Transport in Plants

1
Transport in Plants
2
Plants require
CO2
Light
Sugar
H2O
O2
Minerals Macronutrients (besides C, H,
O) Ca, K, Mg, N, P, S Micronutrients B,
Cl, Cu, Fe, Mn, Mo, Ni, Zn
Fig. 36.2
3
Here we explore how these substances are
transported within plants
but first we need some more background
Fig. 36.2
4
Background
Cell wall helps maintain a cells shape, but the
plasma membrane regulates the traffic of
molecules into and out of a cell
Fig. 36.8
5
Background
Plasmodesmata provide cytosolic connections among
cells
Cytosol cytoplasm minus organelles
Fig. 36.8
6
Background
Vacuoles often account for 90 of a plant cells
volume, but they are never shared by adjacent
cells
Fig. 36.8
7
Background
Substances can move from cell to cell via the
symplastic, apoplastic, or transmembrane routes
Fig. 36.8
8
Background
Solutes tend to diffuse down concentration
gradients
Passive transport is diffusion across a membrane
Passive transport is generally slow, unless
solutes travel through transport proteins in the
membrane
Some transport proteins are selective channels
Some selective channels are gated
environmental stimuli open or close them
9
Background
Active transport requires energy to move solutes
up their concentration or charge gradient
E.g., proton pumps are very common active
transport proteins
Fig. 36.3
Proton pumps create membrane potentials
potential energy can be used to perform cellular
work
10
Background
The membrane potential provides the energy to
uptake some minerals, e.g., K ions
The K ions pass through transport proteins
Fig. 36.4
11
Background
The membrane potential provides the energy for
cotransport of ions up their concentration
gradients, as H ions move down theirs
Fig. 36.4
The coupled ions pass through co-transport
proteins
12
Background
The membrane potential provides the energy for
cotransport of some neutral molecules (e.g.,
sugar) up their concentration gradients, as H
ions move down theirs
Fig. 36.4
13
Background
Osmosis is diffusion of water across a membrane
To predict the movement of water across a
membrane alone (e.g., an animal cell), it is
sufficient to know whether the inside solute
concentration is lt or gt the outside solute
concentration
To predict the movement of water across a
membrane plus cell wall (e.g., a plant cell), we
must know both the solute concentration
difference and the pressure difference
14
Background
Water potential is a combined measure of solute
concentration and pressure
? (psi) ? measured in MPa (megapascals 1 MPa
10 atmospheres)
Pure water in an open container ? 0 MPa
Solutes reduce the value of ?
Pressure increases the value of ?
Negative pressure (tension) decreases the value
of ?
15
Background
Water potential pressure potential solute
(osmotic) potential ? ?P ?S
?P can be positive, 0, or negative ?S is always ?
0
Water will always move across a membrane from
higher to lower ?
16
Background
Lets consider 4 examples
Fig. 36.5
17
Background
Lets consider 4 examples
Fig. 36.5
18
Background
Lets consider 4 examples
Fig. 36.5
19
Background
Lets consider 4 examples
Fig. 36.5
20
Background
Lets consider real cells
Flaccid cell ?P 0
Fig. 36.6
21
Background
Lets consider real cells
Flaccid cell ?P 0
Fig. 36.6
22
Background
Lets consider real cells
Flaccid cell ?P 0
If a cell loses water to the environment, it
plasmolyzes
Fig. 36.6
23
Background
Lets consider real cells
Flaccid cell ?P 0
Fig. 36.6
24
Background
Lets consider real cells
Flaccid cell ?P 0
Fig. 36.6
25
Background
Lets consider real cells
Flaccid cell ?P 0
If a cell gains water from the environment, it
becomes turgid
Turgor pressure that keeps cell membrane
pressed against cell wall
Fig. 36.6
26
Background
Lets consider real cells
Flaccid cell ?P 0
If a cell gains water from the environment, it
becomes turgid
Aquaporins are transport proteins that form
channels for water
Fig. 36.6
27
Armed with this background How do roots absorb
water and minerals?
28
How do roots absorb water and minerals?
Solutes pass into roots from the dilute soil
solution
Fig. 36.9
29
How do roots absorb water and minerals?
Symplastic route
Active transport occurs through proton pumps,
that set up membrane potentials, that drive the
uptake of mineral ions
Fig. 36.9
30
How do roots absorb water and minerals?
Apoplastic route
Some water and dissolved minerals passively
diffuse into cell walls
Fig. 36.9
31
How do roots absorb water and minerals?
Solutes diffuse through the cells (or cell walls)
of the epidermis and cortex (the
innermost layer of which
is the endodermis)
Fig. 36.9
32
How do roots absorb water and minerals?
At the endodermis, only the symplastic route is
accessible, owing to the Casparian strip
Chapt. 35 observed that the endodermis regulates
the passage of substances into the vascular stele
Fig. 36.9
33
How do roots absorb water and minerals?
The final layer of live cells actively transports
solutes into their cell walls
Solutes then diffuse into xylem vessels to be
transported upward
Fig. 36.9
34
How do roots absorb water and minerals?
The final layer of live cells actively transports
solutes into their cell walls
The final layer may be an endodermal cell
Fig. 36.9
35
How do roots absorb water and minerals?
The final layer of live cells actively transports
solutes into their cell walls.
or a cell of the pericycle (outermost layer of
stele)
Fig. 36.9
36
How do roots absorb water and minerals?
Note In this figure the pericycle is drawn as
a continuous layer of cells
37
How do roots absorb water and minerals?
Mycorrhizal mutualism (fungus roots)
Fungus helps plant obtain water and minerals
(e.g., P) Plant feeds sugars to the fungus
No fungus
With fungus
38
How do roots absorb water and minerals?
Some species (including many legumes) have root
nodules that house N-fixing bacteria
Bacteria convert N2 to NH4 (ammonium), providing
plant with fixed N Plant feeds sugars to the
bacteria
39
How does xylem transport xylem sap?
40
How does xylem transport xylem sap?
In some species of trees the process moves water
and nutrients gt 100 m upwards!
Fig. 36.13
41
How does xylem transport xylem sap?
Nearly all of the energy to drive the process
comes from the sun
Evapor-ation at the top pulls water up from the
bottom
Unbroken chains of water molecules (held together
by cohesive H-bonds) fill xylem vessels
Fig. 36.13
42
How does xylem transport xylem sap?
The evaporation of water out of leaves is called
transpiration
Fig. 36.12
43
How does xylem transport xylem sap?
Water vapor escapes through stomata
Fig. 36.12
Fig. 36.15
44
How does xylem transport xylem sap?
Transpiration creates a water pressure gradient
Fig. 36.13
45
How does xylem transport xylem sap?
Transpiration creates a water pressure gradient
Water flows upward through xylem vessels by bulk
flow down the pressure gradient
Lower ? at the top is the tension that pulls
water up from the bottom
Fig. 36.13
46
How does xylem transport xylem sap?
The Transpiration-Cohesion-Tension Mechanism
Fig. 36.13
47
How do plants regulate the transport of xylem
sap?
48
How do plants regulate the transport of xylem
sap?
Stomata
K is actively transported into and out of guard
cells
49
How do plants regulate the transport of xylem
sap?
Stomata
When K is high, the amount of H20 is high,
and guard cells open stomata
50
How do plants regulate the transport of xylem
sap?
Stomata
When K is low, the amount of H20 is low, and
guard cells close stomata
51
How do plants regulate the transport of xylem
sap?
Stomata
Light stimulates the uptake of K by guard
cells, opening stomata
52
How do plants regulate the transport of xylem
sap?
Stomata
Low CO2 stimulates the uptake of K by guard
cells, opening stomata
53
How do plants regulate the transport of xylem
sap?
Stomata
Low H2O availability inhibits the uptake of K by
guard cells, closing stomata
54
How does phloem transport phloem sap?
55
How does phloem transport phloem sap?
Sugars manufactured in leaves diffuse to phloem
companion cells
Fig. 36.17
56
How does phloem transport phloem sap?
Companion cells actively transport sugars into
sieve-tube members (elements)
Fig. 36.17
57
How does phloem transport phloem sap?
Food (sugars) are then translocated from sources
to sinks according to the Pressure-Flow Theory
1. At sources, sugars are actively transported
into phloem
Fig. 36.18
58
How does phloem transport phloem sap?
Food (sugars) are then translocated from sources
to sinks according to the Pressure-Flow Theory
2. Water follows by osmosis from source cells and
xylem
this creates high pressure
Fig. 36.18
59
How does phloem transport phloem sap?
Food (sugars) are then translocated from sources
to sinks according to the Pressure-Flow Theory
3. At the sink, sugars diffuse out of the phloem
and water follows by osmosis
this creates low pressure
Fig. 36.18
60
How does phloem transport phloem sap?
Food (sugars) are then translocated from sources
to sinks according to the Pressure-Flow Theory
Sugar solution flows from high to low pressure
Fig. 36.18
61
How does phloem transport phloem sap?
Food (sugars) are then translocated from sources
to sinks according to the Pressure-Flow Theory
4. Water may be taken up by the transpiration
stream in the xylem
Fig. 36.18
62
How does phloem transport phloem sap?
Pressure-Flow Theory
63
Not all herbivores chew leaves
Some exploit sap
E.g., aphids tap sieve-tube elements for phloem
sap