Chapter 6: Solute Transport p125 Membrane Transport

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Chapter 6 Solute Transport p125Membrane
Transport

• nature of plant cell membranes
• lipid bi-layer that separates cell contents from
  the aqueous surrounding environment 6.7
• self-assembling bilayer orients fatty acids to
  fatty acids and polar glycerol and
  phosphoglyderol to aqueous environments
• functions of cell membranes
• maintain relatively constant milieu inside cell
• control entry of ions and other solutes
• exclude external environment and export foreign
  substances
• separate various cell compartments
• maintain compartmentalization of cell functions
  within each mitochondria, chloroplasts,
  vacuoles, golgi and endplasmic reticulum

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Membrane Transport

• two major factors in membrane transport
• permeability- property of ease of passage of a
  species through a membrane
• permeability promoted by membrane transport
  proteins 6.6
• motive force- the force that moves molecules or
  ions through the membrane
• transport of one species is coupled to chemical
  potential of another
• properties correspond to conductivity and driving
  force that comprise the factors in equations for
  flow across a membrane
• membrane proteins are involved in both 6.7
• channels- selective pores enhance diffusion
  permeability
• pumps- transporter for primary active transport,
  transport that is directly connected to energy
  source ATP or light
• transporter- for secondary active transport
  coupled to transport of another species such as
  protons

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6-3,
aquaporin
ion specific carriers
permeability enhanced by membrane proteins

• Figure 6.6 Membrane permeability for substances
  diffusing across

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• Figure 6.7 Three classes of membrane transport
  proteins

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Passive and Active Transport

• transport- movement of ions, molecules between
  the various compartments in biological systems
• passive transport- net movement of solute
  molecules by diffusion down a its electrochemical
  potential gradient
• no transport occurs at equilibrium i.e. there is
  no net movement of a species without a driving
  force being supplied
• active transport- net movement of a specific
  substance against its own electrochemical
  potential gradient
• transport is driven by a driving force supplied
  in some manner
• species can be changed in the course of or after
  transport
• transport by coupling the transport of one
  species down its gradient to the movement of
  another species against its gradient

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Energy and Transport

• chemical potential- potential energy source that
  moves a substance against a gradient
• sum of potential energy, generally of three
  components
• ?j ?j RT lnCj zj FE
  Vj P (eq. 6.1)
• solute concentration Cj conditioned by the
  absolute temperature T that moves the solute
• electrostatic charge zj conditioned by
  electrical potential E that moves the charged
  particles
• electrochemical potential involves both C and z
• hydrostatic pressure P that moves the solute
  particles
• R, F and Vj are constants for the respective
  terms
• ?j is a way of handling the value of standard
  water potential, which is not able to be
  calculated

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Energy and Transport

• when each of the three applies to transport
• P for osmotic water movement, bulk flow
• Cj and T for membrane transport of non-ionic
  solute species
• E and zj for membrane transport of ionic
  species
• ?? j for membrane transport of species between
  solutions inside and outside ?? j (? ji
  - ? jo )
• uncharged charged ions
• ?? j RT ln Ci ?? j RT ln Ci
  zF(Ei - Eo)
• Co
  Co
• for the first equation, transport is due to ?
  concentration
• for the second equation, transport is due to the
  resultant of the sum of ? concentration and ?
  charge
• in each case, the sign gives the direction of net
  movement

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Energy and Transport

• passive and active transport are compared in
  6.1
• note the sign of the inequality
• (1) ? jA gt ? jB ?? j ? jA - ? jB ()
  spontaneous A to B
• (2) ? jA ? jB ?? j ? jA - ? jB (0)
  at equilibrium, A , B
• (3) ? jA lt ? jB ?? j ? jA - ? jB (-)
  spontaneous B to A
• passive transport diffusion moves solute
  molecules spontaneously down the potential energy
  gradient
• (1) ? jA gt ? jB spontaneous A to B
• active transport transport of solute against a
  chemical potential gradient
• (1) ? jA gt ? jB active transport needed to go
  from B to A
• (2) ? jA ? jB active transport needed to go
  either way

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• Figure 6.1 Relationship between chemical
  potential transport

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Membrane Potential

• concentration gradients of ions across membranes
  generate electrical potential across the membrane
• typically the inside of the cell is more negative
  than the external solution
• proton transport is the main determinant of
  membrane potential
• a charged solute is at equilibrium of takes into
  account both the charge gradient and the chemical
  concentration gradient
• at equilibrium the difference in concentration is
  balanced by the difference in voltage, the
  Nernst potential
• two sources of force to tranport species across
  the membrane
• Nernst potential is given by the Nernst equation
• ?En (Ei - Eo)
  RT ln Co
• 
  zjF Ci
• potential difference and ion
  concentration gradient across the membrane
  balance each other

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Table 6.1 Comparison of observed and predicted
ion concentrations in pea root tissue based on
the observed membrane potential.In which cases
does observed predicted? Why or why not? see
6.4

• __________________________________________________
  ____________________
• Concentration
• in external medium Internal Concentration
  (m mol L-1)__
• Ion (m mol L-1)
  Predicted Observed
• __________________________________________________
  ____________________
• K 1 74 75
• Na 1 74 8
• Mg2 0.25 1,340 3
• Ca2 1 5,360 2
• NO3- 2 0.0272 28
• Cl- 1 0.0136 7
• H2PO3- 1 0.0136 21
• SO42- 0.25 0.00005 19
• __________________________________________________
  ____________________________
• Data from Higinbotham et al. 1967
• Note membrane potential was mesured as 110 mV

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Membrane Potential

• in T6.1 note the differences between internal
  concentration predicted from the Nernst equation
  and the observed concentration
• those at predicted levels are at equilibrium
• only K is at equilibrium
• anions NO3-, Cl-, H2PO3-, SO42- are higher than
  predicted showing that they enter by active
  transport
• cations Na, Ca2, Mg2 enter by diffusion down
  their electrochemical gradient but are actively
  extruded
• T6.1 is oversimplified since cell has several
  compartments

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contributions of ions to membrane potential

• Figure 6.4 Ion concentrations transport.
  Figure 6.5 Proton potential

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a voltage step triggers the selective flow of
ions of a given species as determined by the
selective filter

• Figure 6.8 Model of a voltage-gated K channel

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• Figure 6.9 secondary active transport of sucrose
  against its concentration gradient driven by
  transport of protons down their concentration
  gradient

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night
night
day
Photosynthetic carbon reduction cycle
chloroplast transporters that supply carbon for
synthesis of sucrose for export during day and
night
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protein loops that regulate transport of glucose
through the inner chloroplast envelope
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