13.7 Exchange And Transport Flashcards
(34 cards)
What are the root hairs of a plant responsible for?
The absorption of water and mineral ions.
What are the exchange surfaces of a plant?
Root hairs
By which process do plants constantly lose water?
Transpiration.
How much water is lost by plants per day by transpiration?
As much as 700 dm^3 per day in a large tree.
Why are root hair cells efficient surfaces for the exchange of water and mineral ions?
- they provide a large surface area as they are very long extensions and occur in thousands on each of the branches of a root.
- they have thin surface layer across which materials can move easily.
What is the water potential of the soil solution surrounding root hair cells?
Slightly less than zero (mostly water) it contains small quantities of mineral ions.
How does water move from the soil into the root hair cells?
By osmosis.
What do root hair cells contain which make them have a low water potential?
Sugars, amino acids and mineral ions are dissolved in them. Water moves by osmosis down the water potential gradient.
How does water continue it’s journey across the root after moving into it from the soil by osmosis?
- the apoplastic pathway
- the symplastic pathway
Describe the apoplastic pathway.
As water is drawn into endodermal cells it pulls more water along behind it due to cohesive properties of water molecules. Therefore water is drawn along the cell walls of the cells of the root cortex.
(Little or no resistance to the pull of water because of cellulose cell walls with water filled spaces.)
Describe the symplastic pathway.
This takes place across the cytoplasm of the cells of the cortex due to osmosis.
The water passes through the cell walls along tiny openings called plasmodesmata which are each filled with a thin strand of cytoplasm. So, there is a continuous column of water from the root hair cell to the xylem at the centre of the root.
How does water move through the column in symplastic pathway?
Water entering by osmosis increases the water potential of the root hair cell.
The root hair cell now has higher water potential than the first cell in the cortex.
Water therefore moves from the root hair cell to the first cell in the cortex by osmosis down, the water potential gradient.
The first cell now has a higher water potential than its neighbour to the inside of the stem.
This chain continues, a water potential gradient is set up across all the cells of the cortex, which carries water along the cytoplasm from the root hair cell to the endodermis.
What blocks water travelling by the apoplastic pathway after it reaches the endodermis?
The waterproof band that makes up the casparian strip in the endodermal cells.
Where does the blocked water on the apoplastic pathway go?
It is forced into the living protoplast of the cell where it joins water that has arrived there by the symplastic pathway.
What is the most likely way that water gets into the xylem after it has reached the living protoplast?
The active transport of salts - carrier proteins actively transport salts into the xylem from the endodermal cells. This creates a lower water potential in the xylem. Water moves by osmosis down a concentration gradient into the xylem. (Helps create root pressure).
Give evidence for root pressure.
- the pressure increase with a rise in temperature and decrease at lower ones.
- metabolic inhibitors such a cyanide prevent most energy release by respiration and also cause root pressure to cease.
- a decrease in the availability of oxygen or respiratory substrates causes a reduction in root pressure.
Learn the root picture on page 190
Root hair Cortex Endodermis Phloem Xylem
What is the main force that pulls water up a stem?
The evaporation of water from the leaves. Transpiration.
The humidity of the atmosphere is usually less than that of the…
Air spaces next to the stomata. Water vapour molecules diffuse out of the air spaces into the surrounding air.
By which method is water lost from the mesophyll cells of the leaf?
By evaporation from their surfaces to the air spaces of the leaf. This is replaced by water reaching the mesophyll cells from the xylem by apoplastic or symplastic pathways.
(Remember symplastic method or water potential gradients etc).
Name the two main factors responsible for the movement of water up the xylem.
Root pressure
Cohesion-tension theory
Describe cohesion-tension theory.
Water evaporates from the leaves as a result of transpiration.
Water molecules form hydrogen bonds between one another and stick together (cohesion).
Water forms a continuos pathway across the mesophyll cells and down the xylem.
As water evaporates, more molecules of water are drawn up behind it as a result of cohesion.
Water is hence pulled up the xylem as a result of transpiration - transpiration pull - this puts the xylem under tension.
Give evidence for the cohesion-tension theory.
- change in diameter of tree trunks according to the rate of transpiration. During the day when transpiration is at it’s greatest there is more tension in the xylem. The tree trunk shrinks in diameter.
- if a xylem vessel is broken and air enters it, the tree can no longer draw up water. The column is broken- water can’t stick together.
- when a xylem vessel is broken, water does not leak out, instead air is drawn in which is consistent with it being under tension. (If it was under pressure, water would leak out).
What type of process if transpiration?
Passive.
(Xylem vessels are dead anyway). (Heat does come from the sun to cause evaporation though).
