3.3.4.2 Mass Transport in Plants Flashcards
How does water out through the stomata?
Humidity of the atmosphere is less than that of the air spaces next to the stomata
When stomata are open, water vapour molecules diffuse out of the air spaces into the surrounding air
Water lost by diffusion from the air spaces is replaced by water evaporating from the cell walls of surrounding mesophyll cells
By changing the size of the stomatal pores, plants can control their rate of transpiration
What is the function of xylem tissue?
It transports water and soluble materials from the roots to the leaves
How does water move across the cells of a leaf?
Mesophyll cells lose water to the air spaces by evaporation due to heat
Cells have a lower water potential so water enters via osmosis from neighbouring cells
Loss of water from neighbouring cells lowers water potential
They take water from neighbouring cells
This creates a water potential gradient where water moves to the mesophyll then into the atmosphere
How does water move up the xylem stem?
Water evaporates from mesophyll cells due to heat
Water molecules form hydrogen bonds and stick together- cohesion
Water forms a continuous column across mesophyll cells and down the xylem
As water evaporates, more molecules are drawn up via cohesion (transpiration pull)
Transpiration puts the xylem under tension which creates negative pressure (cohesion tension theory)
What is the structure of the xylem?
Long cells
Have thick cells containing lignin
Lignin waterproofs walls of cells and strengthens them
Cells die from lignin which forms a long tube
Lignification is not complete and pores (called pits or bordered pits) allow water to move between vessels or into living parts
Lignin forms rings or spirals around the vessel and strengthen the tube
What is the apoplastic pathway?
Movement between walls of neighbouring cells
What is the symplastic pathway?
Movement through plasma membranes and plasmodesmata to cytoplasms from cell to cell
What is the vacuolar pathway?
Same as the symplastic pathway but also through vacuoles
What are the effects of the cohesion tension theory?
In daytime there is greater transpiration which creates a higher tension thats pulls the xylem walls inward and shrinks the trunk Therefore the trunk is wider at night
Xylem vessel is broken which allows air to enter it. No water is drawn up because the continuous column of water is broken and cohesion is stopped
Or the xylem vessel is broken so water does not leak because air is drawn in instead which creates tension
Is transpiration pull an active/passive process?
Passive process and xylem tissue is dead so cannot actively move water
Is transpiration an active/passive process?
An active process as heat from the sun evaporates water from the leaves
How does xylem tissue withstand the pressure of cohesion tension?
It is strengthened with lignin
What is transpiration?
The loss of water vapour from upper parts of the plant
What is the endodermis?
A layer of cells surrounding the xylem
Cells move minerals by active transport into the xylem
What is capillary action in plants?
Adhesion of water to xylem vessels as they are narrow
What affects the rate of transpiration?
Leaf number, number/size/position of stomata, cuticle, light, temperature, humidity, wind, water availability