Mass Transport in Plants A1 Flashcards
What labels does the cross section of a root have?
(6)
- phloem
- xylem
- epidermis
- root hair
- endodermis
- cortex
Describe the features of a root and how they help its function.
(2)
- extensions to increase surface area to increase uptake of water
- thin cell wall for a shorter diffusion pathway
Explain how roots absorb minerals, ions and water.
(3)
- actively transport ions from soil across their membrane from soil across their membrane into cytoplasm, via carrier proteins
- this lowers the water potential of the cytoplasm, to below the water potential in the soil
- water enters the root hair cells via osmosis down a water potential gradient (high to low) through a partially permeable membrane
How does water get into xylem from root hair cell?
Water passes from root hair cells, across the cortex cells to the xylem.
Describe the symplastic pathway.
(5)
- water passes through the cytoplasm of the cell
- across the cell membranes
- through plasmodesma (hole in cell wall) from one cell to another
- via osmosis down a water potential gradient
- it is relatively slow as there is resistance to the movement of water (cell membrane and solutes)
Describe the apoplastic pathway.
(4)
- water passes through the adjoining cell walls from one cell to another
- movement is relatively fast (less resistance to water molecules as larger gaps in between cellulose molecules)
- this is via diffusion
- the water then reaches the endodermis
What do the cells of the endodermis do to water and ions?
(6)
- the endodermis has a waterproof strip (casparian strip) which prevents water movement through walls of the endodermis
- water is forced to pass through cytoplasm of cell of the endodermis
- the cells actively transport ions to xylem
- this lowers water potential in xylem
- so water passes into xylem from endodermal cells down a water potential gradient
- this enables movement of water from root hairs to centre of root, down a water potential gradient.
Describe the structure and function of the xylem.
(4)
- dead cells which form hollow tubes - this allows water potential to not be affected, so water stays within the xylem vessels and doesn’t leave
- end walls break down so xylem forms a continuous tube - so water can form a continuous column
- cell walls are strengthened with lignin - this makes the xylem waterproof
- there are pits in the xylem - these are little holes which allow water to move between xylem vessels.
Explain how water enters xylem from the endodermis in the root and is then transported to the leaves.
(8)
(in the roots)
1. casparian strip blocks apoplastic pathway/only allows symplastic pathway
2. active transport by endodermis
3. of ions/salts into xylem
4. lowers water potential in xylem/water enters xylem by osmosis/down a water potential gradient
(xylem to leaf)
5. evaporation / transpiration (from leaves)
6. creates cohesion/tension (H-bonding) between water molecules (negative pressure)
7. adhesion/water molecules mind to xylem
8. creates continuous column of water
Root pressure moves water through the xylem.
Describe what causes root pressure.
(4)
- active transport by endodermis
- ions/salts into xylem
- lowers water potential (in xylem)
- water enters by osmosis
Name a factor that can affect transpiration.
light / temperature / air movement / humidity
Describe the process of transpiration.
(2)
- open stomata (so CO2 can enter leaf) causes water to diffuse from air spaces in leaf to out leaf from high-low water potential (transpiration)
- this causes a continuous concentration gradient through cells in the plant causing a continuous column in xylem vessels
What is the cohesion tension theory of water movement up a plant?
(6)
- water moves out stomata from a high to low concentration gradient
- water forms continuous column in narrow xylem vessel
- water moles have forces of attraction so they stick together - they have cohesion
- they are also attracted to the walls of the xylem (adhesion)
- pulling force is great and column is under tension
- this is called transpiration stream
Root pressure moves water through the xylem.
Describe how a high pressure is produced in the leaves.
(3)
- water potential becomes lower/becomes more negative (as sugar enters phloem)
- water enters phloem by osmosis
- increased volume (of water) caused increased pressure
What type of pressure aids water movement from root to top of tree?
Hydrostatic pressure is higher in roots than top of tree so aids water movement upwards