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transport in plants Flashcards

1
Q

transpiration

A

the evaporation of water from a plants surface

usually through stomata , which allows entry of C02 for photsynthesis

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2
Q

environmental factors affecting transpiration

A
  • light
  • temperature
  • humidity
  • air movement
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3
Q

how does light affect transpiration

A
  • during daylight stomata opens to allow CO2 to enter for photosynthesis
  • increasing rate of transpiration
  • as water evaporates from the mesophyll cells + diffuses out the leaf
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4
Q

how does temperature affect transpiration

A
  • increased temp
  • increases rate of transpiration
  • as water molecules will have more KE
  • allowing them to evaporate more
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5
Q

humidity

A

water vapour content of the air surrounding the plant

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6
Q

how does air movement affect transpiration

A
  • air movement removes water vapour from leafs surface
  • increasing water potential gradient
  • and increasing rate of transpiration
  • in still air, water vapour builds up around the leaf
  • decreasing water potential gradient and rate of transpiration
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7
Q

how does humidity affect transpiration

A
  • increased humidity
  • increases water potential of air
  • leads to a decrease in rate of transpiration.
  • as water potential gradient for diffusion of water decreases
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8
Q

xerophytes

A

plants which have adaptations to limit water loss via transpiration

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9
Q

adaptations of xerophyte
which reduce rate of transpiration

A
  • thick waxy cuticle
  • hairs on leafs surface
  • rolling up of leaves
  • reduced SA: volume ratio
  • stomata positioned in ‘epidermal pit or grooves’ beneath plant surface
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10
Q

how does thick waxy cuticle on xerophytes reduce rate of transpiration

A

long diffusion pathway
to reduce rate of evaporation/ transpiration

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11
Q

how do hairs on surface of xerophyte decrease rate of transpiration

A

trap layer of still air
which becomes saturated with water vapour
reducing water potential gradient for water loss
due to increase in humidity
reducing transpiration

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12
Q

how do rolling up of leaves reduce rate of transpiration

A

trap layer of still air
which becomes saturated with water vapour
reducing water potential gradient for water loss
due to increase in humidity
reducing transpiration

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13
Q

how does a reduced SA: volume ratio reduce transpiration

A

reduce surface area for water loss

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14
Q

how do stomata positioned in epidermal pits/ grooves beneath leafs surface
reduce rate of transpiration

A
  • reduces exposure to air currents
  • trapped air becomes saturated with water vapour meaning
  • increased humidity around the pits
  • so WP gradients reduced
  • reducing transpiration
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15
Q

what occurs in the xylem

A

movement of water + dissolved ions

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16
Q

transpiration stream

A

movement of water + dissolved ions from root hairs to leaves

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17
Q

whats cohesion- tension theory used to explain

A

movement of water + dissolved ions from root hairs in xylem to stomata in leaves

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18
Q

structural features of xylem

A
  • tissue in xylem is dead - no cell contents - hollow tubes- minimal resistance to flow of water + ions
  • lignin in cell wall- strength - more rigid + provides support
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19
Q

Describe cohesion- tension theory

A
  1. Solar heat energy causes evaporation/ transpiration of water from leaves, lowering WP
  2. Water moves from cell to cell across the leaf by osmosis down a water potential gradient
  3. Waters drawn from xylem, creating a tension (‘negative pressure’) by “pulling up” the water and dissolved ions.
  4. water columns maintained in the xylem by cohesive forces and adhesive forces
  5. Cohesion = the attraction of the water molecules to each other by hydrogen bonding.
  6. Adhesion = to the attraction of the water molecules to the xylem walls.
  7. upward movement of the water maintains water potential
    gradient across root cells, ( soil has higher wp than root cells) allowing water uptake from the soil via osmosis.
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20
Q

why is water ‘pulled up’ as a column of water

A

water molecules joined tg by hydrogen bonds
cohesion + adhesion occurs

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21
Q

role of lignin

A

provides strength + rigidity to cell walls e.g of xylem

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22
Q

how does water move from cell to cell across the leaf

A
  • water moves from xylem to cell by osmosis for use in photosynthesis
  • water moves cell to cell by osmosis
  • water moves from cell wall to sub stomatal air space by evaporation
  • water moves from stomata to air by evaporation
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23
Q

evidence for cohesion- tension theory

A
  • rate of transpiration increases, diameter of tree trunk decreases
  • evaporation of water from leaves draws water up xylem by osmosis creating tension
  • tension pulls in walls of xylem so diameter decreases
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24
Q

potometer

A

used to measure rate of transpiration by tracking the movement of an air bubble in a capillary tube

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25
Q

What assumption does the potometer method make about water uptake?

A

assumes that the rate of water uptake is the same as rate of transpiration
However, some water is used for turgidity maintenance and photosynthesis

26
Q

What measurements are required to calculate the rate of transpiration using a potometer

A
  1. The distance (d) the air bubble moves in cm.
  2. The time (t) taken for the bubble to move distance d.
  3. The radius (r) of the capillary tube’s lumen.
27
Q

what is the unit for rate of transpiration

A

cm3 hr-1

28
Q

How is the volume of water taken up by the plant calculated?

A

Volume= ((pie x radius) squared) x diameter

r = radius of the capillary tube (cm).
d = distance the air bubble moves (cm)

29
Q

How is the rate of transpiration calculated using a potometer in cm 3 h -1

A

Rate= (πr) squared x diameter
divided by time

​

30
Q

Why must there be no air bubbles in the potometer?

A

If air bubbles are present, the cohesion of water is lost, and the water will not be drawn up correctly from the potometer

31
Q

What role does the rubber bung play in the potometer?

A

provides an air-tight seal to prevent air from entering the system, ensuring accurate measurements

32
Q

How should the shoot be prepared for insertion into the potometer?

A

The end of the shoot should be cut under water
before being inserted into the potometer under water
to prevent air from entering the xylem and disrupting the measurement

33
Q

Why is it necessary to fill the potometer with water before use?

A

to prevent air from entering so there are accurate transpiration measurements

34
Q

What is the function of the reservoir in a potometer?

A

allows you to push the air bubble back to the start of the scale, enabling repeat readings to measure transpiration over time

35
Q

2 precautions student should take when setting up the potometer to obtain reliable measurements

A
  • cut shoot underwater
  • ensure there are no air bubbles
36
Q

2 reasons potometer doesnt truly measure rate of transpiration

A

all the water taken up by the plant, is not all used in transpiration

  1. waters used in photosynthesis
  2. waters produced in respiration
37
Q

evidence for movement of ions in xylem

A
  1. using radioactive isotopes which can be used as tracers
  2. separated xylem + phloem in a section of the stem of a plant using wax cylinder to prevent lateral (sideways) transport
  3. supplied roots of the plant with radioactively labelled potassium ions 42K
  4. left plants for few hours + then measured and compared the amount of radioactivity in the xylem and phloem tissues in the wax cylinder
  5. amount of radioactivity in xylem is greater indicating that transport of the potassium ions occurs in xylem
  6. the small amnt of radioactivity in phloem tissue is due to lateral transport from the xylem in the area where the wax cylinder is not present
38
Q

what are radioactive isotopes used for in terms of movement of ions in xylem

A

can be used as tracers to provide evidence that the transport of ions througha plant occurs mainly in the xylem

39
Q

What is translocation in plants?

A

transport of photosynthetic products (carbs, proteins, and lipids)
through the phloem
from leaves (source) to sinks (growing areas)

40
Q

What forms do the photosynthetic products take during translocation?

A

transported as their monomers:

  • Carbohydrates are transported as sucrose.
  • Proteins are transported as amino acids.
  • Lipids are transported as fatty acids and glycerol
41
Q

Where are the photosynthetic products produced and where are they transported?

A

produced in the leaves (the source) during photosynthesis
transported to various sinks :

  • Growing areas (young leaves, shoot tips, root tips)
  • Roots
  • Developing fruits or storage areas (e.g., potatoes)
42
Q

What are the main components of the phloem?

A

consists of living cells, including:

  • Sieve elements (form sieve tubes)
  • Companion cells (adjacent to sieve elements, contain cytoplasm and many mitochondria)
43
Q

how do sieve elements form sieve tubes?

A
  • joined end to end to form sieve tubes
  • the end walls of sieve elements are called sieve plates as they possess pores
44
Q

What are the characteristics of mature sieve elements?

A
  • No nucleus or organelles ( glucose can pass through more easily)
  • Have cytoplasm around the edge of the cell with few organelles
45
Q

What is the function of the companion cells in the phloem?

A

next to each sieve element
They have dense cytoplasm and many mitochondria, supplying energy required for the active transport of substances into and out of sieve elements

46
Q

what is the mass-flow hypothesis

A

hypothesis abt method of translocation in phloem

47
Q

describe process of translocation in the phloem

A

(1. photosynthesis occurs in the leaf
2. condensation reaction occurs between glucose + fructose to form sucrose)

  1. sucrose is actively transported into phloems sieve tubes by companion cells
  2. this lowers the WP of the sieve tubes (in phloem) causing water to enter from xylem by osmosis
  3. this increases the hydrostatic pressure in the sieve tubes
  4. sucrose transported from phloem to sink
  5. in sink: sugars are unloaded - being used in respiration for growth or stored as insoluble starch
  6. increasing WP in sieve tubes so water moves out these cells into the xylem by osmosis
  7. lowering hydrostatic pressure at the sink
  8. photosynthetic products are transported down pressure gradient from source to sink by mass flow
48
Q

what does xylem + phloem transport

A

xylem = water + dissolved ions
phloem = photosynthetic products e.g sugars

49
Q

evidence for translocation in phloem
ringing experiments

A

1st experiment of this method:
- involved the removal of a complete ring of phloem from the trunk of a tree during the summer
- few months after ringing tree trunk, a slight swelling developed above the ring
- swelling was due to build-up of photosynthetic products from leaves, which were prevented from being transported past the ring due to the removal of the phloem

50
Q

What was the first evidence that translocation occurs in the phloem?

A

Ringing experiments provided the first evidence for translocation in the phloem.

51
Q

What is the process of ringing (girdling) in plants?

A

involves removing a complete ring of phloem around part of a plant, (usually the stem) which prevents transport through the phloem at that point

52
Q

What did the swelling above the ring in a ringing experiment indicate?

A

indicated a build-up of photosynthetic products that could not be transported past the ring due to the removal of the phloem

53
Q

How do radioactive isotopes like 14C help in studying translocation?

A

used as tracers to track the movement of photosynthetic products, allowing more sophisticated experiments to demonstrate translocation in the phloem

54
Q

the use of radioactive isotopes as evidence for transport in the phloem
method

A
  1. Two plants of the same species are used.
  2. One plant (A) is ringed, and the other (B) is left intact.
  3. A leaf below the ring in plant A and a similar leaf in plant B are supplied with radioactive 14CO₂ or injected with 14C₆H₁₂O₆.
  4. The plants are left in sunlight for a few hours
  5. the transport of the radioactive photosynthetic products is tracked using autoradiography
    • describe autoradiography*
55
Q

What is autoradiography and how is it used in translocation experiments?

A

involves placing each plant between X-ray film. The radioactive compounds in the plant expose the film, showing the location of the transported photosynthetic products

56
Q

What did the autoradiograms of plant A and B show in translocation experiments with radioactive isotopes

A

plant B (control) = photosynthetic products were transported throughout the plant, especially to growing regions (shoot/root tips , young leaves + storage areas)

plant A (ringed) = photosynthetic products were unable to be transported/ move past the ring, remaining below the ring

57
Q

Why does small amount of photosynthetic products appear above the ring in plant A, and why?

A

small amount of photosynthetic products appeared above the ring

due to lateral transport between the phloem and xylem tissue below the ring
demonstrating that some movement can occur through the xylem

58
Q

evidence supporting the mass flow hypothesis in phloem translocation

A
  • Phloem sap is released when the stem is cut, indicating hydrostatic pressure in the sieve tubes (supporting that transport in phloems due to pressure changes)
  • Lowering the temperature/ using respiratory inhibitors reduces the rate of translocation, suggesting active transport involved
59
Q

evidence against the mass flow hypothesis

A
  • specific structure of sieve tubes and sieve plates is not essential for mass flow
  • In young phloem tissue, substances have been observed moving in opposite directions within the same sieve tube. ( so process may not be solely due to pressure gradients)
60
Q
A

B (7 decks)

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