Membrane

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• Lecture 9
• Membrane
• transport
• processes

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Protein-mediated transport can be active or
passive
chemical potential for a given
solute j
Requires energy (hydrolysis of ATP)
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Electrochemical gradient
Electrochemical gradient exists when there is a
concentration and an electrical gradient.
Concentration Gradient exists when there is
unequal concentration across a membrane. 
Electrical Gradient results when there is
unequal distribution of charges across a
membrane. When an anion or a cation moves in one
direction, and there is no accompanying flux of
another ion that neutralizes the charge.
NERNST EQUATION PREDICTS THE DISTRIBUTION OF IONS
AT EQUILIBRIUM
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Passive and active transporters
Passive transport Active transport
In general Cation uptake passive Cation efflux
active Anion uptake active Anion efflux passive
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Channels, Carriers and Pumps mediate the
transport of solutes across membranes
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Channel transporters enhance diffusion across
membranes passive transport
Example K channel, Ca2 channels, anion
channels (Cl_, malate2-, NO3_) K enters
cells via gated channels
One subunit
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Pumps carry out PRIMARY ACTIVE TRANSPORT
- Primary active transport is coupled to a source
of energy primarily ATP hydrolysis - Most
pumps transport H or Ca2 or large organic
molecules (anthocyanins, flavonoids ABC
(ATP-binding cassette) transporters) -
Plasma membrane H-ATPase generates the gradient
of electrochemical potential of H across the
membrane - Vacuolar H-ATPase and
H-pyrophosphatase (H-PPase) pump protons
into the lumen of the vacuole and Golgi
cisternae
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H-Pumps generate a proton electrochemical
gradient
Extrusion of protons from the cytosol ? creation
of a membrane potential and pH gradient This
gradient of electro- chemical potential of H is
the proton motive force PMF stored free
energy in form of the H gradient
PM H-ATPase Vacuolar H-ATPase Vacuolar H-PPase

Cytoplasm
H
pH 5.5
H
Vacuole
pH 7.3
pH 5.5
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Proton transport is a major determinant of the
membrane potential
Membrane potential of a pea cell collapses when
CN_ is added to the external solution
Energy required for active transport through ATP
hydrolysis Study effect of CN_ on membrane
potential CN_ poisons mito-chondria, blocks ATP
production
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Model of a H-ATPase at work
? regulation of cytoplasmatic pH ? control of
cell turgor, driving organ movement,
stomatal opening, cell growth ? expressed in
guard cells, where they energize the plasma
membrane to drive solute uptake during
stomatal opening
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Two-dimensional representation of the PM
H-ATPase from yeast
10 transmembrane domains 100 kDa Arabidopsis
12 genes
(Autoin-hibitory domain)
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SECONDARY ACTIVE TRANSPORT is mediated by carriers
PMF generated by H transport is used in
secondary active transport to drive the transport
of many other substances against their gradients
of electrochemical potential.
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Two types of secondary active transport
Symport Antiport
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Transport processes on the plasma membrane
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Transport processes on the plasma membrane
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Transport processes on the tonoplast
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Ions are actively taken up into root cells
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Summary

• CONCEPTS
• LIPID BILAYERS ARE IMPERMEABLE TO IONS AND
  CHARGED MOLECULES.
• There are 3 main classes of transport proteins.
  Channels Pumps Carriers
• PROTEINS CATALYSE THE TRANSPORT OF SOLUTES ACROSS
  MEMBRANES, JUST AS ENZYMES CATALYZE CHEMICAL
  REACTIONS (Carriers).     High specificity High
  affinity, low Km     Fast Rate Vmax
• PROTEIN-MEDIATED TRANSPORT CAN BE ACTIVE OR
  PASSIVE. Passive transport is movement down an
  electrical and a concen-tration gradient.Active
  transport is movement against an electrical and
  concen-tration gradient.

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Summary
What is an electrochemical gradient? How is a
gradient formed? Electrochemical gradient exists
when there is a concentration and an electrical
gradient. NERNST EQUATION PREDICTS THE
DISTRIBUTION OF IONS AT EQUILIBRIUM
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Summary

• H PUMPS ARE THE MAJOR ION PUMPS IN PLANTS. They
  generate an electrical (- inside) and pH gradient
  (acid outside).     a) PLASMA MEMBRANE H-ATPase
  EXTRUDE H out of the cell.     b) VACUOLAR
  H-ATPase ACIDIFIES THE VACUOLE     c) VACUOLAR
  H-PPase ALSO ACIDIFIES THE VACUOLE The stored
  energy generated by H pumps is used for active
  transport of other ions and metabolites.
• ACTIVE TRANSPORT OF MANY NUTRIENTS AND
  METABOLITES IS DEPENDENT ON H-COUPLED
  CO-TRANSPORT.     Symport and Antiport
• CHANNELS ALLOW RAPID, PASSIVE TRANSPORT OF IONS,
  METABOLITES AND WATER.         e.g. Light
  stimulated opening of stomatal pore.
•  Water is transported via AQUAPORINS in membranes
  that conduct large volumes of water rapidly.
• Guard cell movement controls stomatal
  aperture.Example of how pumps, carriers,
  channels and water transport work together to
  regulate stomatal aperture (Opening and Closing)