Unit 4- Plant Transport Flashcards
What makes up the vascular bundle of roots and stems?
Phloem
Cambium
Xylem
Pericycle (root only)
What is the function of the pith and cortex?
Pith- used for food storage (centre of root stem structure)
Cortex- contains ‘packing cells’ which provide the stem strength, flexibility and in roots contains stored starch.
How do the epidermis’ differ in roots and stems?
Stem- secretes a waterproof cuticle and may be replaced by bark in trees
Root- the single layer of cells contains root hairs which increase surface area enormously
What is the purpose of the endodermis and where is it found?
In the root
- a single layer of cells surrounding the vascular tissue, it is waterproofed with the casparian strip
Describe the structure of a xylem:
Structure:
Long narrow tubes made of dead cells containing lots of lignin, before the cells die they form thick cell walls which contain the added lignin
Structure of the phloem: 3
Long columns of sieve tube cells (no nucleus, and alive) with holes at either end (sieve plates)
The cytoplasm has been reduced to cytoplasmic strands which move up and down the column
Each sieve tube cell has an adjacent companion cell connected by plasmodesmata (food holes) so proteins and nutrients can be fed to the companion cells
What is the difference between symplast and apoplast pathway?
Symplast - through living cytoplasm
Apoplast - through non-living cell walls
Describe the symplast pathway from root hairs to xylem:
- water is absorbed by osmosis into root hair cells (down water potential gradient)
- water then diffuses through the cell cytoplasms which are connected by plasmodesmata (holes in the cell wall)
- all down a water potential gradient and no membranes need to be crossed
Describe the apoplast pathway from root to casparian strip (waterproof layer in endodermis, surrounding the xylem):
- water is absorbed by osmosis into root hair cell wall
- cell walls are thick and very open so water can easily diffuse through them down the water potential gradient
- no cell membranes to cross (diffusion only) but this all stops just before reaching the xylem
- water then needs to cross the cell membrane by osmosis joining the symplast pathway
What is root pressure?
The pressure produced from osmosis that pushes water up the xylem
Why do the xylem vessels shrink during the day?
Transpiration is occurring most
Causes mass flow of water up xylem
The pressure pulls in the xylems
Describe mass flow:
- Transpiration in leaves causes low pressure
- water is sucked up the stem to replace lost water
- water has a high tensile strength due to hydrogen bonds
- because the water molecules are cohesive the whole column is pulled up the xylem
- cohesion tension model
What makes up the walls of xylem?
Strong lignin of dead cells
How does water diffuse through the leaf?
Water diffuses from the leaf vein through the cells (symplast and apoplast pathway) and evaporates into the sub-stomata air space
What do we use to measure water uptake/rate of transpiration?
Potometer
Why do potometers not work for much larger plants? (If it could fit)
We assume rate of water uptake = rate of transpiration
For larger plants this is not so much the case
Rate of transpiration during the day compared to night?
Day - more transpiration so lose more water and wilt
Night - less transpiration than water uptake so cells become turgid
How does temp increasing affect rate of transpiration?
High temp -> increase in rate of evaporation -> due to more KE-> water molecules diffuse from sub stomata air space through stomata faster
How does humidity affect transpiration?
High humidity means a high water potential in surrounding air -> lower water potential gradient between sub stomata air space and outside-> less evaporation
Winds affect on transpiration?
Wind blows away water molecules replacing with drier air -> lowers water potential of surrounding air -> water potential gradient and transpiration
Lights effect on transpiration?
Light stimulates plants to open their stomata for photosynthesis -> more light, more transpiration
What factors affect the rate of transpiration?
Light, humidity, temp, wind
What is in the phloem?
A very concentrated solution of sucrose, amino acids and hormones (sap)
What is glucose converted to for translocation?
Sucrose
Describe translocation:
- sucrose is loaded into phloem from companion cells (AT), lowering WP at top of phloem
- ions are being actively transported out the xylem to the leaf, increasing WP in xylem
- water diffuses from xylem to phloem (osmosis) down the WP gradient
- increased hydrostatic pressure at top of phloem, sap is forced down the phloem
- sucrose diffuses out the phloem in the roots and the rest is actively transported out
- WP at bottom of phloem increases, while ions are being actively transported into xylem from soil
- WP is now lower in phloem than xylem so water moves by osmosis into xylem
What does puncturing a phloem and watching sap ooze out tell us?
There is a high pressure inside the phloem
