⭐️Exchange:Mass transport in plants

Question 1

Why does water vapour move out of the stomata?

Answer 1

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

Question 2

What do root hair cells do?

Answer 2

They are the exchange surfaces of plants responsible for the absorption of water by osmosis and mineral ions by active transport

Question 3

What is the xylem?

Answer 3

Dead hollow lignified tubes

Question 4

Give the steps to the cytoplasmic route of water movement across a leaf

Answer 4

1. Mesophyll cells lose water to the air spaces by evaporation due to the heat supplied by the sun
2. These cells now have a lower water potential so water enters by osmosis from neighbouring cells
3. The loss of water from the neighbouring cells lowers their water potential
4. They in turn take in water from their neighbours by osmosis
5. This establishes a water potential gradient that pulls water from the xylem, across the leaf mesophyll and into the atmosphere

Question 5

What are the two ways water us drawn up the xylem?

Answer 5

• 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

Question 6

What are the 6 factors affecting water loss?

Answer 6

• humidity
• temperature
• wind
• surface area
• stomatal density
• thickness of the waxy cuticle

Question 7

Give one way root hair cells are adapted to their function

Answer 7

They have a large surface area to maximise active transport

Question 8

What does transpiration do?

Answer 8

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

Question 9

What are the steps of the cohesion tension theory?

Answer 9

1. Water evaporates from mesophyll cells due to the heat from the sun in transportation
2. 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
3. as more water evaporates more water is drawn up to replace it
4. Transpiration pull (KEY PHRASE) creates tension and negative pressures within the xylem
5. Therefor, movement of water up the xylem is a passive process mostly driven by transpiration

Question 10

What is a potometer?

Answer 10

A piece of equipment used for measuring rate of transpiration

Question 11

Why is it important that xylem vessels have no end walls?

Answer 11

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

Question 12

Give three pieces of evidence that support the cohesion tension theory

Answer 12

• 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

Question 13

What are phloem cells made up of?

Answer 13

• sieve tube elements which have perforated end walls creating sieve plates
• companion cells which provide ATP required for active transport of organic substances

Question 14

What are sources?

Answer 14

The sites of production of the organic substance e.g. sucrose which is created in the leaf so the leaf is the sink

Question 15

What are sinks

Answer 15

The location where the organic substance is directly used or stored for future use, for sucrose, this while be respiring cells

Question 16

Give 2 organic molecules the phloem transports

Answer 16

Sucrose and amino acids

Question 17

What are the phases of the mass flow theory?

Answer 17

• translocation 1, transfer of sucrose into sieve elements from photosynthesising tissue
• translocation 2, mass flow of sucrose into sieve tube elements

Question 18

What happens in the first stage of the mass flow hypothesis: transfer of sucrose into sieve elements from photosynthesising tissue

Answer 18

1. Photosynthesis occurring in the chloroplasts of leaves creates inorganic substances
2. The sucrose diffuses down the concentration gradient by facilitated diffusion from the photosynthesising cells into companion cells
3. Hydrogen ions are actively transported from companion cells into the spaces within cell walls using ATP
4. The H+ then diffuse down a concentration gradient through carrier proteins into the sieve tube elements.
5. Sucrose is transported along with the hydrogen ions in co transport using co transporter proteins

Question 19

What happens in the second stage of the mass flow hypothesis:
mass flow of sucrose into sieve tube elements

Answer 19

1. As sucrose has been actively transported into the sieve tubes by co transport the sieve tubes now have a more negative water potential causing the xylems water potential to be much less negative so water moves out of the xylem and into the sieve tubes by osmosis causing the sieve tubes to have a high hydrostatic pressure
2. Because the sink cells (respiring cells in this case) use sucrose, they have a lower concentration of sucrose so sucrose has to be actively transported into them from the sieve tubes which causes the sink cells to have a lower water potential
3. This then causes water to also move into the respiring cells from the sieve tubes by osmosis causing the hydrostatic pressure of the sieve tubes in this region to be lowered
4. Therefore because of water enters the sieve tubes at the source and leaves at the sink, the hydrostatic pressure is high at the source and low at the sink
5. This means there’s a mass flow of sucrose solution down thus hydrostatic gradient into the sieve tubes

Question 20

Give three pieces of evidence that question the mass flow theory

Answer 20

• the function of the sieve plates is unclear as they would seem to hinder mass flow
• not all solutes move at the same speed but they should do so if the movement is by mass flow
• sucrose is delivered at more or less the same rate to all regions rather than going more quickly to the ones with the lowest sucrose concentration which the mass flow theory would suggest

Question 21

How does the use of traces support the mass flow theory?

Answer 21

1. Plants are provided with radioactively labelled carbon dioxide and over time the sugars produces as a result of photosynthesis using the radioactive co2 means they are also radioactively labelled
2. Thin slices from the stem are cut out and placed on x ray film
3. The section of the stem that contains the labelled sugars then turns black which highlights where the phloem are and shows that sugars are transported in the phloem alone.

Question 22

How do ringing experiments support the mass flow theory?

Answer 22

1. A ring of bark and phloem are peeled and removed off a tree trunk
2. This causes the part of the trunk above the ring swells
3. Analysis of the liquid in the swelling shows it contains sugars
4. This means when the phloem is removed, the sugars can no longer be transported and so proves that the phloem transports sugars

Question 23

Give 4 pieces of evidence that support the mass flow theory

Answer 23

• when phloem is cut, a solution of organic molecules flows out
• plants provided with radioactive co2 can be shown to have radioactively labelled carbon in the phloem after a short time
• the needle like mouth part of aphids can be used to penetrate the phloem and extract the contents of sieve tubes which show daily variation in sucrose content of leaves that are mirrored a little later on by identical changes in the sucrose concentration of the phloem
• the removal of the phloem around the whole circumference of the stem leads to accumulation of sugars above the ring and their disappearance from below it