A7 Mass transport in plants Flashcards
What are potometers used to measure?
Transpiration
What are the steps in using a potometer?
- Cut a shoot underwater to prevent air getting into xylem and cut at a slant to increases the surface area for water uptake.
- Assemble the potometer under water and insert the shoot
- Keep the end of the capillary tube in water
- Check the apparatus is water and airtight
State and explain the 5 properties of water.
- Water is cohesive which means it sticks to other water molecules due to its hydrogen bonds. It is also adhesive which means it sticks to other substances forming a hydration shell
- Water can act as a solvent so it can dissolve many different kinds of molecules.
- Water is a reactive molecule as it is used in condensation reactions and released in hydrolysis reactions. It is also a metabolite as it is involved in important metabolic reactions ( photosynthesis and respiration).
- Water has high latent heat for vaporisation ( requires heat energy to convert from a liquid to a gas which is called evaporation which involves sweat)
- Water has high specific heat capacity as it acts as a temperature buffer (maintains temp change)
What is the theory called in the xylem where water molecules are cohesive and that there is a pull force from evaporation and diffusion?
Cohesion-tension theory
What is transpiration?
The evaporation and diffusion of water molecules out of the stomata
Describe the Cohesion- tension theory. 5 marks
-Water lost from leaf because of transpiration / evaporation and diffusion of water out of the stomata
-Lowers water potential of mesophyll / leaf cells
-Water pulled up xylem (creating tension)
-Water molecules cohere / ‘stick’ together by hydrogen bonds;
-(forming continuous) water column;
-Adhesion of water (molecules) to walls of xylem;
State and explain the factors that affect transpiration.
- Light intensity (high) -More stomata are open so there is more diffusing of water out
- Temperature (high) - More kinetic energy so there is a higher rate of diffusion and transpiration
- Wind speed (high) - wind carries away diffused water molecules so there will be a high concentration gradient therefore a higher rate of diffusion
- Humidity (low)- High water concentration gradient which increases the rate of diffusion
What is humidity?
How much water is in the air
Give two example of Xerophytes.
Cactus and Marram grass
How do Xerophytes slow down the rate of transpiration by their adaptations?
Marram grass:
- Waxy, Waterproof cuticles on leaves which prevents evaporation
- Leaf hair which trap water reducing the rate of water loss
- Stomata sunk in pits reduces the concentration gradient which reduces diffusion and evaporation
What are the differences between the Xylem and the Phloem?
Xylem Phloem
-Transports water - Transports sugars
and minerals
- Transport is up the plant - Transport is up and down
- No end walls - End walls with pores
- Dead hollow vessels - Cells are alive ( AT)
- Strengthened with lignin - Companion cells help it
live - No subcellular structures
What is the definition of Translocation?
- The movement of organic substances from source to sink in the Phloem
What is a source cell?
A cell where sucrose is made
What is a sink cell?
A cell where sucrose is transported to be used, for example it could be stored as starch or used as cellulose for growth
What is sucrose made of?
Glucose and Fructose
How does sucrose move out of the source cell?
Sucrose is actively transported by companion cells into the seive tube
What cause the movement of sucrose from the top, near the source cell and the bottom, near the sink cell?
There is a high hydrostatic pressure near the source cell as water is entering the Phloem from the Xylem via osmosis this creates a pressure gradient (mass flow)
Why is there a low water potential of water near the source cell?
There is a high concentration of sucrose as it is exiting the source cell, this decreases the water potential
Why does osmosis occur near the source cell?
There is a low water potential in the sieve tubes so water moves from high water potential to low water potential.
How does the source cell form sucrose?
Photosynthesis or hydrolysing starch
Why is there a low water potential near the sink cell?
There is a lower concentration of sucrose as it is being actively transported by the companion cells into the sink cells
What lowers the water potential in the Xylem?
Minerals (salt and ions)
State and explain features of the Xylem.
Xylem vessels are stacked on top of each other to ensure a continuous column of water.
Lignin in the walls makes the walls waterproof and allows them to withstand tension
Thick cell walls withstand tension
Narrow tubes increase the surface are for adhesion
Xylem cells have pores which enables the sideways flow of water and exit or entry of water
Xylem has no end walls to ensure a continuous column of water
