⭐️Exchange:Mass transport in plants Flashcards
Why does water vapour move out of the stomata?
Because the humidity of the atmosphere is less than that of the air spaces next to the stomata so there is a downwards water potential gradient
What do root hair cells do?
They are the exchange surfaces of plants responsible for the absorption of water by osmosis and mineral ions by active transport
What is the xylem?
Dead hollow lignified tubes
Give the steps to the cytoplasmic route of water movement across a leaf
- Mesophyll cells lose water to the air spaces by evaporation due to the heat supplied by the sun
- These cells now have a lower water potential so water enters by osmosis from neighbouring cells
- The loss of water from the neighbouring cells lowers their water potential
- They in turn take in water from their neighbours by osmosis
- This establishes a water potential gradient that pulls water from the xylem, across the leaf mesophyll and into the atmosphere
What are the two ways water us drawn up the xylem?
- Cohesion due to H bonds between water molecules forming a continuous unbroken column of water
- adhesion due to h bonding between the water molecules and the walls of the xylem vessels
What are the 6 factors affecting water loss?
- humidity
- temperature
- wind
- surface area
- stomatal density
- thickness of the waxy cuticle
Give one way root hair cells are adapted to their function
They have a large surface area to maximise active transport
What does transpiration do?
It causes the continuous unbroken column of water to be stretched and put under tension which causes negative pressures that cause the xylem walls to pull inwards hence the xylem must be thick in order to prevent collapsing
What are the steps of the cohesion tension theory?
- Water evaporates from mesophyll cells due to the heat from the sun in transportation
- due to cohesion between water molecules, water forms a continuous unbroken column (KEY PHRASE) across mesophyll cells in the leaf and all down the xylem
- as more water evaporates more water is drawn up to replace it
- Transpiration pull (KEY PHRASE) creates tension and negative pressures within the xylem
- Therefor, movement of water up the xylem is a passive process mostly driven by transpiration
What is a potometer?
A piece of equipment used for measuring rate of transpiration
Why is it important that xylem vessels have no end walls?
Because it means the xylem forms a series of continuous unbroken tubes from root to leaves which is essential for cohesion tension theory of water flow up the stem
Give three pieces of evidence that support the cohesion tension theory
- the diameter of tree trunks change during the day as during the day when transpiration is at it’s greatest, there is more tension in the xylem which pulls the xylem wall inwards causing the diameter to shrink. Vice versa at night
- if a xylem vessel is broken and air enters it, a plant can no longer draw up water because the continuous column of water is broken so molecules can’t stick together
- a xylem vessel that’s broken doesn’t cause water to leak out of it, instead, air is drawn into it which is consistent with it being under tension
What are phloem cells made up of?
- sieve tube elements which have perforated end walls creating sieve plates
- companion cells which provide ATP required for active transport of organic substances
What are sources?
The sites of production of the organic substance e.g. sucrose which is created in the leaf so the leaf is the sink
What are sinks
The location where the organic substance is directly used or stored for future use, for sucrose, this while be respiring cells