Mass transport systems in plants

Question 1

What is transpiration + explain the factors affecting transpiration

Answer 1

The loss of water vapour from stomata by evaporation

Factors affecting it:

Light intensity: Positive correlation- more light, more stomata open= large SA for evaporation

Temp: Positive correlation- more heat, more kinetic energy, faster molecules so more evaporation

Humidity: Negative correlation- more water vapour in air, water potential more positive outside leaf so decreases water potential gradient

Wind: Positive correlation- More wind blows away humid air containing water vapour, maintaining water potential gradient.

Question 2

Explain cohesion- tension theory

Answer 2

-Water is a diploar molecule so hydrogen bonds can form between the hydrogen + oxygen on different water molecules - creates cohesion so water travels up xylem as a continuous column
- Surface tension also creates sticking effect
-Adhesion is when water sticks to other molecules - water adheres to xylem walls. Narrower the xylem, bigger the impact of adhesion ( pulls water up)
-As water moves into roots by osmosis it increases vol of liquid in root so pressure increases in root ( root pressure). This increase in pressure forces water above it upwards.

Question 3

Describe the features of xylem vessel

Answer 3

-Transport water + minerals + provide structural support
-Long cylinders of dead tissue with open ends to form continuous column
-Contain pits enabling water to move sideways
-Thickened with lignin which has spiral patterns making plant flexible

Question 4

Explain how water moves up xylem

Answer 4

1- Water vapour evaporates out of stomata on leaves + loss in water vol creates lower pressure
2- When water is lost by transpiration, more water is pulled up to replace it (moves due to negative pressure)
3- Due to hydrogen bonds they are cohesive- creates column of water within xylem
4- Water molecules adhere to walls of xylem- helps pull water column upwards
5-As column of water is pulled upwards, it creates tension, pulling xylem in to become narrower ( increasing impact of adhesion)

Question 5

Name the 2 components of phloem vessels

Answer 5

-Sieve tube elements- form a tube to transport sugars (e.g. sucrose) in dissolved form of sap. Living cells, no nucleus + few organelles, hollower tube so maximum flow. End walls are perforated to form sieve tube plates

-Companion cells- involved in ATP production for active loading of sucrose into sieve tubes

Question 6

Translocation- mass flow theory- how sucrose transports from source to sieve tube element

Answer 6

-Sucrose is manufactured from products of photosynthesis in cells with chloroplasts
-High conc of sucrose at site of production so sucrose diffuses down conc gradient into companion cells via facilitated diffusion
-Hydrogen ions are actively transported from companion cells into spaces within cell walls using ATP
-This creates a conc gradient so hydrogen ions diffuse down conc gradient through carrier proteins into sieve tube elements
-Co-transport of sucrose with H+ via co-transporter proteins to transport sucrose into sieve tube element

Question 7

Translocation- mass flow theory- movement of sucrose within phloem sieve tube element

Answer 7

-Increase in sucrose in sieve tube elements lowers water potential
-Xylem has higher water potential so water enters sieve tube elements from surrounding xylem vessels via osmosis
-Increase in water vol in sieve tube elements increases hydrostatic pressure causing liquid to be forced towards sink

Question 8

Translocation-mass flow theory- transport of sucrose to the sink ( respiring cells)

Answer 8

-Sucrose is used in respiration at the sink or stored as insoluble starch
-Cells have a low sucrose content so sucrose is actively transported into them, lowering their water potential
-Due to lowered water potential, water moves into sink cell from sieve tubes by osmosis
-Removal of water decreases vol in sieve tube element so hydrostatic pressure decreases
-High hydrostatic pressure at source + low hydrostatic pressure at sink
-There’s a mass flow of sucrose solution down hydrostatic gradient in sieve tubes

Question 9

Give evidence for + against the mass flow hypothesis of translocation

Answer 9

For:
- Pressure within sieve tubes, as shown by sap being released when cut
-Higher sucrose conc in leaves than roots
-Increases in sucrose levels in leaf are followed by similar increases in sucrose levels in phloem

Against:
-The function of sieve plates is unclear as they would seem to hinder mass flow
-Not all solutes move at same speed ( as they would in mass flow)
-Sucrose is delivered at same rate rather than to lower sucrose concs

Question 10

How can tracing experiments be used to investigate transport in plants?

Answer 10

-Tracing involves radioactively labelling carbon
-Plants are grown in environment containing radioactively labelled CO2 + this is absorbed into plant + used in photosynthesis to create sugars containing radioactively labelled carbon
-Thin slices from stems are cut + placed on x-ray film that turns black when exposed to radioactive material
-Section of stem containing sugars turn black highlighting where the phloem are + shows sugars are transported in phloem

Question 11

How can ringing experiments be used to investigate transport in plants?

Answer 11

-A ring of bark + phloem is peeled + removed off a tree trunk
-Result of removing phloem is trunk swells with sucrose solution above removed section + tissue below dies
-Shows sucrose is transported in phloem