Chapter 2
Plant nutrients uptake and
transport Mechanisms
N u trie n t u p t a ke s te p s
nu t
M ove m en t t h r o ug h so il
C ell w all C ell m embran e
C ell to cell tran sp ortvascu l a r tissue
un loa d i n g
nutrient
(plasmodesmata )
THE CELL WALL
? Before traversing the plasma
membrane (p.m.),ions must first cross
the cell wall and contact the p.m,The
bulk flow of soil solution could
theoretically carry inorganic ions into
the cell wall and through the apoplastic
pathway,The thickness of a cell wall is
in the range of 500 to 1000 nm,It is
porous.
表 1-13一些物质的颗粒直径（毫微米 nm）
颗粒
分子或
离子晶
体直径
水化离
子直径 颗粒
分子或
离子晶
体直径
水化离
子直径
葡萄糖 0.89 Mg2+ 0.13 0.92
Na+ 0.19 0.60 Ca2+ 0.20 0.88
K+ 0.27 0.53 CI- 0.36 0.50
NH4+ 0.30 0.54 NO3- 0.41
Compared to cell walls(500-1000nm),
the nutrient ions or molecular are very small,
Cell wall (细胞壁）
? The cell wall of parenchyma cells(薄
壁细胞 ) is made up of a middle lamella
(M.L.)(胞间薄层 ),which separated the
daughter cells after cell division,and
the primary wall(初生壁） which was
deposited on to the M.L
? While the M.L,is made up of pectic
substances(果胶物质 ),the primary wall
also contains hemicelluloses and
cellulose,Pectins(果胶质 ) are based on
branched chains of a sugar polymer
made up of galacturonic acid(半乳糖醛
酸 ),This is galactose (半乳糖 )in which
the alcoholic group(醇羟基 ) at C6 is
replaced by a carboxyl group(羰基 ).
Cell wall(细胞壁）
Free space
? Clearly,ionization of carboxyl groups
creates a negative charge which
makes the cell wall a cation exchange
resin,Indeed much Ca2+ is associated
with the pectins,In soils with high
quantities of free Al3+,this toxic ion
is bound in large quantities to the cell
wall,
Free space
? Thus when ions enter the cell wall,
cations can exchange with the cell wall
according to their binding
affinity,typically
? trivalents>divalents>monovalents,
? Cation exchange capacity (CEC)
? Anions should not be bound at all,
Cell membranes(生物膜 )
? Allow for controlled intracellular
environment
? Overton in 1890 demonstrated that
penetration of many solutes across
the membrane was a function of their
lipid solubility,He proposed
therefore,that the membrane itself
was lipid（脂质）,
Cell membranes(生物膜 )
? Unfortunately scientists inferred from
this that the permeability（通透性） and
diffusion gradient of a substance
determined its penetration of the p.m,
? Most biologically important molecules are
not lipid soluble and must penetrate the
plasma membrane through proteinaceous
transport systems,
Cell membranes
? Structure based on hydrophilic-hydrophobic
interaction of phospholipids (磷脂） with aqueous
phases
? Contain approx,50 % protein whose function is
probably mostly transport
Cell membranes are highly selective
The basic structure of a cell membrane is the
phospholipid bilayer,which has very low
permeability to most nutrients,
Uptake is made faster by transport proteins
embedded in the membrane.
Permeability of membrane
? high permeability to molecules that are:
gases e.g,O2,CO2
? lipid soluble e.g,ethanol
? non-polar e.g,urea
? low permeability to ions e.g,K+,NO3-
? very low permeability to divalent ions e.g,
Mg2+,SO42-
10-2
10-4
10-6
10-8
10-10
P
cm/s
Cl-
K+
Na+
H2O
Ethanol
Boron
Cl-
K+
Na+
Boron
H2O
Ethanol乙醇
Lipid Bilayer Biological Membrane
MEMBRANE PERMEABILITY
气体
小的不带电荷
的极性分子
带电荷的
极性分子 离子 大的不带电荷
极性分子
Membrane permeabilityPermeability of membrane
The characteristics of movement of molecular
across membrane
Movement of molecules across membranes
4 mechanisms
1,Diffusion directly through the lipid bilayer
2,Transport via carrier proteins
3,Transport through ion channels
4,Active transport
- ATPases
- co-transport
facilitated
diffusion
Passive
transport
Passive transport
? Passive,the flux is driven by the
free energy associated with a
gradient of concentration or
pressure or electrical potential or
even a combination of these driving
forces,
Active transport
? Active transport is the result of
coupling energy from biochemical or
biophysical reactions to drive
transport against the prevailing free
energy gradient,
Passive transport and Active
transport
? Early workers commonly invoked
evidence from effects of metabolic
inhibitors or anoxia to demonstrate
active transport,but there is a
difference between active transport
and energy -dependent transport,
from Knox,Ladiges & Evans
m o l e c u l e t o b e t r a n s p o r t e d
c h a n n e l
p r o t e in
c a r r ie r
p r o t e in s
e x t r a c e l l u l a r
s p a c e
l ip id
b ilaye r
c y t o p l a s m
s im p l e
d if f u s io n
c h a n n e l - m e d ia t e d
t r a n s p o r t
c a r r ie r - m e d ia t e d
t r a n s p o r t
e n e r g y
e l e c t r o c h e m ic a l
g r a d ie n t
p a s s iv e t r a n s p o r t
( f a c ilit a t e d d if f u s io n )
a c t iv e t r a n s p o r t
Types Membrane Transport Mechanisms
Carriers and ion channel
? Carrier（载体）, this is a general term for
a protein which carries an ion across the
membrane,Equivalent terms are
transporter（运输蛋白）,or permease (透
性酶） (used by microbiologists),
? Channel(离子通道） A channel is a protein
pore through which ions move,-
i,very rapidly ( 106 ions per sec can pass
through a single channel
ii down their free energy gradients,
Facilitated Diffusion
? Three Essential Characteristics:
– Specific (selective for single nutrient molecule or ion)
– Passive (requires no input of energy)
– Saturates (non-linear dependence on concentration)
Driving forces for membrane transport,
concentration differences
Molecules will diffuse until the
concentration is the same everywhere
Active Transport
? Requires input of energy
? For uncharged substances active transport moves
molecules against their concentration gradients
? Enables cells to accumulate molecules to higher
concentrations than in the extracellular fluid.
ATP
ATP
ATP
Driving forces for membrane transport,
metabolic energy
? For charged substances (ions),active transport
moves molecules against concentration + electrical
gradients
Active Transport
Types of nutrient molecules
Example
Neutral solute glucose,urea,
boratic acid
Cation NH4+,H+
Divalent cation Ca2+,Zn2+
Anion NO3-,Cl-
Divalent anion SO42-
K+
Cl-
Zn2+
Mn2+
Mg2+
NO3-
NH4+ PO4-Ni2+
Cu2+
SO42-
Fe3+Ca2+
Fe2
+
B
OH
OH OH
All essential mineral nutrients are
absorbed as ions except boron
Therefore all nutrients except Boron need membrane transporters
Voltages across membranes
? All cells have voltage differences across their membranes
(commonly in the range –50 to – 200 mV)
? This voltage can be used to drive membrane transport
? The voltage is mainly generated by pumping of ions
membrane voltage = membrane electrical potential = PD
V
V
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
++
+ +
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
++
+
+
Driving forces for membrane
transport,VOLTAGE
0 mV-120 mV
Negative membrane voltage will cause:
? Accumulation of cations
? Depletion of anions
Coupled Transport（藕联运输）
The use of a downhill gradient of one molecule to move a second molecule
up a gradient.
Two steps
1,Establish the down gradient.
? Sodium-potassium or proton pump.
2,Use this gradient to drive a molecule up a gradient
Cotransport
-Symport
-Antiport
Both molecules move
in same direction
in opposite directions
Basic principles of membrane transport
Uncharged molecules move in response to:
? differences in concentration (chemical gradient)
Charged molecules move in response to:
? differences in concentration (chemical gradient)
? differences in voltage (electrical gradient)
= electro-chemical gradient
What happens when electrical and concentration
differences oppose each other?
The NERNST equation:
E (mV) = RTzF ln CoC
i
x
Simplified version:
E (mV) = 60z x log C0C
i
R = gas constant
(8.31 J K-1 mol-1)
T = oK
z = valence (e.g,+1,-2)
F = Faraday’s constant
96,500 J mol-1
Co = external concentration
Ci = internal concentration
(E = electrical potential difference
or voltage across the membrane)
Nernst Equation
Principles:
A neutral solute will distribute itself so that at
equilibrium,concentrations are equal
(including across membranes)
A charged solute (ion) will distribute itself according
to both concentration differences and electrostatic
Attraction(静电吸引） to surfaces or regions of opposite
charges
The Nernst equation predicts the concentration of ions
at electrochemical equilibrium
Na+ 1 mM 10 mM
-60 mV
Predicting direction of passive movement
E (mV) = 60z x log C0C
i
Cl- 1 mM 10 mM (observed)
0.1 mM (predicted)
? Higinbotham and his co-workers made
use of this Nernst relationship in
1967 to evaluate the distribution of
cations and anions in media containing
1 mM concentrations of various ions
using pea and oat roots,
Nernst Equation
Nernst Equation
? They first measured the
concentration of ions in the roots and
then measured the electrical
potential difference,Then they
predicted the concentration of ions
in the root according to the Nernst
(equilibrium) relationship for each ion,
Distribution of ions in roots grown in nutrient medium
external
concentration
(mM)
cytoplasmic
concentration
(mM)
accumulation
ratio
K+ 0.5 90 180x
Na+ 1 10 10x
Ca2+ 1 0.1 0.1x
Mg2+ 0.5 2 4x
Cl- 1 10 10x
PO4- 0.1 10 100x
NO3- 1 5 5x
pH 5.5 7.5 (0.01)
total osmotic solute (approx.)
20 300 15x
? They concluded that all the anions
were actively maintained within the
root and only K+ was at equilibrium
in pea roots (not oat),Most cations
were at much lower concentrations
than predicted,All of the anions
were at much higher concentrations
than predicted.
Nernst Equation
Distribution of ions in roots grown in nutrient medium
external
concentration
(mM)
cytoplasmic
concentration
(mM)
Predicted concentration
at –120 mV
K+ 0.5 90 50
Na+ 1 10 100
Ca2+ 1 0.1 10,000
Mg2+ 0.5 2 5,000
Cl- 1 10 0.01
PO4- 0.1 10 0.001
NO3- 1 5 0.1
pH 5.5 7.5 3.3
active uptake
active uptake
active uptake
active uptake
active efflux
active efflux
active efflux
active efflux
Energetic considerations - Cations
Mn2+
Mn2+Cu2+
Zn2+
Co2+Cd2+
X2+ -120 mV
X2+
ATP
Cations are attracted into the cell by the negative membrane PD.
Uptake of nutrient cations therefore does not generally require
energy.
Efflux of cations from the cell usually requires energy (i.e,active
transport)
Energetic considerations - Anions
Anions are repelled from the cell by the negative membrane PD.
Uptake of nutrient anions therefore generally requires energy,in most
cases provided by the electrochemical gradient for H+ (H+-cotransport)
Efflux of anions from the cell usually does not requires energy and
may occur through ion channels.
H+ -120 mV
ATP
H+
H+
NO3-
PO4-
Cl-
SO4-
Cl-
Passive transport (facilitated diffusion)
= movement in direction of electrochemical gradient
Active transport (requires energy)
= movement against electrochemical gradient
Two mechanisms
1,ATPase
2,Co-transport
Active and Passive transport
Two types of
active transport
1,ATPase (e.g,H+-ATPase,Ca2+-ATPase)
2,Co-transport
The proton ATPase (H+-ATPase)
cytoplasm ext,medium
ATP
ADP + Pi
H+
? regulates intracellular pH
? generates membrane PD
- drives co-transport
? acidifies external medium
+ ++ + + ++ +
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ + + + +
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
l l l
l l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l l
l l
l l l l
l l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l l
l l
l
A TP
A D P +P i
H
+H
+
2H
+
2H
+
- 1 0 0 m V
Ca
2+
NO
-
3
NO
-
3
Zn
2+
1
2
3
4
4 types of transporter
1,ATP-driven pumps (active)
2,Co-transporters (active/passive)
3,Channels (passive)
4,Carriers (passive)
M
2 +
( C a )
2 +
K
+
K
+
M
2 +
( K OR C )
( K IR C )
( D A C C )
( V IC C )
( H A C C )
( IR T 1 )
( Z IP - n )
Fe
2 +
Zn
2 +
Fe
2 +
F e - P S
3 +
( M n )
2 +
( M n )
2 +
( Z n )
2 +
( C u )
2 +
nH
+
nH
+
nH
+
nH
+
nH
+
nH
+
( N r a m p - n )
P O ( M o O )
4 4
- -
S O ( M o O )
4 4
2 - -
C l ( N O )
- -
3
N O ( C l )
3
- -
M o O
4
-
Cl
-
Cl
-
N H
4
+
N H
4
+
A T P
A T P
H
+
H B O
3 3
H B O
3 3
NO
3
-
1
2
3
4
5
6
7
8
10
11 12
13
14
15
16
17
18
Ca
2 +
9
19
2021
A
-
A T P a se s
C a tion c h a n n e ls
A n ion cha n n e ls
U n ipo r te r s
C a tion c o tr a n sp o r te r s
A n ion cotr a n spo r te r s
Putative
plasma membrane
transporters
Substrate concentration (mM)
Velocity of
reaction (v)
0 2 4 6 8 10 12
0
20
40
60
μmol product
mg protein x h
Vmax
Km = [S] when v = ? Vmax
= 1 mM
Most transporters display enzyme-like properties and can be
described by their affinity for the transport substrate (Km),
and the maximum rate (Vmax)
Many high affinity transporters are induced by starvation
Nutrient induced by,increase
phosphate starvation 4x (wheat,barley)
sulphate starvation 500x (Lemna)
nitrate **nitrate 3x (spruce)
Inducible（诱导的） /repressible（可抑制的） high affinity
systems for
all macronutrients except Ca and Mg
No evidence for micronutrient cations (Zn,Mn,Cu,Fe,Ni)
0 1 2 3 4 5
40
30
20
10
0
T ra ns po rt
ra t e
( nm ol / g,h)
s t a r v a t i on ( d a y s )
P i = 1 0 礛
0 1 2 3 4 5
160
120
80
40
0
s t a r v a t i on ( d a y s )
P i = 1 0 0 礛
2 transport systems:
1,high affinity system
induced by starvation
2,low affinity system
little response to starvation
Induction of high affinity phosphate transport by starvation
(plants starved of P,then capacity for uptake of P measured at low and high concentrations)
(note different scales)
Inducible Constitutive (always there)
Nutrient uptake – basic considerations
? The cell membrane is a barrier to uptake of most nutrients
? To increase uptake,the cell synthesises membrane transporters
? Synthesis of transporters responds to nutrient deficiency & toxicity
? Nutrient transporters behave like enzymes
? Transport can be driven by
(a) concentration and electrical gradients
(passive transport)
(b) metabolic energy (active transport)
Plant nutrients uptake and
transport Mechanisms
N u trie n t u p t a ke s te p s
nu t
M ove m en t t h r o ug h so il
C ell w all C ell m embran e
C ell to cell tran sp ortvascu l a r tissue
un loa d i n g
nutrient
(plasmodesmata )
THE CELL WALL
? Before traversing the plasma
membrane (p.m.),ions must first cross
the cell wall and contact the p.m,The
bulk flow of soil solution could
theoretically carry inorganic ions into
the cell wall and through the apoplastic
pathway,The thickness of a cell wall is
in the range of 500 to 1000 nm,It is
porous.
表 1-13一些物质的颗粒直径（毫微米 nm）
颗粒
分子或
离子晶
体直径
水化离
子直径 颗粒
分子或
离子晶
体直径
水化离
子直径
葡萄糖 0.89 Mg2+ 0.13 0.92
Na+ 0.19 0.60 Ca2+ 0.20 0.88
K+ 0.27 0.53 CI- 0.36 0.50
NH4+ 0.30 0.54 NO3- 0.41
Compared to cell walls(500-1000nm),
the nutrient ions or molecular are very small,
Cell wall (细胞壁）
? The cell wall of parenchyma cells(薄
壁细胞 ) is made up of a middle lamella
(M.L.)(胞间薄层 ),which separated the
daughter cells after cell division,and
the primary wall(初生壁） which was
deposited on to the M.L
? While the M.L,is made up of pectic
substances(果胶物质 ),the primary wall
also contains hemicelluloses and
cellulose,Pectins(果胶质 ) are based on
branched chains of a sugar polymer
made up of galacturonic acid(半乳糖醛
酸 ),This is galactose (半乳糖 )in which
the alcoholic group(醇羟基 ) at C6 is
replaced by a carboxyl group(羰基 ).
Cell wall(细胞壁）
Free space
? Clearly,ionization of carboxyl groups
creates a negative charge which
makes the cell wall a cation exchange
resin,Indeed much Ca2+ is associated
with the pectins,In soils with high
quantities of free Al3+,this toxic ion
is bound in large quantities to the cell
wall,
Free space
? Thus when ions enter the cell wall,
cations can exchange with the cell wall
according to their binding
affinity,typically
? trivalents>divalents>monovalents,
? Cation exchange capacity (CEC)
? Anions should not be bound at all,
Cell membranes(生物膜 )
? Allow for controlled intracellular
environment
? Overton in 1890 demonstrated that
penetration of many solutes across
the membrane was a function of their
lipid solubility,He proposed
therefore,that the membrane itself
was lipid（脂质）,
Cell membranes(生物膜 )
? Unfortunately scientists inferred from
this that the permeability（通透性） and
diffusion gradient of a substance
determined its penetration of the p.m,
? Most biologically important molecules are
not lipid soluble and must penetrate the
plasma membrane through proteinaceous
transport systems,
Cell membranes
? Structure based on hydrophilic-hydrophobic
interaction of phospholipids (磷脂） with aqueous
phases
? Contain approx,50 % protein whose function is
probably mostly transport
Cell membranes are highly selective
The basic structure of a cell membrane is the
phospholipid bilayer,which has very low
permeability to most nutrients,
Uptake is made faster by transport proteins
embedded in the membrane.
Permeability of membrane
? high permeability to molecules that are:
gases e.g,O2,CO2
? lipid soluble e.g,ethanol
? non-polar e.g,urea
? low permeability to ions e.g,K+,NO3-
? very low permeability to divalent ions e.g,
Mg2+,SO42-
10-2
10-4
10-6
10-8
10-10
P
cm/s
Cl-
K+
Na+
H2O
Ethanol
Boron
Cl-
K+
Na+
Boron
H2O
Ethanol乙醇
Lipid Bilayer Biological Membrane
MEMBRANE PERMEABILITY
气体
小的不带电荷
的极性分子
带电荷的
极性分子 离子 大的不带电荷
极性分子
Membrane permeabilityPermeability of membrane
The characteristics of movement of molecular
across membrane
Movement of molecules across membranes
4 mechanisms
1,Diffusion directly through the lipid bilayer
2,Transport via carrier proteins
3,Transport through ion channels
4,Active transport
- ATPases
- co-transport
facilitated
diffusion
Passive
transport
Passive transport
? Passive,the flux is driven by the
free energy associated with a
gradient of concentration or
pressure or electrical potential or
even a combination of these driving
forces,
Active transport
? Active transport is the result of
coupling energy from biochemical or
biophysical reactions to drive
transport against the prevailing free
energy gradient,
Passive transport and Active
transport
? Early workers commonly invoked
evidence from effects of metabolic
inhibitors or anoxia to demonstrate
active transport,but there is a
difference between active transport
and energy -dependent transport,
from Knox,Ladiges & Evans
m o l e c u l e t o b e t r a n s p o r t e d
c h a n n e l
p r o t e in
c a r r ie r
p r o t e in s
e x t r a c e l l u l a r
s p a c e
l ip id
b ilaye r
c y t o p l a s m
s im p l e
d if f u s io n
c h a n n e l - m e d ia t e d
t r a n s p o r t
c a r r ie r - m e d ia t e d
t r a n s p o r t
e n e r g y
e l e c t r o c h e m ic a l
g r a d ie n t
p a s s iv e t r a n s p o r t
( f a c ilit a t e d d if f u s io n )
a c t iv e t r a n s p o r t
Types Membrane Transport Mechanisms
Carriers and ion channel
? Carrier（载体）, this is a general term for
a protein which carries an ion across the
membrane,Equivalent terms are
transporter（运输蛋白）,or permease (透
性酶） (used by microbiologists),
? Channel(离子通道） A channel is a protein
pore through which ions move,-
i,very rapidly ( 106 ions per sec can pass
through a single channel
ii down their free energy gradients,
Facilitated Diffusion
? Three Essential Characteristics:
– Specific (selective for single nutrient molecule or ion)
– Passive (requires no input of energy)
– Saturates (non-linear dependence on concentration)
Driving forces for membrane transport,
concentration differences
Molecules will diffuse until the
concentration is the same everywhere
Active Transport
? Requires input of energy
? For uncharged substances active transport moves
molecules against their concentration gradients
? Enables cells to accumulate molecules to higher
concentrations than in the extracellular fluid.
ATP
ATP
ATP
Driving forces for membrane transport,
metabolic energy
? For charged substances (ions),active transport
moves molecules against concentration + electrical
gradients
Active Transport
Types of nutrient molecules
Example
Neutral solute glucose,urea,
boratic acid
Cation NH4+,H+
Divalent cation Ca2+,Zn2+
Anion NO3-,Cl-
Divalent anion SO42-
K+
Cl-
Zn2+
Mn2+
Mg2+
NO3-
NH4+ PO4-Ni2+
Cu2+
SO42-
Fe3+Ca2+
Fe2
+
B
OH
OH OH
All essential mineral nutrients are
absorbed as ions except boron
Therefore all nutrients except Boron need membrane transporters
Voltages across membranes
? All cells have voltage differences across their membranes
(commonly in the range –50 to – 200 mV)
? This voltage can be used to drive membrane transport
? The voltage is mainly generated by pumping of ions
membrane voltage = membrane electrical potential = PD
V
V
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
++
+ +
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
++
+
+
Driving forces for membrane
transport,VOLTAGE
0 mV-120 mV
Negative membrane voltage will cause:
? Accumulation of cations
? Depletion of anions
Coupled Transport（藕联运输）
The use of a downhill gradient of one molecule to move a second molecule
up a gradient.
Two steps
1,Establish the down gradient.
? Sodium-potassium or proton pump.
2,Use this gradient to drive a molecule up a gradient
Cotransport
-Symport
-Antiport
Both molecules move
in same direction
in opposite directions
Basic principles of membrane transport
Uncharged molecules move in response to:
? differences in concentration (chemical gradient)
Charged molecules move in response to:
? differences in concentration (chemical gradient)
? differences in voltage (electrical gradient)
= electro-chemical gradient
What happens when electrical and concentration
differences oppose each other?
The NERNST equation:
E (mV) = RTzF ln CoC
i
x
Simplified version:
E (mV) = 60z x log C0C
i
R = gas constant
(8.31 J K-1 mol-1)
T = oK
z = valence (e.g,+1,-2)
F = Faraday’s constant
96,500 J mol-1
Co = external concentration
Ci = internal concentration
(E = electrical potential difference
or voltage across the membrane)
Nernst Equation
Principles:
A neutral solute will distribute itself so that at
equilibrium,concentrations are equal
(including across membranes)
A charged solute (ion) will distribute itself according
to both concentration differences and electrostatic
Attraction(静电吸引） to surfaces or regions of opposite
charges
The Nernst equation predicts the concentration of ions
at electrochemical equilibrium
Na+ 1 mM 10 mM
-60 mV
Predicting direction of passive movement
E (mV) = 60z x log C0C
i
Cl- 1 mM 10 mM (observed)
0.1 mM (predicted)
? Higinbotham and his co-workers made
use of this Nernst relationship in
1967 to evaluate the distribution of
cations and anions in media containing
1 mM concentrations of various ions
using pea and oat roots,
Nernst Equation
Nernst Equation
? They first measured the
concentration of ions in the roots and
then measured the electrical
potential difference,Then they
predicted the concentration of ions
in the root according to the Nernst
(equilibrium) relationship for each ion,
Distribution of ions in roots grown in nutrient medium
external
concentration
(mM)
cytoplasmic
concentration
(mM)
accumulation
ratio
K+ 0.5 90 180x
Na+ 1 10 10x
Ca2+ 1 0.1 0.1x
Mg2+ 0.5 2 4x
Cl- 1 10 10x
PO4- 0.1 10 100x
NO3- 1 5 5x
pH 5.5 7.5 (0.01)
total osmotic solute (approx.)
20 300 15x
? They concluded that all the anions
were actively maintained within the
root and only K+ was at equilibrium
in pea roots (not oat),Most cations
were at much lower concentrations
than predicted,All of the anions
were at much higher concentrations
than predicted.
Nernst Equation
Distribution of ions in roots grown in nutrient medium
external
concentration
(mM)
cytoplasmic
concentration
(mM)
Predicted concentration
at –120 mV
K+ 0.5 90 50
Na+ 1 10 100
Ca2+ 1 0.1 10,000
Mg2+ 0.5 2 5,000
Cl- 1 10 0.01
PO4- 0.1 10 0.001
NO3- 1 5 0.1
pH 5.5 7.5 3.3
active uptake
active uptake
active uptake
active uptake
active efflux
active efflux
active efflux
active efflux
Energetic considerations - Cations
Mn2+
Mn2+Cu2+
Zn2+
Co2+Cd2+
X2+ -120 mV
X2+
ATP
Cations are attracted into the cell by the negative membrane PD.
Uptake of nutrient cations therefore does not generally require
energy.
Efflux of cations from the cell usually requires energy (i.e,active
transport)
Energetic considerations - Anions
Anions are repelled from the cell by the negative membrane PD.
Uptake of nutrient anions therefore generally requires energy,in most
cases provided by the electrochemical gradient for H+ (H+-cotransport)
Efflux of anions from the cell usually does not requires energy and
may occur through ion channels.
H+ -120 mV
ATP
H+
H+
NO3-
PO4-
Cl-
SO4-
Cl-
Passive transport (facilitated diffusion)
= movement in direction of electrochemical gradient
Active transport (requires energy)
= movement against electrochemical gradient
Two mechanisms
1,ATPase
2,Co-transport
Active and Passive transport
Two types of
active transport
1,ATPase (e.g,H+-ATPase,Ca2+-ATPase)
2,Co-transport
The proton ATPase (H+-ATPase)
cytoplasm ext,medium
ATP
ADP + Pi
H+
? regulates intracellular pH
? generates membrane PD
- drives co-transport
? acidifies external medium
+ ++ + + ++ +
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ + + + +
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
l l l
l l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l l
l l
l l l l
l l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l
l l
l l
l
A TP
A D P +P i
H
+H
+
2H
+
2H
+
- 1 0 0 m V
Ca
2+
NO
-
3
NO
-
3
Zn
2+
1
2
3
4
4 types of transporter
1,ATP-driven pumps (active)
2,Co-transporters (active/passive)
3,Channels (passive)
4,Carriers (passive)
M
2 +
( C a )
2 +
K
+
K
+
M
2 +
( K OR C )
( K IR C )
( D A C C )
( V IC C )
( H A C C )
( IR T 1 )
( Z IP - n )
Fe
2 +
Zn
2 +
Fe
2 +
F e - P S
3 +
( M n )
2 +
( M n )
2 +
( Z n )
2 +
( C u )
2 +
nH
+
nH
+
nH
+
nH
+
nH
+
nH
+
( N r a m p - n )
P O ( M o O )
4 4
- -
S O ( M o O )
4 4
2 - -
C l ( N O )
- -
3
N O ( C l )
3
- -
M o O
4
-
Cl
-
Cl
-
N H
4
+
N H
4
+
A T P
A T P
H
+
H B O
3 3
H B O
3 3
NO
3
-
1
2
3
4
5
6
7
8
10
11 12
13
14
15
16
17
18
Ca
2 +
9
19
2021
A
-
A T P a se s
C a tion c h a n n e ls
A n ion cha n n e ls
U n ipo r te r s
C a tion c o tr a n sp o r te r s
A n ion cotr a n spo r te r s
Putative
plasma membrane
transporters
Substrate concentration (mM)
Velocity of
reaction (v)
0 2 4 6 8 10 12
0
20
40
60
μmol product
mg protein x h
Vmax
Km = [S] when v = ? Vmax
= 1 mM
Most transporters display enzyme-like properties and can be
described by their affinity for the transport substrate (Km),
and the maximum rate (Vmax)
Many high affinity transporters are induced by starvation
Nutrient induced by,increase
phosphate starvation 4x (wheat,barley)
sulphate starvation 500x (Lemna)
nitrate **nitrate 3x (spruce)
Inducible（诱导的） /repressible（可抑制的） high affinity
systems for
all macronutrients except Ca and Mg
No evidence for micronutrient cations (Zn,Mn,Cu,Fe,Ni)
0 1 2 3 4 5
40
30
20
10
0
T ra ns po rt
ra t e
( nm ol / g,h)
s t a r v a t i on ( d a y s )
P i = 1 0 礛
0 1 2 3 4 5
160
120
80
40
0
s t a r v a t i on ( d a y s )
P i = 1 0 0 礛
2 transport systems:
1,high affinity system
induced by starvation
2,low affinity system
little response to starvation
Induction of high affinity phosphate transport by starvation
(plants starved of P,then capacity for uptake of P measured at low and high concentrations)
(note different scales)
Inducible Constitutive (always there)
Nutrient uptake – basic considerations
? The cell membrane is a barrier to uptake of most nutrients
? To increase uptake,the cell synthesises membrane transporters
? Synthesis of transporters responds to nutrient deficiency & toxicity
? Nutrient transporters behave like enzymes
? Transport can be driven by
(a) concentration and electrical gradients
(passive transport)
(b) metabolic energy (active transport)
