Mass Transport: Plants

Question 1

Xylem

Answer 1

-Tissue that transports water in the stem and leaves of plants
-Provides structural support to stem

-One direction
-Dead cells
-Hollow tube
-Walls lined with lignin, waterproof and structural support

Question 2

Transpiration

Answer 2

Humidity of atmosphere less than in the air spaces of the mesophyll inside stomata.
Water diffuses out of stomata down a water potential gradient.
This lowers the water potential of the air spaces in the mesophyll

Question 3

Cohesion-tension theory

Answer 3

Water molecules form hydrogen bonds between one another and stick together, cohesion.
Water forms a continuous column from the mesophyll cells down the xylem.

Water moves from mesophyll cells into air spaces down water potential gradient following transpiration

Pulling up water from the neighbouring cells and thus the entire water column due to the cohesion, and so water moves up the xylem
Transpiration pull puts xylem under tension.

Question 4

Evidence for Cohesion-tension

Answer 4

Trunks shrink during day, when transpiration is at its highest, showing xylem under tension.

Broken xylem vessels break cohesion and water is no longer pulled up.

Broken xylem vessels cause air to be drawn in (tension) rather that water being pushed out (pressure)

Question 5

Apoplast vs Symplast Pathway

Answer 5

Apoplast: Water moves from root hair cells through the cell walls of the cells in epidermis and cortex, faster, moves by diffusion, can not cross the Casparian strip in the cell walls of the endodermis, so takes symplast through endodermis into xylem

Symplast: Water moves from root hair cells through the cytoplasm and plasmodesmata (connects the cytoplasm of cells) of cells in epidermis, cortex and endodermis into xylem, moves by osmosis, slower.

Question 6

Factors affecting Rate of Transpiration

Answer 6

-Light Intensity
-Temperature
-Humidity
-Wind

Question 7

Light Intensity

Answer 7

The greater the light intensity the greater the photosynthesis, so the stomata stay open for longer for gas exchange as a result there is more water loss

Question 8

Temperature

Answer 8

Higher temperatures more water in the mesophyll evaporates, greater loss of water through stomata

Question 9

Humidity

Answer 9

More humid means there is more water in the air around the stomata.
Shallower water potential gradient between inside mesophyll and environment.
Slower rate of diffusion, less water loss

Question 10

Wind

Answer 10

Wind blows water out of stomata and away from the leaf.
Less water in the air around stomata.
Steeper water potential gradient, greater loss of water

Question 11

Translocation

Answer 11

the transport of assimilates from source to sink and requires ATP

Question 12

Phloem

Answer 12

-bidirectional
-living cells

-Sieve tube elements: form a tube for the transport of sap (dissolved assimilates). Cells have thick walls (withstand pressure), few organelles, a small cytoplasm, so sap moves easier

-Companion cells: produce ATP for the active loading of sucrose into sieve tube. Many mitochondria and infolding to increase SA for active transport

-Plasmodesmata
-Sieve plates

Question 13

Source and Sink

Answer 13

Source: photosynthesizing cells which produce sugars e.g. cells in green leaves, storage organs
Sink: respiring cells which use sugars or cells which store them e.g. cells in meristems, roots, fruits

Question 14

Movement of Sucrose into Sieve Elements

Answer 14

Sucrose form green leaves moves via apoplastic and symplastic pathways to phloem.
Companion cells use ATP to transport hydrogen ions from cytoplasm into their cell walls by active transport
Hydrogen ions diffuse back into the companion cell down a concentration gradient via a cotransporter and the sucrose is co transported in.
Sucrose diffuses into the sieve tube elements from companion cells via the plasmodesmata

Question 15

Mass Flow of Sucrose through Sieve Tubes

Answer 15

Sucrose moves in, water potential decreases, water from xylem moves in via osmosis, increases the hydrostatic pressure in the phloem, sucrose dissolved in water moves through the phloem down a pressure gradient to areas of lower hydrostatic pressure and sucrose is unloaded at sinks.

Question 16

Movement of Sucrose into Sink

Answer 16

Sucrose either diffuses out of the phloem or exits by active transport into the sinks, where it is then converted to storage molecules like starch to maintain the water potential gradient.
Water moves back into the xylem at the sink via osmosis

Question 17

Evidence For Mass Flow Hypothesis

Answer 17

Cut stem releases sap, showing pressure in the phloem.
Higher sucrose concentration in leaves than roots
Increased sucrose in the leaves results in increased sucrose in the phloem
Downward flow in daylight, stops in sunlight
Companion cells have a lot of mitochondria

Question 18

Evidence Against Mass Flow Hypothesis

Answer 18

Function of sieve plates unclear, appear to hinder mass flow.
Not all solutes move at the same speed
Sucrose delivered to all regions at same rate rather than ones with lower sucrose concentration

Question 19

Ringing Experiments

Answer 19

Bark and phloem of a woody stem are removed, xylem remains, area above ring swells due to accumulation of sucrose, showing phloem, not xylem, transports sucrose.

Question 20

Tracing Experiments

Answer 20

Plant is exposed to a radioactive tracer, carbon-14 CO2, which is used in photosynthesis to produce radioactive sucrose, its movement is traced, X-rays of tissue containing the carbon-14 are blackened.

The regions with the radioactive carbon correspond to the regions of the phloem and other tissues do not contain it, showing the phloem is solely responsible for translocation.