3.4.2 mass transport in plants Flashcards
What environmental factors INCREASE the rate of transpiration?
Increasing temperature, Decreasing humidity, Increasing light intensity, Increasing wind speed
What environmental factors DECREASE the rate of transpiration?
Decreasing temperature, Increasing humidity, Decreasing light intensity, Decreasing wind speed
Why does the rate of transpiration increase with increasing light intensity?
Causes stomata to open
Why does the diameter of the xylem decrease as the rate of transpiration increases?
Water lost from the leaf in transpiration creates more tension and pulling force on water molecules in xylem
What does water lost from the leaf in transpiration creating more tension and pulling force on water molecules in xylem create?
The diameter of the xylem to decrease
What device is used to measure the rate of transpiration?
Potometer
What are the vital procedures to setting up a potometer?
Cut stem underwater, and at a slant; Ensure no air bubbles are present; Seal joints with vaseline to ensure airtight
Why do potometers not truly measure the rate of transpiration?
Water used for support, Water used in photosynthesis, Water produced in aerobic respiration, Apparatus not fully sealed
What is the benefit of xylem cells being dead, and therefore empty with no cytoplasm contents?
Allows water to flow easier
What xylem specialization allows water to flow easier?
Cells are dead, and therefore empty with no cytoplasm contents
What is the benefit of xylem cells being long, forming tubes, with no end walls, meaning it’s hollow?
Allows for continuous columns of water
What xylem specialization allows continuous columns of water to flow?
Cells are long, forming tubes, with no end walls meaning they are hollow
What is the benefit of xylem cells having a thickened cell wall with cellulose and lignin?
Support, and waterproofing
What xylem specialization allows for support, and waterproofing?
Thickened cell walls with cellulose and lignin
What is the benefit of xylem cells having pits in walls?
Allows lateral movement of water
What xylem specialization allows for lateral movement of water?
Pits in walls
Describe and explain the cohesion-tension theory.
Water lost from the leaves by evaporation, due to transpiration; Lowers the water potential of mesophyll cells, so water drawn out of xylem; Water pulled up xylem creating tension; Due to cohesive forces between water molecules, caused by hydrogen bonding; Allows a continuous column of water to be pulled; Water molecules adhere to the xylem walls
In the cohesion-tension theory how is water lost from the leaves?
By evaporation, due to transpiration
In the cohesion-tension theory why is water drawn out of the xylem?
The water potential of mesophyll cells has been lowered
In the cohesion-tension theory what creates tension?
Water being pulled up xylem
In the cohesion-tension theory what causes cohesive forces between water molecules?
Hydrogen bonding
In the cohesion-tension theory what allows a continuous column of water to be pulled?
Cohesive forces between water molecules, due to hydrogen bonding
In the cohesion-tension theory what do water molecules adhere to?
Xylem walls
In the phloem solution, what key molecules are transported?
Sucrose and amino-acids
What key component are within the sieve tube element of the phloem?
Sieve plates
What is the specialization of companion cells in the phloem?
Have lots of mitochondria, to actively transport sucrose into phloem from source
What is the most general way of describing the route of the mass-flow hypothesis?
From source to sink
Describe and explain the Mass-Flow Hypothesis.
Sucrose actively transported into sieve tube element, from source, by companion cells; This lowers the water potential in the sieve tube element, so water moves in by osmosis from the xylem; Produces a high hydrostatic pressure; Mass flow of sucrose from source to sink; At sink, sucrose is unloaded and used or converted to starch for storage
In the Mass-Flow Hypothesis what actively transports sucrose into the sieve tube element from the source?
Companion cells
In the Mass-Flow Hypothesis what does a low water potential in the sieve tube element allow?
Water to move in by osmosis from the xylem
In the Mass-Flow Hypothesis what does the movement of water into the sieve tube element by osmosis from the xylem create?
High hydrostatic pressure
In the Mass-Flow Hypothesis where is the mass flow?
From source to sink
In the Mass-Flow Hypothesis what happens at the sink?
Sucrose is unloaded and used or converted to starch for storage
Explain how the following provides evidence for the Mass-Flow Hypothesis: Translocation slows if respiration inhibitors used.
No active transport of sucrose from companion cells at source
Explain how the following provides evidence for the Mass-Flow Hypothesis: If phloem punctured, contents flow out.
Occurring at high pressure
Explain how the following provides evidence for the Mass-Flow Hypothesis: Plants can be supplied with radioactive CO2 and traced.
Sucrose moves from source, up and down stem, to sinks
Explain how the following provides evidence for the Mass-Flow Hypothesis: Ring of bark removed, so that xylem remains, but phloem removed.
Bulging above ring due to build-up of fluid in phloem; Tissues die below, as no sucrose for respiration
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.
- Sucrose actively transported into phloem; 2. Lowering/reducing water potential; 3. Water moves in by osmosis
Describe the mass flow hypothesis for the mechanism of translocation in plants.
- In source sugars actively transported into phloem; 2. By companion cells; 3. Lowers water potential of sieve tube and water enters by osmosis; 4. Increase in pressure causes mass movement towards sink
