Exam qs

• Suggest reasons why the rate of water uptake by a plant might not be the same as the rate of transpiration

  • water used for support/turgidity
  • water used in photosynthesis
  • water used in hydrolysis
  • water produced during respiration
• Describe how water is moved through a plant according to the cohesion-tension hypothesis (4)

  • water evaporates from leaves
  • reduces water potential in cell
  • water is drawn out of xylem
  • creates tension
  • cohesive forces
  • water pulled up as a column
• Use your knowledge of leaf structure to explain why less water is lost through the upper surface of leaves than is lost through lower surface (2)

  • more stomata on lower surface
  • thicker waxy cuticle on upper surface
• Describe the cohesion-tension theory of water transport in the xylem (5)

  • water is lost from leaf because of transpiration
  • lowers water potential of mesophyll/leaf cells
  • water pulled up xylem
  • water molecules cohere/‘stick’ together by hydrogen bonds
  • this forms a continuous water column
  • adhesion of water molecules to walls of xylem
• Describe and explain the relationship between humidity and transpiration rate (3)

  • high humidity leads to decreased transpiration
  • high humidity means increased water potential
  • reduces water potential gradient
  • less evaporation
• Why is it important that the xylem tissue contains hollow tubes? (1)

  maintains transpiration stream
• Why is lignin present in xylem cell walls? (1)

  provides support
• The student cut the shoot and put it into the photometer under water. Explain why (1)

  prevent air entering
• The student wanted to calculate the rate of water uptake by the shoot in cm3 per minute. What measurements did she need to make? (2)

  • distance and time
  • radius of capillary tube
• Suggest how a reservoir allows repeat measurements to be made (1)

  returns bubble to start
• If the enzymes of the mitochondria of the root cells are inhibited, eg with cyanide, then root pressure drops. 

  Suggest why this happens. (6 marks)

  • Aerobic respiration occurs in mitochondria
  • Supplies root cells with energy
  • Mitochondria cannot produce ATP if enzymes inhibited
  • Energy required for active transport of ions into the xylem
  • If no ions actively pumped into xylem then water potential stays high
  • No/reduced water potential gradient for water to move by osmosis into root cells
  • Less water means lower hydrostatic pressure
• Why is transpiration important in a plant?

  • It provides a means of cooling the plant via evaporation
  • The transpiration stream is helpful in the uptake of mineral ions
  • The turgor pressure of the cells provides support to leaves and the stem of non-woody plants
• Describe the transport of carbohydrates in plants (5)

  • sucrose is co-transported with hydrogen ions into phloem
  • by companion cells
  • lowers water potential in phloem and water enters from xylem by osmosis
  • produces higher hydrostatic pressure
  • mass flow to respiring cells
  • carbohydrates unloaded from phloem by active transport
• The mass flow hypothesis is used to explain the movement of substances through phloem.

  Use your understanding of the mass flow hypothesis to explain how pressure is generated inside this phloem tube (3)

  • sucrose actively transported into phloem
  • lowering the water potential
  • water moves into phloem by osmosis from xylem
• One theory of translocation states that organic substances are pushed from a high pressure in the leaves to a lower pressure in the roots.

  Describe how a high pressure is produced in the leaves (3)

  • water potential becomes lower as sugar enters phloem
  • water enters phloem by osmosis
  • increased volume of water = increased pressure
• Describe the mass flow hypothesis for the mechanism of translocation in plants (4)

  • in source/leaf sugars actively transported into phloem
  • by companion cells
  • lowers water potential of sieve tube and water enters by osmosis
  • increase in pressure causes mass movement towards sink/root
  • sugars used in root for respiration
• The diameter of a tree is less during the day, when the tree is transpiring, than it is at night. Explain how this supports the cohesion-tension theory (2)

  • evaporation from leaves during daytime mainly
  • tension pressure on water in xylem creates inward pull on walls of xylem vessel
• Describe one other piece of evidence that supports the cohesion-tension theory and explain how it supports this theory (2)

  • break in xylem vessel (introducing air) prevents water movement
  • because the continuous water column is broken
• Describe the processes involved in the transport of sugars in plant stems (5)

  • (At source) sucrose is actively (transported) into the phloem
  • By companion cells
  • Lowers water potential in phloem and water enters by osmosis
  • (Produces) high (hydrostatic) pressure
  • Mass flow/transport towards sink/roots
  • At sink/roots sugars are unloaded