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Mass Transport in Plants Flashcards

1
Q

Where is water absorbed from?

A

Root hair cells in the roots

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2
Q

Where is the majority of water transported through in plants?

A

Xylem vessels

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3
Q

What are xylem vessels like? (3 points)

A

Hollow
Thick walled
Tubular

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4
Q

What is the main force that that pulls water through the xylem vessels?

A

Transpiration

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5
Q

Is transpiration active or passive? Why?

A

Passive as the energy required is supplied by the sun

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6
Q

How is a water potential gradient formed between the stomata and the atmosphere?

A

The humidity of the atmosphere is less than that of the air spaces next to the stomata

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7
Q

What happens when there is a water potential gradient between the atmosphere and the stomata and the stomata is open?

A

Water vapour molecules diffuse out of the air spaces in the stomata into the surrounding air

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8
Q

What happens after water vapour is lost by diffusion from the air spaces in transpiration?

A

Replaced by water evaporating from the cell walls of the surrounding mesophyll cells

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9
Q

How can plants control their rate of transpiration?

A

By changing the size of the stomatal pores

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10
Q

How is water lost from mesophyll cells?

A

By evaporation from their cell walls to the air spaces of the leaf

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11
Q

How is water lost from the mesophyll cell replaced?

A

From water reaching the mesophyll cells from the xylem via cell walls or the cytoplasm.

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12
Q

How does water reach the mesophyll cell through the cytoplasm? (5 steps)

A
  1. Mesophyll cells lose water to evaporation
  2. This reduces their water potential
  3. Water enters the mesophyll cells by osmosis from neighbouring cells
  4. The neighbouring cells now have lowered water potential
  5. This in turn takes in water from their neighbouring cells
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13
Q

What is the main factor for the movement of water up the xylem?

A

The cohesion-tension theory

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14
Q

How does water have cohesion in the cohesion-tension model?

A

The water molecules form hydrogen bonds between one another so stick together

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15
Q

What does water form across the mesophyll cells down through the xylem in the cohesion-tension model?

A

A continuous, unbroken column

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16
Q

What happens as water evaporates from the mesophyll cells into the spaces beneath the stomata in the cohesion-tension theory?

A

More molecules of water are drawn up due to the cohesion between the molecules

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17
Q

What is the process of the column of water being pulled through the xylem called in the cohesion-tension theory?

A

The transpiration pull

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18
Q

What does transpiration pull put on the xylem in the cohesion-tension theory? What pressure is exerted within the xylem?

A

Puts the xylem under tension
Negative pressure

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19
Q

What are the 3 pieces of evidence there is to support the cohesion-tension theory?

A
  • The change in diameter of tree trunks according to the rate of transpiration
  • If air enters the xylem vessel
  • When a xylem vessel is broken, water doesn’t leak out
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20
Q

Why does a change in diameter of tree trunks according to the rate of transpiration support the cohesion-tension theory? (3 points/steps)

A

When transpiration is at its highest, there is more tension (negative pressure) in the xylem.
This pulls the xylem walls in, causing the trunk to shrink in diameter.
When the rate of transpiration is lower, there is less tension so the diameter is wider

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21
Q

How does when air enters a xylem vessel, the tree no longer pulls up water support the cohesion-tension theory?

A

The continuous column of water is broken so water molecules can no longer stick together

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22
Q

How does when a xylem vessel is broken, no water leaks out support the cohesion-tension theory?

A

Proves that there is negative pressure. Air also moves in, which is consistent with it being under tension

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23
Q

Is transpiration pull a passive or active process?

A

Passive, so doesn’t require metabolic energy to take place

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24
Q

Why cant xylem cells actively move the water in the xylem?

A

Xylem cells are dead

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25
What does the xylem not have that allows the xylem to form a series of **continuous, unbroken tubes** from roots to leaves
They have **no end walls** which means there is no obstruction to the flow, so water can move smoothly
26
What is translocation?
The process by which organic molecules and some mineral ions are transported from one part of the plant to another
27
What is the tissue that translocates biological molecules called?
Phloem
28
What is phloem made up of?
Sieve tube elements, which are long thin structures arranged end to end
29
What feature do the end walls in the phloem have? What do they form?
Perforated to form sieve plates
30
What are associated with the sieve tube elements in the phloem?
Cells called companion cells
31
When are sugars produced?
In photosynthesis
32
Where are **sources** in the phloem system?
The site of the sugar's production
33
What are **sinks** in the phloem system
The places where the sugars will be used directly or stored for future use
34
Which direction can translocation move in and why?
Up **and** down because the sinks could be above or below the sources
35
What **molecules** can be transported by translocation? (2)
**Sucrose** Amino acids
36
What **inorganic ions** can be transported by translocation? (4)
Potassium Chloride Phosphate Magnesium
37
Why is it accepted that materials are transported in the phloem?
The rate of movement is **too fast** to be diffusion
38
What is the current theory of translocation?
The **mass flow** theory
39
What are the 3 phases of the mass flow theory of translocation?
- Transfer of sucrose into sieve elements from photosynthesising tissue - Mass flow of sucrose through sieve tube elements - Transfer of sucrose from the sieve tube elements into storage or other sink cells
40
What is the first phase of the mass flow theory of translocation called?
Transfer of sucrose into sieve elements from photosynthesising tissue
41
What is the second phase of the mass flow theory of translocation?
Mass flow of sucrose through sieve tube elements
42
What is the third phase of the mass flow theory of translocation?
Transfer of sucrose from the sieve tube elements into storage or other sink cells
43
What is the first step of the first phase of the mass flow theory of translocation? (1st)
Sucrose is manufactured from the products of photosynthesis in cells with chloroplasts
44
What is the step after **sucrose is manufactured from the products of photosynthesis in cells with chloroplasts** in the first phase of the mass flow theory of translocation? (1st)
The sucrose diffuses down a concentration gradient by **facilitated diffusion** from the photosynthesising cells into companion cells
45
What is the step after **the sucrose diffuses down a concentration gradient by facilitated diffusion from the photosynthesising cells into companion cells** in the first phase of the mass flow theory of translocation? (1st)
Hydrogen ions are actively transported from companion cells into the spaces within cell walls using ATP
46
What is the step after **hydrogen ions are actively transported from companion cells into the spaces within cell walls using ATP** in the first phase of the mass flow theory of translocation? (1st)
The hydrogen ions then diffuse down a concentration gradient through **carrier proteins** into the sieve tube elements
47
What is the step after **the hydrogen ions then diffuse down a concentration gradient through carrier proteins into the sieve tube elements** in the first phase of the mass flow theory of translocation? (1st)
Sucrose molecules are transported along with the hydrogen ions into the sieve tube elements by co-transport through protein carriers called **co-transport proteins**
48
What is mass flow?
Bulk movement of a substance through a given channel or area
49
What does the sucrose and hydrogen ions moving into the sieve tubes do to the sieve tubes? (2nd phase)
Makes the sieve tubes have a lower water potential
50
What happens after the sieve tubes have a lower water potential in the second phase of translocation?
As the xylem has a higher water potential, water moves into the sieve tubes from the xylem, creating a high hydrostatic pressure within the sieve tubes
51
What is sucrose being used for in the sink in translocation? (2 points)
- Used during respiration - Converted into starch for storage
52
What happens when the respiring cell has used its sucrose?
Sucrose is **actively transported** by companion cells into respiring cells then into from the sieve tubes
53
What happens to the respiring cells after sucrose has been actively transported into them by the companion cells in the second phase of the mass flow theory of translocation?
Their water potential is lowered so water also moves in from the sieve tubes via osmosis
54
What happens to the hydrostatic pressure of the sieve tubes close to the respiring cells after sucrose and water have been lost from them? (2nd phase)
The hydrostatic pressure is lowered
54
How does mass flow occur between source and sink in the second phase of translocation?
As there is high hydrostatic pressure in the source and low hydrostatic pressure in the sink, so sucrose is pushed from source to sink by mass flow
55
Is mass flow passive or active? What does this mean mass flow can be affected by?
Happens as a result of the active transport of sucrose so is overall active. Therefore mass flow can be affected by temperature and metabolic poisons
55
What are 3 pieces of evidence against mass flow?
- The functions of sieve tubes are **unclear** as they would seem to hinder mass flow - Not all solutes move at the same speed, they should do so if movement is by mass flow - Sucrose is delivered at the same rate to all regions, instead of favouring regions with the lowest sucrose concentration
56
What are 6 pieces of evidence supporting mass flow?
- There is pressure within sieve tubes, as sap is released when they are cut - The concentration of sucrose is higher in the source than in sinks - Downward flow in the phloem occurs in daylight, when sucrose can be produced, but ceases when in shade or at night - Increases in sucrose levels in the leaf are followed by increases of sucrose levels in the phloem - Metabolic poisons and/or lack of oxygen inhibits the translocation of sucrose - Companion cells contain lots of mitochondria and readily produce ATP
57
What are 2 types of experiments that can be used investigate transport in plants?
- Ringing experiments - Tracer experiments
58
What do woody stems have? (ringing exp)
Outer protective layer of bark, inside is a layer of phloem which extends all around the stem
59
What is done at the start of a ringing experiment?
A section of the outer layers (protective later and phloem) is removed around the circumference.
60
What happens a period of time after a section of the outer layers is removed around the circumference of the woody stem in a ringing experiment?
The region of the stem immediately above the removed region begins to swell with liquid
61
What do samples of the liquid that swells the woody stem above the removed layers contain? (ringing exp)
Rich in sugars and other dissolved organic substances
62
What happens to the regions above and below the removed section? (ringing exp)
Above - Continues to grow Below - Withers and die
63
What is the conclusion drawn from a ringing experiment?
The phloem, not the xylem is the tissue responsible for the transport of sugars in plants
64
What are used in tracer experiments?
Radioactive isotopes
65
Which radioactive isotope is used in most tracer experiments?
14C of carbon which can make radioactively labelled carbon dioxide (14CO2)
66
What happens in a tracer experiment?
The plant is grown in an atmosphere containing 14CO2 The plant uses the 14C isotope in the sugars produced by photosynthesis This can be traced by autoradiography as they move within the plant
67
What does the tracing of the 14C incorporated sugars show about translocation?
That the phloem is the only tissue responsible for translocation of sugars

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