Importance of Ion Transport Processes
Hundreds of millions of years ago, when the first cell
emerged from the primordial ooze, it was enclosed by
a membrane that was impermeable to many of the
substances needed to sustain life. To overcome this
obstacle, the cell had to invent transporters, exchangers,
pumps and ion channels. An early trick developed by
the first cells was to exploit the ion gradients created
by proton pumps as batteries of potential energy.
Mutations in ion transport proteins are associated
with disease. Defects in other properties of ion transport
can cause ion leakage, with equally devastating effects.
Membrane Transport
Passive Transport
movement of molecule down its
electro-chemical gradient
Active Transport
movement of molecule against its
electro-chemical gradient
Richard Horn, Nature (2007) 446: 32
Figure 6.7 Three classes of membrane transport proteins:
channels, carriers, and pumps
Membrane Transport Proteins
Passive transport
Channels
- trans-membrane proteins that function as selective pores
- pore opening may be controlled by a ‘gate’
- transport is passive as long as the ‘gate’ is open
Carriers
- binding of molecule to be transported results in a
conformational change in membrane protein
which exposes the molecule to the other side of
the membrane
- slower transport than channel because of binding ions
and protein carrier conformational change
Membrane Transport Proteins
Active transport
Carriers
- carrier must couple the ‘uphill’ transport of a molecule
with another energy releasing event
(occurs in symports and antiports)
Secondary Active Transport
- couple transport of a molecule to ‘down hill’
transport of another molecule (e.g. H+)
- process is driven by a proton motive force created
by a separate proton pump
Primary Active Transport (proton pump, Ca+2 pump)
- process is coupled directly to ATP hydrolysis
Secondary Active Transport
- couple transport of a molecule to ‘down hill’ transport
of another molecule (e.g. H+)
- process is driven by a proton motive force created
by a separate proton pump
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Plant Proton Pump - (Cell Membrane type)
Various transport proteins in plasma membrane and tonoplast
Two examples of secondary active transport coupled to a primary proton gradient
Hypothetical model of secondary active transport (SYMPORT)
Chemical Potential of Ions in Cells
•  - chemical potential of ion of interest
• * - chemical potential under standard conditions
• solute concentration component: RT ln[ion]
or 2.3 RT log[ion]
• electrical potential component: zFE
• can ignore gravity effects and pressure effects for ions
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Factors that alter Chemical Potential of an Ion
from that of standard conditions
• Concentration of ion in cell
• Electrical charge of the ion
Factors included in the Nernst Equation:
E 
2.3RT  C out 
log

zF  C in 

After development of the theory for Ion Transport
• Researchers wanted to test the theory (Nernst equation) and make
measurements of:
(i) membrane potential (electrical difference across membrane)
(ii) concentrations of ions inside cells under controlled conditions
Initial experiments used giant algal cells in glass vessels
Figure 6.3 Diagram of a pair of microelectrodes used to measure membrane potentials
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Early experiments showed the following patterns
Questions generated from the early experiments
(i) Membrane potential (E) = -110 mV
(ii) K+ was shown to have exactly the concentration as
was predicted by the Nernst equation
(i) What was the precise cause of the magnitude of
the membrane potential?
(ii) Do all nutrient ions contribute significantly to the
membrane potential?
(iii) All other cations (+ charge) had lower concentrations
inside the cell than was predicted by the Nernst equation
It was realized that only three ions (K+, Na+, Cl-) had high
enough concentrations to influence the membrane potential
(iv) All anions (- charge) had higher concentrations
inside the cell than was predicted by the Nernst equation
Modified Nernst equation > Goldman equation considers the
influence of K+, Na+, Cl- simultaneously
Since most ion concentrations were displaced from equilibrium,
this implied that most ions were actively transported against
chemical potential gradients:
anions into cells
cations out of cells
Calculations using the Goldman equation consistently produced
values of E = -50 to -80 mV
Measurements consistently showed
values of E = -100 to -200 mV > existence of proton
pump predicted
Prediction of the membrane proton pump was made before
its existence was experimentally confirmed
• cyanide poisoning causing changes to the cell membrane
potential was consistent with proton pump functioning
• subsequently the H+ pump proteins were isolated and their
functional attributes were described and studied
• membrane proton pumps and secondary active transport
using symports and antiports (or gated channels) play
major roles in ion uptake (and ion extrusion) and mineral
nutrition along with many other growth & development
processes in plants
Figure 6.5 Plasma membrane potential of a pea cell collapses when cyanide is added
Nature (2007)
450: 1111
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