Transport In Plants Flashcards
What is transpiration?
- The loss of water vapour from the leaves.
- Evaporation of water from the surface of a plant
What is the transpiration stream?
Transpiration causes water to be ‘pulled up’ the xylem in the stem and roots in a continuous flow.
What is osmosis?
The net diffusion of water across a partially permeable membrane, from a solution with a high water potential to one with a lower water potential
Why do multicellular organisms need transport systems?
- Need substances to live e.g. plant cells need water, minerals and sugars and they need to get rid of wastes
- In unicellular organisms, these substances can diffuse directly in and out of the cell across the cell membrane, the diffusion rate is quick because of the short distances substances have to travel
- However in multicellular organisms e.g. animals and plants direct diffusion from the outer surface would be too slow because substances would have to travel large distances to reach every single cell, therefore they need transport systems to move substances to and from individual cells quickly
What is the role of the xylem?
- The xylem transports water and mineral ions from the roots up to the shoots/other parts of the plant in the transpiration stream
- Because of the pressure difference
- Xylem contains dead cells arranged end-to-end forming continuous vessels. When mature have no cytoplasm instead lumen through which water passes, and walls of xylem contain lignin
- When they begin life as living cells they contain a cytoplasm and cellulose cell walls
- As they develop they become elongated and gradually their original cellulose cell walls become impregnated with lignin made by the cytoplasm
- As this happens the cells die forming hollow tubes, liginifcation makes them very strong and enables them to carry up water to very tall plants without collapsing. Lignin is also impermeable to water
What is the role of the phloem?
- The phloem transports sugars (for energy), like sucrose and amino acids (for building proteins) from where they are made in the leaves to other parts of the plant
- Sugar may also be taken to the roots and converted into starch for storage
- This movement of food substances around the plant is known as translocation
- Consists of living cells, tubes formed by cells arranged end-to-end but have cell walls made of cellulose and have no cytoplasm
- The end of each cells is formed by a cross-wall of cellulose with holes called a sieve plant
- The living cytoplasm extends through the holes of the sieve plates, linking each cell with the next forming a sieve tube - Phloem sieve tubes have no nucleus and so are controlled by companion cells, the companion cells have a nucleus to make proteins
- This makes it easy to transport
Describe root hair cells
- The cells on the plant roots grow into long root hairs
- Each branch of a root will be covered in millions of the microscopic hairs
- This gives the plant a bigger surface area for absorbing water from the soil
How is the water taken into the root hair cells?
- By osmosis
- Since there is usually a higher water potential in the soil than inside of the cell, so the water is drawn into the root hair cell by osmosis
- Root hair cells may also take up mineral ions by diffusion or active transport
How would you measure the rate of photosynthesis using a weight potometer?
- A potometer is a piece of apparatus that measures the rate of water uptake
- A ‘weight’ potometer is used to measure the rate of loss of mass from a potted plant usually over several hours.
- The polythene bag around the pot prevents loss of moisture by evaporation form the soil. Most of the mass lost by the plant will be due to water evaporating from the leaves during transportation, although there will be a small change in mass due to respiration and photosynthesis, since both of these processes exchange gases with the air
How would you measure the rate of photosynthesis using a volume potometer?
A volume potpmeter is used to find the rate of uptake of water by a leafy shoot
1. Cut a shoot underwater to prevent air from entering the xylem. Cut it at a slant to increase the surface area available for water uptake
2. Assemble the potometer in water and insert the shoot underwater so that no air can enter
3. Remove the apparatus from water but keep the end of the capillary tube submerged in a beaker of water
4. Check that the apparatus is watertight and airtight, use Vaseline to seal any joins
5. Dry the leaves, and allow time for the shoot to acclimatise and then shut the tap
6. Remove the end of the capillary tube form the beaker of water until one air bubble has formed and then put the end of the tube back in the water .
7. Record the starting position of the bubble
8. Start the stopwatch and record the distance moved by the bubble per unit time e.g. per hour
9. Keep the conditions constant throughout e.g. temperature and humidity
If the bubble moves further per unit time then this shows a faster rate of transpiration
-If there are air bubbles in the apparatus or the plant’s xylem it will affect your results
How could you change the experimental conditions?
- Light intensity: place a lamp or put in cupboard
- Temperature: put in hot or cold room
- Humidity: spray a little water into a clear plastic bag before sealing it around the plant
- Wind speed: use a fan to increase the wind speed around the plant
What factors affect the rate of transpiration?
- Light intensity
- Temperature
- Wind speed
- Humidity
What is the transpiration stream? What is the point of it?
- Transpiration causes water to be ‘pulled up’ the xylem in the stem and roots in a continuous flow
- Since transpiration means evaporation which creates a slight shortage of water in the leaf, and so more water is drawn up from the rest of the plant through the xylem vessels to replace it
- This in turn means moe water is drawn up from the roots, and so there is a constant transpiration stream of water through the plants
- Functions include:
- supplying water for the leaf to carry out photosynthesis
- carrying mineral ions dissolved in water
- providing water to keep the plant cells turgid
- allows evaporation form the leaf surface, which cools the leaf
* Transpiration is a side-effect of the way leaves are adapted for photosynthesis, as they have stomata so that gases can be exchanged easily, because there is more water inside of the plant than in the outside air, the water escapes from the leaves through the stomata by diffusion
Why are turgid cells in a plant important?
- Supports the non-woody parts of the plant such as young stems and leaves
- Holds stem upright, so that the leaves can carry out photosynthesis properly
How does wind speed affect transpiration?
- The rate of transpiration increases with faster air movements across the surface of the leaf
- This is because the moving air sweeps away any water vapour which might remain near the stomata
- The moist air would otherwise reduce the diffusion gradient since there would be a high concentration of water particles outside of the leaf as well as inside it, and slow diffusion down
