transpiration high on a dry, windy day?

  Low humidity so low water potential around stomata

  Air movement blows away water molecules

  Maintains a steep water potential gradient
• Why is the plant left for a while before beginning the experiment into rate of transpiration?

  So the shoot can acclimatise

  Excess water evaporates from leaves
• How is the potometer reset?

  Opening reservoir

  Moving bubble to initial position
• Why is the surface area of leaves measured, when investigating the rate of transpiration in different plants?

  Different plants have different sized leaves

  Rate of transpiration also depends on surface area of leaves

  Allows comparison of results
• Describe how a potometer works

  There is a tube connecting the plant shoot to a beaker of water

  There is an air bubble and ruler

  As the plant takes up water, the air bubbles moves

  The distance moved in a given time can be measured
• Why must leaves be dried before starting the rate of transpiration experiment?

  Excess water means the potometer doesn't work properly
• Why is it important to assemble the potometer underwater?

  So that no air enters the apparatus

  This prevents plants from taking up water effectively so affects transpiration

  So there are no (more) air bubbles
• What factors can affect the rate of transpiration?

  Light intensity

  Humidity

  Temperature

  Wind
• What is transpiration?


  The evaporation of water vapour, mainly through the stomata in the surface of a leaf
• How are xylem vessels adapted to withstand negative pressure (tension) ?

  Thick cell walls

  Lignin in cell walls
• Why is it important that the xylem vessels have no cytoplasm?

  Maintains continuous column of water

  Less resistance to the flow of water/mineral ions
• How are the xylem vessels adapted for maintaining a continuous column of water?

  No end walls

  Stacked on top of each other

  No cytoplasm
• What is the name of the theory for how water moves through a plant?

  Cohesion-tension theory
• From which cells in the leaf does water evaporate?

  Spongy mesophyll cells