B5. Transportation Flashcards
What is the function of a xylem?
Transports water and mineral salts.
What is the function of a phloem?
Transports sugar, amino acids and hormones.
In what directions do phloem and xylem transport in?
Xylem = unidirectional; upwards from roots in the soils via stem, leaves, fruits and flowers
Phloem = transports in 2 directions
- Upwards from leaves to fruits and flowers
- Downwards from leaves to roots
State the functions and adaptations of root hair cells.
Functions:
- Increases the SA of the root for absorption of water and mineral ions.
- Provides anchorage for the plant.
Adaptations:
- Thin, permeable cell walls for water to easily diffuse in (and partially permeable membrane)
- Many of them: large SA to absorb
- Concentrated solution (cell sap contains mineral ions and some sugars) for higher water potential gradient.
State the pathway taken by water through root, stem and leaf.
Root hair cell -> cortex -> xylem of root -> xylem of stem -> xylem of leaf -> mesophyll cells.
Define ‘transpiration’.
The evaporation of water at surfaces of the mesophyll cells, followed by the loss of water vapour from the plant leaves, through the stomata.
What is cohesion?
When water molecules attract each other, sticking together.
What is adhesion?
Water molecules stick to xylem vessel.
How is water vapour lost in relation to cell surfaces, air spaces and stomata?
- Transpiration
- Water evaporates into the air spaces in the spongy mesophyll
- High concentration of water molecules in the air spaces
- Water vapour diffuses out of the leaf into the surrounding air, through the stomata, by diffusion.
Describe the effects of temperature on transpiration rate.
Increase in temp = increases the kinetic energy of the water molecules, so they diffuse faster.
Describe the effects of light on transpiration rate.
Increase in light intensity = stomata open to allow gas exchange for photosynthesis, so water vapour can diffuse out of the leaf.
Describe the effects of humidity on transpiration rate.
Decrease in humidity = lower concentration outside the leaf -> steeper concentration gradient for diffusion.
Describe the effects of air movement on transpiration rate.
Wind = water blown away from surface of leaf, increases rate of transpiration.
Explain the mechanism of water uptake in terms of transpiration.
- Water enters root hair cells by osmosis.
- As the water enters the cell, its water potential becomes higher than in the cell next to it.
- So the water moves by osmosis, into the next cell.
- This process if repeated until water reaches the xylem.
Explain the mechanism of water movement through a plant.
- Transpiration continuously removes water from the leaf;
- Thus water is constantly being taken from the top of the xylem vessels, to supply the cells in the leaves;
- This reduces the effective pressure at the top of the xylem vessels;
- This creates a transpiration stream or ‘pull’, pulling water up;
- Water molecules have a strong tendency to stick together. This is called cohesion;
- When the water is ‘pulled’ up the xylem vessels, the whole column of water stays together;
- Roots also produce a root pressure, forcing water up the xylem vessels.
