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A Level Biology > Mass Transport In Plants > Flashcards

Mass Transport In Plants Flashcards

1
Q

What is xylem

A

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

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2
Q

Describe structure of xylem vessels

A

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

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3
Q

Why no end walls in xylem

A

Makes uninterrupted tube so continuous water coloumn

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4
Q

What helps water pass up xylem

A

Mass flow
Cohesion tension theory
Adhesion

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5
Q

Describe adhesion

A

Same bond between water molecules

Stick to side and edges of xylem vessels

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6
Q

How is water in the roots move up the stem

Cohesion tension theory in xylem

A

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
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7
Q

More water pulled up the xylem more —-

A

Transpiration

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8
Q

What is transpiration ?

A

The evaporation of water from the surface of the leaves

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9
Q

What is the downside of stomata being open

A

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

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10
Q

Describe transpiration

A

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

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11
Q

Explain transpiration stream

A

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

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12
Q

Explain movement of water across the cells of a leaf

A

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

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13
Q

How does light intensity affect transportation rate

A

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

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14
Q

How does temperature affect transpiration rate

A

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

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15
Q

How does humidity affect transpiration rate

A

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

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16
Q

How does wind affect transpiration rate

A

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
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17
Q

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

A

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

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18
Q

Name the 2 possible pathways across the cortex to the endodermis

A

Apoplastic

Symplastic

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19
Q

How does water travel through apoplastic way

Cell walls

A

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

Cohesive forces between water molecules pulls more water along

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20
Q

Where and why is the water in the apoplastic pathway stopped

A

Stops at the endodermis

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

21
Q

Describe how does water travel through the symplastic way

A

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

22
Q

Explain how from both pathways water gets to the xylem

A

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

23
Q

What is the function of the phloem

A

Phloem tissue transports solutes around plants

24
Q

What is the phloem formed from

A

It is formed from cells arranged in tubes

25
Q

Name the 2 cells the phloem is formed from

A

Sieve tube elements

Companion cells

26
Q

Describe sieve tube elements

A

Living cells that form tube for transporting solutes

No nucleus

27
Q

Describe companion cells

A

Companion cell for each sieve tube element

Carry out living functions for sieve cells

28
Q

What is translocation

A

Movement of solutes / assimilates from source to sink

29
Q

Translocation requires ——-

A

Energy

30
Q

Why does translocation require energy and where does it come from

A

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

31
Q

What is a source and what is a sink

A

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

32
Q

Describe the mass flow hypothesis

A

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

33
Q

Name 4 pieces of experimental evidence of translocation

A

Ringing experiment
Tracer experiment
Use of aphids
Metabolic inhibitors

34
Q

Describe ringing experiment

A

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

35
Q

Describe tracer experiment

A

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

36
Q

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

A

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

37
Q

How to determine what plant is shown in ringed experiment

A

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

38
Q

How are aphids used to find evidence of translocation

A

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
39
Q

Describe how metabolic inhibitor is used to find evidence for translocation

A

Add a metabolic inhibitor to phloem

  • Translocation stops
  • Proves active transport is involved
  • As it requires ATP to move against a concentration gradient
40
Q

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

A

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

41
Q

Describe One piece of evidence that supports the root pressure theory

A

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

42
Q

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

A

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

43
Q

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

A

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

44
Q

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

A

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

45
Q

Why is pressure inside xylem negative

A

Water is under tension

46
Q

Why more lost through upper surface than lower surface

A

More stomata o lower surface

Waxy cuticle on upper

47
Q

Describe adaptions of xerophytic plants to reduce water loss

A 
•	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
48
Q

How does lignin realest to function of xylem

A

Provides support

A Level Biology (16 decks)

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