Mass Transport In Plants

Question 1

What is xylem

Answer 1

Tissue that transports water in the stem and leaves of the plant
Allows substances to move up the plant from roots to leaves

Question 2

Describe structure of xylem vessels

Answer 2

Long
Hallow - lignin no cytoplasm or nucleus (doesn’t slow water flow)
Ridgid so less likely to collapse
Dead cells
Arranged end to end to form continuous coloumn

Question 3

Why no end walls in xylem

Answer 3

Makes uninterrupted tube so continuous water coloumn

Question 4

What helps water pass up xylem

Answer 4

Mass flow
Cohesion tension theory
Adhesion

Question 5

Describe adhesion

Answer 5

Same bond between water molecules

Stick to side and edges of xylem vessels

Question 6

How is water in the roots move up the stem

Cohesion tension theory in xylem

Answer 6

Water evaporates from the leaves via the (open) stomata due to transpiration

• Reducing water potential in the cell and increasing water potential gradient
• Water drawn out of xylem
• Creating tension
• Cohesive forces between water molecules pull water up as a column
• Water is also adhesive (adhesion) so sticks to the edges of the column

Question 7

More water pulled up the xylem more —-

Answer 7

Transpiration

Question 8

What is transpiration ?

Answer 8

The evaporation of water from the surface of the leaves

Question 9

What is the downside of stomata being open

Answer 9

When stomat open , water moves out of leaf down water potential gradient because more water in leaf down wp gradient
Because more water inside the leaf than in air water

Question 10

Describe transpiration

Answer 10

Water evaporates from moist cell walls and accumulates air spaces in leaf

When stomata open
Water vapour diffuses out of leaf down water potential gradient

Higher concentration of water vapour inside the leaf than outside

Question 11

Explain transpiration stream

Answer 11

Water evaporates from cell walls of mesophyll
Water from xylem replaces this
Water moving out of xylem reduces pressure as it is high pressure in xylem to move up

Question 12

Explain movement of water across the cells of a leaf

Answer 12

Mesophyll cells lose lose water to air spaces by evaporation

These cells have lower wp so water enters by osmosis from neighbouring cells

Loss of water in neighbouring cells lowers their water potential
Take water from their neighbouring cells by osmosis
Water is replaced by neighbouring mesophyll cells
Water moves down gradient
Until wp gradient is established that pulls water from xylem

Question 13

How does light intensity affect transportation rate

Answer 13

The higher the light intensity the faster transpiration rate
Positive correlation
Stomata open when there is light in co2 for photosynthesis
More water to evaporate faster
Stomata close when dark -> low transpiration rate

Question 14

How does temperature affect transpiration rate

Answer 14

The higher the temperature, the faster the transpiration rate (positive
correlation)
- Water molecules gain kinetic energy as temperature increases
- Move faster
- Water evaporates faster as wp gradient increased

Question 15

How does humidity affect transpiration rate

Answer 15

The lower the humidity, the faster the transpiration rate (negative correlation)
- Because as humidity increases, more water is in the air so it has a higher water
potential
- Decreasing the water potential gradient from leaf to air
- less evaporation / slower

Question 16

How does wind affect transpiration rate

Answer 16

The windier, the faster the transpiration rate (positive correlation)

• Wind blows away water molecules from around the stomata
• Decreasing the water potential of the air around the stomata
• Increasing the water potential gradient
• Water evaporates faster

Question 17

Describe how water gets from the soil to the root through to the endodermis

Answer 17

Mineral ions actively transported into root hair cells
Creates wp gradient
Wp in root hair cell is more negative and lower
Water from soil enters root hair cell by osmosis
Water moves thought the cortex by osmosis down wp gradient
(First cortex cell has more negative wp than root hair cell , water moves in , then second cortex)
Water diffuses out through s plastic or a o plastic pathway

Question 18

Name the 2 possible pathways across the cortex to the endodermis

Answer 18

Apoplastic

Symplastic

Question 19

How does water travel through apoplastic way

Cell walls

Answer 19

Water moves through water filled spaces between cellulose molecules in the cell wall

Cohesive forces between water molecules pulls more water along

Question 20

Where and why is the water in the apoplastic pathway stopped

Answer 20

Stops at the endodermis

Once water has reached endodermis it is forced int the symplastic by the casparian strip

Question 21

Describe how does water travel through the symplastic way

Answer 21

Cells in cortex are joined by plasmodesmata
Water moves by osmosis through cytoplasm and vacuoles of ach cell and through plasmodesmata between cells

Continuous flow of water moving down osmotic gradient

Question 22

Explain how from both pathways water gets to the xylem

Answer 22

Endodermis cells actively transport ions in the the xylem
Lowers the xylem water potential
More negative than endodermis cells
Water enters the xylem by osmosis

Root pressure also takes place - influx of water from the soil creates a hydrostatic pressure this is enough to move water up xylem

Question 23

What is the function of the phloem

Answer 23

Phloem tissue transports solutes around plants

Question 24

What is the phloem formed from

Answer 24

It is formed from cells arranged in tubes

Question 25

Name the 2 cells the phloem is formed from

Answer 25

Sieve tube elements

Companion cells

Question 26

Describe sieve tube elements

Answer 26

Living cells that form tube for transporting solutes

No nucleus

Question 27

Describe companion cells

Answer 27

Companion cell for each sieve tube element

Carry out living functions for sieve cells

Question 28

What is translocation

Answer 28

Movement of solutes / assimilates from source to sink

Question 29

Translocation requires ——-

Answer 29

Energy

Question 30

Why does translocation require energy and where does it come from

Answer 30

Companion cells produce ATP to actively load assimilates into and out of sieve tube elements

Question 31

What is a source and what is a sink

Answer 31

Source - area where sucrose is moved into phloem (high conc)

Example source for sucrose is leaves

Sink - area where sucrose is removed from phloem

Example - food storage organs , roots, stems

Question 32

Describe the mass flow hypothesis

Answer 32

Active transport used to actively load solutes from companion cells into sieve tubes of phloem at source

Water potential inside the sieve tubes is lowered , water enters sieve tubes by osmosis from xylem and companion cells

Pressure inside sieve tube increases

At the sink end:
- Low concentration of solute
- Solutes removed to be used up e.g. enzymes hydrolyse
- Increasing the water potential inside the sieve tubes
- Water leaves tubes via osmosis
- Lowering pressure inside sieve tubes
Pressure gradient pushes solute from source to sink

Question 33

Name 4 pieces of experimental evidence of translocation

Answer 33

Ringing experiment
Tracer experiment
Use of aphids
Metabolic inhibitors

Question 34

Describe ringing experiment

Answer 34

Ring of bark/phloem removed from woody stem
= bulge forms above ring
Fluid from bulge has higher concentration of sugars than fluid from below ring
= evidence that there’s downward flow of sugars

Tissue below ring will die because no organic substances can be transported

Question 35

Describe tracer experiment

Answer 35

Supply part of plant with carbon-14
Carbon 14 incorporated into organic substances produced by leaf
These substances move round plant
Movement of substances can be tracked using autoradiography
Results will show where the organic substances have moved via translocation from source to sink

Question 36

In tracer experiment, describe how the movement of substances can be tracked using autoradiography ?

Answer 36

plant killed and placed in a photographic film, film turns black where
the radioactive substance is present

Question 37

How to determine what plant is shown in ringed experiment

Answer 37

Autoradiography show plants killed at different times

So for example if a plant is ringed at a certain point, there should not be any conc of isotope above it

Question 38

How are aphids used to find evidence of translocation

Answer 38

Aphids pierce the phloem using mouthpiece

• Releasing sap from plants
• Flow of sap higher and quicker at leaves/source than further down/ sink
• Evidence of a pressure gradient; higher pressure near source

Question 39

Describe how metabolic inhibitor is used to find evidence for translocation

Answer 39

Add a metabolic inhibitor to phloem

• Translocation stops
• Proves active transport is involved
• As it requires ATP to move against a concentration gradient

Question 40

Why does an air bubble in xylem vessel block movement of water through that xylem

Answer 40

When water is evaporated it is under tension, this causes water molecules to stick together in continuous column
Air bubble breaks column

Question 41

Describe One piece of evidence that supports the root pressure theory

Answer 41

Sap is discharged from cut stem, only upwards force could make this happen

Question 42

The diameter of a tree is less during the day than it is at night. Explain how this supports cohesion theory

Answer 42

Evaporation of water is mainly during the daytime
So the tension in the xylem creates an inwards pull
Xylem vessels become narrower due to adhesion of water molecules to the walls of the xylem vessels

Question 43

The diameter of a tree is less during the day than it is at night

Answer 43

Root pressure give outward force on the walls of xylem so trees would become wider not narrower as the xylem should become wider

Question 44

Use cohesion theory to explain why the circumference of trees changes throughout the day

Answer 44

Stomata open
Water evaporates
Water moves in through osmosis
Evaporation exerts force -> creates tension
Cohesion holds water column together
Adhesion between water molecules and walls of xylem equals pulling force wich causes water columns decrease in volume

Question 45

Why is pressure inside xylem negative

Answer 45

Water is under tension

Question 46

Why more lost through upper surface than lower surface

Answer 46

More stomata o lower surface

Waxy cuticle on upper

Question 47

Describe adaptions of xerophytic plants to reduce water loss

Answer 47

•	Stomata sunk in pits
	◦	Increases humidity in pits/reduces exposure to the wind
	◦	Reduced water potential gradient
	•	Layer of 'hairs'on epidermis
	◦	Traps layer of moist airaround stomata
	◦	Reduced water potential gradient
	•	Curled leaves
	◦	Reduced SA for water loss / stomata covered
	•	Reduced number of stomata
	◦	Reduced SA for water loss
	•	Thickwaxycuticleon leaves &stems
	◦	Waterproof = reduces evaporation

Question 48

How does lignin realest to function of xylem

Answer 48

Provides support