Topic 3: Mass Transport In Plants Flashcards
Define transpiration
Transpiration is the loss of water vapour from the stomata by evaporation.
Describe how four different factors affect the rate of transpiration
1- light intensity
There is positive correlation between light intensity and transpiration. This is because the higher the light intensity, the more stomata that open and this provides a larger surface area for evaporation
2- Temperature
There is a positive correlation between temperature and transpiration. The more heat there is the more kinetic energy, and therefore faster-moving molecules. This increases evaporation.
3- Humidity
There is a negative correlation between humidity and transpiration. The more water vapour in the air, the more positive the water potential is outside of the leaf. This reduces the water potential gradient and therefore reduces evaporation.
4- Wind
There is a positive correlation between wind (air movement) and transpiration. The windier it is, the more humid air containing the water vapour that is blown away. This maintains the water potential gradient, increasing evaporation.
Describe how water moves up the xylem (5 marks)
1.Water vapour evaporates out of stomata on leaves. This loss in water volume creates a lower pressure.
2.When this water is lost by transpiration more water is pulled up the xylem to replace it (moves due to negative pressure).
3.Due to the hydrogen bonds between water molecules, they are cohesive (stuck together). This creates a column of water within the xylem.
4.Water molecules also adhere (stick) to the walls of the xylem. This helps to pull the water column upwards.
5.As this column of water is pulled up the xylem it creates tension, pulling the xylem in to become narrower.
What is the function of the phloem?
Phloem is the tube responsible for the transport of organic substances in plants, such as sugars.
Describe the structure of:
Sieve tube elements
These are living cells, but they contain no nucleus and very few organelles. This is to make the cell more hollow and therefore provide less resistance to the flow of sugars.
Describe the structure of Companion cells -
As the sieve tube elements have few organelles, they depend on the companion cell for resources. The companion cells provide ATP required for active transport of organic substances.
Briefly outline mass flow hypothesis
Organic substances, such as sucrose, move in solution from the leaves where they are created in photosynthesis, to respiring cells. The site of production is called the ‘source’ and the site of use is called the ‘sink’ in the mass flow hypothesis.
Describe source to sink mass flow hypothesis
Sucrose lowers the water potential of the source cell. This causes water to enter by osmosis. This increases the hydrostatic pressure in the source cell.
The respiring cell is using up sucrose, and therefore it has a more positive water potential. Therefore water leaves the sink cell by osmosis. This decreases the hydrostatic pressure in the sink cell.
This results in the source cell having a higher hydrostatic pressure than the sink cell, so the solution is forced towards the sink cell via the phloem.
Write the process of translocation.
Photosynthesis occurring in the chloroplasts of leaves creates organic substances, e.g. sucrose.
This creates a high concentration of sucrose at the site of production, therefore sucrose diffuses down its concentration gradient into the companion cells via facilitated diffusion.
Active transport of H+ occurs from the companion cell into the spaces within the cell walls using energy.
This creates a concentration gradient and therefore the H+ move down their concentration gradient via carrier proteins into the sieve tube elements.
Co-transport of sucrose with the H+ ions occurs via protein co-transporters to transport the sucrose into the sieve tube element.s
The increase of sucrose in the sieve tube element lowers the water potential, so water enters the sieve tube elements from the surrounding xylem vessels via osmosis. The increase in water volume in the sieve tube element increases the hydrostatic pressure causing the liquid to be forced towards the sink.
Sucrose is used in respiration at the sink or stored as insoluble starch. More sucrose
is actively transported into the sink cell, which causes the water potential to decrease. This results in the osmosis of water from the sieve tube element into the sink cell (some water also returns to the xylem). The removal of water decreases the volume in the sieve tube element and therefore the hydrostatic pressure decreases.
The movement of soluble organic substances is due to the difference in hydrostatic pressure between the source and sink end of the sieve tube element.
Transport in plants can be investigated by using tracer and ringing experiments. These two experiments are used to prove that it is the phloem that transports the sugars and not the xylem. Describe the process of tracing:
Tracing involves radioactively labelling carbon. Plants are provided with only radioactively labelled carbon dioxide and over time this is absorbed into the plant and used in photosynthesis to create sugars which all contain radioactively labelled carbon. Thin slices from the stems are then cut and placed on x-ray film that turns black when exposed to radioactive material.
Transport in plants can be investigated by using tracer and ringing experiments. These two experiments are used to prove that it is the phloem that transports the sugars and not the xylem. Explain how tracing provides evidence of translocation.
When the stems are placed on the x-ray film the section of the stem containing the sugars turn black, and this highlights where the phloem are and shows sugars are transported in the phloem.
Transport in plants can be investigated by using tracer and ringing experiments. These two experiments are used to prove that it is the phloem that transports the sugars and not the xylem. Describe ringing experiments:
A ring of bark and phloem is peeled and removed off a tree trunk.
Transport in plants can be investigated by using tracer and ringing experiments. These two experiments are used to prove that it is the phloem that transports the sugars and not the xylem. Explain how ringing experiments provide evidence of translocation.
The result of removing the phloem is that the trunk swells above the removed section.
Analysis of the liquid in this swelling shows it contains sugar. This shows that when the phloem is removed, the sugars cannot be transported and therefore proves the phloem transports sugars.
