3.3.4.2 Mass Transport in plants Flashcards
Explain how the active transport of mineral ions into the xylem vessels in the roots results in water entering these vessels and then being moved ho the xylem tissue (hint root pressure)
The water potential in the xylem is reduced by the entry of ions
So a water potential gradient is established between the xylem and surrounding cells
The plasma membranes of the surrounding cells are partially permeable
So water enters the xylem via osmosis
So the volume of water in the xylem increases
And so the pressure in the xylem increases as water cannot move back down due to the concentration gradient
The presence of an air bubble in a xylem vessel in the stem blocked the movement of water through that vessel. Use the cohesion tension theory to explain why
Water evaporates from the leaves
So the water in the xylem is under tension causing the water molecules to stick together
So the water is stuck together in a single column, the air bubble breaks the column which prevents cohesion
Explain how water enters a plant root from the soil and travels through to the endodermis
Water enters root hair cells by osmosis (this is because the active transport of mineral ions has a created a water potential gradient causing water to move into the root hair cell)
Then water moves through the cortex by osmosis down a water potential gradient
The water diffuses through the symplastic pathway (through the cells cytoplasm) and apoplastic pathway (through the cell walls)
Describe one piece of evidence that supports the root pressure theory and explain how it supports the theory
Sap is discharged from a cut stem. Only an upward force could make this happen
The diameter of a tree is less during the day than it is at night. Explain how this supports cohesion tension theory
Evaporation of water through the leaves mainly during the daytime
So the tension in the xylem creates an inward pull
So the xylem vessels become narrower due to the adhesion of water molecules to the walls of the xylem vessels
The diameter of a tree is less during the day than it is at night. Explain how this does not support root pressure theory
Root pressure give out an OUTWARD force pushing on the walls of the xylem
So trees would become wider not narrower as the xylem should become wider
Using the cohesion tension theory explain the changes in the circumference of a tress throughout the day
Increase of light, at the start of the day, causes the stomata to open.
So water evaporates out of the stomata causing water to move across the leaf by osmosis.
Evaporation exerts force causing tension in water columns
Cohesion holds water column together
Adhesion between walls of the xylem and the water molecules results in a pulling force which causes the water columns to decrease in volume
Hence the xylem vessels and the tree trunk both become narrower
Explain how the rate of water movement through the apoplast pathway is affected by an increase humidity of air around the leaves
High humidity decreases the water potential gradient so no net movement of water in or out the leaf.
This mean that the rate of water movement decreases as less water is lost by transpiration
Which part of the cortex forms the apoplast pathway?
The cell walls
Explain how the structure of the endodermis affects the passage of water by the apoplast pathway
The Casparian bands are waterproof so lower the water potential in the cytoplasm of the endodermis cell.
So water enters the symplastic pathway by osmosis
Describe how water is moved through a plant according to the cohesion tension theory
Water evaporates from leaves
Reducing the water potential in cells
So water is drawn out of xylem
This creates tension
So cohesive forces between water molecules pull water up as a column
Explain how water enters the xylem from the endodermis in the root and is then transported to the leaves
In the root the casparian strip blocks the apoplast pathway.
Active ta sport by the endodermis of ions into the xylem lowers the xylems water potential.
This results in water entering the xylem via osmosis.
Then evaporation from the leaves creates tension in the xylem.
So water molecules bind to the xylem and create a continuous water column
What is translocation?
Translocation is the transport of organic substances in the phloem from source to sink
give 2 features of xerophytic plants
hairs
stomata in pits
thick way cuticle
rolled leaves
spines/fewer leaves
why do some leaves have hairs on them?
trap a layer of water vapour so WP gradient reduced
