chapter 4 Flashcards
(95 cards)
1
Q
How does water move up to the top of a tree?
A
*Water first enters the roots.
*Then moves to the xylem, the innermost vascular tissue.
*Water rises through the xylem because of a combination of factors.
*Most of that water exits through the stomata in the leaves
2
Q
Unidirectional Transport
A
The movement through the body of plants, from roots to shoots(top of tree)
3
Q
What is the result of local changes?
A
Local changes result in long-distance movement of materials.
4
Q
Where are the greatest distances traveled by water molecules and dissolved minerals?
A
The xylem
5
Q
To where does water move into across cell membranes of root cells?
A
Water moves across cell membranes of root cells, into the xylem in the stele.
6
Q
Once water enters the xylem, how does it move?
A
Once water enters the xylem, it can move upward
7
Q
What is the force that moves the water?
What is it caused by?
A
Most of the force that moves the water is ‘pulling’ caused by transpiration: evaporation of water from stomata.
8
Q
Transpiration
A
evaporation of water from stomata
9
Q
Why does pulling occur?
A
The pulling occurs because water molecule stick to each other (cohesion) and to the walls of the tracheids or xylem vessels (adhesion) due to the hydrogen bonding.
10
Q
What do cohesion and adhesion result in?
A
Both cohesion and adhesion result in the water in a stable, unbroken column.
11
Q
Where does long system transport also occur?
A
the phloem
12
Q
What is the type of phloem transport?
A
unlike xylem transport, phloem transport (of sugars) is bidirectional (up and down the plant).
13
Q
How does water diffuse?
A
Water can diffuse down its concentration (from high concentration to low water concentration) across a plasma membrane by osmosis.
14
Q
How is the rate of water movement into or out of cells enhanced?
A
by membrane water channels called aquaporins.
15
Q
Aquaporins
A
—membrane water channels
—speed up water movement across a membrane, but do not change its direction
16
Q
What is water potential used for
A
Water potential (Ψw) is used to predict which way water will move.
Water moves by osmosis from:
high water potential ———> low water potential
high water concentration —> low water concentration
low mineral concentration –> high mineral concentration
17
Q
What is water potential measured in
A
Water potential (Ψw) is measured in units of pressure called Mpa (megapascals).
18
Q
In wet soil, how does water move?
A
In a wet soil: water moves by osmosis to the roots (low Ψw) to leaves (lower Ψw) to atmosphere (even more negative Ψw ).
19
Q
what are the 2 components of water potential?
A
Pressure potential
Solute potential
20
Q
Pressure potential
A
1) (Ψp ) Pressure potential or turgor P:
Physical forces, like gravity or pressure on a plant cell wall.
—The effect of gravity is small (at level of water movement in the cell), but turgor pressure is significant. (it is usually a positive pressure).
—As turgor P increases, the value of Ψp increases
21
Q
Solute potential
A
(Ψs) Solute potential:
*Is the concentration of solutes in each solution. (It is usually a negative P).
*As solute pressure (Ψs) increases, it becomes more negative.
22
Q
Total water potential
A
Total water potential is the sum of its pressure potential and solute potential.
23
Q
What is the turgor pressure of solutions that are not contained within a membrane?
A
Solutions that are not contained within a membrane do not have a turgor P and their Ψp = 0 Mpa.
24
Q
What does the turgor Pressure generated from fluid within a cell pushing against the cell wall give?
A
Turgor Pressure generated from fluid within a cell pushing against the cell wall gives a turgid cell a Ψp > 0 MPa (positive value).
25
What is the solute potential of pure water?
0 mpa
26
What happens to solute potential as solute are added to water?
As solutes are added to the water, the value of Ψs decreases: Ψs < 0 Mpa
Fewer free water molecules are available to move, which decrease the water potential.
27
What happens when a plant cell is placed in a hypotonic solution of water?
Water moves into the cell by osmosis.
The cell expands and becomes turgid(swollen) as turgor P of the cell increases
28
What happens when a plant cell is placed in a hypertonic solution of water? ex. high concentration of sucrose
Water leaves the cell and turgor P drops.
The cell shrinks -> plasmolysis.
29
What happens when a plant cell is placed in an isotonic solution of water?
No net movement of water between the cell and the solution as Ψw of the cell equals that of the solution.
The volume of the cell remains constant.
30
what happens whenΨw > Ψw of solution?
WhenΨw > Ψw of solution ----> water moves out of plant cells, causing plasmolysis and the plant wilts
31
How does water potential regulate the movement of water through the whole plant?
Water moves from the soil into the plant only if water potential of the soil is greater than in the root
Water in a plant moves along a Ψwgradient from the soil to successively more negative water potentials in the roots, stems, leaves, and atmosphere.
32
Root hairs
Root hairs increase the surface area for water and mineral absorption
33
In some plants, how is the surface area increased?
In some plants, this S/A is further increased by root association with mycorrhizal fungi
34
Why are root hairs always turgid?
their water potential is more negative than the soil because they have a proton pump in their cell membrane, and they are always actively pumping ions from the soil to their tissues
35
What happens once water and absorbed minerals are observed?
they have to move through the cortex, then endodermis, until they reach the xylem.
36
Once water is in the root, how can it move inside the cortex?
Once water is inside the root, it can move in the cortex via 3 transport mechanisms:
Apoplast route
Symplast route
Transmembrane route
37
Apoplast route
-Rapid movement through the cell walls and the space between cells
-It is non-selectivemmovement (avoiding membrane transport).
38
Symplast route
-Water moves from cytoplasm to cytoplasm passing through plasmodesmata.
-This movement is selective
39
Transmembrane route
-It involves membrane transport between cells and also across the membranes of vacuoles within cells.
-Most selective movement: it allows greatest control over what substances enter and leave the cells.
40
What happens when water reaches the endodermis?
Apoplast route is blocked through by suberin(waterproof substance) in Casparian strip.
Water and dissolved ions are forced to go inside the endodermal cells.
41
Why is transport in endodermis very selective?
-It controls water and minerals flow to the xylem.
-It blocks harmful ions.
42
What is the pathway of water and mineral transport in roots?
---Water and dissolved minerals are absorbed by root hairs.
---They travel in the cortex via apoplast or symplast routes.
---Once the endodermis is reached: apoplast movement is blocked by Casparian strip.
---Water and dissolved minerals are forced to pass through the membrane and cytoplasm of endodermal cells before reaching the xylem.
---Endodermal cells controls the transport of harmful solutes through the rest of the plant.
43
Is Mineral ion concentration in the soil water higher/lower than in the plant?
lower than in the plant
44
How are minerals taken up by the root?
Minerals are often taken up by the roots via active transport, then across the endodermis to increase the solute concentration in the stele
45
What do plasma membranes of endodermal cells contain?
Plasma membranes of endodermal cells contain a variety of protein transport channels, through which proton pumps actively transport specific ions against their concentration gradient (from low to high concentrations).
46
What happens once the ions are inside the stele?
Once inside the stele, the ions (plant nutrients) are transported via the xylem throughout the plant.
47
Root pressure
---Water accumulates in the xylem ---> creating a pressure called root pressure.
---Root P is caused by the continuous accumulation of ions in the roots at times when transpiration rate from leaves is low or absent.
---This root P pushes water up the xylem for a short distance, only.
---Root P alone is not enough to explain water transport in xylem.
---So, there must be another mechanism for pushing water up the xylem for a longer distance
48
Transpiration
Transpiration provides the main force for pulling water and ionic solutes from roots to leaves.
Evaporation of water from leaves create negative P (tension) on the entire water column of xylem ---> pulling water up the xylem.
The adhesive and cohesive properties of water (tensile strength) create an unbroken column.
Water has an inherent tensile strength that arises from the cohesion of its molecules (their tendency to form H bonds with one another).
The tensile strength is inversely proportional with the diameter of the column:
The smaller diameter of the column ---> the greater the tensile strength.
49
Transport in xylem
Evaporation from leaves create tension, responsible for pulling the water up the xylem.
Cohesion and adhesion properties of water create an unbroken column.
Tracheids and vessels have very small diameter, resulting in great tensile strength/strong cohesive force to pull water up (> than gravity).
Vessels have larger diameter than tracheids; more mass of water can pass through them
50
Cavitation
If bubbles of air enter the xylem, the continuous water column and cohesion would fail.
A gas-filled bubble can expand and block the tracheid or vessel; this is called cavitation (or embolism).
Cavitation stops water transport and can lead or dehydration and death of part or all the plant.
To compensate for cavitation, other pathways develop. Tracheids and vessels are connected by pits in their walls; bubbles cannot pass through the pits as they are larger and cannot block them.
51
Mineral transport through the plant
Most minerals enter the roots by active transport then move in the xylem (tracheidsand vessels).
They are relocated to different plant parts.
There is one exception: once calcium is deposited in tissues, it cannot be transported elsewhere.
52
Guttation
It is loss of water in the liquid form, along openings in the leaf margins (not via stomata!)
It is not dew!
It occurs at night, when humidity and soil moisture are high, and when there is no transpiration.
As roots continue to actively pump ions, water keeps entering the roots.
Because there is no transpiration, extra water is forced out through the leaf margins.
The cause of guttation is root pressure.
53
Transpiration simple definition
is water loss in the vapor form through the stomata.
Most of the water (90%) taken by the roots is lost to atmosphere.
Stomata open and close to balance water and CO2 needs
54
Stomatal openings and closings are controlled by
changes in the turgor P of the guard cells.
55
Guard cells
are the only epidermal cells with chloroplasts
have thicker walls on the inside and thinner walls on the outside
can bulge and bow outward, when they are turgid:
---> the stoma shows (open)
become flaccid when they lose water: stomata close.
56
What does the unequal cell-wall thickening on guard cells result in?
the opening of stomata when the guard cells are turgid and expand.
57
What does blue light trigger?
a proton pump in plasma membrane of guard cell.
58
What is pumped inside and outside the guard cell?
H+ are actively pumped outside guard cells
Creating a proton gradient/voltage that drives K+ inside guard cells through specific channels
Increase in K+ concentration in guard cells
Chloride (-) and malate (2-) move inside guard cells
59
What happens to the concentration of solutes-water potential?
Increase in concentration of solutes inside guard cell and decrease in water potential
60
What happens after water moves by osmosis inside guard cell?
Guard cells swell, becoming turgid.
They separate and the stomata open
61
What happens when K+ ions leave (passively) the guard cells?
Chloride and malate follow to outside
Increase in concentration of solutes outside the guard cells
Water moves out by osmosis
Guard cells become flaccid →stomata close
62
What happens when a plant is exposed to drought or too much wind?
the stress hormone, abscisic acid (ABA) is released from chloroplasts (in leaves).
ABA binds to specific receptors on plasma membrane of guard cells, triggering signaling pathway that opens K+, Cl-and malate ion channels.
➢These ions move out of guard cell
➢Water follows
➢Guard cells become flaccid
➢Stomata close.
This leads to reduction of water loss.
63
How do transpiration rates change with temperature and wind velocity? Why?
Transpiration rates increase with temperature and wind velocity because water molecules evaporate more quickly
64
At which temp does stomata close?
high temperature > 34°
65
In some plants, what do they do when CO2 concentrations in leaves is high?
when the CO2 concentration in leaves is high, guard cells close to conserve water.
Even at night, a low level of CO2 induces stomata to open
66
What type of photosynthesis does succulent plants have?
CAM photosynthesis:
Stomata open at night and close during the day ---> CO2 is taken at night and stored; it is fixed during the day to decrease water loss in dry regions.
67
What are the adaptations that have helped plants respond to environmental fluctuations?
1.Adaptations to drought
2.Adaptations to flooding
3.Adaptations to live in fresh water
4.Adaptations to live in salty water
68
How did plants adapt to reduce water loss?
a)Some plants become dormantduring the dry season
Annual plants die but leave their seeds dormant in soil.
b)Deciduous plants shed their leaves in regions with harsh winter. As water becomes locked in ice, it cannot be taken by roots (form of drought). To decrease water loss, a tree gets rid of its leaves in winter.
c)Thick, hard leaves with few stomata on lower epidermis
d)Stomata hidden in crypts (pits or depression) in lower epidermis
e)Leaves with wooly trichomes
Trichomes reflect light off plant; reducing heat and transpiration
69
How do plants protect themselves from drought?
Deeply embedded stomata, extensive trichomes, and multiple layers of epidermis minimize water loss in this leaf, shown in cross section.
70
How did plants adapt to flooding?
*Flooding depletes available oxygen in soil
*It interferes with minerals and carbohydrate transport in roots
*It results in hormone changes: Ex. increase in ethylene hormone that suppresses root elongation.
*It may lead to abnormal growth
*O2deprivation is the most important problem since it leads to:
-Reduced cellular respiration
-Physical changes in the roots
-Interference of water flow through the plant
-Leaves may dry out
➢One adaptation to flooding: Stomata close to maintain leaf turgor.
-Standing water has much less O2than moving water; it is harmful to plants.
71
How did plants adapt to live in fresh water?
Plants living in fresh water like water lilies have aerenchyma.
Aerenchyma= loose parenchymal tissue with large air spaces.
O2 gets transported from plant part above the water to submerged plant by passing in aerenchyma.
Some plants normally have aerenchyma.
Others, if subjected to frequent flooding, they can form aerenchyma when necessary. Ex. Corn.
Trees also respond to flooding by former larger lenticles (help in gas exchange) and more adventitious roots
72
Aerenchyma
---loose parenchymal tissue with large air spaces.
---facilitates gas exchange in aquatic plants.
---Water lilies float on pond surface, collecting oxygen and then transporting it to submerged portions of the plant.
---Large air spaces in the leaves add buoyancy (help in floating).
Gas exchange occurs through stomata found only on the upper surface of the leaf.
73
How did plants adapt to live in salty water?
Plants like mangroves grow in areas flooded with saltwater.
They need to supply O2 to their submerged parts and control their salt balance.
Mangroves have pneumatophores= long, spongy, air-filled, roots that emerge above the mud/water.
Roots have large lenticelson their non-submerged portion, so that O2 enters and moves to air-spaces (aerenchyma) inside pneumatophores.
Mangroves have succulent leaves with high water content to dilute the salts.
At root level, salt uptake is blocked, or excess salts is pumped (secreted) out.
74
pneumatophores
pneumatophores= long, spongy, air-filled, roots that emerge above the mud/water.
75
Mangroves
The black mangrove grows in areas that are commonly flooded, and much of each plant is usually submerged.
Modified roots called pneumatophores supply the submerged portions of the plant with oxygen because these roots emerge above the water and have large lenticels. Oxygen diffuses into the root through the lenticels, passes into the abundant aerenchyma, and moves to the rest of the plant.
76
Halophytes
Halophytes(= salt lovers) are plants that can tolerate soils with high salt concentrations.
Some produce high concentrations of organic molecules within their roots to alter the water potential between soil and roots ---> so that water flows in roots.
77
Translocation
---Most carbohydrates produced in leaves are distributed through phloem to rest of plant.
---Most carbohydrates produced in leaves are distributed through phloem to rest of plant.
78
Phloem transport
Bidirectional
79
What is the phloem responsible for transporting?
carbohydrates (CHOs), hormones, (in addition to proteins, organic acids and some ions).
80
Sap
CHO rich fluid in the phloem
81
CHO rich fluid in the phloem
Sap
82
Demonstrating phloem transport
Aphids feed on the nutrient-rich content of the phloem, which they extract through their piercing mouthparts called stylets.
When an aphid is separated from its stylet and the cut stylet is left in the plant, the phloem fluid oozes out of it and can then be collected and analyzed
83
Pressure flow/mass flow/bulk flow
CHOs, produced in leaves or stored in storage organs (bulbs, roots..) as starch, are first converted to sucrose before translocation.
Translocation happens through the pressure-flow hypothesis.
This is the most widely accepted model of CHOs movement in phloem.
It states that dissolved CHOs flow from a source to a sink.
84
Sources
Sources are regions where sugars are produced(photosynthetic leaves) or stored (bulb, root…).
85
Sinks
Sinks are regions where sugars are used (growing leaf, stem, fruit, developing bulb…).
Note: Food-storage tissue (ex. Bulb, root) can be sources or sinks.
86
What must happen before sugar transport in the phloem?
sucrose must be loaded in the phloem
87
Where does phloem loading occur? And what does it require?
Phloem-loading occurs at the source, and it requires ATP
88
Phloem loading process
Some sucrose move passively from mesophyll cells to companion cells and then into sieve cells via the symplast( from cytoplasm to cytoplasm via plasmodesmata).
Most sucrose is moved actively into companion cells and into sieve tube members. Sucrose molecules arrive through apoplast(outside cell walls and spaces between cells) then are moved across the membrane and into the sieve cells via (2°transport) a sucrose and H+ transporter. This is energy driven and needs ATP
ATP is provided by companion cells and parenchyma cells that are adjacent to sieved tubes
89
Sucrose loading summary
Companion cell actively transports H+ into the surrounding mesophyll cells.
This creates a H+ gradient between the surrounding and the companion cell: more H+ ions outside the companion cell.
H+ ions move back into the companion cell down their concentration gradient through a co-transporter protein (H+/sucrose symporter).
When a H+ ion moves through the co-transporter, a sucrose molecule is also transported into the companion cell, against its concentration gradient (from low to high concentration).
The same process occurs to transport sucrose from the companion cell into the sieve tube element.
90
Mass flow
During loading, sucrose moves from companion cells into sieve tube elements by active transport.
Accumulation of sugar reduces the water potential of the sieve tube element.
There is a difference between water potentials in sieve tubes and adjacent xylem cells ---> water flows into the sieve tube by osmosis, from neighboring/adjacent xylem (from high water potential in xylem to low water potential in phloem).
The incoming water increases the hydrostatic (turgor) pressure in sieve tubes
This P pushes the sugars inside the sieve tubes.
Movement of sucrose in sieve tubes does not need energy.
It occurs from high hydrostatic pressure near the source cells to low hydrostatic pressure near the sink cells.
91
How is sucrose uploading at the sink?
Sucrose molecules are actively removed/unloaded
Water follows by osmosis
Turgor pressure drops inside the sieve tubes (at the sink end)
This causes a mass flow from the stronger P at the source to the weaker pressure at the sink.
92
What only needs ATP?
Loading and unloading of sucrose in phloem need ATP
93
How does mass flow of water occur?
Mass flow of water occurs from stronger (more positive) turgor P at the source to weaker (more negative) turgor P at the sink.
94
What only needs ATP?
Only loading and unloading of sugars need ATP
95
What does not require ATP?
Translocation inside the sieve tubes
