BY2 - Transport in Plants
- Created by: beth-marie2511
- Created on: 22-03-16 21:21
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- Transport in Plants
- Transpiration
- DEFINITION: The evaporation of eater from the inside of the leaves and into the atmosphere.
- Evaporation of water molecules creates the transpiration stream - water is lost from the leaves which causes more water molecules to be 'pulled' up the xylem from the roots.
- All plants have to balance water loss and water uptake.
- If a plant loses more water that it absorbs then it will wilt.
- If a plant loses an excess of water it cannot retain its turgor and cells become flaccid, causing the plant tot die.
- As long as there is a concentration gradient between the inside of the leaf and the atmosphere water vapour will diffuse out.
- If a plant loses more water that it absorbs then it will wilt.
- All plants have to balance water loss and water uptake.
- Factors Affecting the Rate of Transpiration
- The rate at which water is lost is called the transpiration rate.
- It is dependent upon environmental factors.
- Any factor that causes an increase in the concentration gradient between the inside of the leaf and the atmosphere will increase the rate of transpiration.
- It is dependent upon environmental factors.
- Higher Temperatures
- Increases the rate of transpiration.
- Water molecules have increased kinetic energy so more water molecules evaporate from the air space, through the stomata and into the surrounding atmosphere.
- Increases the rate of transpiration.
- Humidity (Percentage of water vapour in the air)
- High Humidity
- Large percentages of water vapour in the air.
- Causes the rate of transpiration to decrease.
- Low Humidity
- Little to no water vapour in the air.
- Causes the rate of transpiration to increase due to a large concentration gradient between the inside and outside of the leaves.
- High Humidity
- Air Movement
- Increased air movement increases the rate of transpiration.
- Water vapour is carried away from the diffusion shell and this increases the diffusion concentration gradient.
- Increased air movement increases the rate of transpiration.
- Light Intensity
- Increasing light intensity causes the rate of transpiration to increase.
- This is because it results in stomatal opening.
- Increasing light intensity causes the rate of transpiration to increase.
- The rate at which water is lost is called the transpiration rate.
- Potometer
- Measures the loss of water vapour (or the evaporation of water) from the leaves.
- Set Up
- 1. Shoot cut is placed under the water to prevent air entering.
- Avoid wetting the leaves because the rate of transpiration will be reduced due to the concentration gradient changing.
- 2. Place shoot into the bung and seal with Vaseline to prevent air entering.
- 3. Allow time for the apparatus to settle and adapt to the new conditions.
- 4. Ensure the bubble is set at an appropriate position on the scale.
- 5. Record the movements of the bubble per unit of time (minute/hour/day etc.) and this will give and approximation of the transpiration rate.
- When explaining the method, remember to state how you would alter the environmental conditions.
- E.G. Number of Fans
- E.G. Number of Lamps
- E.G. Temperature (number of heaters).
- When explaining the method, remember to state how you would alter the environmental conditions.
- 5. Record the movements of the bubble per unit of time (minute/hour/day etc.) and this will give and approximation of the transpiration rate.
- 4. Ensure the bubble is set at an appropriate position on the scale.
- 3. Allow time for the apparatus to settle and adapt to the new conditions.
- 1. Shoot cut is placed under the water to prevent air entering.
- Measures the distance travelled by the bubble over a set period of time.
- Control Variable = Removing shoot leaves.
- Adaptation of Different Plants to Differing Water Availability
- Plants can be classified depending on water availability.
- Xerophytes
- Plants that live in conditions where water is scarce.
- Low water availability.
- E.G. Marram Grass (grow in sand dunes).
- Curled Leaf
- Stomata are inside the leaf.
- This decreases surface area so less water is lost.
- Thick Waxy Cuticle
- Reduces water loss.
- Sunken Stomata
- Found deep inside the leaf.
- This reduces the concentration gradient between the leaf and the atmosphere (a less significant difference).
- Hairs
- Traps water vapour.
- Reduces concentration gradient between the inside and outside of the leaf.
- Roots
- Really large roots to absorb water.
- Plants that live in conditions where water is scarce.
- Hydrophytes
- Plants that live in water.
- They grow submerged or partially submerged in water.
- E.G. Water Lily
- Large Air Spaces
- Provides buoyancy.
- Stomata
- On the upper surface of the leaf to allow gas exchange to take place.
- Thin Cuticle
- Little water vapour is lost.
- Little Support Tissue
- Not needed as the plant is supported by the water.
- Plants that live in water.
- Mesophytes
- Plants that live where there is an adequate supply of water.
- Xerophytes
- Plants can be classified depending on water availability.
- Translocation
- Takes place in the phloem.
- Phloem Structure
- Consists of several types of cells.
- Main ones are Sieve Tube Cells and Companion Cells.
- Sieve Tube cells are stacked on top of one another.
- End walls of these cells contain pores known as sieve plates.
- These cells do not contain a nucleus.
- Organelles have disintegrated.
- Each Sieve Tube cell is linked to one or more companion cells by the plasmodesmata.
- Companion cells contain dense cytoplasm, nucleus and mitochondria.
- Sieve Tube cells are stacked on top of one another.
- Main ones are Sieve Tube Cells and Companion Cells.
- Consists of several types of cells.
- Phloem Structure
- DEFINITION: It is the transport of soluble organic material (e.g. sucrose or amino acids).
- The SOURCE is the site of photosynthesis and the synthesis or organic materials.
- The SOURCE will always be the leaf.
- Sucrose / Amino Acids will be transported away from the source and they will be transported to all other parts of the plant where they will be used for storage and growth.
- These areas are known as SINKS.
- Theories of Translocation
- Main theory is the Mass Flow theory.
- This suggests that there is a passive flow of sugars from the phloem to the leaf, where it is of high concentrations to the growing regions.
- This is because the source (leaf) has a higher water potential / higher hydrostatic pressure than in the roots.
- Consequently the sucrose moves on mass from source to sink.
- This is because the source (leaf) has a higher water potential / higher hydrostatic pressure than in the roots.
- Arguments Against Mass Flow Theory
- It doesn't explain the existence of sieve plates.
- Rate of transport in the phloem is 10,000 times faster than by diffusion.
- Sucrose and amino acids flow at different rates and in different directions - this is not explained by mass flow theory.
- Companion cells contain mitochondria and produce energy - the role of these cells is not explained in the theory.
- This suggests that there is a passive flow of sugars from the phloem to the leaf, where it is of high concentrations to the growing regions.
- Recent Theories
- An active process is involved.
- Cytoplasmic streaming is responsible for the bi-directional flow.
- Solutes travel along different routes.
- E.G. Protein filaments.
- Main theory is the Mass Flow theory.
- Experimental evidence to prove that organic substances are transported in the phloem:
- 1. Ringing Experiment
- Bark was removed and the contents of the phloem were examined.
- This proved the phloem transports sucrose.
- Bark was removed and the contents of the phloem were examined.
- 2. Aphid Experiment
- Aphid attaches to phloem - body is cut away leaving the mouthpart and phloem sap is analysed.
- Proved the sap is sucrose / amino acids.
- Also proved that the sucrose / amino acids are transported at a rapid rate - too rapid to be explained by diffusion.
- Proved the sap is sucrose / amino acids.
- Aphid attaches to phloem - body is cut away leaving the mouthpart and phloem sap is analysed.
- 3. Radioisotopes - C14
- C14 is supplied to the leaf. C14 is a fixed part of glucose which is converted into sucrose in the leaves and is translocated from the leaves to the sink (e.g. roots).
- This proved that sucrose is transported bi-directionally to the sinks.
- C14 is supplied to the leaf. C14 is a fixed part of glucose which is converted into sucrose in the leaves and is translocated from the leaves to the sink (e.g. roots).
- 1. Ringing Experiment
- Takes place in the phloem.
- Transpiration
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