Week 10 Flashcards
1
Q
Plasmolysis would occur when
A
a cell is placed in a very concentrated solution.
2
Q
Water will move
A
from a location with a higher water potential to a location with a lower water potential.
3
Q
A cell that is swollen with water is said to be
A
turgid
4
Q
The hydrostatic pressure that builds as water enters plant cells and presses on the cell wall, is called _________
pressure.
A
turgor
5
Q
Choose all components of the water potential of a solution in a plant cell.
A
Gravity
Pressure
Solute concentration
6
Q
If a cell loses water, the cell membrane pulls away from the wall in a process called
A
plasmolysis
7
Q
If the water potential outside a cell is -0.3 MPa and the water potential inside the cell is -0.5 MPa, will water move and in what direction?
A
Water will move into the cell.
Reason: Water moves from a high water potential to a low water potential. The water potential outside the cell is higher (a larger number) than inside the cell.
8
Q
The solute potential of pure water is
A
Zero
9
Q
A turgid cell is
A
stiff
10
Q
Under mild drought conditions, plants may be stunted because
A
stomata are closed so carbon dioxide is not taken in for photosynthesis.
11
Q
Turgor pressure requires
A
cell walls to constrain the expansion of cells as they take up water
12
Q
The water potential of a solution has two components: _________
forces (such as pressure, or gravity), and the concentration of ________
in the solution.
A
Blank 1: physical
Blank 2: solutes
13
Q
Select all functions of stomata.
A
Minimize water loss
Admit carbon dioxide
14
Q
The addition of solutes to water
A
decreases the water potential.
15
Q
The cells that border stomata are called _________
cells.
A
Blank 1: guard
16
Q
Plants are always dealing with the trade-off between open stomata, in which CO2 is taken in but _________
is lost, and closed stomata, in which the same compound is retained, but CO2 is not taken in.
A
water
17
Q
The gravitropic response is
A
negative in shoots and positive in roots.
18
Q
The first step in the root gravitropic response is the perception of gravity. The last step is differential ______.
A
cell elongation
19
Q
In gravitropism, a ________
signal is converted into a ________
signal.
A
mechanical
physiological
20
Q
Structural features on leaf epidermal cells, called _______
, have evolved to minimize water loss, while allowing carbon dioxide uptake.
A
stomata
21
Q
In shoots, gravity appears to be sensed along the length of the stem in ______.
A
endodermal cells
22
Q
What feature of guard cells allows them to open stomata when turgor pressure in them changes?
A
Their cells are thicker on the inside and thiner elsewhere.
23
Q
In negative gravitropism, how do cells respond to auxin?
A
Cells on the lower side of a stem grow more rapidly than cells on the upper side
24
Q
Plant roots are ________
gravitropic, and plant stems are ________
gravitropic.
A
Blank 1: positively
Blank 2: negatively
25
The roots and stems of plants bend in response to gravity due to ______.
differential growth
26
Which type of signal transduction occurs in gravitropism?
A mechanical signal is transduced into a physiological signal.
27
In roots, gravity is sensed by cells in which area?
Root cap
28
When a stem is placed on its side, auxin
causes the lower cells to elongate.
29
What happens due to low turgor pressure?
stomata closes
30
What is tropism?
directional growth response to external cue
31
What is negative phototropism?
Negative- growing away from the cue.
32
What is the difference between negative and positive gravitropism?
Negative gravitropism
(away from gravity vector)
Positive gravitropism
(towards gravity vector)
33
What happens due to shoot gravitropism in trees?
```
• Trees on steep mountain slopes
can be pushed sideways by snow
and avalanches
• To regain vertical shoot growth,
lower side puts on more growth
– visible in growth rings
```
34
What is the process of Shoot gravitropism?
Signal to Signal perception to Signal transduction to Response.
35
What is required for differential growth?
Differential growth:
Auxin (plant hormone) is
required for shoot growth
36
What does auxin lead to?
Auxin leads to cell elongation in the coleoptile
37
Where is signal perception in shoot gravitropism?
Signal perception is in a specific cell layer
| containing amyloplasts
38
When do plants show no shoot gravitropism?
Plants without endodermis show no shoot gravitropism
39
Outline shoot gravitropism: Signal transduction
Undisturbed shoot growth:
• High auxin concentration at the tip of the
shoot (shoot apical meristem)
• Auxin is transported equilaterally from the tip
to the bottom of the shoot (coleoptile)
• Growth of shoot is driven by auxin in a dosedependent relationship
40
What happens due to Undisturbed shoot growth?
auxin transported
| from the tip of the coleoptile to the bottom
41
What happens due to Gravitropism signal transduction?
Statolith
| movement leads to change in auxin flow.
42
Experiment with coleoptile tip on two agar
| blocks.
Auxin transported from the shoot tip
accumulates in agar blocks.
• Gravistimulated shoot: Auxin flow is redirected
laterally, more auxin flows to the lower half of
the shoot.
• Agar block placed on coleoptile with tip
removed (no source of auxin).
• Coleoptile with agar block from lower half of tip
shows increased differential growth (bending).
43
Differential growth is needed for
gravitropic
| bending
44
How does the shoot bend?
Cells on the lower side of the root grow more
| than on the upper side: shoot bends
45
What is the amount of growth dependent on?
Amount of growth depends on amount of auxin:
| in shoot tissue, more auxin = more growth
46
Auxin leads to
cell growth
47
Outline the Acid-induced growth of plant cells
```
• Acidification of cell wall leads to
loosening of cross links between
cellulose microfibrils
• If cell wall strength is reduced, turgor
pressure can drive cell growth
```
48
What is the Acid growth hypothesis?
Auxin causes the acidification of cell walls
This is still somewhat controversial and
there are probably other factors that
contribute to cell wall acidification.
49
Gravity signal perception is through
movement of
| amyloplasts in the endodermis layer
50
Signal transduction is through
changes in auxin
transport: more auxin is transported to the lower side of
the shoot
51
Changes in auxin transport lead to
differential growth:
| more growth on lower side of shoot
52
Acid growth hypothesis links
links auxin to increased cell wall acidification, which leads to changes in cellulose microfibril crosslinks, which allow turgor pressure to
drive cell growth
53
What happens if you have an acidic cell wall?
If you have an acidic cell wall you have some plant cells that have a better ability to grow as it affects how the cellulose microfibrils are bond/ crosslinked together- high tensile strength which enables them to withstand the turgor pressure.
54
Plants are acutely aware of their _____ and produce very specific _______.
Blank 1- environment
| Blank 2- responses
55
What are some biotic plant responses to their environment?
Herbivory diseases causes a signal transduction making defence response compounds.
56
What are some abiotic plant responses to their environment?
light, water, gravity, nutrients and temperature lead to a signal transduction that causes responses such as
57
What are all plants surrounded by?
A cell wall
58
What is the cell wall?
• Cell wall is extracellular matrix:
outside the plasma membrane
• Rigid structure that provides support and protection
• Cells are fixed in their position and cannot migrate
59
The cell wall has
several layers
60
What do the cell wall layers suggest?
Cell wall is built in layers, oldest layer is
| furthest away from the cell
61
What is the middle lamella?
oldest layer of cell wall
(derived from cell plate, cell division).
contact zone between cell
walls of two cells, Pectin (polysaccharide)
62
What happens during cytokinesis in plants?
* Rigid cell wall prevents migration of cells or division by cleavage of plant cells
* Golgi-derived vesicles move along microtubules towards middle of cell
* Vesicle coalesce to form cell plate (new compartment surrounded by membrane)
* Cell wall forms inside cell plate
63
What is the primary cell wall?
second oldest layer of cell
| wall
64
What is the secondary cell wall?
(not all cells have this):
| youngest layer of cell wall
65
What is the main structural component of the primary and secondary cell wall?
Main structural component of primary and secondary cell wall are cellulose fibres
66
What is a major component of a cell wall?
Cellulose fibres
67
Explain the cellulose cell walls?
* Made of glucose molecules
* Long chains of glucose molecules
* Cellulose molecules are tightly packed together in cellulose microfibrils
68
What are cellulose microfibrils made by?
made by cellulose
| synthase complexes in the plasma membrane
69
Where are the new cellulose microfibrils deposited?
deposited outside the cell
70
What are the enzyme complex (multimer) linked to?
Enzyme complex (multimer) is linked to microtubule in the cytosol and is pushed along it during synthesis
71
What is the orientation of cellulose microfibril?
Orientation of cellulose microfibril is parallel to microtubule
72
What does the orientation of cellulose microfibrils determine?
Orientation of cellulose microfibrils determines growth and cell shape
responses to the environment
73
The cell wall is a
complex structure
74
What are the properties of the complex structure of the cell wall?
• Cellulose microfibrils
crosslinked with other glycans (rigidity) several layers orientation of fibres is important
• Proteins (e.g. extensins, change rigidity)
• Lignin (polymer, improves rigidity)
• Suberin (wax, water proofing)
75
Cell walls give plant cells
specific shapes and structures
76
Cellulose microfibril orientation in the cell wall is controlled by
the cell
77
What are the several layers that the plant cells are built from?
middle lamella, primary and secondary walls
78
Cellulose microfibrils synthesized from
glucose are the major
| structural component of the cell wall
79
Cellulose microfibrils are synthesized by
a multimeric cellulose
| synthase enzyme complex in the plasma membrane
80
Association of the cellulose synthase enzyme complex with microtubules determines the
the orientation of cellulose microfibrils
81
What is essential for plant cell shape?
Water
82
What happens when plants cannot take up water?
If plants cannot take up water from their growth medium (environment), cells (and whole plant) will lose their shape.
83
Water movement across a semi-permeable
| membrane depends on
solute concentration
84
What is a property of the plasma membrane?
The plasma membrane is semi-permeable: water moves more easily across than larger molecules (solutes).
85
What happens if teh solute concentration is different between compartments separated by plasma membranes?
water will move to
| compartment with higher solute concentration.
86
Rigid cell walls lead to
to turgor pressure in plant cells
87
What happens when plants are under normal growth conditions?
Under normal growth
conditions, plant cells are
pressurised.
88
What does Turgor pressure contribute to?
```
Turgor pressure contributes
to cell (and plant) shape
```
89
What happens during Plasmolysis?
• Cell volume shrinks
• plasma membrane
detaches from cell
wall
90
What happens when there is turgor pressure?
• Cell volume increases
• Volume increase is
resisted by rigid cell wall
91
What is water potential used to describe?
Water potential (ψ, psi) is used to describe the
direction of water movement in plants (and
environment)
92
Where does water always move to?
Water always moves from cells or parts of the plant
| with high ψ to cells or part of the plant with lower ψ.
93
What does water potential also determine?
Water potential also determines water uptake from soil and transpiration.
94
How is water potential measured?
Measured as a pressure (megapascals, MPa)
95
Water movement through the plant is driven by
a water potential gradient
96
Water potential of the air is ______ ______ than
| that of the plant.
much lower
97
What does Water potential of the air is much lower than
| that of the plant drive?
This drives transpiration of
| water from the leaves.
98
Water potential of roots is ____ than that of the
| soil.
lower
99
What does Water potential of roots is lower than that of the soil drive?
This drives water uptake into the roots.
100
Where is there water potential?
There is a water potential gradient within the
| plant.
101
Shoots and leaves have a ______ water
| potential than roots.
lower
102
What does Shoots and leaves have a lower water
| potential than roots drive?
This drives water transport
from the roots via the xylem to the shoots and
leaves.
103
What does water potential of a cell depended on?
Water potential of a cell (ψW) depends on turgor
pressure and the concentration of solutes in the cell:
ψW = ψP + ψS
104
Low water potential outside the cell leads to
to loss of
| turgor pressure
105
Turgor pressure is required
| to
to keep plant cells in shape.
106
What happens when the turgor pressure is 0 MPa?
plant wilts
107
When does the solute potential of the cell become more negative?
Solute potential of cell goes more negative
because of cell volume shrinkage
Turgor pressure is completely lost
108
What is osmosis?
Water movement across a semi-permeable membrane
| depends on solute concentration, water flows towards compartment with higher solute concentration
109
What is water potential?
Water flows towards cells (or the environment) with lower
(more negative) water potential. Concentration of solutes in the cell and pressure of the cell
contribute to water potential.
110
What is turgor pressure?
Under normal growth conditions plant cells are pressurised, this pressure is important for cell (and plant) shape.
111
Under normal growth conditions plant cells are ______, this pressure is important for cell (and plant) shape
pressurised (turgor pressure),
112
Concentration of solutes in the cell and pressure of the cell contribute to
water potential
113
Water flows towards cells (or the environment) with
with lower (more negative) water potential
114
Turgor pressure is required to
keep plant cells in shape
115
What does turgor pressure drive?
drives cell growth
116
Growing plant cells increase their volume
10 – 1000 times
117
Turgor pressure in growing plant cells is
| between
0.3 and 1 MPa
| 1 MPa = 145 psi; car tyre ~ 30 psi
118
What does turgor pressure drive?
• Turgor pressure drives cell wall expansion (if
cell wall material allows)
• Expansion of cell wall reduces turgor pressure
• Reduction in turgor pressure decreases water
potential
• Water flows into the cell
119
How do most plant cells grow?
grow by diffuse growth
120
What happens during tip growth?
• Cell wall expansion is
highly localised (at the
tip of the cell)
• Root hairs, pollen tubes
121
What happens during diffuse growth?
```
• Cell wall expansion is dispersed
across the whole cell
• Most other plant cells
• Even though growth is diffuse,
cells usually do not expand
isodiametrically.
```
122
What is isodiametric growth?
Cells expand and grow equally in all dimensions.
| Random orientation of cellulose microfibrils
123
What is Anisotropic growth?
Cells preferentially expand and grow in one dimension
Parallel orientation of cellulose microfibrils
Anisotropic growth
(direction of growth orthogonal to fibril direction)
124
Cell growth direction depends on
cellulose microfibril orientation
125
Turgor pressure (driving force for growth) is
is the same in all directions
| of the cell.
126
What can cellulose microfibrils not do?
Cellulose microfibrils cannot change
| length
127
What can be changed between fibrils?
Spacing between fibrils can be changed
(cross-linking glycans) → Highly regulated
process
128
What does the Orientation of cellulose microfibril
| determine?
determines the direction of growth or
| shape changes
129
Turgor pressure is important to maintain
plant cell shape
130
Turgor pressure is the driving force of
plant cell growth,
| but does not provide a direction for the growth.
131
Orientation of cellulose microfibrils determines
the direction of plant cell growth
132
What is movement of water through the plant controlled by?
stomata
133
Where is water taken up?
• Water taken up by the roots transpires from the plant through stomata (plural) in leaves
134
What does the water potential gradient drive?
Water potential gradient drives water transport
| Soil (high ψ) – plant – air (low ψ)
135
What does the stoma consist of?
Stoma consist of two guard cells that surround pore between them
136
When does the stomatal pore open or close?
Stomatal pore is open or closed to control transpiration
137
What does the control of the stomata need to balance?
needs to balance the needs of photosynthesis and water
138
What needs to be tightly regulated?
Opening and closing of stomatal pores needs to be tightly regulated
139
Why do the stomatal pores need to be opened?
Need to be open to let in CO2 for photosynthesis
140
Why do the stomatal pores need to be closed?
Need to be closed to reduce amount of
| water lost through transpiration
141
What changes to help the stomata open or close?
Cell shape needs to change to open or close stomata
142
Why is the cell wall of guard cells there?
The cell wall of guard cells is built to allow opening and
| closing of the stomatal pore
143
How are the Microtubules and cellulose microfibrils organised?
Microtubules and cellulose microfibrils fan
| out radially from the stomatal pore.
144
What are the properties of the cell walls next to the pore?
Cell walls next to pore are much stronger
(= resist turgor pressure better) than outer
walls or where the two guard cells meet.
145
What happens if the guard cell volume increases?
If guard cell volume increases ( = increased
turgor pressure), cells shape changes are
larger along outer walls and where the
two guard cells meet.
146
What does the influx of water lead to?
Influx of water into the guard cells leads to
| stomata opening
147
Pore closed =
Turgor pressure low.
Transport of solutes
into guard cells lowers
water potential
148
Pore open=
Turgor pressure high.
Cell volume increase leads to cell shape change that
increase pore width
149
When does water flow into the guard cell?
Water potential of guard
cells is lower than
surrounding cells, water
flows into the guard cells
150
What regulates guard cell shape and pore opening and closure?
Environmental factors
151
What will an increase in abscisic acid lead to?
leads to stomata closure
152
Explain the process that causes the stomata to close due to an increase in abscisic acid?
```
1)ABA concentration
increases
2)ABA perception leads to
efflux of solutes
→ water potential of
guard cells less negative
than surrounding cells
3)Water efflux from guard
cells due to changed
water potential
4)Due to water efflux,
turgor pressure of
guard cells decreases
and stoma closes
```
153
What is stomatal density?
Stomatal density: number of stomata per surface area
154
What is Low stomata density?
reduced transpiration
| and uptake of CO2
155
What is high stomata density?
increased
| transpiration and increased uptake of CO2
156
When does the density on mature leaves not change?
Density of stomata on mature leaf does not
change, but new leaves can be made with
more or fewer stomata
157
Stomata density is regulated by
CO2 levels in the
| atmosphere and other environmental factors
158
What do fossil records tell you about stomatal density?
```
Fossil record: stomata density in periods with high CO2 have low
stomata density (can be replicated under lab conditions)
```
159
What are some other r factors regulating stomata density?
* Water – reduced water availability decreases stomata density
* Temperature – increase leads to reduction in density
160
Width of stomatal aperture needs to be regulated to
satisfy the competing needs of photosynthesis
| and water preservation
161
Light, low CO2 concentration and high humidity lead to
stomata opening
162
Darkness, high CO2
concentration, low humidity
and abscisic acid lead to
stomata closing
163
Turgor pressure and changes in cell shape
| regulate
opening and closure of stomata
164
Regulating stomata density is a
a long-term
response to regulate transpiration and CO2 influx.
Long-term response to long-term
environmental changes
165
Touch response is dependent on
on turgor pressure
166
Touch response in Mimosa pudica
• Compound leaf consisting of many leaflets
• Leaflets fold when touched
• Pulvinus: group of cells at base of leaflet
• Response is dependent on change in
turgor pressure and cell shape change
167
What happens in a plants response to touch stimulus?
* Ions flow from cells on the inside of pulvinus (cells close to petiole)
* Increase in water potential (less negative) leads to water flowing from those cells
* Turgor pressure high on outside, low/lost on inside of pulvinus
* Leaflet folds towards petiole
168
Other plants use the same mechanism to move their leaves, but much _____ than Mimosa pudica
slower
169
Changes in water potential and turgor pressure
| lead to
cell shape changes
170
Why is turgor pressure important?
Turgor pressure is important to maintain plant cell shape
Turgor pressure is the driving force of plant cell growth, but does not provide a direction for the growth.
Turgor pressure and changes in cell shape regulate opening and closure of stomata
171
Orientation of cellulose microfibrils determines
the direction of plant cell growth
172
Width of stomatal aperture needs to be
regulated to satisfy the competing needs of photosynthesis and water preservation
173
Regulating stomata density is a
is a long-term response to regulate transpiration and CO2 influx.
174
What helps leaf movements?
Changes in water potential and turgor pressure lead to cell shape changes
This can be used as a mechanism to drive leaf movements in plants
175
What is a cell wall?
extracellular structure which surrounds the cell. Is a rigid structure protecting the plants cells but also gives structure and shape. Cells cannot migrate like animal cells can.
176
What are the layers that the cell wall consist of?
and as all cell wall material is secreted from the cell, the oldest layer is always furthest away from the plasma membrane. First part of the cell wall is the middle lamella- is an equal distance to both cells. And the middle lamella is actually derived from the cell plate. Next to the middle lamella you have the primary cell wall, which gets deposited after the ML and finally closest to the plasma membrane, we have the secondary cell wall. Not all cell have a secondary cell. But the secondary cell wall is usually laid down to increase the strength of the cell wall and this also mainly why you will find lignification. So all things that increase the strength of the cell. The main structural component that gives cell walls their strength are cellulose fibres.
177
What is the most important component of the cell wall?
cellulose
178
What is cellulose?
Cellulose is the most important component of the cell wall, and it is a polymer made from glucose binding blocks (long chain of glucose molecules). In the cell wall the cellulose molecules, and most likely 18 of them, are packed tightly together into cellulose microfibrils. So in the cell wall you wont find single cellulose chains, you'll always find them packed together into cellulose microfibrils.
179
What is cellulose synthesized by?
So cellulose is synthesized by an enzyme called cellulose synthase. Cellulose microfibrils are made by a complex consisting of several cellulose synthase molecules. And the whole complex is located in the plasma membrane. The newly formed microfibril comes out of the end of the enzyme complex facing the outside of the cells.
180
What is on the cytosolic side of the plasma membrane?
On the cytosolic side of the plasma membrane, the enzyme complex is linked to microtubules. During the microfibrils synthesis process, the enzyme complex gets pushed along the microtubule, which means that cellulose microfibrils align parallel to the orientation of these microtubules which are in the cytosol.
181
Why is it important that cellulose microfibrils align parallel to the orientation?
This is important because the link between the microtubules and the cellulose synthase complex provides a mechanism to direct the orientation in which microfibrils are laid down in the cell wall. The orientation of cellulose microfibrils in the cell wall determines the growth direction and also the shape that cells will take on.
182
What are some of the other polysaccharides that are present apart from cellulose?
the middle lamella, consists mainly of pectins and glycans are used to cross-link the cellulose microfibrils. This cross linking between the cellulose microfibrils gives the cell wall rigidity. In the cell wall there are also proteins present, for example one important class of proteins in the cell walls is called extensins. These extensins change the crosslinking of the sugar polymers, so they change how the cellulose microfibrils are linked together and thereby they control how rigid the cell wall is.
183
In addition to the rigidity you get from the cellulose microfibrils, you can also further increase what?
you can also further increase how stiff the cell wall is by adding lignin to it ( another polymer). You can also make the cell wall waterproof by adding waxes like suberin. Once they’re incorporated into the cell wall, they make the cell wall waterproof.
184
What do most plats in order to maintain themselves?
In order to maintain most plants will need to take up water on a regular basis
185
What are cells surrounded by?
Cells are surrounded by a plasma membrane, which are semi permeable. Meaning that water molecules move more easily across the plasma membrane, than larger molecules dissolved in the water.
186
What is osmosis?
Osmosis describes the process by which water moves across a semi-permeable membrane. So it gives you the direction in which water will move. For example, the solid concentrations of the molecules is higher to the left of the SP membrane, which means that water will move from the right compartment to the left and will be an increase in the volume of solution on the left side of the SPM.
187
What happens when you put a plant into a hypertonic solution?
If you put a plant cell into a hypertonic solution where osmosis will lead to water flowing outside of the cell. The cell volume will shrink and the plasma membrane will detach from the cell wall- called plasmolysis.
188
What will happen if you put a plant into a isotonic solution?
In an isotonic solution, you dont have any net water movement in either direction. So the cell volume stays the same.
189
What happens when you put plants in a natural environment?
But the most natural environment for a plant cell is actually under normal conditions, which is a hypotonic environment. Meaning there are more solutes on the inside of the cell than on the outside and water flows into the cell. But because plant cells are surrounded by a cell wall, this water influx is not unlimited. So the cell wall restrains the cell volume increase, you get through the influx of water and the cell contents become pressurised. This pressure that builds up is called turgor pressure and is a very important contributor to plant cells and overall plant shape.
190
What is water potential?
Water potential describes the direction of water movement in plants, and also in relation to the surrounding environment. In plants water always moves from areas with higher water potential to areas with a lower water potential. That usually means a more negative water potential so water potential also determines transpiration and whether plants can take up water from the soil. It is formally described or measured in megapascals.
191
What happens if we measure the water potential of a plant?
If we measure the water potential of a plant, say a tree, and its surrounding soil and air, you will get water potential curves. It shows that the water potential gets more negative and the air usually has a low water potential. It can easily reach a water potential of minus a 100 megapascals and this is a lot lower than the water potential in the leaves. The curve takes its steepest change or has its biggest decrease in water potential at the boundary between the plant, so between the leaves and the surrounding air. This is what drives evapotranscrption of water from the leaves. The water potential of the soil is usually quite high. So soil saturated with water has a water potential at the approaching 0 and within the plant itself, theres also a water potential gradient. So the roots have a lower water potential than the soil, which drives the water uptake by the roots. The shoots and leaves have gradually decreasing water potential the further away you get from the roots, which contributes to water transport to the leaves.
192
What does water potential also depend on?
Water Potential also depends on turgor pressure and the concentration of solutes in the cell. Let's assume we have a plant cell and this is sitting in a beaker with a solution. So there are more solute molecules inside cells, so these red dots represent solute molecules than there are on the outside of the cell. The solute potential is directly related to the concentration of molecules in a solution. The more solute molecules you have in a solution, the more negative your solute potential gets. So in our example, the solute potential of the cytosol is -0.7 megapascals. And the solute potential of the solution, which has fewer solid molecules, is only -0.2 megapascals. So now osmosis dictates that water is going to flow in the direction of the compartment with more solute molecules. So water is going to flow into the cell until an equilibrium is reached. And equilibrium in this case means water flows until the water potential of the cell is the same as the solute potential of the surrounding solution.
193
Osmosis is the initial driver for what?
So osmosis initially drives water flow into the cell, which means that there is an increase in the cell volume. Which also means because we have the cell wall that is resisting this cell volume increase, turgor pressure starts to build at equilibrium, when water stops flowing between the cell and the surrounding solution. The water potential of the same as, the water potential of the cell is the same as the solute potential of the surrounding solution. So both are -0.2 megapascals, so we can use the water potential equation to calculate the turgor pressure that our cell is building up in this solution.
194
What does water potential of the cell depend on?
So we know that the water potential of the cell depends on the sum of the pressure potential and solute potential. If we solve for the pressure potential, we get a pressure potential or turgor pressure of 0.5 megapascals as water potential of the cell needs to be -0.2 megapascals, same as the solute potential.
195
What happens when we If we transfer our cell with a water potential of -0.2 megapascals and turgor pressure of 0.5 MPa into a solution with a higher?concentration of solutes.
So now the solute potential of the solution is lower because it contains more molecules. So in this case, we have the water potential of the cells is -0.2 megapascals, but the solute potential of the solution is -0.9 MPa. At equilibrium when the osmosis driven waterflow has finished and that there is no more additional water flow. The water potential of the cell and the solute potential of the solution surrounding the cell will have the same volume. In this case, the water potential of the solution, which is the same as the solute potential, as solutions have no pressure potential is lower than the water potential of the cell, which means that the direction of the water flow in this case is from the cytosol into the surrounding solution in order to reach the equilibrium.
196
What occurs when water flows from the cytosol into the surrounding solution in order to reach equilibrium?
Results in two things- first as the water flows out of the cell, the cell volume shrinks, and due to the reduced cell volume, the solid potential of the cell starts to decrease because there’s the same amount of solutes still in the cell, but there's a smaller volume, which means that you haven’t increased solute concentration. And the solute potential will decrease. The reduced solute volume also means that the plasma membrane is no longer pressed against the cell wall and the turgor pressure is completely lost. So once equilibrium is reached, this means that the cell's pressure potential is 0 megapascals. So there's no turgor pressure and solute potential is -0.9 MPa. What this means in terms of whole plants is that if the water potential outside of the plant is more negative than the plant itself, turgor pressure is lost and the plant wilts and loses its shape. Turgor pressure you get through the presence of water, is what gives plants cells shape and keep plants upright.
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What does turgor pressure pressurize?
Turgor pressure pressurizes the content of plant cells and is resisted by stiff cell walls and it is this pressure that gives plants cells shape and keeps herbaceous plants like these upright.
198
Turgor pressure not only gives plant cells their shape, what does it also drive?
it also drives growth of plant cells. So when a plant cells grow, they increase the volume by about ten to maybe even 1000 times. So you get massive increases and turgor pressure in growing plants cells is usually somewhere between 0.3 and 1 megapascals. So plant cells are under quite a lot of pressure. This is what drives growth.
199
Turgor pressure also drives what?
So if the cell wall material allows it and this is a regulated process, turgor pressure can drive the expansion of cell wall materials. So the turgor pressure pushes the cellulose mircofibrils away from each other. And this expansion of the cell wall material, which leads to a volume increase of the cell, actually reduces the turgor pressure. A reduction in turgor pressure means a reduction in water potential.
200
What happens when you decrease the water potential?
So you decrease the water potential because you have positive turgor pressure and the negative solute potential. So if you decrease the turgor pressure, the water potential of the cell becomes more negative. This lower, more negative water potential leads to water flow into the cell. And this is volume increase, which is usually taken up into the vacuole. So this is how plant cells grow.
201
What does tip growth decribe?
Tip growth describes growth that happens only in one specific area of the cell. Not many plants actually show this type of growth but root hairs and pollen tubes grow in this fashion. Most other plant cells grow by diffuse growth. This means that the cell wall expansion happens across the whole body of the cell. The whole cell is gray, but this doesn't mean that the plant is growing in all directions equally. How and why plants cells can show directional growth- in plant cells for instance where new cells are produced by cell division. Cells are small and isometric which means they are like a cube or sphere with the same dimensions in all directions.
202
How can these meristematic cells grow?
grows in two ways
isodiametric
Anisotropically
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What is isodiametric growth?
meaning that the cells expand equally in all directions or cells could grow.
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What is Anisotropically?
meaning that cells preferentially grow in one direction only. In the meristem where the cells divide regularly, the cells are small and isodiametric.
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What does Isotrophic growth of plant cells give?
Isotrophic growth of plant cells is what gives plant roots their ship and allows them to grow directionally.
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How is directional growth achieved?
turgor pressure doesn't give a direction to help plant cells grow as turgor pressure is actually acting with the same force on all cell was of the plant. It doesn't have any directionality, the directionality of plant cell growth depends instead on the orientation of cellulose mircofibrils in the cell
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What happens when the cellulose microfibrils are orientated randomly in the cell wall?
plant cells will grow, isodiametrically, they will expand to the same, extent in all directions.
208
What happens when cellulose microfibrils are arranged in a parallel fashion?
owever, if cellulose microfibrils are arranged in a parallel fashion, cells grow and isotropocially and preferentially in one direction. And the direction in which they preferentially grow, in anisotropic growth is orthogonal or at 90 degrees to the direction, the orientation of the cellulose microfibrils in the cell wall.
209
The direction of the cellulose microfibrils shape
shape the direction of cell growth
210
What happens when cellulose fibers are made?
hey cannot change their length, so the length cannot be altered but the spacing between cellulose microfibrils, that can be changed.
211
What can happen to the cross- links?
The cross links can be changed, broken and reformed.
212
Why is the forming, breaking and changing of the cross links regulated?
This is a highly regulated process, which is regulated by the extensive than those proteins in the cell. Because you have this difference in flexibility along the 2 dimensions of cellulose microfibrils. So there is not much change in the length, but a lot of flexibility in the spacing between 2 microfibrils.
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The cellulose microfibrils can determine the
the growth direction of plant cells.
| drives the direction of plants cell growth.
214
Where does the water travel in the plant?
Water is taken up by plant roots from the soil, moves through the xylem and the vasculature and leaves the plant via transcription through the stomata in the leaves.
215
What drives water transport?
Water potential gradient between the soil , plant and air drives this water transport.
216
Where does water vapor leave?
Water vapor leaves the leaves through the stomata and each stoma consists of two guard cells that surround the stomata pore between them. This pore can be open or closed to control the amount of water that a plant loses through transpiration. This process in the stomata is regulated.
217
What is a tightly controlled?
Opening and closing of stomata is such a tightly controlled process because it needs to integrate the demands of two very important processes of plants, photosynthesis and the need to keep hydrated.
218
What do plants not have?
Plants usually don't have unlimited access to water and need to control how much water they lose. Do that by decreasing the stomata aperture or closing stomata altogether. Co2 which is required for photosynthesis needs to enter the leaf through the stomata.
219
How do plants produce energy?
To produce energy plants need to open their stomata to let CO2 and the demands of preventing excessive water loss and photosynthesis are competing - this is why stomata aperture has to be tightly controlled in order to achieve a compromise between these two competing interests.
220
What happens when guard cells are close together?
When guard cells are very close together along the side and are touching the stomata is closed.
221
What does the cell shape look like when the the guard cells are open?
When open the guard cells have a distinct curvature to them so they have bowed apart from each other. Opening and closing of stomata is achieved by these changes in cell shape.
222
Why is there a fan shaped pattern of cellulose and microtubules?
radiating out from the stomata pore. Microtubule orientation predicts how cellulose microfibrils are laid down on the cell wall. We know that the cellulose fibrils are arranged in this pattern. Fan shaped pattern of cellulose microfibrils means that there are distinct differences in the strength of the cell wall within guard cells. Region around the pore is much stiffer than the outside of the cells, or where the two guard cells meet. These areas are much less able to resist deformation by turgor pressure, than the stomatal pore itself.
223
What happens if the guard cell volume increases?
If the guard cell volume increases so that there’s an increased turgor pressure. Means that the changes in cell shape are mainly along the outer cell walls and where the two cells meet. Two guard cells will bow away from each other and that the stomatal pore opens.
224
How is turgor pressure controlled?
Turgor pressure is controlled by the solid concentration or the water potential of the cell. Starting with a closed stomata the turgor pressure in the guard cells is actually quite low. But if solutes like potassium and Chloride ions and malate are taken up into the guard cells , the water potential of the guard cell is lowered. Lowered more negative water potential of the guard cell leads to water flowing into the guard cells. Then the volume, cell volume increases, which means the turgor pressure increases because of the cell wall properties (so the way that the cellulose microfibrils are arranged in the cell wall) this leads to cell shape changes that increase the width of the stomatal pore.
225
What does the influx of water and increased turgor pressure do?
will open stomata.
226
Explain stomatal closure?
starting at a point where turgor pressure of the guard cells is high and the pore is open. If a plant loses too much water through transcription and becomes drought stress it starts to make ABA. This increase in ABA concentration is perceived by the guard cells and leads to the efflux of solutes . Ions like potassium and chloride and solutes like malate will leave the cell. This increases the water potential of guard cells so gets less negative and is higher than the surrounding epidermis cells. Water then flows from the guard cells because it moves to the cells with lower WP. This efflux of water means that the turgor pressure of the guard cells decreases, cell volume decreases and the cell shape changes and the pore gets closed.
227
Another mechanism to control the flow of water vapor from the leaf and CO2 into the leaf is the
the density of stomata on the leaf surface.
228
Why is the control of the flow of water vapor a long term response?
This is a long term response to long term environmental changes because the number of stomata in a mature leaf cannot be changed. However, a plant can produce new leaves with fewer or more stomata .
229
What happens if the number of stomata on the leaves is changed?
If the number of stomata on the leaves is changed, transcription and CO2 uptake will also change. And there a low stomatal density means that you reduce transcription but also means that less CO2 can be taken up.
230
What does high stomatal density increase?
Whereas high stomatal density increases transcription and the uptake of CO2.
231
What is the main environmental factor for plants?
s the CO2 concentration in the atmosphere.
232
What happens when you compare the stomatal density in current growing plants to fossil records?
you can see strong links between the amount of CO2 in the atmosphere and stomatal density. Low stomatal densities were found in fossil records from 100 to 200 million years ago. Looking at CO2 conc between 100 and 200 million years ago, co2 conc were a lot higher than they are today. When CO2 conc in the atmosphere are higher you need fewer stomata to get sufficient CO2 into the leaf. This is why the fossil plants overall have a reduced stomatal density compared to currently grown plants . Water and temperature can also influence stomatal density. To preserve water loss.
233
Explain Mimosa.
Mimosa plants have compound leaves, which are made up of leaflets and these leaflets are folding towards the petioles. At the base of each mimosa leaflets where it meets the petiole theres a group of cells called the oulvinus. It is the turgor pressure and cell shape of the pulvini cells that facilitates the bending of the leaflets . so the pulvinis act almost like a hinge.
234
What happens when you touch the Mimosa leaves?
When you touch the mimosa leaflets, ions flow from the pulvinus cells which are closer to the petiole and this leads to an increase in water potential of these cells. Which means that water flows from these cells and turgor pressure is lost and the cell shape changes. Cells become less rigid and smaller. On the other side of the pulvinus the turgor pressure remains high, the cell shape is the same and this is what leads to the leaflet bending and folding . so this process is very similar to what happens in guard cells. Movement of ions and solutes ultimately leads to changes in cell shape.
235
Explain the properties of Bean leaves?
Bean leaves are horizontal and vertical in order to intercept as much light as possible for photosynthesis. But during the night they turn to a vertical position and this movement is due to the turgor pressure changes in the pulvinus of those leaves.
