Module 3 - Mass transport in plants Flashcards
5 marks
Why is the diameter of the trunksmallest at midday
- Midday= more sunlight
- Stomata open in light= more water loss
- Water evaporates more when warm (fall in WP- water is drawn out of xylem)
- Hydrogen bonds between water molecules= cohesive= creates a column of water within xylem
- Adhesion (water molecules stick to wall of xylem) = helps to pull the water column upwards
- Xylem pulled inwards by tension/ faster flow of water
3 marks
Explain the importance of the xylem being kept open as a continuous tube
- There are no barriers to the movement of water
- Due to cohesion between the H bonds
- As water molecules evaporate, tension is created (cohesion-tension theory)
2 marks
Explain one way sieve tube cells are adapted for mass transport
- Cytoplasm only at edges of the cell/ fewer organelles
- Easy flow of the sugars
1 mark
Explain why values for the pressure in the xylem are negative
(Inside xylem) lower than atmospheric pressure / (water is under) tension;
2 marks
One way in which seive cells are adapted for mass transport of sugars
- Cytoplasm only at edge of the cell
- Easy flow of the sugars
2 marks
One way in which companion cells are adapted for mass transport of sugars
- Lots of mitochondria
- Release energy for active transport of sucrose into the phloem
5 marks
Describe mass flow hypothesis
- Sucrose actively transported into phloem using companion cells
- This lower water potential of phloem
- Water enters phloem via osmosis from xylem
- Increase in water volume = increase in hydrostatic pressure= more liquid forced to sink cell
- Sucrose actively transported to sink cell–> used in respiration as insoluble starch
5 marks
Describe the transport of carbohydrate in plants.
- Sucrose actively transported into phloem cell
- By companion cells
- Lowers water potential in phloem and water enters from xylem by osmosis
- Produces higher hydrostatic pressure
- Mass flow to respiring cells
- Unloaded/removed from phloem by active transport
2 marks
Advantage of sunken stomata on xerophytic plants
- Reduces transpiration/ evaporation
- Fall in water potential gradient
2 marks
Use knowledge of structure of leaf to explain why less water is loss through the upper surface of leaves than is lost through the lower surface
- More stomata on the lower surface
- Waxy cuticle on upper surface
4 marks
Explain how xylem tissue is adapted for its function
- Tubes with no end walls- continuous water columns
- No cytoplasm / no organelles - allows easier water flow
- Lignin- Withstand tension
- Pits in walls- allow lateral movement / get round blocked vessels
3 marks
Describe and explain three ways in which the leaves of xerophytic plants may be adapted to reduce water loss.
- Thick, waxy cuticle- waterproof/impermeable
- Sunken stomata or hairy- saturated layer os still air outside
- Small leaves- reduced SA for water loss
- Reduced number of stomata- reduced SA for water loss
- Leaves roll up in dry weather- less SA for water loss and stomata covered
3 marks
Sorghum has few stomata per mm2 of leaf surface area. Explain how this is an adaptation to the conditions in which sorghum grows.
- Grows in arid / dry conditions
- Less surface area
- Transpiration / water loss would be reduced
2 marks
A student found the number of stomata per cm2 on the lower surface of a daffodil leaf. He removed a small, thin piece of lower epidermis and mounted it on a microscope slide. He examined the slide using an optical microscope. Explain why it was important that the piece of the epidermis that the student removed was
thin.
- Single layer of cells / few layers of cells
- So that light that can pass through / cells absorb light
2 marks
The stomata on the leaves of pine trees are found in pits below the leaf surface. Explain how this helps to reduce water loss.
- Water vapour accumulates / increased humidity / reduced air movement (around stomata)
- Water potential gradient reduced
