Chapter 9 - Transport in plants Flashcards
(180 cards)
1
Q
the 3 main elements a plant needs are?
A
C, H,O
2
Q
position of xylem and phloem within vascular bundles?
A
phloem: outside, xylem: inside facing centre of cell/stem
3
Q
what do sclerenchyma fibres do?
A
tend to lie on the outside of VB and in between diff vessels, provide additional support to the VBs and plants as a whole + help to keep vessels upright and open
4
Q
what do parenchyma fibres do?
A
living cells that act as a packing tissue to separate the xylem vessels from phloem, and provide them w support
5
Q
structure of xylem vessels?
A
- ded cells stacked end to end
- xylem cells have no cytoplasm or organelles and no cell walls at their ends (only at sides) that would slow the flow of water
- lined w lignin - a waterproof polysaccharide
6
Q
in VBs, xylem vessels are?
A
continuous hollow tubes that run through the plant
7
Q
formation of xylem vessels?
A
- immature xylem vessels are waterproofed when lignin is deposited on the inside of their cell walls + in a spiral pattern
- the process of lignification kills the cells and allows for max. flow
8
Q
adaptations of xylem ?
A
- holes in the xylem w no lignin - bordered pits - allow H2O to move between vessels to get to diff leaves e.g.
- vessels are narrow enough to ensure water travels upwards in an unbroken column
- no end walls or organelles to impede water flow
9
Q
H2O movement?
A
- in xylem vessels: mass movement
- between plant tissues: osmosis
10
Q
root hair cells are adapted spec for the uptake of water and mineral ions:
A
- mineral ions are actively transported from the soil into the root hair cell
- these minerals reduce the water potenial of the root hair cell cytoplasm
- water enters the root hair cell by osmosis
11
Q
where does the cortex lie?
A
between epidermis (outside) and endodermis (inside)
12
Q
inside the endodermis,…
A
is the Medulla - means centre/ middle - xylem is here
13
Q
Purposes of the Casparian Strip?
A
- creates a checkpoint for plant immune systems b4 transport thru the rest of the plant - acts as a barrier to pathogens
- blocks water from passing back into the cortex from the xylem
14
Q
where does the water exit xylem vessels?
A
the xylem vessels located in the spongy mesophyll where the xylem vessel finishes
15
Q
how does water vapour diffuse out of the stomata?
A
down a water vapour potential gradient
16
Q
what is transpiration?
A
the loss of water through the aerial parts of the plant mainly thru stomata in the leaves
17
Q
what is the transpiration stream?
A
there is a constant stream of water travelling from the roots, through the stem and out of the leaves. Water diffusing out of the roots is replaced by water from the xylem vessels
18
Q
importance of transpiration?
A
a constant supply of water to the leaves has benefits:
- main being maintenance of turgidity - gives support & strength to the cell
- water for metabolic processes
- transport of mineral ions - if water wasn’t present, the ions wouldn’t be able to dissolve in any solution, need to dissolve so they can flow freely & b delivered to cells
- evap H2O keeps plant leaves cool in the sun - maintains temp
19
Q
evidence for transpiration - - pressure?
A
- the column of water pulled up is under - pressure
- leaf = - pressure, drawn up in roots = + pressure
- if there was a + pressure in leaf and trans wasn’t happening, the water would just stay in the leaf and build up, push back down on the xylem vessel, water wouldn’t have any gradient to be sucked up the xylem vessel
20
Q
what is - pressure?
A
where something is being pulled to an area where its under less pressure
21
Q
what does - pressure do?
in day and night
A
squeezes the xylem vessels in the day (sunlight -> transpiration) and relaxes in the night (less trans)
22
Q
what happens if xlem vessels are broken evid)
A
air gets sucked in and they fail to carry water -> air enters causing bubbles, adding to the pressure of the vessels, and so H2O can’t be carried as there are air blockages in the vessels.
if xylem vessels r broken, the plant loses it’s ability to draw up water
23
Q
what in the xylem do water molecules adhere to?
A
hydrophilic polysaccharides (not lignin - hydrophobic)w
24
Q
what allows water to travel long distances up the xylem ?
A
the combination of cohesion and adhesion
25
capillary action?
- the forces of adhesion push H2O along narrow vessels
- each xylem vessel only has a thin cross section but packing thin xylem vessels at high density max. this effect
- overall this force is quite weak and is more useful in V short plants w a high amount of xylem vessels
26
why are phloem necessary?
Plants need to be able to transport water from their roots upwards and sugars and other assimilates both upwards and downwards
27
Phloem tissue consists of 2 types of cell:
sieve tube element which are lined end to end to allow flow of sap & they have little cytoplasm and no nucleus to max. Space for sap but they r still alive, and companion cells which control transport in the sieve tube elements
28
Companion cells contain many mitochondria bc ?
the phloem requires active transport to move sap in translocation
29
Translocation is?
- the transport of assimilates through a plant
30
outline the process of active loading?
Step 1: H+ ions are pumped out from the companion cells to the surrounding leaf tissue creating a diffusion gradient of H+
2: H+ ions diffuse back into the companion cells thru cotransporter proteins bringing sucrose w them
3: High concs of sugar in companion cells cause sucrose to diffuse into the sieve tube elements
31
what is active loading?
The process of loading sucrose into the sieve tube elements
32
what are sources in plants?
Sources are parts of plants that load materials into the transport system (can be where something is absorbed or created)
The leaves are a source of assimilates that are transported in the phloem
33
what are sinks in plants?
Sinks are parts of the plant where materials are removed from a transport system
The roots are a common e.g. of a sink in plants - they remove the sucrose from the phloem to use it in their own cells
34
The sources and sinks of plants are connected by ?
the phloem allowing assimilates to be transported to where they are needed
35
the process of translocation
- When the assimilates are actively loaded into the sieve tube element, the water potential at the source decreases
- Water from the xylem vessels in the source then follows into the sieve tube element by osmosis
- Loading of assimilates and water at the source creates a high hydrostatic pressure
- Sucrose removed at the sink reduces the water potential of the surrounding cells
- Water then leaves the sieve tube element by osmosis
- Unloading of assimilates and water reduces the hydrostatic pressure at the sink
- The difference in HP between source and sink creates a pushing force hat moves sap to the location it is needed
- This whole process is known as mass flow - bc we’re talking about moving the sugar and water in large amounts from areas of high to low pressure
36
what are xerophytes?
Xerophytes are plants adapted to living in dry conditions
37
what do xerophyte leaves tend to be like?
Leaves tend to be small or needle shaped -> reduces their SA:V -> less area for water to be lost from
38
why do some xerophytes have hair on their leaves?
Hair on leaves = slows air movement around the top of the leaf, traps air around top of leaf which means more humid air so less strong water vapour potential gradient
39
adaptations of cacti?
- have many additional adaptations to help them cope w extremely dry conditions:
- fleshy body for water storage,
- spines that protect the cactus against animals who might eat it for water,
- slow growth & very little flowering = V low metabolic rate so lower demand for water,
- thick stems = reduces SA:V
- Water collection: wide & extensive roots = inc SA for them to gather water
40
Marram grass adaptations?
- most obvious feature it has evolved is the rolled leaf = reduced SA:V -> less water loss due to transpiration
- & in the rolled up area can maintain humidity = reduced water vapour potential gradient
- Other: sunken stomata, hairs, thick waxy cuticle
41
hydrophytes adaptations?
Are plants that are spec adapted to live in water rather than soil
42
Challenges faced by hydrophytes?
- have to get O2 to submerged tissues,
- have to keep afloat to get sunlight,
- high humidity above the surface of the water (water is constantly evaporating) greatly reduces the rate of transpiration
43
adaptations of hydrophytes?
- large air spaces in spongy mesophyll to give buoyancy (ability to float in water),
- leaf stems have many air spaces to allow O2 to diffuse to roots,
- many hydrophytes increase their transpiration rates by using hydathodes
44
what are hydathodes?
structures in plants that release water droplets which then may evaporate from the leaf surface
45
The transport system in plants is divided into 2 types of vascular tissue:
xylem tissue and phloem tisSue
46
xylem function is?
Function is to transport water and dissolved ions from the roots up the stem and into the leaves of plants.
47
Xylem structure?
- Made of 2 main cell types: xylem vessels and xylem parenchyma, but also tracheids.
- Xylem parenchyma are living ‘packing cells’ found in between the xylem vessels.
- Xylem vessels are formed from dead cells that have lost their top and bottom cell walls forming long, hollow tubes.
- The vessel walls become lignified - spirals or rings of a hydrophobic biopolymer called lignin are deposited within the cell walls. This gives the vessels mechanical strength
48
how does water flow thru xylem?
- Sideways movement of water and ions through pits which connect vessels laterally.
- Allows the movement of water from soil to leaves by mass flow, not diffusion.
49
phloem tissue is made up of?
2 main cell types:
| sieve tube elements and companion cells.
50
Sieve tube elements?
- Sieve tube elements (STEs) distribute the assimilates of photosynthesis around the plant. This is translocation.
- STEs are living cells forming continuous columns.
- A modified cell wall called a sieve plate is found at each end of each STE..
- Very little cytoplasm and few organelles, leaving room for the transport of sugars.
51
companion cells?
- Companion cells are ‘normal’ plant cells.
- Many mitochondria and ribosomes - very active.
- Give metabolic support to STEs.
- Closely associated with STEs.
- Involved in loading and unloading sucrose into and out of STEs.
52
how are STEs and companion cells connected?
Many plasmodesmata connecting STEs and companion cells.
53
what are the assimilates of photosynthesis?
The assimilates of photosynthesis are the organic molecules made from the sugars the plant makes during photosynthesis. Sucrose is the main transport sugar, but amino acids, hormones and other organic molecules are also transported.
54
where are VBs fouund in a root?
If you look at a micrograph of a section of a plant’s root, the vascular tissue is central - in the middle, in a structure called the stele
55
where are VBs found in the stem?
If you look at a micrograph of a section of a plant’s stem, the vascular tissue is peripheral - round the outside, in structures called the vascular bundles
56
Epidermis? (root)
The outer layer of cells is the epidermis.
| In the root, many epidermis cells are specialised and have root hairs to increase SA for absorption.
57
cortex? (root)
The thick layer of unspecialised cells between the epidermis and the stele is the cortex - a ‘packing’ layer made of parenchyma cells
58
Endodermis? (root)
The outer layer of the stele is the endodermis. These cells control the entry of ions into the xylem
59
where is phloem tissue found in a plant? (root)
In between the ‘arms’ of the cross is phloem tissue.
60
where is xylem found in a root?
Xylem tissue forms a cross shape in the centre of the stele
61
pericycle? (root)
The pericycle is a layer of cells between the endodermis and the phloem.
62
(stem) collenchyma cells?
just behind the epidermis are a few layers of strengthening cells called collenchyma cells.
63
(stem) cortex?
The thick layer of unspecialised cells in the centre of the stem is the cortex - made of parenchyma cells.
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(stem) each VB has?
Each vascular bundle has 4 distinct regions.
65
(stem) sclerenchyma cells?
Sclerenchyma cells are mechanically strong, supporting the stem.
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(stem) order?
xylem ➡ cambium ➡ phloem ➡ sclerenchyma
67
(stem) lignified fibres & pith?
The lignified fibres are sclerenchyma.
The pith is loosely packed parenchyma cells.
68
Waxy cuticle function?
Minimises water loss from top surface of leaf.
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Upper epidermis function?
Structural cells forming top surface of leaf.
70
Palisade mesophyll function?
Main site of photosynthesis. Packed with chloroplasts.
71
Spongy mesophyll fucntion?
Gas exchange and evaporation of water vapour.
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Air spaces function?
Form part of spongy mesophyll tissue.Continuous with atmosphere.
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Lower epidermis function?
Structural cells forming under surface of leaf.
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Guard cell function?
Work in pairs, changing turgidity to open and close stomata.
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Stoma (pl. stomata) function?
Pores allowing CO2 in, O2 out and water vapour out of leaf.
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Phloem tissue in leaf vein function?
Transports sugars away from the leaf, to other plant cells.
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Xylem tissue in leaf function?
Transports water and ions to leaf cells.
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gas exchange & transpiration?
- On a bright, sunny day, the rate of photosynthesis within the leaf > the rate of respiration.
- This means more O2 is being produced in the leaf than being used and more CO2 is being used than being produced.
- So, to allow photosynthesis to happen (which is essential for the plant to make respiratory substrate for its cells), the stomata have to be open, allowing CO2 into and O2 out of the leaf.
- Because of this, evaporation of water from the spongy mesophyll cells, into the air spaces, and the loss of water vapour through stomata into the atmosphere is inevitable.
- This (transpiration) maintains a constant stream of water through the plant, ensuring photosynthesising cells are supplied with water, and all cells have a high water potential outside them, maintaining their turgidity and therefore support for the plant
- Only when light intensity is very low (eg at night), or the plant is under ‘water stress’ will the stomata close.
79
Why do plants need water?
1. For photosynthesis but they don’t need much for this compared to…
- To ensure a constant stream of water through the plant, ensuring all cells have a high water potential outside them, maintaining their turgidity and therefore supporting the plant.
80
If a plant runs out of water?
If a plant runs out of water, then this stream stops and cells lose their turgidity. The whole plant wilts. Leaves lose their orientation to light so photosynthesis become less efficient. Eventually the plant will die.
If the plant is watered, cells can recover their turgidity quite quickly.
81
Why must a plant control ion entry?
- A key point is that a plant must control the entry of ions into its xylem - eg NO3-, PO43-, K+
- Too few ions, and the plant will not be able to synthesise molecules like proteins and nucleic acids, and too many may be toxic, interfering with metabolic reactions in ce
82
what is the only way ion entry can be controlled?
The only way a plant can control ion entry is when ions cross a membrane. This means ions can be prevented from getting into cells by closing channel or carrier proteins, or actively transported into cells through carrier proteins.
83
how do water and ions get from soil ➡ xylem in root?
- There is a Ψ gradient from the water in the soil, into root hair cells, through the cells of the root, to the xylem vessels at the centre of the root.
- So, water moves down this Ψ gradient from the soil, into the xylem.
84
2 main routes the water can take from soil into roots?
- The symplast route
| - The apoplast route
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what does the symplast route involve?
- water and ions crossing the membrane of a root hair cell (therefore controlling ion entry), moving though the cytoplasm of cells and the plasmodesmata between cells, all the way to the xylem vessels.
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what does the apoplast route involve?
- The apoplast route involves the water and ions moving between cells, through the cell walls between the cells and not crossing a membrane.
- This means ion entry cannot be controlled.
- This continues until the endodermis cells, which have a strip of a waxy material called suberin in their cell walls - the Casparian strip.
- This forces the water and ions across the membrane of the endodermis cells, into the symplast route - therefore controlling ion entry.
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role of the Casparian strip?
The Casparian strip ‘forces’ water and ions across a membrane, from apoplast into the symplast route.
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the apoplast route definition?
water and ions move through the cell walls without entering cells and crossing a membrane
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the symplast route definition?
water and ions cross a membrane, enter the cytoplasm of cells and move from cell to cell through plasmodesmata
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what is root pressure?
an active process ‘pushing’ water and ions from the roots of a plant, upwards.
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demonstrations of root pressure?
= If a plant’s stem is cut just above the ground, water will ooze out of the cut surface, forced up from the roots.
- Some plants show guttation, where pressure from the roots forces water through xylem vessels, up the stem and out of openings at the tip of leaves
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How is root pressure generated?
- Cells in the endodermis actively transport ions from their cytoplasm into the xylem vessel
- this lowers the water potential in the xylem and steepens the water potential gradient between endodermis cells and xylem vessels
- this means more water moves by osmosis from endodermis cells into xylem vessels, increasing the pressure in xylem vessels in the roots
which results in root pressure
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what does root pressure contribute to?
the movement of water into and up the xylem vessels
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What are halophytes?
they are adapted to survive in salty environments where the soil water surrounding their roots has a lower water potential than the cytoplasm of their root hair cells.
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what happens after water has entered the roots?
Water now moves from the roots, up the stem, into the leaves, and evaporates from the leaves through stomata, into the atmosphere. This process is transpiration
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how does water get to the top of huge trees?
root pressure, transpiration, cohesion-tension
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Root pressure?
Water is ‘pushed’ up plants from the roots, as you have recently learned. This is actually pretty insignificant - the other 2 factors are much more imp
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Transpiration?
- In the leaves, water moves down a water potential gradient from xylem into spongy mesophyll cells.
- Water evaporates from the cell walls at the surface of these cells into the air spaces.
- Energy for this is provided by heat energy from the sun, and the environment.
- Water vapour molecules diffuse out of the leaf via stomata, into the atmosphere.
- This is transpiration, and provides a ‘pull’ for water from the top of a plant - water that is lost from here needs to be replaced by water below, in the xylem vessels.
- This causes a continuous stream of water through the plant - the transpiration stream.
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Cohesion - tension theory?
- Water molecules H bond with each other - they show cohesion.
- Water molecules ‘stick’ to the inside of xylem vessels - this is adhesion.
- Because water is being pulled from the top of the plant, this means the columns of water in the xylem vessels are stretched a bit - they are in tension.
- These ideas are summarised as the cohesion-tension model.
100
why are cohesion + adhesion significant in xylem?
As xylem vessels have such a small diameter (~50μm), adhesion and cohesion are very significant at this small scale, causing capillary action. This helps to ensure water moves up the vessels.
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summary of how water moves thru plants?
- Water is pulled from the top of a plant by transpiration - the evaporation of water from the leaves
- columns of water in the xylem vessels maintain a constant transpiration stream, because water has the properties of cohesion and adhesion
- water is pushed from the roots of the plant by the active process of root pressure.
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any facto that affects the rate of evaporation affects the?
rate of transpiration
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factors affecting rate of transpiration?
- Temperature
- Humidity
- Wind speed
- Light intensity
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why does temp affect rate of transpiration?
- Higher temp = 🡩 kinetic energy of water molecules in liquid water on surface of spongy mesophyll cells.
- This 🡩 rate of evaporation, adding water molecules to the air spaces inside the leaf.
- Water potential in air spaces of leaf 🡩.
- Ψ gradient between air spaces and atmosphere gets steeper, 🡩 rate of water loss from leaf.`
105
why does humidity affect rate of transpiration?
- 🡩 humidity adds water molecules to atmosphere outside leaf.
- This 🡩 Ψ outside leaf.
- Ψ gradient between air spaces and atmosphere gets less steep, 🡫 rate of water loss from leaf.
106
why does wind speed affect rate of transpiration?
- 🡩 wind speed removes water molecules from atmosphere outside leaf.
- This 🡫 Ψ outside leaf.
- Ψ gradient between air spaces and atmosphere gets steeper, 🡩 rate of water loss from leaf.
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why does light intensity affect rate of transpiration?
- Does not affect the rate of transpiration directly.
| - At low light intensities, stomata close, preventing water loss from the leaf, slowing the rate of transpiration.
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how can we measure rate of transpiration?
potometer
109
How does a potometer work?
- a cut shoot is placed in the potometer and, as water evaporates from the leaves, it is replaced by water from the potometer.
- The rate of this movement of water can be measured as the water moves in the capillary tube.
- Different conditions can be set up to investigate the effect of different factors: a fan, a hairdryer, a plastic bag, etc…
110
potometer precautions?
Precautions need to be taken to ensure the apparatus is completely air and watertight - if a tiny bubble gets into a xylem vessel, the vessel will become blocked, slowing transpiration.
111
potometer ⭐point?
Another key point is that the plant cells are alive, so are producing water from respiration. Leaf cells are also photosynthesising, so are using water for this process. This means that the water movement in the capillary tube is only an estimation of the water being lost through transpiration.
112
units for potometer data?
Data from a potometer is usually expressed as ‘volume of water lost per unit time per unit of leaf area’ for example:
mm3 min-1 cm-2
113
xylem description of structure?
- used to be living, now dead
- walls made of cellulose + lignin
- end walls disappear - continuous tube
- sideways movement of water and ions through pits
114
xylem description of function?
- allows movement of H2O from soil to leaves by mass flow
- flow of materials is roots➡ shoots + leaves
- parenchyma cells store food + contain tannin deposits - protects plant tissue from attacks from herbivores
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phloem description of function?
- transport the assimilates of photosynthesis
| - translocation
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phloem companion cell function?
- 'life support system' for STEs
| - very active
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parenchyma are?
'packing cells'
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sclerenchyma cells are?
strong, lignified cells
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how do VBs look in diff parts of the plant?
- root: xylem is +/ x shaped
- stem: eggs around outside
- leaf: near palisade mesophyll
120
VB order of components in stem?
S
P
C
X
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How is ion entry controlled in symplast route?
ions cross membranes of root hair cells for entry
122
briefly explain how a potometer works?
- Water evaporates from the leaves.
- Replaced by water from the apparatus.
- Air / water meniscus moves along the capillary tube.
- Movement measured and timed.
- Different conditions can be set up with eg a fan, humidifier / plastic bag, light, radiant heater.`
123
Why does the potometer only give an estimate for the rate of transpiration?
- Potometer measures the water being taken up by the shoot.
- Not all this water is lost from the leaves / transpired.
- Some of this water is used for photosynthesis.
- Also, some water produced by respiring cells.
124
How would you use the potometer to obtain valid data?
- Allow the apparatus to settle by leaving for at least 5 minutes.
- Take measurements of distance and time from the capillary tube. Make sure replicate readings are taken.
- Measure SA of leaves and convert data to mm3 min-1 cm-2 to enable comparison of data between groups.
- Investigate 1 factor as the IV at a time, and make sure other factors are controlled.
- Ideally, set up a control potometer, identical in every way apart from no leafy shoot.
125
What precautions do you have to take when setting the potometer up?
- Set the device up underwater to ensure there are no air bubbles in the tubing.
- Ensure an airtight seal between the shoot and the rubber by using vaseline.
- Cut the shoot underwater to eliminate air bubbles from the xylem vessels at the bottom of the shoot.
- Cut the shoot at an angle so any bubbles don’t enter the xylem vessels.
- Ensure the leaves are dry so as not to block stomata.
126
Xerophytic adaptations?
- thick, waxy cuticle
- stomata in pits or deep grooves, and reduced numbers of stomata
- leaves reduced to spines
- hairs
- curled leaves
- swollen, succulent stems
- leaf loss
- other adaptations: root, avoiding...
127
(Xerophytic adaptations) Thick, waxy cuticles?
- in most plants, up to 10% of the water loss by transpiration is thru the cuticle
- helps min H2O loss
- common in evergreen plants
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(Xerophytic adaptations) leaves reduced to spines
- by ⬇ leaf area, H2O loss can be greatly ⬇
- the leaves of conifers are greatly reduced to thin needles. These narrow leaves, which r almost circular in cross section have greatly reduced SA:V, min. H2O loss in trans
129
(Xerophytic adaptations) hairs espec on underside of leaf
- create a microclimate of still, humid air, reducing the water vapour potential gradient and ⬇ loss of H2O by trans from the surface of the leaf
- some plants e..g marram grass even have micro hairs in the sunken stomata pits
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(Xerophytic adaptations) curled leaves
- greatly ⬇ H2O loss by Trans, espec in combination w other adaptations
- confines all of the stomata within a microenvir, of still humid air to ⬇ diffuciom of H2O vapour from the stomata e.g. marram grass
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(Xerophytic adaptations) swollen, succulent stems
- store water in specialised parenchyma tissue in their stems & roots
- often have a swollen/ fleshy appearance hence name
- H2O stored when it's in plentiful supply 7 used in times of drought
- e.g. aloe Vera
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(Xerophytic adaptations) leaf loss
- some plants prevent water loss thru their leaves by simply losing their leaves when water is not available,.
- e.g. palo verde
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(Xerophytic adaptations) other adaptations
- root adaptations - long tap roots, widespread shallow toots w a large SA
- avoiding the prblms - plants may lose leaves & become dormant, die, leaving seeds behind to germinate & grow rapidly when rain falls again, others survive as storage organs
- a few plants can withstand complete dehydration and recover ➡ linked to disaccharide thehalose.
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(Xerophytic adaptations) Stomata in pits or deep grooves, and reduced numbers of stomata ?
- many xerophytes have their stomata located in puits, which ⬇ air movement, producing a micro climate of still, humid air that ⬇ the water vapour potential gradient so ⬇ Trans, e.g. in Marram Grass
- ⬇ no. of stomata which reduce their water loss by transpiration but also reduce their gas exchange capabilities.
135
how can lignin be layed down in xyleem?
Lignin can be laid down in the walls of the xylem vessels in several different ways. It can form rings, spirals or relatively solid tubes with lots of small unlignified areas called bordered pits. This is where water leaves the xylem and moves into other cells of the plant.
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xylem vs phloem
Like xylem, the phloem sieve tubes are made up of many cells joined end to end to form a long, hollow structure. Unlike xylem tissue, the phloem tubes are not lignified. In the areas between the cells, the walls become perforated to form sieve plates, which look like sieves and let the phloem contents flow through. As the large pores appear in these cell walls, the tonoplast (vacuole membrane), the nucleus and some of the other organelles break down. The phloem becomes a tube filled with phloem s^> and the mature phloem cells have no nucleus.
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plasmodesmata ?
- microscopic channels through the cellulose cell walls linking the cytoplasm of adjacent cells
138
phloem tissue also include?
Phloem tissue also contains supporting tissues including fibres and sclcreids, cells with extremely thick cell walls.
139
the apolplast pathway detailed?
- This is the movement of water through the apoplast - the cell walls and the intercellular spaces.
- Water fills the spaces between the loose, open network of fibres in the cellulose cell wall.
- As water molecules move into the xylem, more water molecules are pulled through the apoplast behind them due to the cohesive forces between the water molecules.
- The pull from water moving into the xylem and up the plant along with the cohesive forces between the water molecules creates a tension that means there is a continuous flow of water through the open structure of the cellulose wall, which offers little or no resistance.
140
water moving down the apolplast pathway ....
At CS, water in the apoplast pathway can go no further and it is forced into the cytoplasm of the cell, joining the water in the symplast pathway. This diversion to the cytoplasm is significant as to get to get there, water must pass through the selectively permeable cell surface membranes, this excludes any potentially-toxic solutes in the soil water from reaching living tissues, as the membranes would have no carrier proteins to admit them. Once forced into the cytoplasm the water joins the symplast pathway.
141
Evidence for the role of active transport in root pressure:
• Some poisons, such as cyanide, affect the mitochondria and prevent the production of ATP. If cyanide is applied to root cells so there is no energy supply, the root pressure disappears.
• Root pressure increases with a rise in temperature and falls with a fall in temperature, suggesting chemical reactions are involved.
• If levels of oxygen or respiratory substrates fall, root pressure falls.
• Xylem sap may exude from the cut end of stems at certain times.
In the natural world, xylem sap is forced out of special pores at the ends of leaves in some conditions - for example overnight, when transpiration is low. This is known as guttation.
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how does transpiration work?
* Water molecules evaporate from the surface of mesophyll cells into the air spaces in the leaf and move out of the stomata into the surrounding air by diffusion down a concentration gradient.
* The loss of water by evaporation from a mesophyll cell lowers the water potential of the cell, so water moves into the cell from an adjacent cell by osmosis, along both apoplast and symplast pathways.
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how does adhesion work?
Water molecules form hydrogen bonds with the carbohydrates in the walls of the narrow xylem vessels.
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what is the transpiration pull?
Water is drawn up the xylem in a continuous stream to replace the water lost by evaporation.
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evid for cohesion tension theory?
- Changes in the diameter of trees.
- When a xylem vessel is broken - for example when you cut flower stems to put them in water - in most circumstances air is drawn in to the xylem rather than water leaking out.
- If a xylem vessel is broken and air is pulled in as described in the previous bullet, the plant can no longer move water up the stem as the continuous stream of water molecules held together by cohesive forces has been broken.
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Changes in the diameter of trees?
When transpiration is at its height during the day, the tension in the xylem vessels is at its highest too. As a result the tree shrinks in diameter. At night, when transpiration is at its lowest, the tension in the xylem vessels is at its lowest and the diameter of the tree increases. This can he tested by measuring the circumference of a suitably sized tree at different times of the day.
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It's diff to make direct measurements of transpiration bc?
because of the practical difficulties with condensing and collecting all of the water that evaporates from the surfaces of the leaves and stems of a plant without also collecting water that evaporates from the soil surface. It is also very difficult to separate water vapour from transpiration and water vapour produced as a waste product of respiration
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Factors that affect water loss from the leaf must either?
act on the opening/closing of the stomata, the rate of evaporation from the surfaces of the leaf cells or the diffusion gradient between the air spaces in the leaves and the air surrounding the leaf
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• Temperature affects the rate of transpiration in two ways:
— An increase in temperature increases the kinetic energy of the water molecules and therefore increases the rate of evaporation from the spongy mesophyll cells into the air snares of the leaf
—An increase in temperature increases the concentration of water vapour that the external air can hold before it becomes saturated (so decreases its relative humidity and its water potential).
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hydrophytic adaptations?
- very thin/ no waxy cuticle
- many always open stomata on upper leaf surface
- reduced structural tissue
- wide flat leaves
- small roots
- large SA of stems and roots underwater
- air sacs
- aerenchyma
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hydrophytic adaptations?
- very thin/ no waxy cuticle
- many always open stomata on upper leaf surface
- reduced structural tissue
- wide flat leaves
- small roots
- large SA of stems and roots underwater
- air sacs
- aerenchyma
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(hydrophytic adaptations) very thin/ no waxy cuticle?
hydrophytes do not need to conserve water as there is always plenty available so water loss by Trans is not an issue.
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(hydrophytic adaptations) many always open stomata on upper leaf surface?
- ⬆ no. of stomate ⬆ gas exchange
| - no risk of loss of turgor, so stomata usually open for gas exchange & the guard cells are inactive
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(hydrophytic adaptations) reduced structural tissue ?
the H2O supports the leaves and flowers so there is no need for strong supportive structures.
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(hydrophytic adaptations) wide flat leaves?
some hydrophytes, inclu. water lilies have leaves that spread cross the surface of the water to capture as much light as possible
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(hydrophytic adaptations) small roots?
water can diffuse directly into stem and leaf tissue so there is less need for uptake by roots
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(hydrophytic adaptations) large SA of stems and roots underwater ?
this max. the area for photosynthesises and for O2 to diffuse into submerged plants.
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(hydrophytic adaptations) air sacs ?
enable the leaves and/or flowers to float to teh surface of the water
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(hydrophytic adaptations) aerenchyma ?
- specialised parenchyma tissue forms in the leaves, stems, roots of hydrophytes.
- has many large air spaces, seemed to be formed by apoptosis in normal parenchyma
- functions: making the leaves and stem more buoyant & forming a low resistance internal pathway for the movement of substances such as O2 to tissues below the H20 ➡ helps plant cope w anoxic conditions in the mud by transporting O2 to the tissues
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Aerenchyma is ?
plant tissue found in the stems and roots of some hydrophytes, which allows oxygen to get down into the roots, which could be in poorly oxygenated water or waterlogged soil.
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translocation is =?
Translocation is the transport of the assimilates of photosynthesis (particularly the sugar sucrose) form source cells to sink cells.
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what are source and sink cells?
Source cells are where sugars are produced.
| Sink cells are where sugars are used.
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tech used to analyse phloem sap?
- aphid can be used to take a sample of phloem sap.
- Aphids naturally use their mouthparts (stylets) to penetrate phloem STEs and feed on the sugars.
- If the aphid is removed, leaving the stylet, the phloem sap can be collected from the stylet and analysed.
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phloem sap is mainly?
sucrose
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Is a leaf a source or sink?
During the day - source, because leaf cells photosynthesise, making sugars - at a faster rate than they respire, which uses sugars.
At night - sink, because at night cells don’t photosynthesise, but they still respire, which uses sugars.
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are root hair cells a source or sink?
Sink. No photosynthesis, but always respiration, using sugars.
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are flowers a source or sink?
Flowers don’t photosynthesise, but their cells respire, so sink. Also some flowers produce nectar to attract insects, which is formed from phloem sap, so definitely sink.
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are tubers source or sink?
In the spring / summer - sink. Because sugars are being converted to starch for storage in the tubers.
In early spring, starch is converted back to sugars which are used to synthesise molecules to produce a new plant, so source.
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Translocation - 10 steps
1. Source cells produce sucrose molecules from photosynthesis, or from the breakdown of storage molecules like starch.
2. Sucrose is actively loaded into companion cells by cotransport with H+ ions.
3. Sucrose diffuses from companion cells into phloem STEs through plasmodesmata.
4 . The high concentration of sucrose lowers the water potential in the STEs, and water moves by osmosis from the xylem into the STEs. This raises the hydrostatic pressure in the STEs close to the source cells.
5 .Sucrose is used by sink cells, either as a respiratory substrate, or to synthesise storage molecules like starch, or both.
6. Sucrose passively unloads from companion cells by diffusing out, into sink cells which are using the sucrose.
7. Sucrose diffuses down a concentration gradient from STEs into companion cells near to the sink cells.
8. The lowering of sucrose concentration raises the water potential in the STEs, and water moves by osmosis from STEs into the xylem. This lowers the hydrostatic pressure in the STEs close to the sink cells.
9. Phloem sap moves by mass flow from high to low hydrostatic pressure from source to sink cells in the phloem STEs.
10. Water and dissolved ions also move by mass flow down the hydrostatic gradient in the xylem vessels. This can contribute to the transpiration stream.
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transolactaion is an ? process
active
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The main sources of assimilates in a plant are:
• green leaves and green stems
• storage organs such as tubers and tap roots that are unloading their stores at the beginning of a growth period
• food stores in seeds when they germinate.
Some of these need to transport resources downwards, and some need to move materials up the plant.
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The main sinks in a plant include:
* roots that are growing and/or actively absorbing mineral ions
* meristems that are actively dividing
* any parts of the plant that are laying down food stores, such as developing seeds, fruits or storage organs.
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what is a major problem for hydrophytes?
Water-logging is a major problem for all hydrophytes. The air spaces of the plant need to be lull of air. not water, for the plant to survive.
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The main sources of assimilates in a plant are:
• green leaves and green stems
• storage organs such as tubers and tap roots that are unloading their stores at the beginning of a growth period
• food stores in seeds when they germinate.
Some of these need to transport resources downwards, and some need to move materials up the plant.
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The main sinks in a plant include:
* roots that are growing and/or actively absorbing mineral ions
* meristems that are actively dividing
* any parts of the plant that are laying down food stores, such as developing seeds, fruits or storage organs.
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what is root pressure and what is it caused by?
- there is a pressure pushing water upwards up xylem vessels from the roots
- caused by active transport of ions from endodermis cells into xylem vessels
- water movies down water potential gradient into xylem vessels and thfr 'pushes' up the plant
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what are halophytes?
flowering plants which are naturally found in saline habitats, such as coastal swamps, sand dunes, inland sand flats.... They have evolved a no. of strategies to survive and reproduce under highly saline conditions where most plants cannot.
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resistance of halophytes to salt stress involves 2 diff adaptations:
- salt tolerance - 'salt accumulators' which involves
accumulating salt in plants cells (⬆ conc, thfr WP⬇) & salt avoidance
- 'salt excludes' which involves adaptations to min the concs of salts in the cells or adaptations to bar salts from entering through plant roots
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Evid for transpiration?
+ adaptations of companion cells seen thru advances in microscopes
+ if Mitochondria in companion cells poisoned, translocation stops
+ flow of sugars is faster than would be from diffusion alone, suggesting active
+ Aphid studies - + pressure in Phloem, conc of sucrose is higher near source than sink
+ sucrose moves at same rate regardless of conc at sink
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Evid against trans?
- not all solutes move at same rate
| - sieve plates role?
