Mass Transport In Plants Flashcards
(29 cards)
What is transpiration
loss of water vapour from the stomata by evaporation.
What is the four factors that affect transpiration
Light intensity
Temperature
Humidty
Wind
How does light intensity affect transpiration
Positive correlation
More light causes more stomata to open = larger surface area for evaporation
How does temperature affect transpiration
Posiive correlation
More heat means more kinetic energy, faster moving molecules and therefore more evaporation
How does humidy affect transpiration
Negative correlation
More water vapour in the air will make the water potential more positive outside of the leaf, therefore reduces the water potential gradient
How does wind affect transpiration
Positive correlation
More wind will blow away humid air containing water vapour, therefore maintaining the water potential gradient.
How does water move up the xylem
Water moves up a plant from the roots against gravity.
Via
Cohesion-tension theory
+ Cohesion
What is cohesion
• Water is a dipolar molecule (slight negative oxygen and slight positive hydrogens.
• This enables hydrogen bonds to form between the hydrogen and oxygen of different water molecules.
• This creates cohesion between water molecules - they stick together. Therefore water travels up the xylem as a continuous water column.
What is adhesion
Adhesion of water is when water sticks to other molecules. Water adheres to the xylem walls.
The narrower the xylem the bigger the impact of capillarity.
Root pressure in transpiration
- As water moves into the roots by osmosis it increases the volume of liquid inside the root and therefore the pressure inside the root increases. This is known as root pressure.
• This increase in pressure in the roots forces water above it upwards (positive pressure).
What are the three fcators of the cohesion tension theory
Cohesion, adhesion and root pressure
Steps of water moving up the xylem
- Water vapour evaporates out of stomata on leaves. This loss in water volume creates a lower pressure.
- When this water is lost by transpiration more water is pulled up the xylem to replace it (moves due to negative pressure).
- Due to the hydrogen bonds between water molecules, they are cohesive (stuck together). This creates a column of water within the xylem.
- Water molecules also adhere (stick) to the walls of the xylem. This helps to pull the water column upwards.
- 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 leaves
Transports organic substamces to all cells in a plant via the phloem
All cells need to respire
What are the two key cells phloem tissue
- sieve tube elements
Living cells
Contain no nucleus
Contain few organelles - companion cells
Companion cells
Provide AT required for active transport of organic substances
What is the mass floow hypothesis in plants
Transport of organic substances in a plant
Requires energy - active (co-transport)
Mass flow from the source of production, the leaves, to the sink, the site where organic substances, such as glucose and sucrose are used in up in respiring tissues
Source to sink explanation
Source cell (photosynthesising leaf cell)
Sucrose lowers waters potential of source cell. Water enters by osmosis (increases hydrostatic pressure in source cell)
Through phloem to
Sink cell (respiring cell)
Respiring cell is using up sucrose and therefore it has a more positive water potential. Water leaves the sink cell by omsosis decreases hydrostatic pressure in the sink cell.
How sucrose transports from the source to the sieve tube element
- 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 cell 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 sieve tube element.
Movement of sucrose within the phloem sieve tube element
- The increase of sucrose in the sieve tube element lowers the water potential.
• Water enters the sieve tube elements from the surrounding xylem vessels via osmosis.
• The increase is water volume in the sieve tube element increase the hydrostatic pressure causing the liquid to be forced towards the sink.
Transport of sucrose to the sink (respiring cells)
• Sucrose is used in respiration at the sink, or stored or stored as insoluble starch.
• More sucrose is actively transported into the sink cell, which causes the water potential to decrease.
• This results in 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.
• Movement of soluble organic substances is due to the difference in hydrostatic pressure between the source and sink end of the sieve tube element
What are tracers
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 dioxide
Second step of tracers
Thin slices from the stems are then cut and placed on x-ray film that turns black when exposed to radioactive material.
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.
Ringing experinment
A ring of bark and phloem are peeled and removed off a tree trunk. The result of removing the phloem is that the trunk swells above the removed section. Analysis of the liquid in this swelling shows it contain sugar. This shows that when the phloem is removed, the sugars cannot be transported and therefore proves the phloem transports sugars.
Describe the mass flow hypothesis for the mechanism of translocation in plants
- In source / leaf sugars actively transported into phloem;
- By companion cells;
- Lowers water potential of sieve cell / tube and water enters by osmosis;
- Increase in pressure causes mass movement (towards sink / root);
- Sugars used / converted in root for respiration for storage.
Describe the cohesion-tension theory of water transport in the xylem
Water r lost from leaf because of transpiration / evaporation of water
(molecules) / diffusion from mesophyll / leaf cells;
OR
Transpiration / evaporation / diffusion of water (molecules)
through stomata / from leaves;
Lowers water potential of mesophyll / leaf cells;
Water pulled up xylem (creating tension);
Water molecules cohere / ‘stick’ together by hydrogen bonds;
5. (forming continuous) water column;
Adhesion of water (molecules) to walls of xylem;
