Mass transport in plants – AQA A Level Summary Notes
Xylem
- tissue that transports water in stem and leaves of plant
Features:
- dead, hollow vessels
- cells which form them are arranged in long column
- walls are strengthened by spirals of lignin – prevents them from collapsing under reduced
pressure caused by transpiration pull
- located in parallel close to each other, arranged in vascular bundles (continue up stem, into
midrib and veins of the leaf)
Water transport through a plant
1. Water enters plants into root hair cells by osmosis
2. Water molecules move through root tissue (cortex) into specialised xylem vessels, which run
up stem and into leaves
3. At leaves, water exits xylem and moves into mesophyll cells
4. Water evaporates into air spaces in spongy mesophyll, then diffuses down a water vapour
diffusion gradient out of stomata
Most minerals also travel through plant in xylem
, The cohesion tension theory of water transport in the xylem
The movement of water molecules up the xylem is driven by the proceses that occur in the leaves
Describe cohesion tension theory (5 marks)
1. Water lost from leaf because of transpiration
2. Lowers water potential of leaf/mesophyll cells
3. Water pulled up xylem (creating tension)
4. (So) water molecules cohere by hydrogen bonds
5. Forming a continuous water column
6. Adhesion of water to walls of xylem
Potometer used to investigate effect of environmental variable on rate of transpiration
Transpiration = loss of water vapour from aerial parts of the plant (mesophyll cells) via the stomata
Transpiration pull = water being pulled up the xylem as a result of transpiration
How do we estimate the rate of transpiration in a leafy shoot?
- Use a bubble or mass potometer
Bubble potometer
- Measure distance of bubble moves in fixed period of time
- Calculate rate of bubble movement
- Syringe used to reset bubble, allowing repeats to be taken
MUST BE SET UP VERY CAREFULLY TO ENSURE ACCURATE DATA IS RECORDED:
1. Must cut shoot under water and assemble potometer under water – requires a large sink. If
air gets into xylem vessels of plant, it can form air locks which will prevent the plant taking
up water and prevents steady transpiration
2. Potometer should be left for leaves to dry. OR dry leaves gently with paper towel. Potometer
will not work properly until any excess water on leaves has evaporated or been removed
3. All fittings must be airtight, so only way apparatus can lose water is by plant transpiring
Mass potometer
- Measure change in mass over a fixed period of time
- Calculate rate of mass loss
- Layer of oil used to prevent evaporation of water from beaker, so water loss only from
leaves
Xylem
- tissue that transports water in stem and leaves of plant
Features:
- dead, hollow vessels
- cells which form them are arranged in long column
- walls are strengthened by spirals of lignin – prevents them from collapsing under reduced
pressure caused by transpiration pull
- located in parallel close to each other, arranged in vascular bundles (continue up stem, into
midrib and veins of the leaf)
Water transport through a plant
1. Water enters plants into root hair cells by osmosis
2. Water molecules move through root tissue (cortex) into specialised xylem vessels, which run
up stem and into leaves
3. At leaves, water exits xylem and moves into mesophyll cells
4. Water evaporates into air spaces in spongy mesophyll, then diffuses down a water vapour
diffusion gradient out of stomata
Most minerals also travel through plant in xylem
, The cohesion tension theory of water transport in the xylem
The movement of water molecules up the xylem is driven by the proceses that occur in the leaves
Describe cohesion tension theory (5 marks)
1. Water lost from leaf because of transpiration
2. Lowers water potential of leaf/mesophyll cells
3. Water pulled up xylem (creating tension)
4. (So) water molecules cohere by hydrogen bonds
5. Forming a continuous water column
6. Adhesion of water to walls of xylem
Potometer used to investigate effect of environmental variable on rate of transpiration
Transpiration = loss of water vapour from aerial parts of the plant (mesophyll cells) via the stomata
Transpiration pull = water being pulled up the xylem as a result of transpiration
How do we estimate the rate of transpiration in a leafy shoot?
- Use a bubble or mass potometer
Bubble potometer
- Measure distance of bubble moves in fixed period of time
- Calculate rate of bubble movement
- Syringe used to reset bubble, allowing repeats to be taken
MUST BE SET UP VERY CAREFULLY TO ENSURE ACCURATE DATA IS RECORDED:
1. Must cut shoot under water and assemble potometer under water – requires a large sink. If
air gets into xylem vessels of plant, it can form air locks which will prevent the plant taking
up water and prevents steady transpiration
2. Potometer should be left for leaves to dry. OR dry leaves gently with paper towel. Potometer
will not work properly until any excess water on leaves has evaporated or been removed
3. All fittings must be airtight, so only way apparatus can lose water is by plant transpiring
Mass potometer
- Measure change in mass over a fixed period of time
- Calculate rate of mass loss
- Layer of oil used to prevent evaporation of water from beaker, so water loss only from
leaves
