7.7-7.9 Plant transport Flashcards
(62 cards)
1
Q
What vessels is water transported through?
A
Xylem
2
Q
How does water move out of the leaf?
A
Through the stomata
3
Q
Explain the process of water exiting through stomata
A
- Atmospheric humidity less than air spaces next to stomata
- Water potential gradient from air space through the stomata to the air
- Water vapour molecules diffuse out of the airspaces
- This water is replaced by water evaporating from the cell walls of the surrounding mesophyll cells
- Changing the size of the stomatal pores, plants control rate of transpiration
4
Q
What are the 2 pathways that water can move across cells?
A
Cell wall pathway or cytoplasmic pathway
5
Q
Explain why is water replaced in the mesophyll cells from the xylem?
A
- Mesophyll cells lose water to the air spaces by evaporation due to heat supplied by the sun
- These cells now have a lower water potential, water enters by osmosis from neighbouring cells
- Loss of water from these neighbouring cells lowers their water potential
- Take in water from their neighbours by osmosis
6
Q
Describe the cell wall pathway
A
- Water moves through spaces between cellulose molecules within the cell wall and intercellular spaces
- Water moves due to cohesion between water molecules
7
Q
Describe the cytoplasmic pathway
A
- Water enters the cytoplasm through plasma membrane
- Water moves into the sap in the vacuole through the tonoplast
- It passes from one cell to the other via plasmodesmata down the water potential gradient across the root cortex
8
Q
Explain the process of how water moves up the stem in the xylem
A
- Water evaporates from mesophyll cells due to heat from the sun leading to transpiration
- Water molecules form hydrogen bonds between one another and hence tend to stick together (cohesion)
- Water forms a continuous, unbroken column across the mesophyll cells and down the xylem
- Lower w.p. in mesophyll so more water drawn up due to cohesion
- A column of water is pulled up the xylem due to transpiration (transpiration pull)
- Transpiration pull causes tension (cohesion-tension theory)
9
Q
Summarise the overall movement of water in plants
A
- Water enters root hair cells by osmosis and moves across down water potential gradients via either cell wall or cytoplasmic pathway
- Water enters the xylem
- Water moves up xylem by the cohesion-tension theory
- Evaporation of water molecules from the spongy mesophyll cells followed by their diffusion out of the stomata in the leaf, lowers water potential
- Water moves into spongy cells from the xylem via osmosis
- Draws more water up the xylem (cohesion)
10
Q
What is the function of the xylem?
A
Carries water and soluble minerals from the roots up the plant
11
Q
State the adaptations of the xylem? (7)
A
- Long cells arranged end to end
- Cell contents are killed
- End walls break down
- Cell walls are thickened with lignin
- Wide lumen
- Pits in walls
- Lignin in a spiral, annular or reticulate pattern
12
Q
Explain the adaptation: long cells arranged end to end
A
Forms a continuous column, less resistance to water flow
13
Q
Explain the adaptation: cell contents are killed
A
No cytoplasm or nucleus to impede water flow (less resistance/more water carried)
14
Q
Explain the adaptation: end walls break down
A
No barrier to water flow between adjacent
15
Q
Explain the adaptation: cells walls are thickened with lignin
A
More rigid and less likely to collapse under the tension created by transpiration pull
16
Q
Explain the adaptation: wide lumen
A
More area for water to flow through
17
Q
Explain the adaptation: pits in walls
A
Allows lateral movement of water from one vessel to another
18
Q
Explain the adaptation: lignin in a spiral, annular or reticulate pattern
A
Allows the plant to stretch and bend
19
Q
What are the 3 “forces” moving water up the plant?
A
Root pressure pushes water up
Capillary action draws water up within the xylem
Cohesion-tension pulls water up the xylem (major)
20
Q
Describe how root pressure pushes water up
A
Water moving into the xylem raises the hydrostatic pressure so pushes the water up the xylem
21
Q
What is capillary action? Why does this happen?
A
Process by which water can rise up a narrow tube against the force of gravity
Due to cohesion and adhesion:
Adhesion - water molecules form hydrogen bonds with carbohydrates in the walls of narrow xylem vessels
Cohesion - water molecules form hydrogen bonds with each other
22
Q
What is cohesion-tension theory?
A
- Water molecules form H-bonds between each other (creates cohesion)
- Form a continuous chain across the cells in the leaf and down the xylem
- A column of water is pulled up the xylem by evaporation of water from the spongy mesophyll (transpiration)
- Transpiration pull
- Puts xylem under tension (negative pressure)
23
Q
Is transpiration pull active or passive?
A
Passive - does not require metabolic energy to take place
- Water cannot be actively moved by xylem vessels because they are dead
- Have no end walls so continuous hence cohesion-tension theory
- Energy for transpiration
- Thermal energy from the sun that evaporates water from the leaves
24
Q
State the 3 pieces of evidence to support the cohesion-tension theory
A
Change in diameter of tree trunks
Xylem broken tree can not long draw up water
Xylem broken water does not leak out
25
Explain the evidence: Change in diameter of tree trunks
Depends on rate of transpiration
Day:
-Transpiration highest
-More tension in xylem
-Pulls water of xylem vessels inwards and causes the trunk to shrink in diameter
Night:
- Transpiration lowest
- Less tension in xylem so diameter of trunk increases
26
Explain the evidence: Xylem broken tree can not long draw up water
Air enters so continuous column of water is broken so water molecules can no longer stick together
27
Explain the evidence: Xylem broken water does not leak out
Not under pressure - water does not leak out
Air is drawn in - shows it is under tension
28
Describe the adaptation of root hair cells
Microscopic - can penetrate between soil particles easily
Many root hairs - large SA:V ratio
Thin - short diffusion distance
Cytoplasm and vacuolar sap contains many solute - lower water potential
29
Describe how water moves into the root hair cells
- Minerals are absorbed from the soil by active transport using ATP
- Reduces water potential inside the cell
- Water moves into cell by osmosis
30
What is transpiration?
Loss of water evaporation from the leaves of a plant via the stomata
Consequence of gas exchange
31
State the factors affecting the transpiration rate (8)
Number of leaves
Number, size and position of stomata
Presence of cuticle
Light
Temperature
Relative humidity
Air movement/wind
Water availability
32
Explain the factor: Number of leaves
Higher r.o.t
More water vapour lost as larger SA
33
Explain the factor: number, size and position of stomata
More stomata = water vapour lost more quickly
Larger stomata = water vapour lost easier
Stomata on underside lower r.o.t
34
Explain the factor: presence of cuticle
Lower r.o.t
Thicker cuticle
Waterproofing
35
Explain the factor: Light
Higher r.o.t - light needed for photosynthesis
Increasing number of open stomata, increasing rate of water vapour diffusing out so increases evaporation from surface of the leaf
36
Explain the factor: Temperature
- Increase KE of water molecules so increases rate of evaporation from the spongy mesophyll cells into the air spaces of leap. Increase diffusion grad
- Increase conc of water vapour that the external air can hold before it becomes saturated (decreases humidity)
Both increase diffusion gradient = higher r.o.t
37
Explain the factor: Relative humidity (amount of water in air compared to total conc of water the air can hold)
Increase relative humidity, decrease rate
- Reduced water vapour potential gradient between the inside of the leaf and the outside air
- Very dry air has the opposite effect and increases the rate of transpiration
38
Explain the factor: air movement/wind
Higher r.o.t
Each leaf has layer of still air and hairs Water vapour that diffuses out of the leaf accumulates, water potential increases so diffusion gradient decreases
Air movement = higher rate
Long period of still air = lower transpiration
39
Explain the factor: water availability
If soil is very dry the plant will be under water stress, stomata will close and the rate of transpiration will be reduced
40
What assumption do you make using a potometer?
Measuring water uptake - assumes all water that is taken up from the plant is lost by transpiration
41
Describe how you would use a potometer
1. Healthy shoot cut under water
- Prevent air bubbles
- Cut at a slant to increase surface area
2. Potometer filled with water (no air bubbles)
3. Insert shoot into potometer under water to prevent any air bubbles
4. Remove potometer from water and ensure the joints around the shoot are airtight
- All joints should be sealed with waterproof jelly (ensures any water loss measured is a result of transpiration)
5. Dry off leaves
6. Allow time for shoot to acclimate
7. Keep conditions constant
8. Shut screw clip
9. Keep ruler fixed and record position of air bubble on scale
10. Start timing and measure the distance moved in a given time
Rate of water uptake = distance moved/time taken
42
What is the function of phloem?
Transports the dissolved products of photosynthesis in various directions around the plant
43
What is translocation?
The process by which organic molecules and some mineral ions are transported from one part of a plant to another
44
State 2 features of phloem
Sieve tube elements
Companion cells
45
How is the structure of sieve tube elements related to function?
Long thin structures arranged end to end - continuous column
End walls are perforated to form sieve plates - lets contents pass through
Contains little cytoplasm and no nucleus - more room for content to travel so nothing to impede the flow
46
How is the structure of companion cells related to function?
Contain numerous mitochondria, nucleus and all organelles
- ATP for active transport
- organelles for other cells therefore functions for both cells
Contains plasmodesmata
- cytoplasm of adjacent cells are linked to allow sharing of substances
47
What is transported during translocation?
Products of photosynthesis:
- Leaves produce large amounts of glucose, converted to sucrose for transport
-Reaches cells, converted to glucose for respiration or starch for storage/to produce other compounds
-Amino acids
- Inorganic ions such as potassium, chloride, phosphate and magnesium
48
Why is sucrose good for transport?
Soluble so easily transported in sap
Metabolically inactive so unlikely to be used during transport
49
What is a source?
Produces and loads sugars into the sieve tube
- Green leaves/stems
- Storage organs
- Food storages in seeds
50
What is a sink?
Sugar is removed from the phloem sieve tubes to be used directly or stored for future use
- Roots
- Meristem (tips of roots)
- Storage parts
Above or below source - translocation in either direction
51
What is mass flow?
Movement of fluids from a high to low pressure (too fast to be diffusion)
52
What are the 3 stages of translocation via mass flow theory?
1. Transfer of sucrose into sieve tube elements from photosynthesising tissue
2. Mass flow of sucrose through sieve tube elements
3. Transfer of sucrose from sieve tube elements into storage or other sink cells
53
Describe how sucrose is transferred into sieve tube elements from photosynthesising tissue (stage 1)
- Sucrose manufactured from the products in the cells with chloroplasts
- Sucrose diffuses down a concentration gradient by facilitated diffusion the photosynthesising cells-->companion cells
- H+ actively transported (ATP used) from companion cells--> spaces within cell walls
- H+ ions diffuse down a concentration gradient through carrier proteins (co-transport proteins) into the sieve tube elements
- Sucrose molecules are transported along (co-transport)
54
Describe the mass flow of sucrose through sieve tube elements
-The sieve tubes have a lower water potential so water moves in from xylem (higher water potential) by osmosis, creating a high hydrostatic pressure
- At the respiring cells (sink), sucrose is either used up during respiration or converted to starch for storage
- Hence, these cells have low sucrose content so sucrose is actively transported into them from sieve tubes
- This lowers their water potential so water also moves into these respiring cells, from the sieve tubes, by osmosis
- Hydrostatic pressure of the sieve tubes in this region is therefore lowered
- So water entering the sieve tube elements at the source (high hydrostatic pressure) and leaving at the sink (low h.p.)
- Mass flow of sucrose solution down this hydrostatic gradient
55
Is mass flow passive or active?
Mass flow is a passive process but occurs due to active transport of sugars - overall process is active
Therefore is affected by temperature and metabolic poisons
56
How is sucrose transferred from sieve tube elements into storage or other sink cells?
Sucrose is actively transported from the sieve tubes into sink cells by companion cells
57
What is the evidence supporting mass flow hypothesis?
- Pressure within sieve tubes
- Conc of sucrose is higher in leaves (source) than in roots (sink)
- Downward flow in phloem only in daylight
- Increase in sucrose in phloem after increase in sucrose in leaf
- Metabolic poisons and/or lack of oxygen stop translocation
- Companion cells have many mitochondria and readily produce ATP
58
What is the evidence questioning the mass flow hypothesis?
- Function of sieve plates as seem to hinder mass flow (suggest they help prevent tubes from bursting under pressure)
- Not all solutes move at the same speed
- Sucrose delivered at same rate to all regions, rather than more quickly to lower concentration areas
59
Describe the ringing experiment
Background:
Woody stems have an outer protective layer of bark on the inside of which is a layer of phloem that extends all round the stem. Inside the phloem layer is xylem
At the start, a section of the outer layers (protective layer and phloem) is removed around the circumference of a woody stem while it is still attached to the rest of the plant
After some time, the region of the stem immediately above the missing ring swells. Samples of liquid in this region are rich in sugars and other dissolved organic substances
Some non-photosynthetic tissues below the ring wither and die, above the ring tissues still grow
60
What can be inferred and concluded from the ringing experiment?
Inferences:
Sugars of the phloem accumulating above the ring, leading to swelling in this region
Interruption of flow of sugars to the region below the ring and death of tissues in this region
Conclusion:
Phloem is the tissue responsible for trans locating sugars in plants
As the ring of tissue removed had not extended into the xylem, it’s continuity had not been broken - if it were responsible, sugars would not have accumulated above ring and died below
61
Describe the tracer experiment
Background:
Radioactive isotopes useful for tracing movement of substances in plants.
Eg the isotope 14C can be used to make radioactively labelled carbon dioxide.
If a plant grown in 14CO2, the 14C isotope will be incorporated into the sugars produced during photosynthesis, can be traced using autoradiography.
In our example:
Taking think cross sections of plant stem and using X ray film to see regions where sugars are, therefore where phloem tissue is.
62
Describe the evidence that translocation of organic molecules occurs in phloem
- When phloem is cut, a solution of organic molecules flow out
- Plants provided with radioactive CO2 can be shown to have radioactively labelled carbon in phloem
- Aphids are insects that feed on plants. Have needle-like mouthparts which penetrate phloem so extract contents of sieve tubes. These contents show daily variations in sucrose content of leaves that are mirrored a little later by identical changes in the sucrose content on the phloem
- Removal of ring of phloem from around circumference of a stem leads to accumulation of sugars above the ring and none below.
