Transport in Plants Flashcards
Transpiration.
Loss of water vapour from the stomata by evaporation.
How Light Intensity Effects the Rate of Transpiration.
There is a positive correlation between light intensity and rate of transpiration. Greater light intensity causes more stomata to open, which increases the surface area for evaporation.
How Temperature Effects the Rate of Transpiration.
There is a positive correlation between temperature and rate of transpiration. More heat gives the water more kinetic energy. The molecules moves faster, which results in more evaporation.
How Humidity Effects the Rate of Transpiration.
There is a negative correlation between humidity and the rate of transpiration. Increased humidity means that there is more water vapour in the air surrounding the leaf. This means that the water potential of the air is greater than that of inside the leaf. This reduces the water potential graidient for the water vapour to diffuse down.
How Wind Effects the Rate of Transpiration.
There is a positive correlation between wind and the rate of transpiration. More wind blows away humid air that contains water vapour. This keeps the water potential of the air surrounding the leaf lower than inside the leaf, which maintains the water potential gradient for the diffusion of water vapour.
Movement of Water Through the Leaf.
- When stomata are open, water vapour diffuses from air spaces in the leaf, out through the stomata, down a water potential gradient. This is transpiration.
- To replace this, water evaporated from the walls of mesophyll cells into tha air spaces, forming water vapour, which continues to build up in the air spaces.
- The water from he mesophyll cells is replaced by water from xylem vessels in the leaf, which are continuous with the xylem in the stem.
Cohesion-Tension Theory.
Cohesion - Water is a dipolar molecule, meaning there is a slightly positive region (the two hydrogens) and a slightly negative region (the oxygen). This means that water molecules can form hydrogen bonds with other molecules, between a hydrogen and an oxygen from different water molecules. The creates cohesion between water molecules, so water travels up the xylem as a continuous water collumn.
Capillarity - Adhesion of water is when the water binds to other molecules. In this case the water is binding to the xylem walls. The narrower the xylem, the greater the effect of caplliarity is. This is because more water is in contact with the xylem walls.
Root pressure - As water moves into the roots by osmosis, it increases the volume of liquid inside the roots, therefore the pressure inside the root increases. This pressure forces water upwards (the water moves by positive pressure).
The Movement of Water Up the Xylem.
- Water evaporated out of the stomata on leaves. This loss in water volume creates a lower pressure in the top of xylem in the stem.
- As a result of this transpiration, more water is pulled up by the xylem to replace the water lost (the water moves due to negative pressure).
- Due to the hydrogen bonds between water molecules, they are cohesive. This creates a collumn of water in the xylem.
- Water molecules also adhere to the walls of the xylem. This helps pull the water collumn upwards. This is capillarity.
- As this collumn of water is pulled up the xylem it creates tension, which pulls the xylem in to become narrower, which further increases the impact of capillarity.
Evidence for the Cohesion-Tension Theory.
Tension has been measured in the xylem as plants transpire.
If a collumn of water in the xylem is broken, air bubbles form and this stops any further upward movement of water in that xylem vessel because air bubbles prevent cohesion.
Respiratory inhibitors do not inhibit this process.
The diameter of trees decrease when they are transpiring, and more so when the temperature and light intensity are high. This can be measure using a dendrometer.
Structure of the Xylem.
Cell walls contain lignin - The lignin strengthens xylem walls against tension and makes them waterproof.
Lignified vessel walls cause the cell contents to die - Leaves a hollow lumen with no cytoplasm, which offers little to no resistance to the mass flow of water and minerals.
Walls of the xylem vessel contain tiny holes called pits - If a vessel becomes blocked or damaged, the water can be diverted laterally so the upwards movement of water can continue in the adjacent vessels.
Vessles lose their end walls - They form a continuous collumn for water movement from root to leaves.
The thickening of the cell walls in xylem vessels is oftem spiral - Uses less material, so it is less wasteful, allows the xylem to have less mass and allows the xylem to be flexible.
The Phloem.
Resposible for transporting organis substances that are made during photosynthesis up and down the plant to all respiring cells.
Sieve tube elements - Living cells that contain very few organelles and no nucleus. This provides a more hollow lumen to offer less resistance for mass flow of organic substances. The end walls of sieve tube elements are perferrated with holes, this allows for a continuous flow of sugar solution.
Companion cells - Living cells with all of their organelles, responsible for providing the ATP required for active transport of organis substances.
The Mass Flow Hypothesis.
Mass flow from the source of production (the leaves) to the sink (the site where organic substances, such as sucrose, are used up in respiring tissues.
Increased concentration of sucrose lowers the water potential of source cell.
Water enters source cell from surrounding cells, down a water potential gradient, via osmosis.
Respiring cells (sink cells) are using up sucrose and therfore have a greater water potential than surrounding cells.
Water leaves the sink, down a water potential gradient, via osmosis.
This increases the hydrostatic pressure in the source cell and decreases the hydrostatic pressure in the sink cells.
The source has a higher hydrostatic pressure than the sink and so the sugar solution is forced towards the sink via the phloem.
Translocation - Movement of Sucrose into the Sieve Tube Element.
- Photosynthesis occuring in the chloroplasts of leaves creates organic substances (sucrose).
- There is a high concentration of sucrose at the site of production, therefore, sucrose diffuses down a concentration gradient into companion cells, by faciliated diffusion.
- Active transport of H+ occurs from the companion cells into the spaces within the cell walls.
- This creates a concentration gradient for H+ and therefore H+ diffuse down the concentration gradient via carrier protiens by facilitated diffusion into the sieve tube element.
- Sucrose is co-transported with H+ into the sieve tube element via protien co-transporters.
Translocation - Movement of Sucrose Through the Phloem.
- There is an increased concentration of sucrose in the sieve tube element, this lowers the water potential inside the sieve tube element.
- This causes water to move into the sieve tube element from surrounding xylem, down a water potential gradient via osmosis.
- The increase in water volume in the sieve tube element increases the hydrostatic pressure, causing liquid to be forced towards the sink (respiring cells).
- Sucrose is used in respiration at the sink or is stores as insoluble starch.
- More sucrose gets actively transported into the sink cells, which decreases the water potential of the sink cells.
- This causes water to move from the sieve tube element into the sink cells, down a concentration gradient, via osmosis. Some water also returns to the xylem by osmosis.
- There is a decrease in water volume in the sieve tube element, meaning hydrostatic pressure decreases.
- Movement of soluble organic substances is due to the difference in hydrostatic pressure between the source and the sink end of the sieve tube element.
Tracers.
Involves tracking radioactively labelled carbon.
Plants are isolated and provided with radioactively labelled carbon dioxide. Over time this is absorbed by the plant and used in photosynthesis to produce sugars containing radioactively labelled carbon.
Thin slices from the stems are placed onto x-ray film that turns black when exposed to radioactive material.
Sections of the stem containing sugars turned black, this highlighted the location of the phloem and shows that sugars are transported by the phloem.
